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Infrared Spectroscopy
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Introduction Spectroscopy is an analytical technique which helps determine structure It destroys little or no sample The amount of light absorbed by the sample is measured as wavelength is varied 2018/9/11
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Electromagnetic Spectrum
Examples: X rays, microwaves, radio waves, visible light, IR, and UV Frequency and wavelength are inversely proportional c = ln, where c is the speed of light Energy per photon = hn, where h is Planck’s constant 2018/9/11
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The Electromagnetic Spectrum
High Frequency Energy Low Micro wave X-ray UV IR Radio uv NMR visible Vibrational IR 200 nm nm nm 2.5 m m 1m m 2018/9/11
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Energy Transitions Table 1. Types of energy transitions of
the electromagnetic spectrum Region of spectrum Energy transition X-rays Bond breaking UV/Visible Electronic IR Vibrational Microwave Rotational Radiofrequencies Nuclear spin (NMR) Electron spin (ESR) 2018/9/11
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The Spectrum and Molecular Effects
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The IR Region Just below red in the visible region
Wavelengths usually mm More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters ( cm-1) Wavenumbers are proportional to frequency and energy 2018/9/11
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Molecular Vibrations Light is absorbed when radiation frequency = frequency of vibration in molecule Covalent bonds vibrate at only certain allowable frequencies Associated with types of bonds and movement of atoms Vibrations include stretching and bending 2018/9/11
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IR Spectrum Baseline Absorbance/Peak No two molecules will give exactly the same IR spectrum (except enantiomers) Simple stretching: cm-1 Complex vibrations: cm-1, called the “fingerprint region” 2018/9/11
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Interpretation Looking for presence/absence of functional groups
Correlation tables A polar bond is usually IR-active A nonpolar bond in a symmetrical molecule will absorb weakly or not at all 2018/9/11
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Carbon-Carbon Bond Stretching
Stronger bonds absorb at higher frequencies: C-C cm-1 C=C cm-1 CC cm-1 (weak or absent if internal) Conjugation lowers the frequency: isolated C=C cm-1 conjugated C=C cm-1 aromatic C=C approx cm-1 2018/9/11
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Carbon-Hydrogen Stretching
Bonds with more s character absorb at a higher frequency sp3 C-H, just below 3000 cm-1 (to the right) sp2 C-H, just above 3000 cm-1 (to the left) sp C-H, at 3300 cm-1 2018/9/11
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An Alkane IR Spectrum 2018/9/11
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An Alkene IR Spectrum 2018/9/11
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An Alkyne IR Spectrum 2018/9/11
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O-H and N-H Stretching Both of these occur around 3300 cm-1, but they look different Alcohol O-H, broad with rounded tip Secondary amine (R2NH), broad with one sharp spike Primary amine (RNH2), broad with two sharp spikes No signal for a tertiary amine (R3N) 2018/9/11
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An Alcohol IR Spectrum 2018/9/11
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An Amine IR Spectrum 2018/9/11
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Carbonyl Stretching The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1710 cm-1 Usually, it’s the strongest IR signal Carboxylic acids will have O-H also Aldehydes have two C-H signals around 2700 and 2800 cm-1 2018/9/11
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A Ketone IR Spectrum 2018/9/11
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An Aldehyde IR Spectrum
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O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, cm-1, due to strong hydrogen bonding 2018/9/11
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Variations in C=O Absorption
