N-Alkanes interpreted in terms of 4 vibrations:- stretching and bending of CH and CC bonds CC bends: ca. 500 cm1 (out of spectral window) CC stretches: 1200800 cm1, weak bands not of value for interpretation (fingerprint)more characteristic CH stretches: occurs from 3000 - 2840 cm1CH3:2962 cm1, asymmetrical stretch2872 cm1, symmetrical stretch

CH2:2926 cm1, asymmetrical stretch2853 cm1, symmetrical stretch

Note precision!

C12H25C-HStretchesCH3 (as) 2964 cm1CH2 (as) 2924 cm1CH3 (s) 2872 cm1CH2 (s) 2853 cm1C-HBendsCH3 (as) 1450 cm1CH3 (s) 1375 cm1CH2 (s) 1467 cm1scissorCH2 (s) 720 cm1RockAlkanes of 7 ormore carbonsn-alkanes

C10H22C12H26SimilarBut NotIdenticalFingerprinting

Unconjugated Alkenes linear alkenes: - C=C stretch: moderate to weak absorption at 1667-1640 cm1 C=CH: - stretch: 3000 cm1

Unconjugated Alkenes1-deceneC=C stretch1642 cm1CH2 rock 720 cm1C-H stretch3049 cm1Out of plane C=CH bends991, 910 cm1 Overtone of 910 cm-1Absorption(1820 cm1)

IR tutorhttp://members.aol.com/charlieabr/downloads.html

cyclic alkenes: - C=C stretch: sensitive to ring strain conjugated alkenes: -the alkene bond stretching vibrations in alkenes w/o a center of symmetry give rise to two C=C stretches -for symmetrical molecules, e.g. butadiene, only the asymmetric stretch is observed cumulated alkenes: - C=C=C stretch (asymmetric): 20001900 cm1

Conjugated Double BondsC-H stretch3090 cm1SymmetricalC=C stretch1640 cm1(weak)AsymmetricalC=C stretch1598 cm1 (strong)Out of plane C=CH bends

990, 892 cm1

Alkynes stretch: weak absorption at 2260-2100 cm1 - not observed for symmetrical alkynes (v. weak for pseudo symmetric alkynes - terminal alkynes (R-C C-H) absorptions are stronger than internal (R-C C-R) absorptions C CH stretch: - 33333267 cm1 - strong, narrow (as compared to OH or NH)

C CH bend: - 700-610 cm1: broad, strong absorption - 1400-1220 cm1, overtone of above

Terminal Alkynes

AlkyneC-H stretch3310 cm1AlkyneCC stretch2119 cm1AlkyneC-H bend630 cm1AlkyneC-H bend overtone1260 cm1

Mononuclear Aromatic Hydrocarbons (benzene) Out of plane bending of aromatic C-H bonds: most informative - 900675 cm1 - intense bands, strongly coupled to adjacent hydrogens on the ring - position and number of bands gives information about the substitution pattern (particularly useful for alkyl substituted aromatics. Substitution of polar groups can give rise to exceptions)

C=CH stretch: 31003000 cm1

C C stretch: 1600-1585; 1500-1400 cm1

C C out of plane ring bending: 600-420 cm1

Mononuclear AromaticsAromaticC-H out ofPlane bends728 cm1 694 cm-1AromaticC-H Stretches3087, 3062,3026 cm1Overtone bands2000-1650 cm1Aromatic C-C Stretches1600-1585; 1500-1400 cm1Aromatic C-H in plane bends1300-1000 cm1

out ofplane ring bending428 cm1

768 cm-1691 cm-1742 cm-795 cm-1

Alcohols and Phenols Strongly dependent on hydrogen-bonding - non-hydrogen bonded OH groups absorb strongly in the 3700-3584 cm1 range. vapor phase; dilute, non polar solvent; very hindered

Intermolecular hydrogen bonding occurs as conc. increases; - accompanied by a shift to lower freq. (3550-3200 cm1), at the expense of the free OH band - Pronounced for intramolecular H-bonding

CO stretching Vibrations Alcohols (1260-1000 cm1) Phenols (1800-1260 cm1)

primary alcohol: 1050-1085 cm-1 secondary alcohol: 1085-1125-1 tertiary alcohol: 1125-1200 cm-1

1073 cm-11110 cm11202 cm1

Ethers COC stretching bands are most characteristic bands - strong because of strong dipole moment

aliphatic ethers: strong band due to asymmetrical stretching, 1150-1085 cm1 (usually 1125 cm1) weak band due to symmetrical stretching (lower freq)

Alkyl aryl ethers: asymmetrical stretch at 1275-1200 cm-1 symmetrical stretch at 1075-1020 cm1

Vinyl alkyl ethers: asymmetrical stretch at 1225-1200 cm-1 symmetrical stretch at 1075-1020 cm1

CarbonylsC=O stretch easily recognized, intense band

Ketones, aldehydes, acids, esters, lactones, acid halides, anhydrides, amides and lactams all show C=O stretching in the region 1870-1540 cm1.

