VAPOR-PHASE ASYMMETRIC-TOP SIMULATION OF THE CARBONYL STRETCH IN SOME SIMPLE ESTERS

The observed vapour-phase infrared contours of the carbonyl stretching in methyl formate, tert-butyl formate and methyl chloroformate have been simulated using harmonic rigid asymmetric top theory. The simulations confirm the s-cis conformation of these compounds. The dipole change vector for nu(C = O) in all cases was found to make an appreciable angle with the C = O bond. The same contours were also simulated using an anharmonic model based on a Morse function for the C = O stretching. For all three molecules the anharmonic model results in a superior simulation of the experimental contour. The anharmonic model also allowed the satisfactory simulation of the first and second overtones of the C = O stretching in methyl formate and of the first overtone of this vibration in methyl chloroformate.