Units Used in IR Spectroscopy the Wavelength of Light in the IR Region

The wavelength of light in the IR region varies from about 2.5 to 40 μ where 1 μ = 10-4 cm.

Since λcmνsec-1 = c and E = h ν, then ν = c/ λ; ν is proportional to 1/ λ, the general convention in IR is to list frequencies proportional to energy. Frequencies in IR are generally expressed in 1/ λ units; ie cm-1 since these are proportional to energy. 1/(2.5*10-4 cm) = 4000 cm-1 ; 1/(40*10-4 cm) = 250 cm-1 To convert to true energy units, cm-1 needs to be multiplied by the speed of light, 3*10-10 cm sec-1, and Planck’s Constant, h =6.624*10-27 erg sec Infrared Spectroscopy

.5 Evib = (n+1/2)h(k/μ) /2 π where μ = m1m2/(m1+m2)

2 Erot = J(J+1)h /8 π2I; where I = moment of inertia

.5 2 2 Evib-rot = (n+1/2)h(k/μ) /2 π + J(J+1)h /(8 π I); n, J = ± 1.

In IR, the change in n is usually by +1, occasionally some + 2 also occurs (overtone). The number of molecules at room temperature that are in an n = 1 state is very small because the energy spacings are large. The rotational energy spacings are considerably smaller; J = ± 1 An example of a molecule with a small moment of inertia: HBr

.5 2 2 Evib-rot = (n+1/2)h(k/μ) /2 π + J(J+1)h /(8 π I); where I = moment of inertia n, J = ± 1. For most large molecules I is large. In this spectrum n = +1. J = ± 1

n =+1 ,J =0 In the condensed phase collision broadening also obcures the n=1, J =0 null A summary of the principle infrared bands and their assignments. R is an aliphatic group. Funct. Type Frequencies Peak Examples Group cm-1 Intensity Figure No. 3 C-H sp hybridized R3C-H 2850-3000 M(sh) 6, 18, 22 sp2 hybridized =CR-H 3000-3250 M(sh) 7, 13, 42 sp hybridized ≡C-H 3300 M-S(sh) 13 aldehyde C-H H-(C=O)R 2750, 2850 M(sh) 14, 15 N-H primary amine, amide RN-H2, RCONH2 3300, 3340 S,S(br) 18, 19 secondary amine, amide RNR-H, RCONHR 3300-3500 S(br) 20, 21 tertiary amine, amide RN(R3), RCONR2 none 22, 23 O-H alcohols, phenols free O-H 3620-3580 W(sh) 17, 24, 25 hydrogen bonded 3600-3650 S(br) 24, 25, 28 carboxylic acids R(C=O)O-H 3500-2400 S(br) 26, 27, 29, 30 C≡N nitriles RC≡N 2280-2200 S(sh) 31 C≡C acetylenes R-C≡C-R 2260-2180 W(sh) 32 R-C≡C-H 2160-2100 M(sh) 13

sp3 hybridized CH

100

(CH2)4 rock 40 % Transmittance CH , CH symmetric CH , CH bending 20 3 2 3 2 and asymmetric stretches vibrations

0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1

Figure IR-6. N-Decane, neat liquid, thin film:

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 sp3 hybridized CH

100 90 80 70 100 NH bend 60 2 90 50 80 40 symmetric70 and % Transmittance asymmetric NH 2 NH bend 60 2 30 stretches 50 20 40 symmetric and % Transmittance asymmetric NH2 10 30 stretches 400020 3500 3000 2500 2000 1500 1000 500 10 Wavenumbers, cm-1 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1 Figure IR-18. Butylamine, neat liquid; thin film:

