Protonation thermochemistry of α,ω-alkane diols in the gas phase: A theoretical study
Résumé
The proton affinities of 1,2-ethane diol (1), 1,3-propane diol (2), and 1,4-butane diol (3) were calculated by ab initio molecular orbital calculations at the G2(MP2) level. The values (PA(1) = 795.3, PA(2) = 851.5, and PA(3) = 882.4 kJ*mol-1) are in agreement with recent determinations using the kinetic method but at variance with previous results obtained from equilibrium constant measurements. Entropy differences, ΔpS°(M) = S°(MH+) − S°(M) (M = 1−3), were estimated by explicitly considering the rotational barriers of the torsional modes in both the neutral and the protonated molecules, M. Absolute values of calculated ΔpS° (−5, −22, and −32 J*mol-1*K-1 for M = 1, 2, and 3, respectively) are lower than that presently available in the literature. Combining the calculated PA(M) and ΔpS°(M) leads to gas-phase basicities GB(M) equal to 761.4, 812.4, and 840.4 kJ*mol-1 for M = 1, 2, and 3, respectively.