Ch 14 - Important IR Signals

includes specific signal wavenumbers and wavenumber ranges for general groups or patterns

wavenumber of C-H

~3000 cm^-1

wavenumber of C-D

~2200 cm^-1

wavenumber of C-O

~1100 cm^-1

wavenumber of C-Cl

~700 cm^-1

(600-800 cm^-1)

wavenumber of C triple bonded to N

~2200 cm^-1

(2200-2300 cm^-1)

wavenumber of C=N

~1600 cm^-1

wavenumber of C-N

~1100 cm^-1

(1000-1200 cm^-1)

wavenumber range of single bonds (excluding H-bonds)

below 1500 cm^-1 (400-1500 cm^-1)

depicted on right side of spectrum (in fingerprint region)

located on right because they are the weakest bonds

wavenumber range of double bonds

1600-1850 cm^-1

wavenumber range of triple bonds

2100-2300 cm^-1

wavenumber range of bonds to H

2700-4000 cm^-1

strongest bonds because H is so small

diagnostic region

region above 1500 cm^-1 (on the left side of the spectrum)

generally has fewer peaks

provides clearest information

contains signals from H-bonds, triple bonds, and double bonds

different compounds may be different to differentiate here (for e.g., see figure 14.8 in textbook)

fingerprint region

region below 1500 cm^-1 (on the right side of the spectrum)

contains signals from single bonds, resulting from stretching and bending (vibrational excitation)

lots of signals, difficult to analyze

every compound has a unique pattern of signals in this region (hence its name)

wavenumber of sp³-hybridized C-H

~2900 cm^-1

wavenumber of sp²-hybridized C-H

~3100 cm^-1

wavenumber of sp-hybridized C-H

~3300 cm^-1

wavenumber of C=O in ketone

1720 cm^-1

wavenumber of C=O in conjugated, unsaturated ketone

1680 cm^-1

wavenumber of C=O in ester

1735 cm^-1

wavenumber of C=O in conjugated, unsaturated ester

1710 cm^-1

wavenumber range and shape of O-H participating in H-bonding

~3200-3600 cm^-1

broad signal

wavenumber and shape of free O-H

~3600 cm^-1

remember: “free” O-H means not participating in H-bonding

sharper signal, usually shallower than broad O-H

wavenumber range and shape of O-H in carboxylic acid

~2200-3600 cm^-1 .

very broad; extends over usual C-H signals

occurs because of dimers resulting from 2 H-bonding interactions for carboxylic acids

wavenumber and shape of C=O in carboxylic acid

just above 1700 cm^-1 (~1750)

broad and deep

wavenumber of primary amines

2 signals

1 signal at 3350 cm^-1, 1 signal at 3450 cm^-1

due to different ways a molecule can stretch/bend

wavenumber of secondary amines

~3300 cm^-1

one signal only

wavenumbers of C-H in aldehyde

2750 and 2850 cm^-1 (2 weak signals)

wavenumber range and signal intensity of C=O bond

~1650-1820 cm^-1

very strong, dips low

wavenumber range and signal intensity of C=C bond

1600-1700 cm^-1

not very intense

does not come down low

wavenumber ranges for benzene

1450-1600 cm^-1 and 1650-2000 cm^-1

wavenumber range of C triple bonded to C

2100-2200 cm^-1

wavenumber of C-Br

500-600 cm^-1