Line assignments and global analysis of the tunneling–rotational microwave absorption spectrum of dimethyl methylphosphonate

Abstract

Line assignments were carried out for about 600 Fourier-transform microwave transitions for dimethyl methylphosphonate involving levels of all six symmetry species in the G 18 molecular symmetry group appropriate for three large-amplitude motions and covering J and K values of 1 ⩽ J ⩽ 6 and 0 ⩽ | K | ⩽ 3 . The assignments are based on combination–difference loops, variations in line shape for different symmetry species caused by the small internal-rotor splitting patterns of the third, high-barrier methyl top, and agreement with theoretically expected positions. A global fit of 609 lines to a phenomenological tunneling–rotational Hamiltonian with 54 constants was performed, yielding a standard deviation of 8.0 kHz, which is close to the experimental measurement uncertainty. The A/ E tunneling splitting for the lowest barrier methyl group internal rotation motion was determined indirectly to be about 34 GHz. The much smaller tunneling splitting for the methoxy interchange motion was determined (with some assumptions) to be 3.3 MHz.