Theoretical studies on proton affinities of H2N-(CH2)n-NH2 (n = 2−10) diamines at gas phase. Good correlation with protonation constants in solution

A theoretical study on complete protonation of a series of diamine molecules with general formula H2N-(CH2)n-NH2 (n = 2−10, L2−L10) has been reported. The gas-phase protonation energies were computed using density functional theory (DFT) calculations. The standard 6-31G∗ and 6-311++G∗∗ basis sets were used in all calculations. Three species, L, HL+ and H2L2+ can be considered in protonation steps of diamine molecules. Among these, the HL+ is involved in both the first, L + H+ → HL+, and second, HL+ + H+ → H2L2+, steps. Two different structures were considered for latter species: (I) a linear-like structure (II) a cyclic structure due to intramolecular hydrogen bonding. The trends for variations of calculated PA1 in the series of these molecules, is very similar to that of their measured protonation constants when we consider the linear structure for all species. Furthermore, for latter structures there are good correlations between the calculated proton macroaffinities in the gas-phase with corresponding protonation constants in solution. The latter observation has led us to predict the stepwise protonation constants for L7 and L9.