CH3CH2CH2CH2NH2 sp3 hybridized CH

100

40 % Transmittance 20

0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1

Figure IR-22. Triethylamine, neat liquid; thin film: (CH3CH2)3N A summary of the principle infrared bands and their assignments. R is an aliphatic group. Funct. Type Frequencies Peak Examples Group cm-1 Intensity Figure No. 3 C-H sp hybridized R3C-H 2850-3000 M(sh) 6, 18, 22 sp2 hybridized =CR-H 3000-3250 M(sh) 7, 13, 42 sp hybridized ≡C-H 3300 M-S(sh) 13 aldehyde C-H H-(C=O)R 2750, 2850 M(sh) 14, 15 N-H primary amine, amide RN-H2, RCONH2 3300, 3340 S,S(br) 18, 19 secondary amine, amide RNR-H, RCONHR 3300-3500 S(br) 20, 21 tertiary amine, amide RN(R3), RCONR2 none 22, 23 O-H alcohols, phenols free O-H 3620-3580 W(sh) 17, 24, 25 hydrogen bonded 3600-3650 S(br) 24, 25, 28 carboxylic acids R(C=O)O-H 3500-2400 S(br) 26, 27, 29, 30 C≡N nitriles RC≡N 2280-2200 S(sh) 31 C≡C acetylenes R-C≡C-R 2260-2180 W(sh) 32 R-C≡C-H 2160-2100 M(sh) 13

sp2 hybridized CH

100

1410 40 1020 860 % Transmittance

0 4000 3500 3000 2500 2000 1500 1000 500

Wavenumbers, cm-1

Figure IR-8. Indene; neat; 0.05 mm cell: sp and sp2 hybridized CH

100

60 carbon-carbon 40 triple bond stretch % Transmittance

0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1

Figure IR-13. Phenylacetylene, neat liquid; thin H film (note that R here is an aromatic group): sp2 hybridized CH

100

40 % Transmittance 20

0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1 CO2Na Figure IR-42. Sodium benzoate, KBr pellet: Can the frequency dependence on hybridization be rationalized?

Table 2. Carbon Hydrogen Bond Strengths as a Function of Hybridization

Type of C-H bond Bond Strength IR Frequency kcal/mol cm-1 3 sp hybridized C-H CH3CH2CH2-H 99 <3000 2 sp hybridized C-H CH2=CH-H 108 >3000 sp hybridized C-H HC≡C-H 128 3300

h k 1 E = ()n + 2 πμ 2

The bond strength seems dependent of the amount of s character in the bond 1

k1 >k2

0 k2 Potential Energy

0123 Internuclear separation What would happen to k (force constant ) if 1 stretching a bond resulted in severe steric interactions?

0 Potential Energy

0123 Internuclear separation Me Me Me Me C Me C H C Me Me Me Me Hindered sp3 C-H

110

100

90 3017.18 80 % Transmission 1478.5

70 2952.5 ATR of tri-t-butylmethane

60 -4000 -3500 -3000 -2500 -2000 -1500 -1000 -500 Wavenumber cm-1 A summary of the principle infrared bands and their assignments. R is an aliphatic group. Funct. Type Frequencies Peak Examples Group cm-1 Intensity Figure No. 3 C-H sp hybridized R3C-H 2850-3000 M(sh) 6, 18, 22 sp2 hybridized =CR-H 3000-3250 M(sh) 7, 13, 42 sp hybridized ≡C-H 3300 M-S(sh) 13 aldehyde C-H H-(C=O)R 2750, 2850 M(sh) 14, 15 N-H primary amine, amide RN-H2, RCONH2 3300, 3340 S,S(br) 18, 19 secondary amine, amide RNR-H, RCONHR 3300-3500 S(br) 20, 21 tertiary amine, amide RN(R3), RCONR2 none 22, 23 O-H alcohols, phenols free O-H 3620-3580 W(sh) 17, 24, 25 hydrogen bonded 3600-3650 S(br) 24, 25, 28 carboxylic acids R(C=O)O-H 3500-2400 S(br) 26, 27, 29, 30 C≡N nitriles RC≡N 2280-2200 S(sh) 31 C≡C acetylenes R-C≡C-R 2260-2180 W(sh) 32 R-C≡C-H 2160-2100 M(sh) 13

aldehyde C-H

100 90 80 70 60 H-C=O 50 Impurity peak, probably O-H 40 % Transmittance 30 20 10 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1

Figure IR-14. Butanal, neat liquid, thin film:

