SPECTRA AND STRUCTURE OF SMALL RING COMPOUNDS .62. CONFORMATIONAL STABILITY, STRUCTURAL PARAMETERS, AB-INITIO CALCULATIONS AND VIBRATIONAL ASSIGNMENT OF CYCLOPROPYLMETHYL KETONE

The conformational stability, barriers to internal rotation, and funda mental vibrational frequencies of cyclopropylmethyl ketone, C-C3H5C(CH 3)O, have been obtained from Hartree-Fock ab initio calculations with the RHF/3-21G and RHF/6-31G basis sets, as well as the 6-31G* basis s et with electron correlation at the MP2 level, and the results are com pared to those obtained from experiment. The data are consistent with the predominant rotamer having the cis conformation (carbonyl bond cis to the ring). A second form, having a ''near'' trans structure, is ca lculated to have a larger total dipole moment than the cis form, which accounts for its increased abundance in the liquid compared to that i n the gas. A complete vibrational assignment is proposed based on expe rimental data and normal coordinate results from the ab initio calcula tions. The asymmetric torsional barrier has been calculated to be appr oximately 2000 cm-1 and this result along with others is compared to t he corresponding data obtained from both experiment and theory for the cyclopropylcarbonyl halides.