Knowledge Bank

Numerous studies have shown that methane and its various deuterated isotopologs CHD $(n=0-4)$ can rotate relatively freely in solid parahydrogen. Less is known, however, of whether methyl groups can also carry out rotation (or internal rotation) in {\it p}-H$2$, and even less is known of whether other large-amplitude motions involving hydrogen atoms (e.g., inversion or H transfer) can occur easily. The present talk, which is part of an attempt to systematically investigate these latter questions in National Chiao Tung University, presents infrared spectra for the nondegenerate ($\nu 1,\nu 2,\nu 3$) and doubly degenerate ($\nu 4,\nu 5,\nu 6$) vibrational fundamentals of CH$3$F, together with a theoretical analysis strongly suggesting that CH$3$F does indeed rotate about its symmetry axis ({\it a} axis) in {\it p}-H$2$, but does not rotate about axes perpendicular to the symmetry axis. The theoretical analysis makes use of the fact that {\it a}-axis rotation is expected to give rise: (i) to energy levels of the form $E=AK2$, (ii) to first-order Coriolis interactions in degenerate vibrational states (E states), and (iii) to different nuclear spin functions for rovibrational A $(K=0)$ and E $(K=1$ and $2)$ states. Experimental values for the relatively slow E $\to$ A conversion rate in {\it p}-H$2$ have also been determined. The results for CH$3$F will be compared with our earlier results for CH$3$OH.