CH3CH2CH2CHO aldehyde C-H

100

40 C-H stretch of -CHO % Transmittance

0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1

O CH Figure IR-15. Benzaldehyde, neat, thin film (R here is an aromatic group): Focusing on –NH2 A summary of the principle infrared bands and their assignments. R is an aliphatic group. Funct. Type Frequencies Peak Examples Group cm-1 Intensity Figure No. 3 C-H sp hybridized R3C-H 2850-3000 M(sh) 6, 18, 22 sp2 hybridized =CR-H 3000-3250 M(sh) 7, 13, 42 sp hybridized ≡C-H 3300 M-S(sh) 13 aldehyde C-H H-(C=O)R 2750, 2850 M(sh) 14, 15 N-H primary amine, amide RN-H2, RCONH2 3300, 3340 S,S(br) 18, 19 secondary amine, amide RNR-H, RCONHR 3300-3500 S(br) 20, 21 tertiary amine, amide RN(R3), RCONR2 none 22, 23 O-H alcohols, phenols free O-H 3620-3580 W(sh) 17, 24, 25 hydrogen bonded 3600-3650 S(br) 24, 25, 28 carboxylic acids R(C=O)O-H 3500-2400 S(br) 26, 27, 29, 30 C≡N nitriles RC≡N 2280-2200 S(sh) 31 C≡C acetylenes R-C≡C-R 2260-2180 W(sh) 32 R-C≡C-H 2160-2100 M(sh) 13

NH2- Stretching Frequencies

100 90 100 80 90 80 70 70 NHNH bend2 bend 60 60 2 50 50 40symmetricsymmetric and and 40 % Transmittance asymmetric NH2 % Transmittance asymmetric NH 30 stretches 2 30 stretches 20 20 10 10 4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 Wavenumbers,2500 2000 cm-1 1500 1000 500 Wavenumbers, cm-1 Figure IR-18. Butylamine, neat liquid; thin film:

CH3CH2CH2CH2NH2 NH2- Stretching Frequencies

120

100

40 % Transmittance symmetric and

20 asymmetric NH2 stretches 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber, cm-1 O CNH 2 Figure IR-19. Benzamide, KBr pellet (R here is an aromatic group): Focusing on –NHR A summary of the principle infrared bands and their assignments. R is an aliphatic group. Funct. Type Frequencies Peak Examples Group cm-1 Intensity Figure No. 3 C-H sp hybridized R3C-H 2850-3000 M(sh) 6, 18, 22 sp2 hybridized =CR-H 3000-3250 M(sh) 7, 13, 42 sp hybridized ≡C-H 3300 M-S(sh) 13 aldehyde C-H H-(C=O)R 2750, 2850 M(sh) 14, 15 N-H primary amine, amide RN-H2, RCONH2 3300, 3340 S,S(br) 18, 19 secondary amine, amide RNR-H, RCONHR 3300-3500 S(br) 20, 21 tertiary amine, amide RN(R3), RCONR2 none 22, 23 O-H alcohols, phenols free O-H 3620-3580 W(sh) 17, 24, 25 hydrogen bonded 3600-3650 S(br) 24, 25, 28 carboxylic acids R(C=O)O-H 3500-2400 S(br) 26, 27, 29, 30 C≡N nitriles RC≡N 2280-2200 S(sh) 31 C≡C acetylenes R-C≡C-R 2260-2180 W(sh) 32 R-C≡C-H 2160-2100 M(sh) 13

NH- Stretching Frequencies

120

100

% Transmittance 20

4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1

Figure IR-20. Diethylamine, neat liquid; thin film:

CH3CH2NHCH2CH3 NH- Stretching Frequencies

100 90 80 70 60 50 40 % Transmittance 30 20 Amide I Amide II 10 4000 3500 3000 2500 2000 1500 1000 500 Wavnumbers, cm-1

Figure IR-21. N-Methyl acetamide, neat liquid; thin film:

