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Created with MindGenius Business 2005® Principles of Spectroscopy Dr. Alison Willows.

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Presentation on theme: "Created with MindGenius Business 2005® Principles of Spectroscopy Dr. Alison Willows."— Presentation transcript:

1 Created with MindGenius Business 2005® Principles of Spectroscopy Dr. Alison Willows

2 Created with MindGenius Business 2005® Introduction Spectral lines occur in spectroscopy through the absorption, emission or scattering of a photon when the energy of a molecule or atom changes. In atomic spectroscopy the change in energy is a result of electronic transitions (see CH115) In molecular spectroscopy the energy of the molecule also changes due to changes of rotational and vibrational state Ultra-violet and visible spectroscopy looks at the change of electron distribution

3 Created with MindGenius Business 2005® Introduction Molecular spectra thus contain more information than atomic spectra, e.g. Bond strengths, lengths and angles and molecular dimensions, shapes and dipole moments These energy changes can be detected using Infra-red spectroscopy (vibration) and microwave spectroscopy (rotation)

4 Created with MindGenius Business 2005® Electromagnetic spectrum

5 Created with MindGenius Business 2005® Introduction Copyright © Houghton Mifflin Company. All rights reserved.

6 Created with MindGenius Business 2005® Infrared spectroscopy Used to determine the types of bonds present in a molecule Infrared radiation is passed through a sample and the radiation transmitted is recorded. Radiation absorbed by the molecule appears as a band in the spectrum Instruments scan from around 700 to 5000 cm -1

7 Created with MindGenius Business 2005®

8 Infrared activity Infrared active if normal mode motion is accompanied by a change of dipole moment Where normal mode motion is an independent, synchronous motion of atoms (or groups of atoms) that may be excited without leading to excitation of any other normal mode

9 Created with MindGenius Business 2005® Infrared spectroscopy Stretching and bending motions of a CH 2 group that requires energies in the infrared region

10 Created with MindGenius Business 2005® Bond stretching frequencies

11 Created with MindGenius Business 2005® Bond stretching frequencies, cont. Vibration of bonds can be described using Hooke’s law Frequency of a stretching vibration is directly proportional to the strength of the bond and inversely proportional to the masses at either end e.g. C-H, N-H and O-H bond stretching vibrations are high frequency (short wavelength) compared to those of C-C and C-O because of low mass of hydrogen compared to carbon or oxygen

12 Created with MindGenius Business 2005® Bond stretching frequencies, cont. Double bonds is generally stronger and ‘stiffer’ so C=C vibration is at a higher frequency than C-C Triple bonds stretch at even higher frequencies  vibration frequency corresponds to the relationship of bond strength

13 Created with MindGenius Business 2005® Spectra characteristics Intensity of IR absorption bands is proportional to the change in dipole moment Intensity is not proportional to the number of atoms causing the dipole moments (unlike NMR where area of peaks is proportional to the number of hydrogen atoms causing them) It is not usually possible to assign all of the peaks in an IR spectrum (unlike NMR)

14 Created with MindGenius Business 2005® Spectra characteristics Peaks with longer wavelengths than 1250 cm -1 are the result of combinations of vibrations so are characteristic of the whole molecule rather than a functional group This is termed the fingerprint region No two organic compounds will have the same IR spectrum, even if the UV and NMR spectra are the same

15 Created with MindGenius Business 2005® Spectra analysis Not practical to calculate theoretical spectra Analysis is done by comparison with other spectra Many modern instruments come with extensive libraries of known compounds The acquired spectrum is compared to those stored in the library and a list of compounds with spectra that have matching or similar absorption signals is generated For simple molecules it is possible to identify functional groups by hand

16 Created with MindGenius Business 2005® Spectra Analysis, cont. Pay most attention to the strongest absorptions Pay more attention to the peaks to the left of 1250 cm -1 (shorter wavelengths) Pay as much attention to the absence of certain peaks as to the presence of others Be wary of O-H peaks as water is a common contaminant of samples

17 Created with MindGenius Business 2005® Spectra comparison Infrared spectra of two similar ketones Show similar functional group bands Differ in fingerprint region

18 Created with MindGenius Business 2005® Spectra feature summary Functional group bands will appear in the same range regardless of details of molecular structure Bands in the fingerprint region will be unique for each compound

19 Created with MindGenius Business 2005® Questions Using IR spectroscopy how could you quickly distinguish between the structural isomers benzyl alcohol and anisole? Benzyl alcohol’s spectrum will show a band in the O-H stretching frequency region (3200 to 3700 cm-1), anisole will not anisole

20 Created with MindGenius Business 2005® These are the infrared spectra of hexanoic acid, 1-pentanol, cyclohexane and 3-pentanone. Match each compound with the correct spectrum, indicating which IR bands you used to make your assignments

