triangle(triangle S-double dagger) and triangle(triangle S) for the competing bond cleavage reactions in (CH3CN)(ROH)H+ [R=CH3, C2H5, C3H7, (CH3)(2)CH]

摘要

Microcanonical variational transition-state theory was used to determine the entropies of activation for hydrogen-bond cleavage reactions leading to CH3CN + ROH2+ in a series of acetonitrile-alcohol proton-bound pairs (CH3CN)(ROH)H+ (where R = CH3, CH3CH2, CH3CH2CH2, and (CH3)(2)CH). In each case, the dissociation potential surface was modelled at the MP2/6-31 + G(d) level of theory. The dissociating configurations having the minimum sums-of-states were identified in each case and the resulting entropies of activation were calculated. Combined with previous work on the competing reaction leading to CH3CNH+ + ROH, the results permitted the determination of the Delta(Delta S-double dagger) in each proton-bound pair. For the (CH3CN)(CH3OH)H+ and (CH3CN)(CH3CH2OH)H+ proton-bound pairs, the entropies of activation for the two dissociating channels are essentially the same [i.e., Delta(Delta S-double dagger) = 0], while Delta(Delta S-double dagger) for the propanol-containing pairs ranged between 40 and 45 J K-1 mol(-1). The latter non-zero values are due to a combination of the location of the dividing surface in each dissociation and the rapidity with the frequencies of the vanishing vibrational modes go to zero as they are converted to product translations and rotations during the dissociation.