AB-INITIO PERSPECTIVE ON THE EXPERIMENTAL IDENTIFICATION OF CHLOROFORMIC ACID BY IR SPECTROSCOPY

An ab initio quartic force field has been determined for the trans con formation of chloroformic acid at the MP2 level of theory using a 6-31 1G* basis set. Anharmonic vibrational frequencies have been determine d for the fundamentals of this rotamer using the standard perturbation theory approach. Harmonic vibrational frequencies are reported for th e cis rotamer and optimised geometries have been obtained for both cis and trans rotamers of each species. The internal rotation barrier for chloroformic acid is found to be 3872 cm(-1) (46 kJ mol(-1)). The IR spectrum obtained by Pimentel and Jensen in 1967 and assigned to trans -chloroformic acid represents the only experimental evidence for the e xistence of chloroformic acid in the vapour phase (J. Phys. Chem., 196 7, 71, 1803). Comparison drawn between their data and the results obta ined here reveal differences which create some doubt regarding the ass ignment of Pimentel's spectrum to chloroformic acid.