o A vibrating molecule with n atoms has 3n-6 modes of vibration
o IR active molecules absorb energy from the rapidly reversing electric field found in an EM wave. This field stretches and compresses the molecule.
o Looser = lower tighter = higher
- IR: Reading the charts
o Conjugated double bonds are looser than normal ones (spread charge)
o Sp3 is looser than sp2 (less s character)
o Hydrogen bonding gives broad peaks for OH, sharper ones for NH
o Primary NH (or amide) – 2 spikes, Secondary NH (or amide) – 1 spike
o Aldehydes give a pair at 2700 and 2800
o Carboxylic acid’s OH peak is shifted down to 3000
o Double bonds conjugate carbonyls, lowering them
o The N in an amide pulls electrons away from the C==O, lowering it
o C-N bonds are stiffer and more polar than C-C
- Mass Spec: The theory
o Only the positive ions are detected by the mass spectrometer
o Because HRMS can get the mass to a good number of decimal places, we can figure out exactly which atoms are in the molecule
- Mass Spec: The charts
o The isotopes of atoms come in predictable percentages, and they give rise to M+1 and M+2 peaks
o Molecular ions usually have even numbers and fragments have odd numbers. When nitrogen is involved, it’s the other way around.
o A methyl radical is weak, but ok. A methyl cation can’t be formed at all.
o A branch carbon atom is a good place to cleave since it leaves a secondary or tertiary carbocation
o Allylic-stabilized carions show strong absorption
o Alcohols lose water readily, so you almost never see their molecular ion. Instead you see the “M-18” peak.
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