Protonation Thermochemistry of α,ω-Alkane Diols in the Gas Phase: A Theoretical Study

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, Δ p S °(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 Δ p S ° (−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 Δ p S °(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.