THE ROTATION-TUNNELING SPECTRUM OF ARGON-ACETALDEHYDE - THEORY AND ANALYSIS

The microwave spectrum of argon-acetaldehyde is split into quartets which are rotation-tunneling in character. Qualitatively, each asymmetric-rotor level is first split into an A and an E component by internal rotation of the methyl group. These A and E components are then each further split into another doublet, as the argon (effectively) tunnels from a position above the acetaldehyde heavy atom plane to an equivalent position below that plane. The observed transitions (220 lines with J less than or equal to 7) have been fitted to nearly experimental accuracy (8 kHz) using 29 parameters. The Hamiltonian was expressly designed for the Ar-acetaldehyde system and was constructed using group-theoretical considerations and principles from the generalized internal axis method formalism originally developed for the water dimer. This is the first application of the method to a case involving an inversion motion and methyl internal rotation. (C) 1995 Academic Press, Inc.