19F Spin–Rotational Interactions in 1- and 2-Trifluoromethylnaphthalenes in the Liquid State

Abstract

The temperature dependences of the ¹H and ¹⁹F spin–lattice relaxation times in 1‐ and 2‐trifluoromethylnaphthalenes, and also in benzotrifluoride have been measured by the adiabatic fast passage method. The effects of dilution in toluene‐d₈ on ¹H and ¹⁹F $T_1\text{'}\mathrm{s}$ in the molecules studied have also been investigated. It has been found that the ¹⁹F relaxation rates in 2‐trifluoromethylnaphthalene and benzotrifluoride are determined by spin–internal‐rotation coupling and independent of intermolecular torques within the temperature region studied. An approximate relationship between the angular velocity correlation time and the barrier to internal rotation of the CF₃ group has been proposed. The estimated barriers to rotation, 0.2 and 0.4 kcal/mole in benzotrifluoride and 2‐trifluoromethylnaphthalene, are in agreement with those calculated by using approximate potential functions. Since there is a steric interaction between the CF₃ group and the hydrogen at the 8 position in 1‐trifluoromethylnaphthalene, the spin–internal‐rotation coupling is quenched to a large extent, and the spin‐rotational mechanism is mainly due to the spin–over‐all‐rotation coupling.