Protonation thermochemistry of alpha,omega-alkyldiamines in the gas phase:A theoretical study

The proton affinities (PA) of the three first members of the series of alph a,omega -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 entro pies, DeltaS(p)degrees = S degrees (MH+) -S degrees (M), were estimated by explicitly considering the rotational barriers of the torsional modes in bo th the neutral molecules M = 1, 2, 3 and their protonated forms MH+. Calcul ated protonation entropy values are -17, -29, and -46 J . mol(-1) K-1 for 1 , 2, 3, respectively. Combining the calculated PA and DeltaS(p)degrees 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.]