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1 CHEM 430 IR SPECTROSCOPY. I NTRODUCTION The method provides a rapid and simple method for observing the functional group species present in an organic.

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Presentation on theme: "1 CHEM 430 IR SPECTROSCOPY. I NTRODUCTION The method provides a rapid and simple method for observing the functional group species present in an organic."— Presentation transcript:

1 1 CHEM 430 IR SPECTROSCOPY

2 I NTRODUCTION The method provides a rapid and simple method for observing the functional group species present in an organic molecule The spectrum is a plot of the percentage of IR radiation that passes through the sample (% transmission) versus some function of the wavelength of the radiation related to covalent bonding Infrared and Raman Spectroscopy 11-1 CHEM 430 – NMR Spectroscopy2

3 I NTRODUCTION Instrumentation. Modern IR spectrometers are based on the Michelson interferometer Fourier transform infrared ( FT– IR) spectrometers: The absorption spectrum is obtained by means of Fourier transformation of an interferogram. Dispersive infrared spectrometers: Earlier instruments based on monochromators that disperse the radiation from an IR source into its component wavelengths - spectrum is obtained by measuring the amount of radiation absorbed by a sample as the wavelength is varied. Raman spectroscopy provides information complementary to that obtained from IR spectroscopy. Infrared and Raman Spectroscopy 11-1 CHEM 430 – NMR Spectroscopy3

4 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. The quantum mechanical energy levels observed in IR spectroscopy are those of molecular vibration When we say a covalent bond between two atoms is of a certain length, we are citing an average because the bond behaves as if it were a vibrating spring connecting the two atoms For a simple diatomic molecule, this model is easy to visualize: Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy4

5 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. There are two types of bond vibration: 1. Stretch – Vibration or oscillation along the line of the bond 2. Bend – Vibration or oscillation not along the line of the bond Infrared and Raman Spectroscopy H H C H H C scissor asymmetric H H CC H H CC H H CC H H CC symmetric rocktwistwag in planeout of plane

6 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. Each stretching and bending vibration occurs with a characteristic frequency Typically, this frequency is on the order of 1.2 x Hz (120 trillion oscillations per sec. for the H 2 vibration at ~4100 cm -1 ) The corresponding wavelengths are on the order of ,000 nm or 2.5 – 15 microns (  m) When a molecule is bombarded with electromagnetic radiation (photons) that match the frequency of one of these vibrations (IR radiation), it is absorbed and the bonds begin to stretch and bend more strongly (emission and absorption) When this photon is absorbed the amplitude of the vibration is increased NOT the frequency Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy6

7 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. The result of the spectroscopic process is a spectrum of the various stretches and bends of the covalent bonds in an organic molecule Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy7 7

8 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. The x-axis of the IR spectrum is in units of wavenumbers,, which is the number of waves per centimeter in units of cm -1 (Remember E = ħ or E = ħc/ ) This unit is used rather than wavelength (microns) because wavenumbers are directly proportional to the energy of transition being observed – chemists like this, physicists hate it High frequencies and high wavenumbers equate higher energy is quicker to understand than Short wavelengths equate higher energy Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy8 8

9 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. This unit is used rather than frequency as the numbers are more “real” than the exponential units of frequency IR spectra are observed for what is called the mid-infrared: cm -1 The peaks are Gaussian distributions of the average energy of a transition Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy9 9

10 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. So how does the IR detect different bonds? The potential energy stretching or bending vibrations of covalent bonds follow the model of the classic harmonic oscillator (Hooke’s Law) Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy10 Potential Energy (E) Interatomic Distance (y) Remember: E = ½ ky 2 where: y is spring displacement k is spring constant

11 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. Aside: Physically here are the movements we are discussing: Stretching vibration: a typical C-C bond with a bond length of 154 pm, the displacement is averages 10 pm: Bending vibration: For C-C-C bond angle a change of 4° is typical, which corresponds to an average displacement of 10 pm. Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy11 10 pm 154 pm 4o4o 10 pm

12 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. The energy levels for these vibrations are quantized as we are considering quantum mechanical particles Only discrete vibrational energy levels exist: Note there is no energy level below = 0, at any temperature above absolute zero there is always the first vibrational energy level Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy12 Potential Energy (E) Interatomic Distance (r) rotational transitions – (in microwave region) Vibrational transitions,

13 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. However, the application of the classical vibrational model fails apart for two reasons: 1. As two nuclei approach one another through bond vibration, potential energy increases to infinity, as two positive centers begin to repel one another 2. At higher vibrational energy levels, the amplitude of displacement becomes so great, that the overlapping orbitals of the two atoms involved in the bond, no longer interact and the bond dissociates We say that the model is really one of an aharmonic oscillator, for which the simple harmonic oscillator model works well for low energy levels Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy13

14 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. Here is the derivation of Hooke’s Law we will apply for IR theory: Vibrational frequency given by: : frequency K: force constant – bond strength  : reduced mass = m 1 m 2 /(m 1 +m 2 ) Reduced mass is used, as each atom in the covalent bond oscillates about the center of the two masses Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy14

15 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. What does this mean for the different covalent bonds in a molecule? Let’s consider reduced mass, , first: The C-H and C-C single bonds differ by only 16 kcal/mole: 99 kcal · mol -1 vs. 83 kcal · mol -1 (similar K) Due to the reduced mass term, these two bonds of similar strength show up in very different regions of the IR spectrum: C─C 1200 cm -1  = (12 x 12)/( ) = 6(0.41) C─H 3000 cm -1  = (1 x 12)/(1 + 12) = 0.92(0.95) A smaller atom therefore gives rise to a higher wavenumber (and  and E) Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy15

16 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. What does this mean for the different covalent bonds in a molecule? When greater masses are added, the trend is similar (K’s here are different) C─I500 cm -1 C─Br 600 cm -1 C─Cl 750 cm -1 C─O 1100 cm -1 C─C 1200 cm -1 C─H 3000 cm -1 A smaller atom therefore gives rise to a higher wavenumber (and  and E) and a larger atom gives rise to lower wavenumbers (and  and E) Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy16

17 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. What does this mean for the different covalent bonds in a molecule? Let’s consider bond strength, K: A C≡C bond is stronger than a C=C bond is stronger than a C-C bond wavenumber, cm -1  H f From IR spectroscopy we find: C≡C ~ C=C ~ C — C ~ Note the good correlation with the heats of formation for each bond! Stronger bonds give higher wavenumbers (and  higher and E) Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy17

