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 conformation of chloroformic acid at the MP2 level of theory using a 6-311G** basis set. Anharmonic vibrational frequencies have been determined for the fundamentals of this rotamer using the standard perturbation theory approach. Harmonic vibrational frequencies are reported for the 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 existence of chloroformic acid in the vapour phase (J. Phys. Chem., 1967, 71, 1803 ). Comparison drawn between their data and the results obtained here reveal differences which create some doubt regarding the assignment of Pimentel's spectrum to chloroformic acid.