STRUCTURE OF METHYL CYANOFORMATE FROM MICROWAVE SPECTROSCOPY AND ABINITIO CALCULATIONS

The microwave spectra of seven isotopic species of methyl cyanoformate, CD3OC(O)CN, CD2HOC(O)CN, (CH3OC(O)CN)-C-13, (CH3OC(O)CN)-O-18, (CH3OC(O)CN)- -13, (CH3OC(O)CN)-C-13 and CH3OC(O)CN)-N-15 have been measured and assigned. A barrier to internal rotation of 407 +/- 1 cm-1 (1164 +/- 3 cal/mol) has been obtained for seven symmetric internal rotor species from the internal rotation analyses. The rotational constants from nine isotopic species have been used to determine both r(s) and r0 structural parameters for the s-trans conformer (methyl group trans to the cyanide group). All independent structural parameters could be determined. Optimized structures were calculated for the s-trans conformer by ab initio Hartree-Fock gradient calculations with both the 3-21G and 6-31G* basis sets as well as with MP2/6-31G* calculations. With the 6-31G* basis set, the barrier to interconversion of the s-trans form to the s-cis form was calculated to be 4055 cm-1 (11.59 kcal/mol) with the s-trans conformer being more stable by 2364 cm-1 (6.76 kcal/mol). The barrier to internal rotation of the methyl group was calculated to be 442 cm-1 (1264 cal/mol).