18 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. The y-axis of the IR spectrum is in units of transmittance, T, which is the ratio of the amount of IR radiation transmitted by the sample (I) to the intensity of the incident beam (I 0 ); % Transmittance is T x 100 T = I / I 0 %T = (I / I 0 ) X 100 IR is different than other spectroscopic methods which plot the y-axis as units of absorbance (A). A = log(1/T) As opposed to chromatography or other spectroscopic methods, the area of a IR band (or peak) is not directly proportional vs. concentration of other functionalities, it can be used vs. itself if standardized!!! Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy18

19 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. The intensity of an IR band is affected by two primary factors: Whether the vibration is one of stretching or bending Electronegativity difference of the atoms involved in the bond: For both effects, the greater the change in dipole moment in a given vibration or bend, the larger the peak. The greater the difference in electronegativity between the atoms involved in bonding, the larger the dipole moment Typically, stretching will change dipole moment more than bending Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy19

20 V IBRATIONS OF M OLECULES The IR Spectroscopic Process. It is important to make note of peak intensities to show the effect of these factors: Strong (s) – peak is tall, transmittance is low Medium (m) – peak is mid-height Weak (w) – peak is short, transmittance is high * Broad (br) – if the Gaussian distribution is abnormally broad (* this is more for describing a bond that spans many energies) Exact transmittance values are rarely recorded Infrared and Raman Spectroscopy 11-2 CHEM 430 – NMR Spectroscopy20

21 21 II. Infrared Group Analysis A. General 1.The primary use of the IR spectrometer is to detect functional groups 2.Because the IR looks at the interaction of the EM spectrum with actual bonds, it provides a unique qualitative probe into the functionality of a molecule, as functional groups are merely different configurations of different types of bonds 3.Since most “types” of bonds in covalent molecules have roughly the same energy, i.e., C=C and C=O bonds, C-H and N-H bonds they show up in similar regions of the IR spectrum 4.Remember all organic functional groups are made of multiple bonds and therefore show up as multiple IR bands (peaks) There are 4 principle regions: 4000 cm cm cm cm cm -1 Bonds to H O-H single bond N-H single bond C-H single bond Triple bonds C≡C C≡N Double bonds C=O C=N C=C Single Bonds C-C C-N C-O Fingerprint Region

22 22 We will pick up next time with peak intensities, width of bands and some simple symmetry rules, as well as instrument design Monday we should finally get to functional groups where we will apply in depth the general topics we have discussed in the introductory material No Problem set for today! But take this time to review some organic: - bond strengths – both inter and intra-molecular - bond distances for more organic-y bonds - hybridization models - Periodic table and properties – you should know the position and EN’s of H, B, C, N, O, F, Si, P, S, Cl, Br and I

23 23 IR Spectroscopy I. Introduction F.The IR Spectrum 4.The intensity of an IR band is affected by two primary factors: Whether the vibration is one of stretching or bending Electronegativity difference of the atoms involved in the bond: For both effects, the greater the change in dipole moment in a given vibration or bend, the larger the peak The greater the difference in electronegativity between the atoms involved in bonding, the larger the dipole moment Typically, stretching will change dipole moment more than bending 5.It is important to make note of peak intensities to show the effect of these factors: Strong (s) – peak is tall, transmittance is low Medium (m) – peak is mid-height Weak (w) – peak is short, transmittance is high * Broad (br) – if the Gaussian distribution is abnormally broad (*this is more for describing a bond that spans many energies) Exact transmittance values are rarely recorded

24 24 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies We have learned: That IR radiation can “couple” with the vibration of covalent bonds, where that particular vibration causes a change in dipole moment The IR spectrometer irradiates a sample with a continuum of IR radiation; those photons that can couple with the vibrating bond elevate it to the next higher vibrational energy level (increase in A) When the bond relaxes back to the 0 state, a photon of the same is emitted and detected by the spectrometer; the spectrometer “reports” this information as a spectral band centered at the of the coupling The position of the spectral band is dependent on bond strength and atomic size The intensity of the peak results from the efficiency of the coupling; e.g. vibrations that have a large change in dipole moment create a larger electrical field with which a photon can couple more efficiently

25 25 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies Remember, most interesting molecules are not diatomic, and mechanical or electronic factors in the rest of the structure may effect an IR band From a molecular point of view (discounting phase, temperature or other experimental effects) there are 10 factors that contribute to the position, intensity and appearance of IR bands 1.Symmetry 2.Mechanical Coupling 3.Fermi Resonance 4.Hydrogen Bonding 5.Ring Strain 6.Electronic Effects 7.Constitutional Isomerism 8.Stereoisomerism 9.Conformational Isomerism 10.Tautomerism (Dynamic Isomerism)

26 26 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 1.Symmetry H 2 O For a particular vibration to be IR active there must be a change in dipole moment during the course of the particular vibration For example, the carbonyl vibration causes a large shift in dipole moment, and therefore an intense band on the IR spectrum For a symmetrical acetylene, it is clear that there is no permanent dipole at any point in the vibration of the C  C bond. No IR band appears on the spectrum

27 27 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 1.Symmetry H 2 O Most organic molecules are fortunately asymmetric, and bands are observed for most molecular vibration The symmetry problem occurs most often in small, simple symmetric and pseudo-symmetric alkenes and alkynes Since symmetry elements “cancel” the presence of bonds where no dipole is generated, the spectra are greatly simplified

28 28 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 1.Symmetry H 2 O Symmetry also effects the strength of a particular band The symmetry problem occurs most often in small, simple symmetric and pseudo-symmetric alkenes and alkynes Since symmetry elements “cancel” the presence of bonds where no dipole is generated, the spectra are greatly simplified

29 29 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 2.Mechanical Coupling In a multi-atomic molecule, no vibration occurs without affecting the adjoining bonds This induces mixing and redistribution of energy states, yielding new energy levels, one being higher and one lower in frequency Coupling parts must be approximate in E for maximum interaction to occur (i.e. C-C and C-N are similar, C-C and H-N are not) No interaction is observed if coupling parts are separated by more than two bonds Coupling requires that the vibration be of the same symmetry

30 30 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 2.Mechanical Coupling For example, the calculated and observed for most C=C bonds is around 1650 cm -1 Butadiene (where the two C=C systems are separated by a dissimilar C-C bond) the bands are observed at 1640 cm -1 (slight reduction due to resonance, which we will discuss later) In allene however, mechanical coupling of the two C=C systems gives two IR bands – at 1960 and 1070 cm -1 due to mechanical coupling For purposes of this course, when we discuss the group frequencies, we will point out when this occurs

31 31 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 3.Fermi Resonance A Fermi Resonance is a special case of mechanical coupling It is often called an “accidental degeneracy” In understanding this, for many IR bands, there are “overtones” of the fundamental (the ’s you are taught) at twice the wavenumber In a good IR spectrum of a ketone (2-hexanone, here) you will see a C=O stretch at 1715 cm-1 and a small peak at 3430 cm -1 for the overtone overtone fundamental