Conjugation of C=O with C=C lowers the stretching frequency to ~1680 cm-1 The C=O group of an amide absorbs at an even lower frequency, cm-1 The C=O of an ester absorbs at a higher frequency, ~ cm-1 Carbonyl groups in small rings (5 C’s or less) absorb at an even higher frequency 2018/9/11
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An Amide IR Spectrum 2018/9/11
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Carbon - Nitrogen Stretching
C - N absorbs around 1200 cm-1 C = N absorbs around 1660 cm-1 and is much stronger than the C = C absorption in the same region C N absorbs strongly just above 2200 cm-1. The alkyne C C signal is much weaker and is just below 2200 cm-1 2018/9/11
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A Nitrile IR Spectrum 2018/9/11
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Summary of IR Absorptions
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Strengths and Limitations
IR alone cannot determine a structure Some signals may be ambiguous The functional group is usually indicated The absence of a signal is definite proof that the functional group is absent Correspondence with a known sample’s IR spectrum confirms the identity of the compound 2018/9/11
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Infrared Regions Region wavelength wavenumber frequency
(mm) (cm-1) (Hz) Near ~ ~ x 1014~ 1.2 x 1014 Middle 2.5 ~ ~ x 1014~ 6.0 x 1012 Far 50 ~ ~ x 1012~ 3.0 x 1011 Most used 2.5 ~ ~ x 1014~ 2.0 x 1013 2018/9/11
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1. Using photometers or spectrophotometer similar to UV spectrometry
IR Application Near IR: ~ 2500 nm 1. Using photometers or spectrophotometer similar to UV spectrometry 2. Application: routine quantitative determination of species, e.g. moisture, proteins, CHs, fats in agricultural, food, and chemical industries. 2018/9/11
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Used largely for qualitative organic analysis
IR Application Mid-IR: Used largely for qualitative organic analysis and structural determination based on absorption spectra. 2. FT-IR has been used for quantitative analysis of complex gaseous, liquid, or solid mixtures. Far-IR: Used for qualitative analysis of pure inorganic or metal organic species. 2018/9/11
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IR Sources An inert solid heated btw 1500 ~ 2000k, The max radiation ~ 5900 cm-1 (1.7 mM); at long wavelength, 1% of max ~ 670 cm-1 (15 mM). Types of Sources ►The Nernst Glower: rare earth oxides, negative temp coeff of resistance, heated to red heat ► The Globar Source: a silicon carbide rod, positive temp coeff, greater output at > 5 mm. 2018/9/11
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Types of Instruments for IR
Dispersive grating photometers: used for qualitative work Fourier transform photometers: used for both qualitative and quantitative works; speedy, reliable, and convenient Nondispersive photometers: for quantitative use 2018/9/11
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IR Absorption Region ν(Hz) = c = c (cm/sec) / (cm)
λ= 2.5 ~ 15 mm (10 -4 cm = 1 mm = 104 Ao). (4000 cm-1 ~ 666 cm-1 ) wavenumber: (cm-1, reciprocal centimeters) (cm-1) = 1 / (cm) ν(Hz) = c = c (cm/sec) / (cm) wavenumber (cm-1) = / wavelength (mm) wavelength (mm) = 104 /(cm-1) 2018/9/11
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Theoretical Introduction
ΔE = h .ν = h c /λ = h c C : velocity of light, 3 x 1010 cm/sec h : Planck’s constant ν : frequency (1/sec) λ : wavelength (mm) : wavenumber (cm-1) 2018/9/11
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IR Absorption Process A Quantized Process:
Only selected frequencies (energies) of infrared radiation will be absorbed by a molecule Bonds have dipole moment, Yes ! Symmetric bonds, No ! (e.g. H2, Cl2) 2018/9/11
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IR Application Infrared spectrum can be a fingerprint used for identification. Infrared spectrum gives the structural information about a molecule. 2018/9/11
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Absorption Transmittance: T = I/Io Absorbance: A = Log Io/I
UV example Transmittance: T = I/Io Absorbance: A = Log Io/I if no absorption, T = 1; A = 0 Most spectrometers display absorbance on vertical axis, and the commonly observed range from 0 (100% T) to 2 (1% T) max: The wavelength of maximum absorbance 2018/9/11
