UNIMOLECULAR REACTIONS OF IONIZED ALKANES - THEORETICAL-STUDY OF THE POTENTIAL-ENERGY SURFACE FOR CH3-CENTER-DOT AND CH4 LOSSES FROM IONIZED BUTANE AND ISOBUTANE
S. Olivella et al., UNIMOLECULAR REACTIONS OF IONIZED ALKANES - THEORETICAL-STUDY OF THE POTENTIAL-ENERGY SURFACE FOR CH3-CENTER-DOT AND CH4 LOSSES FROM IONIZED BUTANE AND ISOBUTANE, Journal of the American Chemical Society, 116(24), 1994, pp. 11078-11088
In connection with the observed losses of CH3. and CH4 from ionized bu
tane (1) and isobutane (2) in the gas phase, ab initio molecular orbit
al 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 surfac
e. The isomerization of 1 to 2 is found to take place via a transition
structure (X) consisting of a nonclassical H-bridged propyl cation co
ordinated 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 addit
ion 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 fou
nd to take place via a transition structure (XI) which can be viewed a
s a sec-propyl cation coordinated to the methyl radical. XI is calcula
ted 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 literatu
re.