13C spin–lattice relaxation times and molecular motion in N-alkyltetrahydroisoquinoline derivatives

Abstract

¹³ C Spin–lattice relaxation times have been measured for N-methyl-1,2,3,4-tetrahydroisoquinoline and several quaternary N-methyl-N-alkyl-1,2,3,4-tetrahydroisoquinolinium salts, and have been analysed in terms of an anisotropic molecular motion. The calculated orientations of the major axes of molecular overall rotation reflect well the delicate differences in conformation of these derivatives. In quaternary salts, the longer alkyl chain restricts the rotational motion about the major axis more than that about the perpendicular axis, and brings the molecular motion close to the isotropic rotational one. Internal rotations of N-methyl groups are also discussed. In these derivatives, the values of the barrier to methyl rotation, V_O, resemble one another, as well as the rotational diffusion rates, D_i, and are quite large owing to steric hindrance by the lone pair or by the N-alkyl groups.