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An Infrared Spectrum 2018/9/11
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IR Spectrum Baseline Absorbance/Peak No two molecules will give exactly the same IR spectrum (except enantiomers) Simple stretching: cm-1 Complex vibrations: cm-1, called the “fingerprint region” 2018/9/11
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Vibration Modes Symmetric Stretch Asymmetric Stretch Symmetric Bend
Symmetric Stretch Asymmetric Stretch Symmetric Bend 2018/9/11
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Vibration Modes of CH2 Group
Gas Phase IR Spectrum of Formaldehyde, H2C=O 2018/9/11
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Vibration Modes Scissoring wagging Sym. Stretching (~ 1250) (~ 2853)
(~ 1450) Asym. Stretching Rocking Twisting (~ 720) (~ 2926) (~ 1250) in plane Out of plane Stretching vibrations Bending Vibrations 2018/9/11
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Basic Principle Eosc h νosc Applying Hooke’s Law K = - k .Δr
As for any harmonic oscillator, the energy of bond vibration: Eosc h νosc Applying Hooke’s Law K = - k .Δr K Restoring force - Arising force opposed to extension k Elasticity constant (unit: N / m = kg . m / sec2 / m) ( ~ bond strength between two atoms) Δr extension 2018/9/11
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Basic Principle νosc = ½ . √k / m (1) E = (n + ½) ½ h. √k / m (2)
The natural frequency of the oscillation νosc = ½ . √k / m (1) Wave equation of quantum mechanics E = (n + ½) ½ h. √k / m (2) h : Planck’s const. n : vibration quantum number Take (1) into (2): Evib = h νosc (n + ½ ) 2018/9/11
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Basic Principle From SchrÖdiger Equation: Evib = h νosc (n + 1/2)
vibration quantum number, n = 0, 1, 2, Ground state (n = 0): E0 = ½ hνosc First excited state (n = 1): E1 = 3/2 hνosc ΔEvib = E1 – E0 = 3/2 hνosc - ½ hνosc = hνosc 2018/9/11
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Basic Principle The frequency of radiation, ν
vibrational frequency, νosc E radiation = h ν = ΔEvib = hνosc = ½ h . √k / m or ν = νosc = ½ . √k / m 2018/9/11
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Basic Principle νosc = 1/2 . √k / m
m = m1. m2 / (m1 + m2) = reduced mass of atoms in AMU (unit: kg) ΔE vib = h . νosc = h / 2 . √k / m ν(cm-1) = E vib / h c = 1 / 2 c .√k / m = 5.3 x 10 –12 sec/cm . √k / m [ unit of √k / m : sec -1 ] 2018/9/11
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Example (cm-1) = 5.3 x 10 –12 s/cm . √1 x 10 3 N /m /2.7 x 10 –26 kg
Calculate the wavenumber and wavelength of the fundamental frequency peak due to the stretching vibration of C=O group. Solution: m1 = 12 x kg / mol / 6.0 x atoms / mol = x 10 –26 kg m2 = 16 x kg / mol / 6.0 x atoms / mol = x 10 –26 kg m = m1. m2 / (m1 + m2) = 1.1 x 10 –26 kg (cm-1) = 5.3 x 10 –12 s/cm . √1 x 10 3 N /m /2.7 x 10 –26 kg = 1.6 x 103 cm-1 2018/9/11
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Example (cm-1) = E vib / h c = 1 / 2 c .√k / m
= 7.76 x 1011/2 c √k / m’ (taking 6.02 x 1023 out of root square) = 4.12√k / m’ ‘ = M1M2/(M1+M2); both are atomic weights. K : force constant in dynes/cm. 1 N = 1 kg m/sec2 1 Dyne = 1 g cm/sec2 2018/9/11
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k (Chemical bond force constant) :
Force Constants k (Chemical bond force constant) : single bond 5 x 102 N/m = 5 x 105 dynes/cm double bond 1.0 x 103 N/m = 106 dynes/cm triple bond 1.5 x 103 N/m = 1.5 x 106 dynes/cm 2018/9/11
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Force Constants C=C bond: = 4.12√k / m K = 106 dynes/cm
m = 12x12/(12+12) = 6 = 4.12√106/6 = 1682 cm-1 (calculated) = 1650 cm-1 (experimental) 2018/9/11
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Force Constants C-H Bond: = 4.12√k / m K = 5 x 105 dynes/cm
m = 12x(1)/(12+1) = 0.923 = 4.12√5 x 105/0.923 = 3032 cm-1 (calculated) = 3000 cm-1 (experimental) C-D Bond: = 2206 cm-1 (experimental) 2018/9/11
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Frequencies of vibration
CC C=C C-C 2150 cm cm cm-1 increasing K 2018/9/11
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Frequencies of vibration
