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

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(2) CH(3) and M = Ar and N(2)). 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 = CH(3). Inclusion of a dinitrogen molecule removes this barrier by permitting COR(+) to deposit R(+) on N(2) 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 = CH(3), 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.