21 Created with MindGenius Business 2005® Structures Hexanoic acid 1-pentanol Cyclohexane 3-pentanone

22 Created with MindGenius Business 2005® Fourier Transform Infrared (FTIR) Computer based: can run multiple scans increasing signal-to-noise ratio Faster and more accurate than traditional double beam instruments Uses a Michelson Interferometer

23 Created with MindGenius Business 2005® FTIR operation IR beam is split so half goes to a fixed mirror and half to a moving mirror The moving mirror changes the phase of the one beam sinusoidally The frequency changes as the mirror moves The beams reinforce or destructively interfere giving an interferogram The digitized interferogram is converted to a function of frequency by a Fourier transform operation creating a spectrum

24 Created with MindGenius Business 2005® FTIR advantages All IR frequencies are scanned simultaneously (spectrum obtained in seconds) High resolution is possible without losing signal strength as there is no slit to select wavelength Small samples or very dilute solutions can be analysed as it is possible to sum hundreds of scans

25 Created with MindGenius Business 2005® Rotational Spectra The energy for small molecules undergoing rotation is in the region 0.1 – 10 cm -1. This is in the microwave region of the EM spectrum Transitions are detected by monitoring net absorption of microwave radiation The rotation of a three dimensional body can be quite complex. Rotation is described as components around three perpendicular axes through the centre of gravity

26 Created with MindGenius Business 2005® Moments of inertia Rotational properties of a molecule can be expressed in terms of moments of inertia around the three perpendicular axes; A, B & C The moment of inertia, I, of a molecule is defined as the mass of each atom multiplied by the square of its distance from the rotational axis through the centre of the molecule Where r i is the perpendicular distance of the atom i from the axis of rotation

27 Created with MindGenius Business 2005® Types of rotation If we regard molecules as rigid motors, bodies that do not distort under the stress of rotation, then we can classify four types: Linear rotors – one moment of inertia equal to zero, I A = 0,(e.g. CO 2, HCl, HC  CH) Symmetric rotors – 2 equal moments of inertia, I B = I C, I A ≠ 0 (e.g. NH 3, CH 3 Cl and CH 3 CN) Asymmetric rotors – 3 different moments of inertia, I A ≠ I B ≠ I C (e.g. H 2 O, H 2 CO and CH 3 OH) Spherical rotors – 3 equal moments of inertia, I A = I B = I C (e.g. CH 4, SiH 4, SF 6 )

28 Created with MindGenius Business 2005® Four types of rotation

29 Created with MindGenius Business 2005® Linear rotors The atoms in the molecule are arranged in a straight line E.g. H — Cl The three directions of rotation are: About the bond axis End-over-end rotation in the plane of the screen End-over-end rotation perpendicular to the screen

30 Created with MindGenius Business 2005® Symmetrical rotors Two of the moments of inertia are equal, the third does not equal zero As with linear rotors, the end-over-end rotations are equal Two types of symmetrical rotor: Prolate – I B = I C > I A Oblate - I B = I C < I A

31 Created with MindGenius Business 2005® Asymmetric rotors Majority of substances belong to this category All three moments of inertia are different I A ≠ I B ≠ I C E.g. H 2 O and vinyl chloride CH 2 = CHCl

32 Created with MindGenius Business 2005® Spherical rotors All three moments of inertia are equal Due to their symmetry they have no dipole moment Rotation alone can not produce a dipole so no rotational spectrum is observed These rotors will not be considered further

33 Created with MindGenius Business 2005® Rotational spectra Rotational energy is quantized, like all other forms of molecular energy Rotational energy levels may be calculated using the Schrödinger equation for the type of molecule J J F(J) 42B 30B 20B 12B 6B 2B 0 Where J is the angular momentum quantum number, F(J) is the rotational term (energy) and B is the rotational constant (cm -1 ) When J = 0 the molecule is not rotating When J = 1 the molecule has its lowest angular momentum Selection rule  J =  1 Allowed rotational energies of a linear or spherical rotor

34 Created with MindGenius Business 2005® Linear and spherical motors The energy of a rotational state is given by: F(J) = BJ(J+1) Where F(J) is the rotational term, a wavenumber (cm -1 ), B is the rotational constant (cm -1 ) and J is the angular momentum quantum number

35 Created with MindGenius Business 2005® Rotational spectra, cont. Determining the rotational constant B allows the bond length to be calculated So the measurement and identification of one spectral line allows calculation of moment of inertia and thus bond length

36 Created with MindGenius Business 2005® Moments of inertia for different types of molecules

37 Created with MindGenius Business 2005® Points to consider As the moment of inertia for end-over-end rotation is greater than that for a diatomic molecule the B value will be smaller and hence the spectral lines will be closer together B values for diatomic molecules are about 10 cm -1. B values for triatomic molecules can be 1 cm -1 or less and smaller still for larger molecules The larger the molecule the closer together the spectral lines