32 32 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 3.Fermi Resonance Ordinarily, most overtones are so weak as not to be observed But, if the overtone of a particular vibration coincides with the band from another vibration, they can couple and cause a shift in group frequency and introduce extra bands If you first looked at the IR (working “cold”) of benzoyl chloride, you may deduce that there were two dissimilar C=O bonds in the molecule

33 33 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 3.Fermi Resonance In this spectrum, the out of plane bend of the aromatic C-H bonds occurs at 865 cm -1 ; the overtone of this band coincides with the fundamental of C=O at 1730 cm -1 The band is “split” by Fermi resonance (1760 and 1720 cm -1 )

34 34 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 3.Fermi Resonance Again, we will cover instances of this in the discussion of group frequencies, but this occurs often in IR of organics Most observed: -Aldehydes – the overtone of the C-H deformation mode at 1400 cm -1 is always in Fermi resonance with the stretch of the same band at 2800 cm -1 -The N-H stretching mode of –(C=O)-NH- in polyamides (peptides for the biologists and biochemists) appears as two bands at 3300 and 3205 cm -1 as this is in Fermi resonance with the N-H deformation at 1550 cm -1

35 35 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 4.Hydrogen Bonding One of the most common effects in chemistry, and can change the shape and position of IR bands Internal (intramolecular) H-bonding with carbonyl compounds can serve to lower the absorption frequency 1680 cm cm -1

36 36 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 4.Hydrogen Bonding Inter-molecular H-bonding serves to broaden IR bands due to the continuum of bond strengths that result from autoprotolysis Compare the two IR spectra of 1-propanol; the first is an IR of a neat liquid sample, the second is in the gas phase – note the shift and broadening of the –O-H stretching band Neat liquid Gas phase

37 37 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 4.Hydrogen Bonding Some compound, in addition to intermolecular effects for the monomeric species can form dimers and oligomers which are also observed in neat liquid samples Carboxylic acids are the best illustrative example – the broadened O-H stretching band will be observed for the monomer, dimer and oligomer

38 38 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 5.Ring Strain Certain functional group frequencies can be shifted if one of the atoms hybridization is affected by the constraints of bond angle in ring systems Consider the C=O band for the following cycloalkanones: cm -1 We will discuss the specific cases for these shifts during our coverage of group frequencies

39 39 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 6.Electronic Effects - Inductive The presence of a halogen on the  -carbon of a ketone (or electron w/d groups) raises the observed frequency for the  -bond Due to electron w/d the carbon becomes more electron deficient and the  -bond compensates by tightening

40 40 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 6.Electronic Effects - Resonance One of the most often observed effects Contribution of one of the less “good” resonance forms of an unsaturated system causes some loss of p-bond strenght which is seen as a drop in observed frequency

41 41 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 6.Electronic Effects - Resonance In extended conjugated systems, some resonance contributors are “out-of-sync” and do not resonate with a group Example:

42 42 IR Spectroscopy I. Introduction G.The IR Spectrum – Factors that affect group frequencies 6.Electronic Effects - Sterics Consider this example: In this case the presence of the methyl group “misaligns” the conjugated system, and resonance cannot occur as efficiently The effects of induction, resonance and sterics are very case- specific and can yield a great deal of information about the electronic structure of a molecule

43 43 IR Spectroscopy III.Group Frequencies and Analysis A.Introduction 1.When approaching any IR spectrum be sure to use the larger-to-smaller region approach- do not immediately focus on any one single peak (even –OH or C=O) 2.From the Hooke’s Law derivation we are using we find that the IR can be conveniently be divided into four major regions: Bonds to HTriple bondsDouble bondsSingle Bonds O-H N-H C-H C≡CC≡NC≡CC≡N C=O C=N C=C C-C C-N C-O C-X “Fingerprint Region” 4000 cm cm cm cm cm -1

44 44 IR Spectroscopy III.Group Frequencies and Analysis A.Introduction 3.If supporting information is available – molecular formula, chemical inferences – (i.e. this was the product of an oxidation reaction), assume this information is correct and the analysis of the IR should support it (later in your careers you can doubt information given to you) 4.If a molecular formula is available, do an HDI! 5.Many texts list various methods for approaching an IR spectrum; use the method that works best for you and stick to it. 6.The most common mistakes in spectral analysis are those of “jumping the gun” to a conclusion (usually based on some small, insignificant peak) or taking a random haphazard approach to the spectrum (gee, here is an IR, oh, let’s start looking for phosphorus this time) Be methodical, develop a scheme and stick to it!

45 45 IR Spectroscopy III.Group Frequencies and Analysis Before we begin – Each functional group will be described as follows: Group General – What is most recognizable? What makes it different from similar groups? Group Frequencies (cm -1 ): Bond observed in cm -1 type of vibrationExceptions and things to watch Scale on bottom summarizes band positions and strengths Strong -Medium - Weak -

46 46 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkanes General – due to the small electronegativity difference between C and H, hydrocarbon bands are of medium intensity at best and give simple spectra Group Frequencies (cm -1 ): C-H StretchStrained ring systems may have higher -CH 2 -~1465Methylene bend (scissor) -CH 3 ~1375Methyl bend (sym) -(CH 2 ) 4 -~720Rocking motion 4 or more –CH 2 - (long chain band) C-CNot interpretively useful, small weak peaks

47 47 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkanes – Dodecane – C 12 H 26

48 48 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkanes – Cyclopentane – C 5 H 10

49 49 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkanes Additional – If the region is free of interference, the presence of certain alkyl groups can be discerned: H H CC Methylene Methyl Scissor 1465 H H CC H Bend asymm 1450 H H CC H Bend symm 1375 usually overlap gem-dimethyl t -butyl

50 50 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkanes Additional – Example: Compare 2,2-dimethylpentane vs. 2-methylhexane: vs.

51 51 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkenes General – slightly more complex than alkanes; asymmetric C=C is observed as well as the sp 2 -C-H stretch. Still, bands are weak to medium in intensity Group Frequencies (cm -1 ): =C-H Stretch - Diagnostic for unsaturation- may be aromatic as well =C-H Out-of-plane (oop) bend - Can be used to determine degree of substitution C=C Stretch - Can be reduced by resonance - Symmetrical C=C do not absorb - trans- weaker than cis-

52 52 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkenes – 1-octene – C 8 H 16 Note – you still have alkane present!

