Nuclear Magnetic Resonance Spectroscopy CHM 504 Nuclear Magnetic Resonance Spectroscopy

NMR spectrometer

Information from 1H NMR Spectrum 1. Number of different kinds of equivalent atoms. – Number of different signals 2. Number of equivalent atoms of each kind (only in 1H NMR; more difficult in 13C NMR). – Integration (area under each signal) 3. Chemical environment of each kind of atom (functional groups) – Chemical shift 4. Connectivity (nearby atoms). – Splitting of signals

1H NMR Spectrum of CH3CO2CH3

Chemical Shift Independent of spectrometer used. Most 1H signals in the range 0-12 ppm (from right to left) Correlates with chemical environment (functional groups)

Regions of 1H NMR spectrum

1H Chemical shifts and functional groups d (ppm) Type of H 0.5 – 1.5 1.5 – 2.5 2.5 – 3.0 Y = O, N, Cl, Br 2.5 – 4.5 4.5 – 6.5 6.5 – 9.0 Aromatic H 9.5 – 10.5 Aldehyde H 1.5 – 6.0 Alcohol OH 9.5 - 12 Carboxylic acid OH

Integration “Integration” is the determination of the area under each peak. The spectrometer measures this in arbitrary units and displays it as a step graph. Area under a peak ∝ number of H atoms giving rise to that peak. Thus the “Integral” allows you to deduce how many equivalent H atoms of each kind are present. Does not work for 13C NMR.

1H NMR Spectrum of Methyl 2,2-Dimethylpropanoate

Signal splitting Coupling to neighbouring H nuclei (2-3 bonds away, i.e., attached to same or adjacent C atoms). Neighbouring H nuclei may be in +1/2 or -1/2 spin state (approx. 1:1) Affects the effective magnetic field felt by the nucleus. Beff = B0 – Bshielding ± Bneighbouring Signal is split into two (doublet) in 1:1 ratio. Implication: 1 neighbouring H.

Signal splitting (contd) Bneighbouring is independent of B0. Hence, the split signals are separated by the same frequency difference regardless of the spectrometer used. This frequency difference is called the coupling constant, J (typically 1-18 Hz). Splitting is mutual. If nucleus A splits the signal of nucleus B, then vice versa with the same J. NO SPLITTING IS OBSERVED BETWEEN EQUIVALENT NUCLEI. Thus, coupling is observed across 2 bonds (H’s attached to the same C) only if they are non-equivalent.

Splitting patterns No. of equivalent splitting H’s Multiplicity Peak intensity ratio 1 Doublet (d) 1:1 2 Triplet (t) 1:2:1 3 Quartet (q) 1:3:3:1 4 Quintet 1:4:6:4:1 5 Sextet 1:5:10:10:5:1 6 Septet 1:6:15:20:15:6:1

1H NMR spectrum of CH3CH2Br

Interpretation A typical pattern for a CH3CH2 (ethyl) group. The 3 H triplet implies a CH3 with a neighbouring CH2 group. The 2 H quartet implies a CH2 with a neighbouring CH3 group.

1H NMR Spectrum of (CH3)2CHI

Interpretation A typical pattern for a (CH3)2CH (isopropyl) group. The 6 H doublet implies 2 CH3 groups with a neighbouring CH (i.e., attached to a CH). The 1 H septet implies a CH with 2 neighbouring CH3 groups.

1H NMR Spectrum of toluene

Information from 13C NMR Spectrum 1. Number of different kinds of equivalent atoms. – Number of different signals 2. . Chemical environment of each kind of atom (functional groups) – Chemical shift

13C Chemical shifts and functional groups d (ppm) Type of H 0 - 60 Alkyl C 30 - 80 C–Y (Y = O, N, Cl, Br) 65 - 85 Alkyne C 100 - 150 Alkene C 110 - 160 Aromatic C 160 - 185 Carbonyl C (carboxylic acids, esters, amides) 180 - 220 Carbonyl C (aldehydes, ketones)

13C NMR spectrum of 6-methyl-5-hepten-2-ol