Large-amplitude motion in 1,4-cyclohexadiene and 1,4-dioxin: theoretical background for joint treatment of spectroscopic, electron diffraction and ab initio data

A flexible self-consistent approach based on the adiabatic separation between large- and small-amplitude motions has been developed for the joint treatment of gas-phase electron diffraction (ED) and spectroscopy data. The Hamiltonian developed gains versatility by directly proceeding from assumed model properties to energy levels and wavefunctions. In addition to the vibrational terms, it explicitly includes rotational effects as well as interactions between overall rotation and intramolecular motion. A particular form of a Hamiltonian is specified by the system to be considered. If the vibrational energy is much higher than rotational energy, the latter can be separated and treated conventionally as perturbation. Six-membered ring planar floppy molecules 1,4-cyclohexadiene and 1,4-dioxin (1,4-dioxacyclohexa-2,5-diene) were selected for illustration. Large-amplitude approach described below and previously developed technique based on the general framework of rigid molecules with account for anharmonicity have been compared in predicting theoretical ED intensities for these molecules. We conclude that no noticeable deviations between ED intensities obtained using the two mentioned theoretical approaches have been observed when large-amplitude vibrations were governed by the approximately quadratic potential function (the case of 1,4-cyclohexadiene) while the case of quartic potential (for 1,4-dioxin) resulted in significantly different ED patterns. (C) 2002 Elsevier Science B.V. All rights reserved.