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, 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.

Gas‐phase basicities of polyfunctional molecules. Part 1: Theory and methods

“…A systematic study shows that if X has a proton affinity between that of O and C of CO the barrier for rearrangement completely disappears (Chalk & Radom, 1997). Other examples of rearrangements of protonated or cationized species involving assisted proton (or even alkyl cation) transport have been reported so far, from small species HNNO + /NNOH + (Ferguson, 1989), CH 3 CO + /CH 3 OC + (Cunje et al, 1998) to 2‐tetrahydropyranol, a model for the study of sugar epimerization (Morpurgo et al, 2000).…”

Electron bombardment matrix isolation of Rg/Rg′/methanol mixtures (Rg= Ar, Kr, Xe): Fourier-transform infrared characterization of the proton-bound dimers Kr2H+, Xe2H+, (ArHKr)+ and (ArHXe)+ in Ar matrices and (KrHXe)+ and Xe2H+ in Kr matrices

“…20 It has also been shown in theory that rare gases can act as neutral bases for proton-transport catalysis in the CH 3 OH˙ϩ/˙CH 2 OH 2 ϩ isomerization 21 and in other systems such as the HCO ϩ to COH ϩ proton transfer isomerization. 22,23 Given the presence of rare gas in these experiments as the carrier/support gas it is assumed that the methanol radical cation can readily isomerize to its distonic isomer ͑reaction 2͒. The observation of both isotopomers of the Rg 2 H ϩ species and the various scrambled isotopomeric by-products listed above in the EBMI experiments with CD 3 OH as the precursor is experimental evidence of this isomerization process.…”

Proton-transport catalysis and proton-abstraction reactions: An ab initio dynamical study of X+HOC+ and XH++CO (X=Ne, Ar, and Kr)

“…[1][2][3][4][5][6][7][8][9][10][11] However, because there is a substantial barrier of about 150 kJ mol Ϫ1 separating the two forms, [1][2][3][4][5][6][7]11 both isomers can be separately observed as stable species in the gas phase. [8][9][10][12][13][14] For example, the microwave spectra of both HOC ϩ and HCO ϩ have been recorded.…”

“…[15][16][17] The barrier for the rearrangement of HOC ϩ to HCO ϩ has been found to be substantially reduced, or in some cases even eliminated, as the result of interaction with an appropriate neutral molecule ͑X͒. 1,10,14,18,11 This is an example of what Bohme has described as catalyzed proton transport, 19 or what may be referred to more generally as ion-transport catalysis. 20 The reason for the barrier lowering is straightforward.…”