Abstract
A series of phenylethylamine and amphetamine derivatives were synthesized in order to determine the nature of the internal and overall molecular dynamics in these systems. ^13^C and ^2^H NMR relaxation times were measured. For the latter measurements, compounds specifically deuteriated in the side‐chains were synthesized. The relaxation data were analysed via a computer model incorporating the spectral density function for axially symmetric anisotropic motion. The motion of these molecules was found to be substantially anisotropic, with rotation about the symmetry axis of the molecule being at least twice as fast as rotation normal to the symmetry axis. The results of more complex models of axially symmetric anisotropic molecular motion, involving internal rotation or conic diffusion of the side‐chains, were found to be sensitive to variation of the input data, but the correlation times for internal rotation of the ring—C‐β bond were longer than might have been expected. Similarly, owing to its flexibility, the effective correlation times of atoms in the side‐chain were found to be faster than those of the ring carbons, but did not always decrease along the side chain. Intermolecular interactions or aggregation states in CDCl~3~ may reduce segmental motion of side‐chains. A particular feature of the current study is that it was demonstrated that deuteriation of the side‐chain gives a unique opportunity to determine independently side‐chain correlation times via the quadrupolar relaxation times of the ^2^H nucleus, and to compare these results with those of the carbon atoms. These correlation times were found to be in very good agreement with those obtained from the ^13^C data.