53 53 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkenes – trans-4-octene – C 8 H 16 Note – absence of C=C band, shouldering of C-H band

54 54 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkenes – cis-2-pentene – C 5 H 10 Note – shouldering of C-H band

55 55 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkenes – cyclopentene – C 5 H 8 Note – increased complexity due to ring vibrations

56 56 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkenes Substitution – The out of plane =C-H bend produces strong bands but interference can come from aromatic rings (similar oop) and C-Cl bonds (~700) monosubstituted cis-1,2 trans-1,2 1,1-disubstitued trisubstituted tetrasubstituted none, with weak C=C

57 57 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkenes Substitution – The monosubstitued band is very reliable; and the variance induced by electronic effects is observed monosubstituted (R-) monosubstitued w/lone pair group (ex. –Cl, -F, -OR) monosubstitued w/conj. group (ex. C=O, C  N) The shifts are similar for 1,1-disubstitued systems overtone usually observed

58 58 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkenes Rings – Incorporation of a double bond endocyclic or exocyclic to a ring may shift the observed band Endocyclic: Ring strain shifts the C=C band to lower (ex. cyclopropene) The adjacent C-C bond couples with the C=C system – if the resulting component vector is along the line of the C=C bond an increase in occurs – this reaches a minima at 90 o for cyclobutene (no net component along C=C bond) and rises again with cyclopropene C=C 1650

59 59 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkenes Rings – Endocyclic: If C=C at a ring fusion, absorption is reduced as if one further carbon was removed from the ring: The presence of additional alkyl groups on the ring dramatically raises C=C C=C 1611 C=C 1656 C=C

60 60 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkenes Rings – Exocyclic: these C=C bonds give an increase in absorption with decreasing ring size: As the angle between the two C-C bonds is reduced – more p character is required (sp = 180°, sp 2 = 120°, sp 3 = 109.5°, “sp >3 ” = <109° The p character of the double bond is reduced, but the stronger  bond is strengthened to a greater degree Think of the allene example (“2-membered ring”) as an extreme example C=C

61 61 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkynes General – can be symmetric, psuedo-symmetric or internal – greatly reducing the number of observed bands Group Frequencies (cm -1 ):  C-H ~3300Stretch - Diagnostic for terminal alkyne CCCC ~2150Stretch - Can be reduced by resonance -Symmetrical and psuedo-sym. C  C do not absorb  C-H Bend (Text does not list) Possible not to observe any bands for the C  C system

62 62 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkynes – 1-hexyne – C 6 H 10 Nice terminal, asymmetric, well behaved alkyne

63 63 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkynes – 3-hexyne – C 6 H 10 A not-so-nice, internal, symmetrical alkyne

64 64 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Alkynes – 1-hexyne – C 6 H 10 Nice terminal, asymmetric, well behaved alkyne

65 65 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Mononuclear aromatic rings General – not true alkenes; most of the small bands associated with them are not of diagnostic value; electronic effects of a single group on the ring can change the observed bands drastically Group Frequencies (cm -1 ): -C-H StretchAlso for alkenes C-H Out of plane (oop) bendCan be used to determine substitution pattern Overtone and combination bands If observed, similar too oop =C-H Ring stretch – observed as two doublets (1600, 1580, 1500 & 1450) Greatly dependent on substituents

66 66 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Mononuclear aromatic rings – toluene – C 7 H 8 Typical mono-substituted (EDG) ring

67 67 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Mononuclear aromatic rings – o-xylene – C 8 H 10 Typical ortho-substituted (EDG) ring

68 68 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Mononuclear aromatic rings – m-xylene – C 8 H 10 Typical meta-substituted (EDG) ring

69 69 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Mononuclear aromatic rings – p-xylene – C 8 H 10 Typical para-substituted (EDG) ring

70 70 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Mononuclear aromatic rings –  -methylstyrene – C 9 H 10 Conjugated mono-substituted ring

71 71 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Mononuclear aromatic rings Substitution – The aromatic out of plane =C-H bend produces strong bands but interference can come from alkenes (similar oop) and C-Cl (~700) Consider this region to only be reliable for alkyl-, alkoxy-, halo-, amino-, and acetyl substituted rings Interpretation is often unreliable for nitro-, carboxylic- and sulfonic groups The overtone of these bands is the dominant source of the combination and overtone bands observed at

72 72 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Mononuclear aromatic rings Substitution – mono ortho meta para 1,2,4 1,2,3 1,3,5

73 73 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Mononuclear aromatic rings Substitution – The aromatic combination and overtone bands are a set of weak absorptions that occur from This is often obscured by C=O mono ortho meta para 1,2,4 1,2,3 1,3,5 The general shape of the pattern is used for determining substitution pattern; typically only a neat liquid sample gives an intense enough set of bands for analysis

74 74 IR Spectroscopy III.Group Frequencies and Analysis B.The Hydrocarbons Polynuclear and Hetero- aromatic rings General – All bands for these aromatic systems are similar to the mononuclear systems; shifts should be assumed, and analysis would be case-by-case

75 75 IR Spectroscopy III.Group Frequencies and Analysis C.sp 3 Oxygen – Alcohols, phenols and ethers Alcohols General – the best recognized group on carefully selected spectra, but H- bonding effects can drastically change the position, intensity and shape of the O-H band Group Frequencies (cm -1 ): O-H (free) StretchSeen in dilute solution or gas phase spectra O-H (H-bond) StretchThe “classic” H-bonded band, seen in addition to the free band in solution C-O-H BendOften obscured by -CH 3 bend C-O StretchCan be used to determine 1 o, 2 o, 3 o or phenolic structure

76 76 IR Spectroscopy III.Group Frequencies and Analysis C.sp 3 Oxygen – Alcohols, phenols and ethers Alcohols – 1-octanol Neat liquid sample gives classic spectrum

77 77 IR Spectroscopy III.Group Frequencies and Analysis C.sp 3 Oxygen – Alcohols, phenols and ethers Alcohols – 1-octanol Same sample in dilute CCl 4 solution (solvent bands deleted for clarity)

78 78 IR Spectroscopy III.Group Frequencies and Analysis C.sp 3 Oxygen – Alcohols, phenols and ethers Phenols – p-cresol Presence of aromatic bands, sharper -OH

79 79 IR Spectroscopy III.Group Frequencies and Analysis C.sp 3 Oxygen – Alcohols, phenols and ethers Alcohols – Substitution – Using the position of the C-O stretching band, it is possible to suggest a 1 o, 2 o, 3 o or phenolic structure to the alcohol; but these should be considered as base values, that may be changed by the effects of conjugation or an adjacent ring system base value phenol1220 tertiary1150 secondary1100 primary1050 C-O 1070 C-O 1017 C-O 1060 C-O 1030

