UNIMOLECULAR REACTIONS OF IONIZED ALKANES - THEORETICAL-STUDY OF THE POTENTIAL-ENERGY SURFACE FOR CH3-CENTER-DOT AND CH4 LOSSES FROM IONIZED BUTANE AND ISOBUTANE

In connection with the observed losses of CH3. and CH4 from ionized butane (1) and isobutane (2) in the gas phase, ab initio molecular orbital calculations at the UMP2, QCISD, and QCISD(T) levels of theory with the 6-31G(d) and 6-31G(d,p) basis sets have been used to investigate the relevant parts of the C4H10.+ ground-state potential energy surface. The isomerization of 1 to 2 is found to take place via a transition structure (X) consisting of a nonclassical H-bridged propyl cation coordinated to the methyl radical. X lies 19.9 kcal/mol above the lowest-energy trans conformer of 1 and 3.6 kcal/mol above the energy of the dissociation fragments sec-propyl cation plus methyl radical. In addition to mediating the 1 --> 2 isomerization, X also mediates the losses of both CH3. and CH4 from 1 through non-minimum energy reaction paths which are energetically accessible. The CH4 elimination from 2 is found to take place via a transition structure (XI) which can be viewed as a sec-propyl cation coordinated to the methyl radical. XI is calculated to lie 12.9 kcal/mol above 2 and 3.1 kcal/mol below the energy of its loosely bound components. These theoretical results are consistent with mass spectrometry experimental findings reported in the literature.