As the atom bonded to carbon increases in mass, the Quantity m increases, the frequency of vibration goes down. C-H C-C C-O 3000 cm cm cm-1 C-Cl C-Br C-I 800 cm cm-1 ~ 500 cm-1 Increasing m 2018/9/11
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Frequencies of vibration
Bending motion easier than stretching motions (The force constant K is smaller) C-H stretching C-H bending ~ 3000 cm-1 ~ 1340 cm-1 2018/9/11
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Frequencies of vibration
Hybridization affects the K values sp sp2 sp3 C-H =C-H -C-H cm-1 2018/9/11
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Example 2018/9/11
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Frequencies of Vibration
C C C = C CC 2150 cm cm cm-1 Increasing K CX stretching CH CC CO CCl CBr CI ~500 cm-1 2018/9/11
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Frequencies of Vibration
Resonance: C=O 1715 cm-1 1675 ~ 1680 cm-1 2018/9/11
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Substituent Effects e nm Hyperchromic Hypsochromic (blue shift)
Bathochromic (red shift) e Hypochromic nm UV Profiles 400 800 blue red 2018/9/11
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Substitution Effects on IR Peaks
Electronic effects Intramolecular factors Inductive effects R-COR C= cm ; R-COH C= cm -1 ; R-COCl C= cm ; R-COF C= cm ; F-COF C= cm ; R-CONH2 C= cm ; 2018/9/11
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Conjugated Effects cm -1 cm -1 cm -1 cm -1 2018/9/11
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C H 1 5 7 6 c m 4 8 Space Effects Steric Hindrance Ring Strain
- 4 8 2 2 2 2 2018/9/11
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2.Intermolecular Factors
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Example 2018/9/11
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Symbols of Vibrations s = symmetric as = asymmetirc
ν = stretching vibrations (bonding vibrations) δ = deformation vibrations (bending vibrations) γ = out-of-plane deformation vibrations τ = torsional vibrations 2018/9/11
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Characterization of IR Spectra
Fingerprint Region: 1450 to 600 cm-1 region Group Frequency Region: 4000 to 1450 cm-1 region 2018/9/11
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Degree of Freedom (DF) According to Cartesian to describe positions:
Each atom has 3 degrees of freedom, (x, y, z). A molecule of n atoms has 3n DF. For nonlinear molecules: 3n-6 are fundamental vibration of DF. For linear molecules: 3n-5 fundamental vibration of DF. fundamental vibration involves no change in the center of gravity of molecules 2018/9/11
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Degree of Freedom (DF) DF of Linear CO2: 2018/9/11
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Degree of Freedom (DF) Three fundamental vibrations of
nonlinear triatomic water molecule: 2018/9/11
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Vibration Modes CH2 group A. Stretching vibrations
Change of bond length B. Deformation vibrations (two types: planar or non-planar) Change of bond angle 2018/9/11
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Fingerprint Regions Localized vibrations: spectra below 1500 cm-1 are
difficult to interpretation, which, we call it “fingerprint region”, are characteristic for the molecule as a whole. Most of them are derived from overtone or combination vibrations. 2018/9/11
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Group Frequency Regions
Absorption bands in the 4000 to 1450 cm-1 region are usually due to stretching vibrations of diatomic units 2018/9/11
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Some General Trends Stretching frequencies are higher than corresponding bending frequencies. (It is easier to bend a bond than to stretch or compress it.) ii) Bonds to hydrogen have higher stretching frequencies than those to heavier atoms. iii) Triple bonds have higher stretching frequencies than corresponding double bonds, which in turn have higher frequencies than single bonds. (Except for bonds to hydrogen). 2018/9/11
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Characterization of IR Spectra
Carbonyl/ethylene groups C=O 1850 ~1630 cm-1, sharp & intensity C=C 1680 ~ 1620 cm-1, generally weak 2018/9/11