80 80 IR Spectroscopy III.Group Frequencies and Analysis C.sp 3 Oxygen – Alcohols, phenols and ethers Ethers General – like alkynes, the simplicity of the spectra may allow them to pass unnoticed – deduce from molecular formula if one should be present Group Frequencies (cm -1 ): C-O Stretch (asymm.)Absence of C=O and O-H will confirm it is not ester or alcohol Simple alkyl ethers usually one band at 1120, aryl alkyl ethers give two bands – 1250 & 1040

81 81 IR Spectroscopy III.Group Frequencies and Analysis C.sp 3 Oxygen – Alcohols, phenols and ethers Ethers – diispropyl ether Spectrum dominated by all other functionality

82 82 IR Spectroscopy III.Group Frequencies and Analysis C.sp 3 Oxygen – Alcohols, phenols and ethers Ethers – Additional Types Aryl and vinyl ethers – The effect of conjugation gives the C-O bond a small amount of double bond character, raising the observed Furthermore, strongly asymmetric systems (aryl alkyl and vinyl alkyl ethers) may show an additional weak C-O band for the symmetric stretch at 1040 and 850 respectively

83 83 IR Spectroscopy III.Group Frequencies and Analysis C.sp 3 Oxygen – Alcohols, phenols and ethers Ethers – Additional Types Epoxides – Most important bands are the ring deformation bands at asym and sym Weaker “breathing mode” band is present at Acetals and Ketals – Give four or five unresolved bands in the region

84 84 IR Spectroscopy III.Group Frequencies and Analysis D.sp 3 Nitrogen – Amines Amines – Once presence is determined, the substitution at nitrogen is easy to determine; only the 3 ° amine may present a problem Group Frequencies (cm -1 ): N-H (-NH 2 ) (2 bands) Stretch (sym. and asym.) Bend N-H (-NHR) (1 band) 1500 Stretch Bend For alkyl amines, very weak – for aromatic 2 ° amines, stronger N-H~800Oop bend N-N StretchRemember 3° amines have no N-H bands

85 85 IR Spectroscopy III.Group Frequencies and Analysis D.sp 3 Nitrogen – Amines 1° Amine – tert-butylamine Two band –NH 2 peak appears as small “w”

86 86 IR Spectroscopy III.Group Frequencies and Analysis D.sp 3 Nitrogen – Amines 2° Amine – dibutylamine Note weakness of –NH- band (can be mistaken as C=O overtone, if carbonyl is present)

87 87 IR Spectroscopy III.Group Frequencies and Analysis D.sp 3 Nitrogen – Amines 3° Amine – tributylamine Difficult to discern from alkane – molecular formula for confirmation almost requisite

88 88 IR Spectroscopy III.Group Frequencies and Analysis D.sp 3 Nitrogen – Amines Ammonium Salts Almost certainly never encountered in neat samples, but an important component of amino acids and many pharmaceuticals Group Frequencies N-H Stretch 1° salts are at the higher end of this band, 3° salts at the lower end Additional band sometimes obs. at 2100 N-H Bend1° as two bands (sym. And asymm.), 2° at the upper end of this range, 3° absorbs weakly

89 89 IR Spectroscopy III.Group Frequencies and Analysis D.sp 3 Nitrogen – Amines Ammonium Salts – anilinium hydrochloride Spectrum is of a KBr disc sample:

90 90 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls General – Along with alcohols, the most ubiquitous group on the IR spectrum. Although it is easy to determine if the C=O is present, deducing the exact functionality and factors that influence the position of the band provide the challenge Base C=O Frequencies (cm -1 ): C=O1810Stretch (sym.)Anhydride band Acid Chloride 1760Anhydride band Ester 1725Aldehyde 1715Ketone 1710Carboxylic Acid 1690Amide

91 91 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls General – The carbonyl C=O frequency is very sensitive to the effects we went over previously – a quick recap Electronic Effects: Inductive vs. Resonance: On first inspection, the ester, amide and acid halide/anhydride all possess lone pairs of electrons that can resonate with the C=O (which should lower ) CNOF ClS

92 92 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls General – Electronic Effects: Inductive vs. Resonance: In the case of an oxygen or chlorine being adjacent to the carbonyl, each of these atoms resist the positive charge in the contributing resonance structure, and the inductive effect becomes a stronger factor CNOF ClS This inductive effect draws in  electrons from the C=O, which strengthens the  bond – these carbonyls appear at higher

93 93 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls General – Electronic Effects: Inductive vs. Resonance: In the case of nitrogen, it is less electronegative than oxygen and has a greater acceptance of the positive charge in the contributing resonance structure, so the carbonyl is lowered in CNOF ClS The inductive effect of nitrogen compared to an sp 2 carbon is negligible by comparison

94 94 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls General – Electronic Effects: Inductive vs. Resonance: Likewise in aldehydes and ketones there is the inductive donation of electrons to the  bond of the carbonyl which slightly weakens and reduces the of the  bond (and explains the small difference between aldehydes and ketones) CNOF ClS The inductive effect of nitrogen compared to an sp 2 carbon is negligible by comparison

95 95 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls General – Electronic Effects: Inductive vs. Resonance: In addition, we discussed this effect in regards to  -halogenated carbonyls as one of the effects that can change group CNOF ClS The inductive effect of chlorine will draw  electrons through  -carbon, weakening the C=O  and strengthening the 

96 96 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls General – Electronic Effects - Resonance: Not only is the C=O lowered by the effects of conjugation, the peak may also be broadened or split by the contribution of the two electronic conformers The s-cis absorbs at higher than the s-trans. Why?

97 97 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls General – Ring Strain Effects: C=O groups that can be incorporated into a ring are sensitive to this effect. As ring size decreases more p-character must be used to make the single bonds take on the smaller angle (re: sp >3 = <109°). The  component of the C=O is weakened, but the  -bond strengthened, raising the overall Cyclic ketones, esters (lactones), amides (lactams) and anhydrides exhibit this behavior To clear up confusion – there are two ways to strengthen the C=O 1)Remove  bond character –  bond becomes more stronger (better overlap) – this is a result inductive w/d 2)Remove  bond character –  bond becomes stronger – ring constraint

98 98 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls General – H-bonding effects: C=O groups are reduced in if some of the electron density is tapped off to form H-bonds: This effect can be inter- or intra-molecular:

99 99 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 1.Ketones – Simplest carbonyl group, for a single carbonyl compound, implied by a lack of any other functionality except hydrocarbon Group Frequencies (cm -1 ): C=O1715Stretch (sym.)  Base, sensitive to change conj. w/C=C C=C reduced to conj. w/Ph ring C=O Decreased ring size raises Bend

100 100 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 1.Ketones – 2-hexanone Typical aliphatic ketone

