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

The proton affinities (PA) of the three first members of the series of α,ω-alkyldiamines, 1,2-ethanediamine ( 1 ), 1,3-propanediamine ( 2 ), and 1,4-butanediamine ( 3 ), were calculated at the G2(MP2) level. [PA(M) = 947.7, 977.7, and 999.8 kJ/mol for M = 1, 2, 3, respectively.] Protonation entropies, Δ S ° p = S °(MH + ) − S °(M), were estimated by explicitly considering the rotational barriers of the torsional modes in both the neutral molecules M = 1, 2, 3 and their protonated forms MH + . Calculated protonation entropy values are −17, −29, and −46 J·mol -1 ·K -1 for 1, 2, 3, respectively. Combining the calculated PA and Δ S ° p lead to calculated gas-phase basicities [GB calc (M) = 910.3, 936.9, and 953.6 kJ/mol for 1, 2, 3, respectively] in excellent agreement with experiment. [GB exp (M) = 912.4, 940.0, and 954.4 kJ/mol for 1, 2, 3, respectively.]