Spectroscopic studies of the order–disorder phase transition in 1,3-dimethyladamantane

Abstract

The order–disorder phase transition in 1,3-dimethyladamantane has been investigated by IR and Raman spectroscopy, and by proton spin–lattice relaxation time measurements, as functions of temperature. The splittings in the internal vibrational modes and number of lattice modes suggest that the structure of the ordered phase is either orthorhombic or monoclinic with at least two molecules per unit cell. Rotation of the methyl groups about their three-fold axes is responsible for relaxation in the ordered phase, and molecular tumbling occurs in the disordered phase. The barriers to molecular motion, determined from the spin–lattice relaxation times, are 14.3 and 15.8 kJ mol^–1, respectively.