101 101 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 1.Ketones – 4-methylacetophenone Typical aromatic ketone,

102 102 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 1.Ketones – Simplest carbonyl group, for a single carbonyl compound, implied by a lack of any other functionality except hydrocarbon Group Frequencies (cm -1 ): C=O1715Stretch (sym.)  Base, sensitive to change conj. w/C=C C=C reduced to conj. w/Ph ring C=O Decreased ring size raises Bend

103 103 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 2.Aldehydes – Presence of the unique carbonyl C-H bond differentiates this group from ketones Group Frequencies (cm -1 ): C=O1725Stretch (sym.)  Base, sensitive to change conj. w/C=C C=C reduced to 1640 conj. w/Ph ring , 2720 Stretch Fermi doublet; Higher  band often obscured by sp 3 C-H

104 104 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 2.Aldehydes – isovaleraldehyde Typical aliphatic aldehyde – note appearance of Fermi doublet and C=O overtone

105 105 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 2.Aldehydes – anisaldehyde Typical aromatic aldehyde, note how C=O obscures the combination and overtone region – oop region would be used to determine substitution

106 106 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 3.Carboxylic Acids – Various H-bonding effects lead to messy spectra, especially in the upper frequency ranges – be aware of the effects of monomeric, dimeric and oligomeric species on the spectrum Group Frequencies (cm -1 ): C=O1710Stretch (sym.)  Base, sensitive to change; conjugation gives reduced C-O Stretch O-H StretchOverlaps C-H region in most cases; multiple “sub-peaks” can be seen for the dimeric and oligomeric species – simplified in non-polar solution or gas phase spectra

107 107 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 3.Carboxylic Acids – propionic acid Aliphatic carboxylic acid – neat sample vs. CCl 4 solution (right)

108 108 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 3.Carboxylic Acids – o-toluic acid Aromatic carboxylic acid, larger non-polar “end” of the molecule cuts down on the hydrogen bonding seen with the smaller, previous propionic acid

109 109 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 3.Carboxylic Acids - Salts Salts are expressed as possesing one single and one double bond – the true picture is one that is isoelectronic with the nitro group, with two bonds to oxygen with a bond order of 1.5 Group Frequencies: Stretch (asymm.) Stretch (sym.)

110 110 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 3.Carboxylic Acids - Salts – ammonium benzoate Here is an example of ammonium and carboxylate moieties:

111 111 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 3.Carboxylic Acids – Amino Acids – L-alanine Amino Acids combine the features of carboxylate and ammonium salts:

112 112 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 4.Esters – Ester oxygen has an electron withdrawing effect that tends to draw in electrons within the C=O system, strengthening it compared to other carbonyls Group Frequencies (cm -1 ): C=O1735Stretch (sym.)  Base, sensitive to change conj. C=C C=C reduced to w/Ph ring conj. of sp 3 O C=O increases with smaller ring C-O Stretch, 2 bands

113 113 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 4.Esters – methyl butyrate Simple aliphatic ester

114 114 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 4.Esters – methyl m-bromobenzoate Conjugation on the carbonyl end:

115 115 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 4.Esters – phenyl acetate Conjugation on the sp 3 oxygen end:

116 116 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 5.Amides – Amide nitrogen acts as a conjugating group with C=O, reducing double bond character; amide nitrogen appears similar to amime, including the effects of substitution Group Frequencies (cm -1 ): C=O1685Stretch (sym.)  Base, sensitive to change Can be as low as 1630 w/conj. N-H~3300StretchSimilar to amines, but typically more intense C=O increases with smaller ring N-H Bend N-H~800oop bend

117 117 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 5.Amides – pivalamide Primary aliphatic amide

118 118 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 5.Amides – 2-pyrrolidone Cyclic secondary amide - lactam

119 119 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 6.Anhydrides – With acid halides, typically the highest n C=O; appears as two bands for the symmetric and asymmetric stretching modes Group Frequencies (cm -1 ): C=O Stretch (asym.)  Base, sensitive to change conj. C=C Stretch (sym.)Two bands of variable relative intensity C=O increases with smaller ring C-O Stretch, multiple bands

120 120 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 6.Anhydrides – iso-butyric anhydride Typical anhydride

121 121 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 7.Acid Halides – Acid bromides and iodides are not often encountered; acid chlorides are the most prevalent (and useful) Group Frequencies (cm -1 ): C=O Stretch (sym.)  Base, sensitive to change conj. w/Ph add. bandFermi resonance with combination and overtone region of aromatic ring C-Cl StretchIf below 600, not observed using NaCl windows C-Br StretchTypically too low to obs. C-I StretchTypically too low to obs.

122 122 IR Spectroscopy III.Group Frequencies and Analysis E.Carbonyls 7.Acid Halides – propionyl chloride Overtones of low peaks can confuse some spectra

123 123 IR Spectroscopy III.Group Frequencies and Analysis F.sp 2 and sp Nitrogen compounds 1.Nitriles – The “other” triple bond group observed in IR, due to the higher dipole change during the stretching vibration, this band is more intense than C  Cs. Group Frequencies (cm -1 ): CNCN 2250Stretch (sym.)  sensitive to change from conjugation; usually stronger than C  C

124 124 IR Spectroscopy III.Group Frequencies and Analysis F.sp 2 and sp Nitrogen compounds Carbonyls 1.Nitriles – benzonitrile

125 125 IR Spectroscopy III.Group Frequencies and Analysis F.sp 2 and sp Nitrogen compounds 2.Imines and Oximes – Often referred to as “derivatives” of carbonyl compounds, these groups are not often encountered in routine IR obs. Group Frequencies (cm -1 ): C=N (imine and oxime) Stretch (sym.)  sensitive to change from conjugation; usually stronger than C  C O-H (oxime) StretchH-bond effects N-O (oxime) Stretch

126 126 IR Spectroscopy III.Group Frequencies and Analysis F.sp 2 and sp Nitrogen compounds 2.Imines and Oximes – acetone oxime

127 127 IR Spectroscopy III.Group Frequencies and Analysis F.sp 2 and sp Nitrogen compounds 3.Isocyanates and Isothiocyanates – Reactive groups, not often observed in routine qualitative IR Group Frequencies (cm -1 ): N=C=O~2270Stretch (sym.) broad  band – coupled vibration N=C=S~2125Stretch (1 or 2 bands) broad  band – coupled vibration

128 128 IR Spectroscopy III.Group Frequencies and Analysis F.sp 2 and sp Nitrogen compounds 3.Isocyanates and Isothiocyanates– tert-butyl isocyanate

