Thermochemical and structural properties of DMAN-“proton sponges”

We report a study on the energetics and structural properties of naphthalene-based proton sponges and their corresponding protonated cations. In particular, we have determined the experimental standard enthalpies of formation in the gas phase at T = 298.15 K, ΔfHmo(g), for the neutral and protonated DMAN [1,8-bis (dimethylamino)-naphthalene], (221.0 ± 7.3) and (729.0 ± 11.1) kJ·mol−1, respectively. A reliable experimental estimation of enthalpy associated with “strain” effect and hydrogen bond intramolecular (included within “enhanced basicity”, EB) contributions to the basicity of DMAN, were deduced from isodesmic reactions, −(29.1 ± 4.6) and (87.1 ± 11.9) kJ·mol−1, respectively. The gas-phase basicities (GB) of naphthalene-based proton sponges are compared with the corresponding aqueous basicities (pKa), covering a range of 149 kJ·mol−1 in GB and 11.5 in pKa. Density functional calculations at the M05-2X/6-311++G(d,p) level of theory were used to check the consistency of the experimental results and also to estimate the unavailable GB values of the considered species.

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► We determined the enthalpy of formation ΔfHmo(g) of neutral and protonated DMAN.
► It has been estimated the “strain” effect and hydrogen bond enthalpies in DMAN-H+.
► GB gas phase basicities of naphthalene proton sponges are compared with their pKa.