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is reflected to the receiver to be processed and recorded. In solid samples, the number

of spin–spin interactions is greatly enhanced due to intermolecular interactions that

are absent in dissolved samples due to translation and rotation movements. As a result,

the resonance signals broaden significantly. However, high-resolution spectra can be

obtained by spinning the solid sample at an angle of 54.7(magic angle spinning).

The sophisticated pulse sequences necessary for multidimensional NMR require

Table 13.1NMR-derived structural parameters of molecules

Parameter Information Example/Comment

Chemical shift Chemical group^1 H,^13 C,^15 N,^31 P

Secondary structure

J-couplings (through
bond)

Dihedral angles^3 J(amide-H, Ha),^3 J(Ha,Hb),...

NOE (through space) Interatomic distances <0.5 nm

Solvent exchange Hydrogen bonds Hydrogen-bonded amide protons are protected
from H/D exchange, while the signals of other
amides disappear quickly

Relaxation/line
widths

Mobility, dynamics,

conformational/chemical

exchange

The exchange between two conformations,

but also chemical exchange, gives rise to two

distinct signals for a particular spin

Torsion angles

Residual dipolar
coupling

Torsion angles^1 H–^15 N,^1 H–^13 C,^13 C–^13 C,...

Computer

Radio

frequency

oscillator

Radio

frequency

detector

Sweep

supply

Magnet pole Magnet pole

Cold N 2

Fig. 13.11Schematic diagram of an NMR spectrometer with cryoprobe.

542 Spectroscopic techniques: II Structure and interactions