Orientational Isomers and Monolayer Structure of CH₃D Physisorbed on NaCl(100)

Abstract

Polarized infrared spectra of CH3D adsorbed on NaCl(100) in the temperature range 5−37 K are reported. Analysis of the ν2 spectral region reveals one transition dipole perpendicular to the surface and two oriented at 70 ± 20° (or 110 ± 20°) from the surface normal. These features are assigned to C−D stretching vibrations of molecules oriented in a tripod conformation with the deuterium in the “top” position (D-up) and in a tripod with the deuterium in a “base” position (D-down). The fraction of D-up orientational isomers is shown to decrease with decreasing temperature. An energy splitting of 100 ± 20 J mol-1 between D-up and D-down orientational isomers is consistent with this temperature dependence. A phase transition near 30 K associated with changes in rotatory motions has been observed. Below this transition, a splitting in the D-down region is shown to be statistically consistent with the position of the deuterium atoms on the surface. Some point toward substrate chloride ions and others point toward ‘gaps' between chloride ions.