METHYL TORSION IN PROPENE INITIATES ETHYLENIC HYDROGEN WAGGING AND TWISTING

We have examined the methyl torsional barrier, potential function, and frequencies in propene by pure ab initio methods using various flexing models, extended basis sets, and systematically increased correlation levels. The principal conclusions are the following: (1) a physically reasonable internal rotation potential function can be obtained only when out-of-plane ethylenic skeletal hydrogen flexing motions which include ethylenic twisting are part of the torsional motion; (2) the internal rotation barrier is calculated using MP4(SDTQ)/6-311G(3df,2p) ab initio theory at 689 cm-1, only 5 cm-1 below the experimental barrier obtained from microwave and far infrared measurements; (3) correlation and basis set effects beyond MP2 and 6-311G(d,p) levels are very important for accurate ab initio calculation of the propene methyl torsional barrier, the correlation effect alone exceeding 85 cm-1; (4) both the fundamental and first overtone frequencies are calculated at the MP4(SDTQ)/6-311G(3df,2p) level within 1 cm-1 of experimental values using a purely theoretical kinetic energy constant (F = 7.1731 cm-1) calculated from MP2/6-311G(3df,2p) fully optimized conformer geometries.