Proton affinity of deuterated acetonitrile estimated by the kinetic method with full entropy analysis

Abstract

Branching ratios for the dissociation of proton-bound dimers of selected alkyl nitriles, [R1CN⋯H+⋯NCR2], have been measured as a function of collision energy, using a triple quadrupole mass spectrometer. The system shows a small collision energy dependence consistent with small differences in entropy requirements for the competing fragmentation channels. The extended form of the kinetic method provides a value for the proton affinity and an approximate value for the relative entropy difference between the two dissociation channels (which corresponds approximately to the relative entropy of protonation of the two bases). A comparison is made between these results and those from earlier standard kinetic method data treatments. The proton affinity of d3-acetonitrile, estimated by the extended kinetic method, is 186.3 ± 1.2 (±1.4) kcal mol−1, where the standard deviation is given, followed by the 90% confidence limits in parentheses. The proton affinity of acetonitrile, estimated using the extended kinetic method as 188.2 ± 1.2 (±1.4) kcal mol−1, is statistically the same as that obtained for d3-acetonitrile. The relative entropies of protonation, Δ(ΔS), for d3-acetonitrile and acetonitrile, referenced to a series of alkyl nitriles, are 1.8 ± 0.3 (±0.3) and −1.1 ± 0.3 (±0.3) cal mol−1 K−1, respectively. Direct comparison of the isotopomers of acetonitrile using the standard and extended kinetic methods was employed to arrive at a more accurate value for the proton affinity difference between d3-acetonitrile and acetonitrile, and this method yielded a difference of ∼0.2 kcal mol−1. The direct comparison was also used to show that the proton affinity difference is a result of isotopic substitution. Normal secondary kinetic isotope effects were observed for the dissociation of the proton-bound dimer, CH3CN⋯H+⋯CD3CN. The branching ratio, kH/kD, was constant at 1.2 over the laboratory collision energy range of 5–50 eV.