Chapter 12
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Frequency of absorption decreases with
Increased atomic mass
Frequency of absorption increases with
Increased bond energy
A molecule is IR active if
Its movement in the electric field creates a change in dipole (must be polarizable)
A molecule is IR inactive if
There is no change in dipole moment, it is either symmetrical or non-polarizable.
IR active water molecules stretch in what way?
Asymmetrical stretching, or bending (towards one another)
Overtones
Occur with very strong vibrations (c=o) and appear as weak absorptions at double the fundamental frequency.
Why do conjugated carbonyl groups vibrate at lower frequencies than non-conjuated carbonyls?
Resonance reduces reduces dipole moment b/c electron density is spread out, which lowers stretch frequency.
Why do Nitriles exhibit stronger absorbency (longer spikes) than Alkynes?
C-N bond is more polarizable.
Functional Groups that vibrate at frequency of 3300cm^(-1) and appearance.
alcohol (O-H) - broad Amines/amide (N-H) - broad, sharp, or with spikes Alkyne (C-C)- always sharp, usually strong
Functional Groups that vibrate at frequency of 3000cm^(-1) and appearance.
Alkane (C-H) - just below 3000 Alkene (C=H) - just above 3000 Acid (O-H) - very broad
Functional Groups that vibrate at frequency of 2200cm^(-1)
Alkyne (C=-H) - just below 2200 Nitrile (C=-N) - just above 2200
Functional Groups that vibrate at frequency of 1710cm^(-1) (very strong)
Carbonyls: ketones and acids: 1710 aldehydes: 1725 + 2700,2800 esters: 1735 + 2500-3500
Functional Groups that vibrate at frequency of ~1685cm^(-1) and appearance.
conjugated carbonyls: ketones: 1685 aldehydes: 1690 acids: 1687
Functional Groups that vibrate at frequency of 1660cm^(-1)
conjugated alkenes inimes (C=N) - stronger than C=C amide (C=O group)
Functional Groups that vibrate at frequency of 1600cm^(-1)
Aromatic C=C bonds
Functional Groups that vibrate at frequency of 1000-1200cm^(-1)
C-O bonds of ethers, esters, and alcohols.
quaternary
methine C-H
methylene CH2
methyl CH3
quaternary
methine C-H
methylene CH2
methylene CH2