129 129 IR Spectroscopy III.Group Frequencies and Analysis F.sp 2 and sp Nitrogen compounds 4.Nitro – Useful, easily incorporated group on aromatic rings, less often encountered on alkyl compounds Group Frequencies (cm -1 ): Stretch (asymm.) Stretch (sym.) Aliphatic nitro Stretch (asymm.) Stretch (sym.) Aromatic nitro

130 130 IR Spectroscopy III.Group Frequencies and Analysis F.sp 2 and sp Nitrogen compounds 4.Nitro – o-nitrotoluene

131 131 IR Spectroscopy III.Group Frequencies and Analysis G.Sulfur 1.Thiols and Sulfides – Sulfur, due to its large size shifts most observed IR bands to lower frequencies – often out of the observed region Group Frequencies (cm -1 ): S-H (thiol) 2550Stretch (sym.)Unique region of IR spectrum C-S-CNo useful information

132 132 IR Spectroscopy III.Group Frequencies and Analysis G.Sulfur 1.Thiol (Mercaptan) – 1,2-ethanethiol

133 133 IR Spectroscopy III.Group Frequencies and Analysis G.Sulfur 2.Sulfoxides and Sulfones – Oxidized sulfur, the S  O bonds are useful for determining oxidation state, if the presence of sulfur is known Group Frequencies (cm -1 ): SOSO1050Stretch (sym.) OSOOSO~1375 ~1150 Stretch (asymm.) Stretch (sym.)

134 134 IR Spectroscopy III.Group Frequencies and Analysis G.Sulfur 2.Sulfoxide (Mercaptan) – di-butyl sulfoxide Be wary of water in the spectrum of sulfoxides

135 135 IR Spectroscopy III.Group Frequencies and Analysis G.Sulfur 2.Sulfone– di-butyl sulfone

136 136 IR Spectroscopy III.Group Frequencies and Analysis G.Sulfur 3.Sulfonic Acids, Sulfonamides and Sulfonates – Sulfur equivalent of the carboxylic acid derivatives; the O or N groups act as we have observed Group Frequencies (cm -1 ): OSOOSO~1375 ~1150 Stretch (asymm.) Stretch (sym.) As for sulfones – groups bound to sulfur identify the group, just as with the carboxylic acid derivatives differ from ketones S-O (acid & sulfonate ) StretchMay appear as several bands O-H & N-H As for the carboxylic acid derivatives

137 137 IR Spectroscopy III.Group Frequencies and Analysis G.Sulfur 3.Sulfonamides – p-toluenesulfonamide

138 138 IR Spectroscopy III.Group Frequencies and Analysis H.Phosphorus 1.Phosphines – Phosphorus in its lowest oxidation state – many bands that overlap with other useful regions; exercise caution in interpretation using IR Group Frequencies (cm -1 ): P-H Stretch (sym.) Bend PH Bend, two bands P-CH Bend, two bands P-CH Bend

139 139 IR Spectroscopy III.Group Frequencies and Analysis H.Phosphorus 1.Phosphines – tri-butylphosphine

140 140 IR Spectroscopy III.Group Frequencies and Analysis H.Phosphorus 2.Phosphine Oxides – More common to observe these phosphorus compounds Group Frequencies (cm -1 ): 3.Phosphate Esters, Acids and Amides – Often encountered in biological systems Group Frequencies (cm -1 ): POPO Stretch (sym.) POPO Stretch (sym.) R-O Stretch, 1 or 2 band P-O Stretch

141 141 IR Spectroscopy III.Group Frequencies and Analysis H.Phosphorus 2.Phosphine Oxides, Phosphate Esters – tri-butylphosphate

142 142 IR Spectroscopy III.Group Frequencies and Analysis I.Halogens 1.Fluorides and Chlorides – Smaller halogen bonds to carbon in observed frequency range Group Frequencies (cm -1 ): Bromides and Iodides – Not often obs. Due to low of C-X stretch Group Frequencies (cm -1 ): C-F Stretch (sym.) Monofluoroalkyl at lower Polyfluoroalkyl at upper Aryl fluorides up to 1450 C-Cl StretchDifferent conformers may give split peaks C-Br StretchBend is obs. at ~1200 C-I StretchBend is obs. at ~1150

143 143 IR Spectroscopy II.Instrumentation and Experimental Aspects A.The IR Spectrometer – Dispersive and Fourier Transform 1.Dispersive IR Spectrometers All spectrometers consist of four basic parts that are coupled with all four parts of the spectroscopic process - irradiation, absorption-excitation, re-emission-relaxation and detection. Irradiation: Spectrometer needs to generate photons h h h Detection-reemission : Spectrometer needs to detect the photons emitted by the sample and ascertain their energy Energy Absorption- Excitation: Spectrometer needs to contain the sample h Relaxation rest state excited state

144 144 IR Spectroscopy II.Instrumentation and Experimental Aspects A.The IR Spectrometer – Dispersive and Fourier Transform 1.Dispersive IR Spectrometers Those four parts are: 1.Source/Monochromator 2.Sample cell 3.Detector/Amplifier 4.Output Dispersive IR spectrometers were the first IR instruments, however their simplicity and longevity allows them to continue in service – for most routine organic analyses their speed and resolution is adequate For the most part, their design is austere and relies on simple mechanics and optics to generate a spectrum, very similar to simply rotating a glass prism to see different bands of visible light

145 145 IR Spectroscopy II.Instrumentation and Experimental Aspects A.The IR Spectrometer – Dispersive and Fourier Transform 1.Dispersive IR Spectrometers Here is a general schematic:

146 146 IR Spectroscopy II.Instrumentation and Experimental Aspects A.The IR Spectrometer – Dispersive and Fourier Transform 1.Dispersive IR Spectrometers Source is a heated nichrome wire which produces a broad band continuum of IR light (as heat) The beam is directed through both the sample and a reference cell A rapidly rotating sector (beam chopper) continuously switches between directing the two beams to a diffraction grating

147 147 IR Spectroscopy II.Instrumentation and Experimental Aspects A.The IR Spectrometer – Dispersive and Fourier Transform 1.Dispersive IR Spectrometers The diffraction grating slowly rotates, such that only one narrow frequency band of IR light is at the proper angle to reach the detector A simple circuit compares the light from the sample and reference and sends the difference to a chart recorder

148 148 IR Spectroscopy II.Instrumentation and Experimental Aspects A.The IR Spectrometer – Dispersive and Fourier Transform 1.Dispersive IR Spectrometers On the older instruments the motor in the chart recorder was synchronized (& calibrated) to the motor on the diffraction grating Because each spectrum is the result of the tabulation of the spectroscopic process at each frequency individually, it is said to record the spectrum in the frequency domain

