Ab initio conformational analysis of N-formyl L-alanine amide including electron correlation

Trifluoromethoxybenzene (TFMB) can occur in a conformation with the OCF3 group in the plane of the phenylgroup ([e]-form) and with the OCF3 group perpendicular to the phenyl plane ([o]-form). Ab initio calculations on the MP2/631G** 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. In contrast, DFT calculations on the B3LYP/cc-pvtz level give the [e]- and [o]-forms as energy minima with a small (<0.3 kcal/mol) energy difference and a small (<0.5 kcal/mol) barrier between them. Extensive vibrational spectroscopic studies (IR and Raman) of the gaseous, the liquid and the solid (glass and crystal) state gave evidence of TFMB occurring in the gas phase, and 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, resulted for room temperature in the ratio [o]/[e]=70%/30%, i.e. the [o]-form being 0.4 kcal/mol more stable than the [e]-form with a barrier of 2.8 kcal/mol between them.