Protonation thermochemistry of α,ω-alkane diols in the gas phase:: A theoretical study

lThe 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, Delta(p)Sdegrees(M) = Sdegrees(MH+) - Sdegrees(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 Delta(p)Sdegrees - (-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 Delta(p)Sdegrees(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.