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Characterization of IR Spectra
Important infrared frequencies: The IR spectra of complicated molecules have many peaks, only a few of which can be easily interpreted. Here are the places to look at: 1500 – 1800 cm-1 : A strong signal for (C=O) or an imine. 2000 – 2200 cm-1 : stretch vibrations of triple bonds and cumulenes (-C≡N, -C≡C , -N=C=O, -C=C=O). 2900 – 3000 cm-1 : stretch vibrations of aliphatic C – H bond. 3000 – 3100 cm-1 : vinyl C – H bond stretches. 3100 – 3600 cm-1 : O – H bond stretches (broad if inter-molecularly at H-bonded). Be careful, since water appears in this region. 2018/9/11
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Characterization of IR Spectra
N-H & O-H regions usually overlap O-H 3650 ~ 3200 cm-1, broad peak N-H 3500 ~ 3300 cm-1, 1 or 2 sharp absorption bands of low intensity, O-H in this region usually gives broad peak. 2018/9/11
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Characterization of IR Spectra
Hydrogen Bonding X— H --- Y : s-orbital of proton overlapping with p- or -orbital of the acceptors. Common proton donor groups: -COOH, -OH, Amines, or amides. Common proton acceptors: O, N, halogens X— H : move to lower frequency with increased intensity. The acceptors, Y ( e.g. C=O), also move to lower frequency but to a less degree. Intermolecular H-bonding: usually in dimers (e.g. RCOOH), or in neat or concentrated solutions of R-OH. Temperature, concentration affect the inter or intra- H-bonding. 2018/9/11
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Characterization of IR Spectra
Intra- H-bond is stronger when a six-membered ring is formed. H-bond is strongest when the bonded structures is stabilized by resonance. 2018/9/11
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Characterization of IR Spectra
Stretching frequencies in hydrogen bonding Intermolecular bonding intramolecular bonding Frequency reduction(cm-1) Frequency reduction(cm-1) X— H --- Y ν OH ν CO comp’ds ν OH ν CO comp’ds Weak R-OH, PhOH < ,2-diols intermol OH to CO most b-OH ketones Medium ~ Strong > RCOOH dimers > 2018/9/11
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1820 ~ 1660 cm-1 : the strongest peak
Analysis of Spectra If C=O 1820 ~ 1660 cm-1 : the strongest peak Acid: is OH present? Broad near 3400~2400 Amide: is NH present? Medium peak ~3500 Ester: is C-O present? Strong at 1300~1000 Anhydride: two C=O ~ at 1810 & 1760 Aldehyde: is C-H present? Two peaks ~ 2850 & 2750 Ketone: the above 5 choices were eliminated 2018/9/11
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Alcohol/phenol: check for OH - broad near 3600~3300
Analysis of Spectra 2. If C=O absent Alcohol/phenol: check for OH - broad near 3600~3300 - confirm by C-O near 1300~1000 Amine: check for NH -medium peak near 3500 Ether: check for C-O near 1300~1000 2018/9/11
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3. Double bonds/or Aromatic rings
Analysis of Spectra 3. Double bonds/or Aromatic rings Alkene: -C=C is weak near 1650 Aromatic: -medium to strong at 1650~1450 Aromatic & vinyl CH: weak peak at 3000 4. Triple bond -CN: medium, sharp near 2250 -CC: weak, sharp near 2150 check also for acetylenic CH near 3300 2018/9/11
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- two strong peaks at 1600-1500 & 1390-1300
Analysis of Spectra 5. Nitro group - two strong peaks at & 6. Hydrocarbon -none of above found -major peaks of CH near 3000 -very simple spectrum, others at 1450 & 1375 2018/9/11
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Summary of IR Absorptions
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Characterization of IR Spectra
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Problem Set 1. Analysis of C5H10O IR Spectrum 2018/9/11
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Problem Set 2. Analysis of C8H8O IR Spectrum 2018/9/11
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Problem Set 3. Analysis of C7H8O IR Spectrum 2018/9/11
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Problem Set 4. Analysis of C8H7N IR Spectrum 2018/9/11
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Problem Set 5. Analysis: C7H6O IR Spectrum 2018/9/11
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Problem Set 6. Analysis: C3H7NO IR Spectrum 2018/9/11
