Infrared Matrix-Isolation Studies of Nuclear-Spin-Species Conversion

Abstract

Studies of the time evolution of intensities in the fine structure of infrared bands of CH3D, H2O, and NH3 isolated in rare-gas matrices in the temperature region 6.5°–20°K have been made. The molecule CH3D exhibits a fine structure at 6.5°K with time-dependent intensities. The data are interpreted in terms of nuclear-spin-species conversion. Because of rapid reconversion at 20°K it is concluded that CH3D cannot be obtained at room temperature with a nonequilibrium distribution of spin isomers by low-temperature equilibration. The time dependence of the fine structure of the H2O bending mode at 6.5°K has been reinvestigated. The essence of the previous work is confirmed. The rate of nuclear-spin conversion is found to be rapid at 30°K. The fine structure of the NH3 umbrella motion exhibits a time dependence which indicates that most of the fine structure observed is not due to rotation.