149 149 IR Spectroscopy II.Instrumentation and Experimental Aspects A.The IR Spectrometer – Dispersive and Fourier Transform 1.Dispersive IR Spectrometers Advantages – simple, easy to maintain – last the life of the source and moving parts Disadvantages – to cover the entire IR band of interest to chemists it is necessary to use two diffraction gratings At high , the component frequencies are more spread out, so the resulting spectra appear to have various regions expanded or compressed The limit to resolution is 2-4 cm -1

150 150 IR Spectroscopy II.Instrumentation and Experimental Aspects A.The IR Spectrometer – Dispersive and Fourier Transform 2.Fourier Transform IR Spectrometers FT-IR is the modern state of the art for IR spectroscopy The system is based on the Michelson interferometer oLaser source IR light is separated by a beam splitter, one component going to a fixed mirror, the other to a moving one and are reflected back to the beam splitter oThe beam splitter recombines the two to a pattern of constructive and destructive interferences known as an interferogram – a complex signal, but contains all of the frequencies that make up the IR spectrum

151 151 IR Spectroscopy II.Instrumentation and Experimental Aspects A.The IR Spectrometer – Dispersive and Fourier Transform 2.Fourier Transform IR Spectrometers The resulting signal is essentially a plot of intensity vs. time Such information if plotted would look like the following: This is meaningless to a chemist – we need this to be in the frequency domain rather than time….

152 152 IR Spectroscopy II.Instrumentation and Experimental Aspects A.The IR Spectrometer – Dispersive and Fourier Transform 2.Fourier Transform IR Spectrometers By applying a mathematical transform on the signal – a Fourier transform – the resulting frequency domain spectrum can be observed FT-IRs give three theoretical advantages: 1.Fellgett’s advantage – every point in the interferogram is information – all wavelenghts are represented 2.Jacquinot’s advantage – the entire energy of the source is used – increasing signal-to-noise 3.Conne’s advantage – frequency precision – Dispersive instruments can have errors in the ability to move slits and gratings reproducibly – FTIR is internally referenced from its own beam

153 153 IR Spectroscopy II.Instrumentation and Experimental Aspects A.The IR Spectrometer – Dispersive and Fourier Transform 2.Fourier Transform IR Spectrometers Justik’s advantage – does it give me what I need 1.Single-beam instrument – collect a background (air has IR active molecules!) 2.Fast – all frequencies are scanned simultaneously 3.No referencing! 4.Computer based – scaling and editing of the spectrum to squeeze out the most data; spectra are proportional (no stretching or squeezing of regions), comparison with spectral libraries Disadvantages – expenisve relative to dispersive instruments, and the components take more expertise and service calls to replace

154 154 IR Spectroscopy II.Instrumentation and Experimental Aspects B.The IR Spectrometer – Experimental aspects 1.Sample size – typically the size of the beam – mm’s  mg’s 2.Non-destructive – sample can be recovered with varying degrees of difficulty 3.Liquid samples – the easiest IR spectra are those of “neat” liquid samples Solid samples are too dense for good IR spectra – inter-molecular coupling of vibrational states occurs and peaks are greatly broadened In the liquid state full 3-D motion is available, and these effects are averaged out and diminished The thickness of a sample can be decreased to reduce these effects further  Thin film liquid samples are best!

155 155 IR Spectroscopy II.Instrumentation and Experimental Aspects B.The IR Spectrometer – Experimental aspects 3.Liquid samples Sample cell cannot possess covalent bonds (SiO 2, or glass is out) The most common cell is a pair of large transparent “windows” of inorganic salts Most common: NaCl – cheap, transparent from 650 – 4000 cm -1, but fragile Less common – AgCl, KBr, etc. – if you need transparency below 650, limit is practically 400

156 156 IR Spectroscopy II.Instrumentation and Experimental Aspects B.The IR Spectrometer – Experimental aspects 4.Solution samples One way solids can be handled is as a solution Key is that the solvent picked will cover the least amount of the spectrum as possible, as it will also be present Common solvents typically are symmetrical, or have many halogenated bonds – low cm -1 : CCl 4, CHCl 3, CH 2 Cl 2, etc. The cell in this case is two NaCl (or other) windows with a spacer, the sample is loaded via a syringe into the cell:

157 157 IR Spectroscopy II.Instrumentation and Experimental Aspects B.The IR Spectrometer – Experimental aspects 4.Solution samples A newer method involves the use of a polyethylene matrix, that will hold allow a solution sample to evaporate, leaving small portions of the sample embedded in the matrix The samples are “liquid-like” The only interference is that of hydrocarbon

158 158 IR Spectroscopy II.Instrumentation and Experimental Aspects B.The IR Spectrometer – Experimental aspects 5.Solid Samples The most common treatment for solid samples is to “mull” them with thick mineral oil (high MW hydrocarbon) - Nujol® Just like with the polyethylene cards, the molecules of the sample are held in suspension within the oil matrix Again, the interference is that of hydrocarbon

159 159 IR Spectroscopy II.Instrumentation and Experimental Aspects B.The IR Spectrometer – Experimental aspects 5.Solid Samples The connoisseurs method (with no organic interference) is to press the solid with KBr into a pellet Under high pressure the KBr liquefies and entraps individual molecules of the sample in the matrix These spectra are the only spectra of solids that are as interference free as liquids

160 160 IR Spectroscopy II.Instrumentation and Experimental Aspects B.The IR Spectrometer – Experimental aspects 6.Differences in Spectral Appearence Compare the following three IR spectra of p-cresol Neat Sample

161 161 IR Spectroscopy II.Instrumentation and Experimental Aspects B.The IR Spectrometer – Experimental aspects 6.Differences in Spectral Appearence Compare the following three IR spectra of p-cresol KBr Pellet

162 162 IR Spectroscopy II.Instrumentation and Experimental Aspects B.The IR Spectrometer – Experimental aspects 6.Differences in Spectral Appearence Compare the following three IR spectra of p-cresol CCl 4 Solution

163 163 IR Spectroscopy II.Instrumentation and Experimental Aspects B.The IR Spectrometer – Experimental aspects 6.Differences in Spectral Appearence Compare the following three IR spectra of m-nitroanisole Nujol Mull

164 164 IR Spectroscopy II.Instrumentation and Experimental Aspects B.The IR Spectrometer – Experimental aspects 6.Differences in Spectral Appearence Compare the following three IR spectra of m-nitroanisole KBr Pellet

165 165 IR Spectroscopy II.Instrumentation and Experimental Aspects B.The IR Spectrometer – Experimental aspects 6.Differences in Spectral Appearence Compare the following three IR spectra of m-nitroanisole CCl 4 Solution


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