⚗️ Spectroscopy — NMR & IR
IR: 3300-3500 broad=OH, 2700-2900 CHO, 1700-1750=C=O
Key IR Absorption Peaks
Broad 3200–3550 = O-H stretch. Broad 3100–3500 = N-H. Sharp 2700–2850 = aldehyde C-H. 1710 = ketone C=O. 1735 = ester C=O. 1715 = carboxylic acid. 1680 = amide. Below 1500 = fingerprint region.
⚗️ Spectroscopy — NMR & IR
NMR chemical shifts: TMS=0, alkyl=0-2, alkene=4.5-6, benzene=6.5-8, aldehyde=9-10, carboxylic acid=10-12
NMR Chemical Shifts
Downfield (high ppm) = deshielded. Upfield (low ppm) = shielded. TMS reference = 0 ppm. Alkyl H: 0–2. Allylic: 2–3. Alkyne: 2–3. Alkene: 4.5–6.5. Aromatic: 6.5–8.5. Aldehyde: 9–10. COOH: 10–12.
⚗️ Spectroscopy — NMR & IR
N+1 rule: n equivalent neighbors = n+1 peaks (splitting pattern)
NMR Splitting Patterns
A proton with n equivalent neighboring H's splits into n+1 peaks. Singlet(1), doublet(2), triplet(3), quartet(4), quintet(5). Integration tells number of H's. Coupling constant J measures peak separation.
⚗️ Spectroscopy — NMR & IR
M+ = molecular ion peak = molecular weight of compound
Mass Spectrometry Basics
M+ peak = molecular weight. M+1 peak: 13C isotope contribution (~1.1% per carbon). M+2 peak: indicates Cl (M:M+2 = 3:1) or Br (M:M+2 = 1:1). Base peak = most abundant fragment.
⚗️ Spectroscopy — NMR & IR
DEPT NMR: CH3 and CH point up, CH2 points down, C = absent
DEPT NMR
DEPT (Distortionless Enhancement by Polarization Transfer) distinguishes C types: CH3 and CH give positive peaks, CH2 gives negative peaks, quaternary C gives no signal. Used with 13C NMR.