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Problem Set 7. Analysis: C4H8O2 IR Spectrum 2018/9/11
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Problem Set 8. Analysis: C7H5OCl IR Spectrum 2018/9/11
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Problem Set 9. Analysis: C6H6S IR Spectrum 2018/9/11
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Problem Set 10. Analysis: C4H6 IR Spectrum 2018/9/11
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Solutions of Problem Set
No. 2: acetophenone No. 3: benzyl alcohol No. 1: 3-pentanone No. 4: benzyl nitrile No. 5: benzaldehyde No. 6: N-methylacetamide No. 8: benzoyl chloride No. 7: ethyl acetate No.9: thiophenol No.10: 1,3-butadiene 2018/9/11
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A Survey of the Important Functional Groups With Examples
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Characterization of IR Spectra
9. Hydrocarbons: Alkanes, Alkenes, and Alkynes A. Alkanes C-H stretch ~ cm-1 CH2 ~ 1450 CH3 ~ 1375 C-C stretch : not useful for interpretation 2018/9/11
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Characterization of IR Spectra
B. Alkene =C-H stretch =C-H out-of-plane (oop) bending, C=C (w) symmetrical type, no absorb symmetrical (cis), stronger 2018/9/11
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Characterization of IR Spectra
Aromatic rings =C-H stretch, 3100 ~ 3000 =C-H out-of-plane (oop) bending, 900 ~ 690 great utility to assign the ring substituted pattern C=C ring stretch, occurs in pair at 1600 & 1475 overtone/combination bands, 2000 ~ 1667 used to assign the ring substituted patterns 2018/9/11
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Characterization of IR Spectra
C. Alkynes C-H stretch ~ 3300 (3.0 m) -CC stretch ~ 2150 (4.65 m) disubstituted or symmetrical: no or weak absorption 2018/9/11
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An Alkyne IR Spectrum 2018/9/11
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Characterization of IR Spectra
Table 2. Physical constants for sp, sp2, sp3 Hybridized carbon and the resulting C-H values Bond C-H =C-H -C-H Type sp- 1s sp2- 1s sp2-1s Length 1.08Å 1.10Å 1.12Å Strength 121Kcal 106Kcal 101Kcal IR freq ~3100 ~2900 2018/9/11
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Characterization of IR Spectra
C-H Stretch Region 3.03m 3.22m 3.33m 3.51m 3.64m acetylenic vinyl =C-H aliph. C-H aldehyde C-H arom. =C-H cyclopropyl -C-H -CH=O sp sp2 sp3 Strain moves absorption to left Increasing s character moves to left 2018/9/11
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Characterization of IR Spectra
Table 3. The stretching vibrations for various sp3 hybridized C-H bonds Stretching vibration group asym. sym. Methyl CH3- Methylene –CH2- Methine –C-H 2962 2926 2872 2853 2890 very weak 2018/9/11
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Characterization of IR Spectra
C-H Bending Vibrations CH2 CH3 Scissoring Bending (as.) bending (sy.) Lone Me g’p Gem-dimethyl Sometimes overlap CH2, ~ 720 rocking band 2018/9/11
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Characterization of IR Spectra
C=C Stretching Vibrations Conjugated effects. cm-1 Ring size effects. The frequency of (endo) double bonds in cyclic compounds is sensitive to ring size. Higher freq. 2018/9/11
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Characterization of IR Spectra
Angle > 90 Angle < 90 ~ 1611 Figure 3. C=C Stretching vibrations in endocyclic Systems 2018/9/11
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Characterization of IR Spectra
External (exo) double bonds H2C=C=CH2 Allene Strain moves the peak to the left Ring fusion moves the absorption to the left 2018/9/11
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Characterization of IR Spectra
C-H Bending Vibrations for Alkenes In-plane scissoring: ~ 1415 Out-of-plane region: 1000 ~ 650 Valuable to indicate the substitution patterns 2018/9/11
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Characterization of IR Spectra
m s s Monosubst. cis-1,2 trans-1,2 1,1-disubst. trisubst. tetrasubst. s s s m cm-1 Figure. The C-H out-of-plane bending vibrations for substituted alkenes 2018/9/11
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11 12 13 14 15 m s s s m s s s m s s m s m 900 800 700 cm-1 Monosubst.
