Analysis of the pure-rotational spectrum in the ν28 = 1 excited torsional state and torsional couplings of trans-ethyl methyl ether

Abstract

The trans-ethyl methyl ether molecule has three low-lying torsional modes, that is, two inequivalent methyl internal rotations and an asymmetric skeletal torsion. The internal rotations of the CCH 3 and OCH 3 methyl rotors and the skeletal torsion correspond to the vibrational modes, ν 28, ν 29 and ν 30 respectively. In this study, the microwave absorption spectrum in the ν 28 = 1 CCH 3 torsional state was analyzed for the first time. Nine hundred fifty seven lines up to J = 48 and K = 4 were assigned, and the rotational, centrifugal distortion and internal rotational tunneling parameters were determined with the use of a tunneling matrix formalism. By combining the present results on the ν 28 = 1 torsional state with those for the ν 30 = 1 skeletal torsional state and the ν 29 = 1 OCH 3 torsional state, torsional couplings are estimated in order to understand quantitatively the inverted A/E sequence patterns observed for those three excited torsional states.