38 Created with MindGenius Business 2005® UV-visible Spectroscopy Absorption of short wavelength, high-energy UV radiation results in electrons moving from one energy level to another with energies capable of breaking chemical bonds Many molecules will produce only one major absorption signal Many combinations of  bonded systems absorb at nearly the same wavelength (C-C, C=O, C=N, triple bonds and aromatic rings) Because of this structure is often found using other techniques and the UV-vis spectrum interpreted based on this

39 Created with MindGenius Business 2005® UV-vis spectroscopy Visible-ultraviolet spectra are most commonly used to detect conjugation Generally, molecules with no double bonds or with only one double bond do not absorb in the uv-vis region Conjugated systems do absorb in this region The greater the conjugation the longer the wavelength of max. Absorption Most of the reliable and useful data is from strongly absorbing chromophores (  >200), mainly indicative of conjugated or aromatic systems

40 Created with MindGenius Business 2005® Beer’s Law revisited Absorbance is related to the concentration and molar absorptivity of a sample Where A is the absorbance,  is the molar absorptivity, c is the concentration (mol dm -3 ), l is the pathlength of the cell  for any peak in a spectrum is a characteristic of that particular molecular structure  varies between 10 and 10 5 for common chromophores, so tables often give log 10  for manageability

41 Created with MindGenius Business 2005® absorption

42 Created with MindGenius Business 2005® Classification of UV absorption bands UV absorption bands have fine structure due to presence of vibrational sub-levels However, this is rarely observed in solution due to collisional broadeningcollisional broadening As transitions are associated with changes of electron orbitals they are often described in terms of the orbitals involved, e.g. Where n denotes a non-bonding orbital and the asterix denotes an antibonding orbital

43 Created with MindGenius Business 2005® UV absorption bands, cont. A molecule can give more than one UV absorption band, because either It contains more than one chromophore, or More than one transition of a single chromophore is observed UV spectra contain fewer features than IR, MS or NMR so less information can be obtained from them

44 Created with MindGenius Business 2005® Important UV chromophores Dienes (2 C=C) and Polyenes (alternating single and double C bonds) Extension of conjugation in a carbon chain is always associated with a shift towards a longer wavelength, and usually towards greater intensity When there are more than 8 conjugated double bonds max is in the visible region Woodward’s rules are available to estimate the position of max

45 Created with MindGenius Business 2005® Effect of extended conjugation on UV absorption

46 Created with MindGenius Business 2005® Carbonyl compounds All carbonyl derivatives exhibit weak absorption although this is not very useful in determining structure Conjugated carbonyl derivatives always exhibit strong absorption (see right)

47 Created with MindGenius Business 2005® Benzene derivatives Exhibit medium to strong absorption in UV region Bands usually have characteristic fine structure Intensity of absorption strongly influenced by substituents

48 Created with MindGenius Business 2005® UV absorption bands in common benzene derivatives Weak auxochromes - CH 3, -Cl, -OCH 3 Groups that increase conjugation: -CH=CH 2, -C(=O)-R, -NO 2 Auxochromes whose absorption is pH dependent: -NH 2, -OH Auxochrome is a group of atoms attached to a chromophore which can increase the wavelength it absorbs light and increase the intensity

49 Created with MindGenius Business 2005® Common solvents for UV-vis For visible spectroscopy any transparent solvent can be used For UV spectroscopy 95% ethanol (absolute ethanol often contains residual benzene which absorbs in UV) methanol water saturated hydrocarbons such as hexane, trimethylpentane, isooctane

50 Created with MindGenius Business 2005® Sample cells Care must be taken to select correct cells for wavelength of interest For UV work can not use glass as it absorbs UV radiation; use only for nm (visible) Lower wavelengths (UV) use silica cells (down to 220 nm) or disposable PMMA ( nm) Below this a special grade of silica is required (down to 185 nm) Cells must be very clean (a fingerprint can give a spectrum of its own)

51 Created with MindGenius Business 2005® Fluorescence spectroscopy Caused by absorption of energy followed by emission of some of it in the form of light Stoke’s law tells us the emitted light almost always has higher wavelength than the absorbed light A molecule absorbs a photon of UV radiation and undergoes a transition to an excited state, one of its electrons is promoted to an orbital of higher energy In fluorescence the excited molecule returns to the ground state immediately ( to s)

52 Created with MindGenius Business 2005® Fluorescence, cont. Two important types of transition for organic molecules: n   * in which an electron in a non-bonding orbital is promoted to a  -antibonding orbital    * in which an electron in a  bonding orbital is raised to a  -antibonding orbital    * leads to significant fluorescence The extent to which a molecule fluoresces depends on its structure In general, intense fluorescence is associated with molecules that have an extensive system of conjugated double bonds with a relatively rigid structure due to ring formation

53 Created with MindGenius Business 2005® Advantages of fluorescence Limit of detection is generally lower in fluorescence than absorbance (  10 3 time lower) Selectivity may also be better as not all absorbing species fluoresce and there is the choice of both excitation and emission wavelength

54 Created with MindGenius Business 2005®


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