Ortho Meta Para 1,2,4- 1,2,3- 1,3,5- s m s s s m s s m s m cm-1 C-H oop C=C oop 2018/9/11
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Characterization of IR Spectra
D. Alcohols and Phenols O-H “free” O-H sharp, 3650~3600 if no H-bonding. H-bonded O-H, broad at 3500~3200 ,usually in neat (pure) liquids. C-O 1250~1000(s) 2018/9/11
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An Alcohol IR Spectrum 2018/9/11
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Characterization of IR Spectra
Table. The C-O and O-H stretching vibrations in Alcohols and Phenol Compound C-O stretch O-H stretch Phenols 3 - OH 2 - OH 1 - OH 1100 1017 2018/9/11
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Characterization of IR Spectra
E. Ethers C-O stretch, 1300 ~ 1000 phenyl & vinyl ethers, shift to higher frequency (increase of double bond character) 1250 (asy), 1040 (sy). Epoxides, 3 bands (1250, 950~815, 850~750) 2018/9/11
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Characterization of IR Spectra
F. Carbonyl compounds acid ester ketone amide chloride Anhydride anhydride aldehyde carboxylic (band 1) (band 2) acid 2018/9/11
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Characterization of IR Spectra
F-1. Factors affect C=O vibration Conjugated effects. a,b-unsaturated, 30 cm-1 to lower freq. 1710 1680 1715 1690 1725 1700 2018/9/11
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Characterization of IR Spectra
2. Ring size effects 1715 1745 1715 1780 1735 1770 1690 1705 2018/9/11
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Characterization of IR Spectra
3. Alpha-substitution effects Axial Cl ~ 1725 Equatorial Cl ~ 1750 2018/9/11
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Characterization of IR Spectra
4. Hydrogen Bonding Effects 1680 2018/9/11
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Characterization of IR Spectra
5. Cyclic ketones Ring strain 2018/9/11
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Characterization of IR Spectra
6. Carboxylic Acids O-H stretch, very broad (strongly H-bond) 3400 ~ 2400 C=O stretch, broad, 1730 ~ 1700 C-O stretch, 1320 ~ 1210 (m) 2018/9/11
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Characterization of IR Spectra
7. Esters (R-C(=O)-O-R’) C=O stretch, 1735 a. conjugation at the R part: ν shift to the right b. conjugation with O at R’ ν part: shift to the left c. ring strain (lactones): ν shift to the left C-O stretch, two or more bands, one stronger and broader, ~ 1000 2018/9/11
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Characterization of IR Spectra
Table. The general effects of a,b-unsaturation or aryl Substitution and conjugation with oxygen on the C=O vibrations 1770 1735 1720 2018/9/11
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Characterization of IR Spectra
Table. The Effects of Ring Size, a,b-unsaturation, and Conjugation with Oxygen in the C=O Vibrations in Lactones 1760 1735 1720 1770 1750 1800 1820 2018/9/11
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Characterization of IR Spectra
8. Amides C=O stretch, ~ 1640 N-H stretch (1 and 2), 3500, 3100 N-H bending, ~ 1550 ~ ~ ~ 1745 2018/9/11