CH3CONHCH3 Focusing on –NR2 A summary of the principle infrared bands and their assignments. R is an aliphatic group. Funct. Type Frequencies Peak Examples Group cm-1 Intensity Figure No. 3 C-H sp hybridized R3C-H 2850-3000 M(sh) 6, 18, 22 sp2 hybridized =CR-H 3000-3250 M(sh) 7, 13, 42 sp hybridized ≡C-H 3300 M-S(sh) 13 aldehyde C-H H-(C=O)R 2750, 2850 M(sh) 14, 15 N-H primary amine, amide RN-H2, RCONH2 3300, 3340 S,S(br) 18, 19 secondary amine, amide RNR-H, RCONHR 3300-3500 S(br) 20, 21 tertiary amine, amide RN(R3), RCONR2 none 22, 23 O-H alcohols, phenols free O-H 3620-3580 W(sh) 17, 24, 25 hydrogen bonded 3600-3650 S(br) 24, 25, 28 carboxylic acids R(C=O)O-H 3500-2400 S(br) 26, 27, 29, 30 C≡N nitriles RC≡N 2280-2200 S(sh) 31 C≡C acetylenes R-C≡C-R 2260-2180 W(sh) 32 R-C≡C-H 2160-2100 M(sh) 13

Tertiary Amines

100

40 % Transmittance 20

0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1

Figure IR-22. Triethylamine, neat liquid; thin film:

(CH3CH2)3N Tertiary Amides

100

40 % Transmittance

0 4000 3500 3000 2500 2000 1500 1000 500

Wavenumbers, cm-1

Figure IR-23. N,N-Dimethylacetamide, neat liquid; thin

film: CH3CON(CH3)2 A summary of the principle infrared bands and their assignments. R is an aliphatic group. Funct. Type Frequencies Peak Examples Group cm-1 Intensity Figure No. 3 C-H sp hybridized R3C-H 2850-3000 M(sh) 6, 18, 22 sp2 hybridized =CR-H 3000-3250 M(sh) 7, 13, 42 sp hybridized ≡C-H 3300 M-S(sh) 13 aldehyde C-H H-(C=O)R 2750, 2850 M(sh) 14, 15 N-H primary amine, amide RN-H2, RCONH2 3300, 3340 S,S(br) 18, 19 secondary amine, amide RNR-H, RCONHR 3300-3500 S(br) 20, 21 tertiary amine, amide RN(R3), RCONR2 none 22, 23 O-H alcohols, phenols free O-H 3620-3580 W(sh) 17, 24, 25 hydrogen bonded 3600-3650 S(br) 24, 25, 28 carboxylic acids R(C=O)O-H 3500-2400 S(br) 26, 27, 29, 30 C≡N nitriles RC≡N 2280-2200 S(sh) 31 C≡C acetylenes R-C≡C-R 2260-2180 W(sh) 32 R-C≡C-H 2160-2100 M(sh) 13

Free and Hydrogen Bonded H-O-R

100 vapor 102 °C

80 liquid film

40 % Transmittance

0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1

Figure IR-24. The liquid and vapor spectra of n-hexanol. Free and Hydrogen Bonded H-O-R

120 Vapor phase spectrum(135 °C) 100

liquid film 60

% Transmittance 40

0 4000 3500 3000 2500 2000 1500 1000 Wavenumbers, cm-1

Figure IR-25. The liquid and vapor spectra of phenol. OH Free and Hydrogen Bonded H-O-R

120

100

80 vapor: 134 °C

40 liquid film

% Transmittance 20

4000 3500 3000 2500 2000 1500 1000 Wavenumber, cm-1

Figure IR-26. The liquid and vapor spectra of hexanoic acid:

CH3(CH2)4 CO2H A summary of the principle infrared bands and their assignments. R is an aliphatic group. Funct. Type Frequencies Peak Examples Group cm-1 Intensity Figure No. 3 C-H sp hybridized R3C-H 2850-3000 M(sh) 6, 18, 22 sp2 hybridized =CR-H 3000-3250 M(sh) 7, 13, 42 sp hybridized ≡C-H 3300 M-S(sh) 13 aldehyde C-H H-(C=O)R 2750, 2850 M(sh) 14, 15 N-H primary amine, amide RN-H2, RCONH2 3300, 3340 S,S(br) 18, 19 secondary amine, amide RNR-H, RCONHR 3300-3500 S(br) 20, 21 tertiary amine, amide RN(R3), RCONR2 none 22, 23 O-H alcohols, phenols free O-H 3620-3580 W(sh) 17, 24, 25 hydrogen bonded 3600-3650 S(br) 24, 25, 28 carboxylic acids R(C=O)O-H 3500-2400 S(br) 26, 27, 29, 30 C≡N nitriles RC≡N 2280-2200 S(sh) 31 C≡C acetylenes R-C≡C-R 2260-2180 W(sh) 32 R-C≡C-H 2160-2100 M(sh) 13

Hydrogen Bonded H-O-R

100

40 % Transmittance

0 4000 3500 3000 2500 2000 1500 1000 500

Wavenumbers, cm-1

Figure IR-27. Decanoic acid, neat liquid, thin film: CH3(CH2)8CO2H Hydrogen Bonded H-O-R

120

100

% Transmittance 40

20 symmetric and asymmetric

-NO2 stretch 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1

NO Figure IR-28. 4-Chloro-2-nitrophenol, KBr pellet: Cl 2 OH Hydrogen Bonded H-O-R

100

40 % Transmittance

0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber cm-1

COOH Figure IR-29. Benzoic acid; KBr disk 100 90 80 70 60 50 % Transmittance 40 30 20 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumbers, cm

Figure IR-30. Benzoic acid, KBr; band distortions and broadening caused by poor grinding, compare to Figure 29. When does one see a free –OH in the condensed phase? Free H-O-R

100 90 80 70 60 50 40

% Transmittance 30 20 10 0 4000 3500 3000 2500 2000 1500 1000 500

Wavenumbers, cm-1

Me Me Me Me C Figure IR-17. Tri-t-butylmethanol, KBr pellet: Me C OH C Me Me Me Me The –CN triple bond A summary of the principle infrared bands and their assignments. R is an aliphatic group. Funct. Type Frequencies Peak Examples Group cm-1 Intensity Figure No. 3 C-H sp hybridized R3C-H 2850-3000 M(sh) 6, 18, 22 sp2 hybridized =CR-H 3000-3250 M(sh) 7, 13, 42 sp hybridized ≡C-H 3300 M-S(sh) 13 aldehyde C-H H-(C=O)R 2750, 2850 M(sh) 14, 15 N-H primary amine, amide RN-H2, RCONH2 3300, 3340 S,S(br) 18, 19 secondary amine, amide RNR-H, RCONHR 3300-3500 S(br) 20, 21 tertiary amine, amide RN(R3), RCONR2 none 22, 23 O-H alcohols, phenols free O-H 3620-3580 W(sh) 17, 24, 25 hydrogen bonded 3600-3650 S(br) 24, 25, 28 carboxylic acids R(C=O)O-H 3500-2400 S(br) 26, 27, 29, 30 C≡N nitriles RC≡N 2280-2200 S(sh) 31 C≡C acetylenes R-C≡C-R 2260-2180 W(sh) 32 R-C≡C-H 2160-2100 M(sh) 13

100

% Transmittance -C≡N →

20 Top: CHCl3 vs CHCl3

Bottom: 5% in CHCl3 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1

Figure IR-31. trans-2-Phenyl-1-cyanoethene, in CHCl3 solution: Ph-CH=CH-CN ClCH2C≡N

The C-C triple bond in terminal acetylenes C≡C Triple Bond Stretch

100

80 → 60 carbon-carbon 40 triple bond stretch % Transmittance

0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1

Figure IR-13. Phenylacetylene, neat liquid; H thin film: ←2120 cm-1 The C≡C triple bond in symmetrical acetylenes C≡C Triple Bond Stretch Where is it?

100

40 % Transmittance 20

0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1 Figure IR-32. Diethyl acetylenedicarboxylate; neat liquid:

C2H5OCO-C≡C-CO2CH2CH3 The C≡C triple bond in internal acetylenes

The C≡C triple bond when next to a heteroatom Spectrum in hexanes (50%)