Ab initio studies of proton sponges. 4. Calculating the strain energy

Quantum chemical methods are applied in order to estimate the strain contribution to basicity in diamine proton sponges. Three methods of estimating the strain induced by nitrogen lone pair-lone pair repulsion, are discussed. Strain virtues in five specific compounds, 1,8-bis(dimethylamino)naphthalene, 4,5-bis(dimethylamino)phenanthrene, 4,5-bis(dimethylamino)fluorene, 1,8-bis(dimethylamino)-2,7-dimethoxynaphthalenc and 1,10-bis(dimethylamino)anthracene, are estimated at the HF/6-31G** and BLYP/G-31G** levels of theory. At the Hartree-Fock (HF) level, the values computed for these five compounds vary widely, from 32 to 84 kJ mol(-1). The BLYP (electron-correlated) calculations indicate that the HF method overestimates strain values by (on average) around 18 kJ mol(-1). None of the three methods discussed here are applicable to every type of proton sponge-certain compounds in which the basic centers are part of heterocyclic systems seem to require some other approach.