Position is determined by (1) physical state (2) electronic andmass of neighboring groups (3) conjugation (4) hydrogen bonding(5) ring strain

Ketones aliphatic: normal position of a neat aliphatic ketone is 1715 cm1

competing effects

Ketones conjugation: shifts position to lower frequency alkene or phenyl group causes absorption in the 1685-1666 cm1 region. For a,b-unsaturated carbonyls, 2 absorptions may be observed

For a,b-unsaturated carbonyls, 2 absorptions may be observed

1685 cm-13350 cm-1Overtone of C=O stretch

1,3-Diketones1725 cm-11630 cm-1OH stretching

Cyclic KetonesThe bond angle influences the absorption frequency of the C=OIn strained rings, interaction with the adjacentC-C bonds increases the frequency of C=O stretching

Aldehydes Aliphatic aldehydes: C=O stretch at 1740-1720 cm1 Electron withdrawing groups shift to higher frequency Conjugative groups shift to lower frequency (1710-1685 cm1)C=O stretchCH stretch28302695 cm1 Often, two bands are observed (the other is a result of an overtone of the CH bend of the aldehyde)

Carboxylic AcidsOH stretch free OH (ca. 3520 cm-1) is observed only in the vapor phase or in very dilute (

EstersC=O stretch saturated aliphatic esters: C=O: 17501735 cm1 formates, a,b-unsaturated, and benzoate esters: 1730-1715 cm1 phenyl or vinyl esters: 1770-1780 cm1CO stretches (strong absorptions; asymmetrical coupled vibrations) saturated aliphatic esters (except acetates): CO: 12101163 cm1 acetates: 1240 cm1 a,b-unsaturated esters: 13001160 cm1 benzoate esters: 13101250 cm1

1749 cm1C=O1243 cm1 CO1763 cm1C=O1199, 1164, 1145 cm1 CO

1770 cm1C=O1823 cm1C=O1730 cm1C=O

Acid HalidesAnhydrides Two carbonyl stretches (symmetrical and asymmetrical)

AmidesNH stretching:3520 (as), 3400 (s) cm1 (dilute)3350 (as), 3180 (s) cm1 (conc)35003400 cm1 (dilute)3330-3060 (s) cm1, multiple bands due to H-bonding (conc)C=O stretching (Amide I band):1690 cm1 (dilute)1650 cm1 (conc) (except CH3CONH2, which absorbs at 1694 cm1 )1680-1700 cm1 (dilute)1640 cm1 (conc) 1680-1630 cm1 (H-bonding not possible, unless in protic solvent)Two bands

AmidesNH Bending (Amide II band):1620-1590 cm1 (dilute)- separate from amide I1655-1620 cm1 (conc)- overlap with the amide I band1550-1510 cm1 (dilute)1570-1515 (s) cm1 (conc)Lactams1650 cm1 1750-1700 cm11760-1730 cm1Secondary lactams do not have an NH bending band (Amide band II) near 1550 cm-1. This is a Characteristic of s-trans lactams.

16621662 cm-1 (I)1655 cm-1 (I)1565 cm-1 (II)(II)

Amines NH stretching - in dilute solution, primary amines (RNH2) display 2 bands, near 3500 and 3400 cm1. These represent free asymmetrical and symmetrical stretches. - in dilute soln, secondary amines (R2NH) display one band near 3350-3310 cm1. - weaker and sharper than OH

- neat primary aliphatic amines (alkylNH2) absorb at 3400-3300 and 3330-3250 cm-1. ArylNH2 absorb at slightly higher frequencies.

Nitriles2260-2240 cm12240-2222 cm1Nitro compounds 2 bands from the asymmetrical and symmetrical stretching of the bond

1523 (as)1347 (s)

Vinyl alkyl ethers: C=C stretch at 1660-1610 cm-1 often a doublet

trans: ~1620 cm1cis: ~1640 cm1 vinyl ethers- wagging shifted to lower frequency