A MICROWAVE AND ABINITIO STRUCTURE DETERMINATION OF METHYL CHLOROFORMATE

The microwave spectra of two isotopic species of methyl chloroformate, CH318OC35ClO and CH318OC37ClO, have been recorded from 18.0 to 39.5 GHz. The rotational constants derived from the spectra were used together with those previously reported for six other isotopic species to determine r0 structural parameters for the s-trans conformer (CH3 trans to Cl). The following heavy atom parameters were obtained: r(CCl) = 1.735(14) Å, r(CO) = 1.341(16) Å, r(OCH3) = 1.448(5) Å, ∠ ClC=O = 125(2)°, ∠OCCl = 108.9(5)° and ∠ COC= 114.4(6)°. The carbon-hydrogen distances were determined from their vibrational frequencies in the infrared spectrum. Optimized structures were calculated for both the s-trans and s-cis conformers by ab initio Hartree-Fock gradient calculations employing the 3-21G* and 6-31G* basis sets. With the 6-31G* basis set, the barrier to interconversion of the s-trans form to the s-cis conformer was calculated to be 3670 cm-1 (10.49 kcal/mol) with the s-trans conformer being more stable by 2220 cm-1 (6.35 kcal/mol). The barrier to internal rotation of the methyl group was calculated to be 492 cm-1 (1407 cal/mol) at the 6-31G* basis set level compared to 428(1) cm-1 (1244(3) cal/mol) obtained from the observed microwave splittings for CH3 18OC35ClO. These results are compared to similar quantities for some related molecules. © 1990.