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

Abstract

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 M···RCO+ 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, M···HCO+, and 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 rearrangements 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.