Infra-red spectroscopy 2

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Infra-red spectroscopy 
 
Vibrational & Rotational 
Spectroscopy 
 
O-H 3400 cm-1 C Ξ C 2150 cm-1 
 
N-H 3400 cm-1 
 
C=O 1715cm-1 
 
C-H 3000 cm-1 
 
C=C 1650cm-1 
 
C Ξ N 2250 cm-1 
 
C-O 1100cm-1 
 
Alkanes 
• C-H stretch around 3000cm-1 
• CH3 methyl groups bends occur at ~ 1375 cm
-1 
• CH2 bending ~ 1460cm
-1. 
Alkanes 
O-H 3400 cm-1 C Ξ C 2150 cm-1 
 
N-H 3400 cm-1 
 
C=O 1715cm-1 
 
C-H 3000 cm-1 
 
C=C 1650cm-1 
 
C Ξ N 2250 cm-1 
 
C-O 1100cm-1 
 
Alkenes -Vinyl compounds 
• =C—H stretch to the left of 3000cm-1 
• C=C stretch 1660-1600 cm-1, move to right with 
conjugation 
• =C—H out of plane (OOP) bending 1000 - 650cm-1 
• C=C stretching 
– ~ 1650 cm-1 (w) when unconjugated. 
– Cis alkenes stronger absorptions than trans – cis 
less symmetric with greater dipole moment. Double 
bonds in rings also weak 
• conjugation with C=O or C=C increases single bond 
character right & increase in intensity 
 
Alkenes -Vinyl compounds 
• Endo - sensitive to ring size 
 
 
 
 
 
absorption frequency decreases as internal angle 
decreases until it reaches minimum at 90o, cyclopropene 
= 60o. 
 
 
 
 
 
C
C
C90o
C
C
C
120o
90o vibrational — C — 
mode not coupled 
to C=C 
greater or less than 
90o — C — vibration 
couples with C=C 
 
 
Alkenes – exo double bonds 
• Exo - increase in absorption frequency with decreasing 
ring size 
alkenes 
• Conjugation of a C=C with either another C=C or 
a C=O confers more single bond character and 
causes a shift to lower frequency 
C-H OOP bending vibrations 
• useful for indicating substitution pattern on double bond 
• monosubstituted double bond: two strong bands 
 @ 990 & 910 cm-1. 
 910 cm-1 810 when electron 
 withdrawing group attached to 
 double bond 
• disubstituted double bonds 
 cis ~ 700cm-1 (s) 
 trans ~ 970 cm-1 (s) 
 1,1 disub ~ 890 cm-1 (s) 
• trisubstituted ~ 815 cm-1 (m) 
C-H OOP – correlation chart 
O-H 3400 cm-1 C Ξ C 2150 cm-1 
 
N-H 3400 cm-1 
 
C=O 1715cm-1 
 
C-H 3000 cm-1 
 
C=C 1650cm-1 
 
C Ξ N 2250 cm-1 
 
C-O 1100cm-1 
 
Alkynes 
Alkynes 
• C-H stretch ~ 3300cm-1 
• C C stretch ~ 2150cm-1 
Alkynes 
• Disubstituted or symmetrically substituted triple 
bonds give weak or no absorption. 
C-H stretch 
Bond C-H =C-H -C-H 
Type sp -1s sp2-1s sp3-1s 
Length 1.08Å 1.10Å 1.12Å 
ir frequency 3300cm-1 ~3100cm-1 ~2900cm-1 
C-H bending 
C-H bending 
Aromatic molecules 
• =C-H stretch occurs just above 3000cm-1 
• =C-H out of plane (oop) bending 690- 900cm-1 
used to assign ring substitution pattern 
• C=C stretch often occurs in pairs at 1600 & 
1475cm-1 
• Overtone & combination bands appear between 
2000 and 1800cm-1. 
Aromatic molecules 
• OOP C-H bending (690 – 1000 cm-1) 
more useful than C-H (1000 - 1300 cm-1) 
• OOP used to assign position of 
substitution on ring. 
• OOP useful for alkyl, alkoxy, halo, amino 
or carboxyl substituted aromatic molecules 
but not for nitro carboxylic acid or sulfonic 
acid derivatives. 
Overtone & combination bands 
O-H 3400 cm-1 C Ξ C 2150 cm-1 
 
N-H 3400 cm-1 
 
C=O 1715cm-1 
 
C-H 3000 cm-1 
 
C=C 1650cm-1 
 
C Ξ N 2250 cm-1 
 
C-O 1100cm-1 
 
Alcohols & Phenols 
Alcohols & Phenols 
• OH stretch: broad band 3500-3200cm-1 due to 
intermolecular H-bonding. As alcohol diluted 
with CCl4 get sharp peak ~ 3600cm
-1. 
• C-O stretching { C-C-O } 
– C-O coupled to C-C so can be used to assign 
1o 2o,3o alcohol or phenol 
– phenols 1220 cm-1 
– 3o alcohols 1150 cm-1 
– 2o alcohols 1100 cm-1 
– 1o alcohol 1050 cm-1 
Alcohols & Phenols 
• unsaturation causes a shift to lower cm-1 
 
CH
2
CH CH CH
3
OH
2o 1100 1060 cm-1 
O-H stretch 
• Undiluted alcohol show broad band ~ 3400 cm-1. 
Broad due to H-bonding. If diluted with CCl4 band 
narrows and moves to higher cm-1 
Ethers 
• strong C –O - C stretch 1300 – 1000 cm-1 . 
Alcohols & esters also absorb in this range so 
must eliminate these by looking for OH (3500 
cm-1) and C=O ( 1735cm-1) 
• Aliphatic ethers - single strong absorption at 
1120cm-1 due to asymmetric stretching 
• Aryl or vinyl ethers 
 
Ethers 
• Aryl or vinyl ethers 
 
two bands at ~ 1250 cm-1 & 1040 cm-1 
• 1250 cm-1 band due to asymmetric stretch 
arising due to strengthening of bond due to 
resonance 
Ar O R CH
2
CH O R
CH
2
CH O R CH2 CH O R..
.. +
..