AB-INITIO STUDIES OF PROTON SPONGES - 4 - CALCULATING THE STRAIN-ENERGY

Quantum chemical methods are applied in order to estimate the strain c ontribution to basicity in diamine proton sponges. Three methods of es timating the strain induced by nitrogen lone pair-lone pair repulsion, are discussed. Strain virtues in five specific compounds, 1,8-bis(dim ethylamino)naphthalene, 4,5-bis(dimethylamino)phenanthrene, 4,5-bis(di methylamino)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, th e 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 e very type of proton sponge-certain compounds in which the basic center s are part of heterocyclic systems seem to require some other approach .