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Characterization of IR Spectra
9. Acid Chlorides C=O stretch at ~ 1800 conjugation moves to the right 10. Anhydrides C=O stretch, two bands, 1830~1800 & 1775~1740 ring strain moves to the left C-O stretch, ~ 900 2018/9/11
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Characterization of IR Spectra
G. Amines N-H stretch, 3500~3300 1-, 2 bands; 2-, 1 band weak, aliphatic amines stronger, aromatic amines N-H bend, 1640~ 1560 2-, near 1500 N-H oop bending, near 800 C-N stretch, 1350 ~ 1000 2018/9/11
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Characterization of IR Spectra
H. Nitriles, Isocyanates and Imines -CN stretch, sharp ~ 2250 -N=C=O a broad, intense band ~ 2270 -N=C=S a broad, intense band ~ 2125 -C=N- in imine or oxime CC give much weak band at ~ 2250 region. 2018/9/11
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Characterization of IR Spectra
J. Nitro Compounds N=O stretch, two strong bands, 1600 ~ 1500 & ~ 1300 Nitroalkanes: ~ 1550, 1380 Nitroarenes: ~ 1530, 1350 Nitroso (R-N=O) one band, ~ 1500 2018/9/11
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Characterization of IR Spectra
J-1. Carboxylate Salts, RC(=O)-O- asym ~1600 (s) sym ~1400 (s) J-2. Amine Salts N-H stretch (broad), 3300 ~ 2600 N-H bend, ~ 1500 (s) 1 (two bands), 1610, 1500 2 (one band), 1610 ~ 1550 2018/9/11
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Characterization of IR Spectra
K. Sulfur Compounds Mercaptans S-H stretch, ~ 2550 (w) Sulfides, R-S-R Sulfoxides, R-S(=O)-R S=O stretch, ~1050 (s) Sulfones, RSO2R S=O (s), (s) Sulfates RSO3R S=O 1375(s), 1200(s) S-O ~750 Sulfonamides RSO2NH2, RSO2NHR S=O (s), (s) N-H , 3250 (1 ) 3250 (2 ) 2018/9/11
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Characterization of IR Spectra
L. Alkyl and Aryl Halides Fluorides C-F stretch, 1400~1000 (s) Chlorides C-Cl stretch, 800~ 600 (s) CH2-Cl bend (wagging), ~ 1200 Bromides/Iodides C-Br/C-I stretch, ~ 660 out of range CH2-Br/CH2-I bend (wagging), ~ 1150 2018/9/11
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Infrared Sources and Transducers
An inert solid with continuous radiation at heated temperature: 1500 ~ 2000 K. Range of radiation intensity: 5000 ~ 670 cm-1 (2 ~ 15 mM) 2018/9/11
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electrically heated (1300 ~ 1500 K)
The Nernst Glower temperature: 1200 ~ 2200 K 2. The Globar Source a silicon carbide rod electrically heated (1300 ~ 1500 K) 2018/9/11
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Sample Handling Techniques
Neat liquids:一滴樣品夾於二鹽片(rock-salt plate)間(厚度約為 ≦ 0.01 mm), a neat spectrum obtained. 一般僅適合用於定性的分析. Solids: KBr pellet: 2 ~ 5 mg sample, mixing with 100~150 mg of powdered KBr, pressing with pressure of 5 ~ 10 Ton to give a transparent disc. KBr is hygroscopic, weak OH band at 3450 cm-1 appeared Nujol mull: grinding the sample with mineral oil (Nujol) to create a suspension and placed between salt plates Solutions: compound dissolved in common solvents, ex. CS2, CCl4, CHCl3, dioxane, cyclohexane, benzene. 2018/9/11
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