REARRANGEMENT OF DIMETHYLCARBENE TO PROPENE - STUDY BY LASER FLASH-PHOTOLYSIS AND AB-INITIO MOLECULAR-ORBITAL THEORY

Laser flash photolysis (Nd:YAG laser, 355 nm, 35 mJ, 150 ps) of dimeth yldiazirine and dimethyldiazirine-d(6) produces dimethylcarbene (DMC) and dimethylcarbene-d(6) (DMC-d(6)), respectively. The carbenes were t rapped with pyridine to form ylides which absorb around 364 nm. It was possible to resolve the growth of the ylides as a function of pyridin e concentration in Freon-113, alpha,alpha,alpha-trifluoromethylbenzene , and perfluorohexane as a function of temperature. The observed rate constant (k(obs)) of ylide formation was linearly dependent on the con centration of pyridine in all solvents and at all temperatures. From p lots of k(obs) versus [pyridine] it was possible to extract values of k(pyr) (the absolute rate constant of reaction of the carbene with pyr idine) and tau, the carbene lifetime in the absence of pyridine, and t heir associated Arrhenius parameters. In Freon-113 and alpha,alpha,alp ha trifluoromethylbenzene the carbenes decay both by rearrangement and by reaction with solvent. In perfluorohexane the carbene decay appear s to be predominantly unimolecular. The experimental results are compa red with ab initio molecular orbital calculations. The experimentally determined barrier to disappearance of DMC in perfluorohexane (2.56 +/ - 0.05 kcal/mol) is much smaller than that calculated (7.4 +/- 2 kcal/ mol) using ab initio molecular orbital theory. The Arrhenius parameter s and isotope effects indicate that the rearrangement of DMC in perflu orohexane has a large component of quantum mechanical tunneling. The a ctivation energy for the disappearance of DMC-d(6) in perfluorohexane (5.63 +/- 0.03 kcal/mol) is consistent with calculations which indicat e that QMT makes only a minor contribution to the deuterated system un der the conditions of this study.