Trifluoromethoxy benzene in the gas phase studied by electron diffraction and spectroscopy supplemented with ab initio calculations

Trifluoromethoxybenzene (TFMB) can occur in a conformation with the OCF3 gr oup in the plane of the phenylgroup ([e]-form) and with the OCF3 group perp endicular to the phenyl plane ([o]-form). Ab initio calculations on the MP2 /6-31G** level give the [o]-form as the energy minimum with the [e]-form as the top of the rotation barrier about 1.4 kcal/mol above the [o]minimum. I n contrast, DFT calculations on the BSLYP/cc-pvtz level give the [e]- and [ o]-forms as energy minima with a small (<0.3 kcal/mol) energy difference an d a small (<0.5 kcal/mol) barrier between them. Extensive vibrational spect roscopic studies (IR and Raman) of the gaseous, the liquid and the solid (g lass and crystal) state gave evidence of TFMB occurring in the gas phase, a nd in the liquid and amorphous solid as a mixture of [o]- and [e]-forms. In the crystal (annealed at 155 K) the [o]-form dominates. Electron diffraction intensities of the gas, fitted to a dynamic model, res ulted for room temperature in the ratio [o]/[e] = 70%/30%, i.e. the [o]-for m being 0.4 kcal/mol more stable than the [e]-form with a barrier of 2.8 kc al/mol between them. (C) 2001 Elsevier Science B.V. All rights reserved.