J. Phys. Chem. A 1998, 102, 478-483

Interconversion of ROC⁺ and RCO⁺ (R = H and CH₃): Gas-Phase Catalysis by Argon and Dinitrogen

Alwin Cunje, Christopher. F. Rodriquez, Diethard K. Bohme and Alan. C. Hopkinson*

Department of Chemistry, York University, Toronto, Ontario, M3J 1P3, Canada

Molecular orbital calculations using density functional theory at the B3LYP/6-311++G(d,p) level have been used to optimize structures for ions COR⁺M and MRCO⁺ and also for the transition structures COR⁺M-(ts) for their interconversion (R = H, CH₃ and M = Ar and N₂).  For the unsolvated ions and for ions COH⁺M, MHCO⁺, COH⁺M(ts) the optimized structures were used for single-point calculations at QCISD(T)(full)/6-311++G(2df,p). Critical points on the COH⁺ and ArCOH⁺ potential energy surfaces were also optimized at MP2(full)6-311++G(3df,3pd). For the uncomplexed ions COR⁺, the barriers to 1,2-migration of R⁺ at B3LYP/6-311++G(d,p) are 35.4 kcal/mol^-1 for R = H and 14.2 kcal mol^-1 for R = CH₃. Inclusion of a dinitrogen molecule removes this barrier by permitting COR⁺ to deposit R+ on N₂ followed by CO retrieving the R⁺ to produce the lower energy isomer, RCO⁺. Argon has a lower R⁺ affinity than the oxygen atom of CO and does not remove R⁺ from COR⁺. Preferential stabilization by argon of the transition structure for the 1,2-migration of R⁺ over stabilization of COR⁺ at the minimum results in a reduction in the barrier to rearrangement. The gas-phase rearrangments of ions COR⁺ via "solvated" transition structures COR⁺Ar(ts) are calculated to have barriers of 8.3 kcal mol^-1 for R = H and 5.7 kcal mol^-1 for R - CH₃, while for COH⁺Ar at MP2(full)/6-311++G(3df,3pd) the barrier is only 2.0 kcal mol^-1. These findings indicate noble gas atoms may catalyze the rearrangement of cations rather than simply cool them by collisions.