AB-INITIO QUANTUM-MECHANICAL STUDY ON THE ORIGIN OF THE PK(A) DIFFERENCES OF THE PROTON SPONGES 1,8-BIS(DIMETHYLAMINO)NAPHTHALENE, 1,8-BIS(DIMETHYLAMINO)-2,7-DIMETHOXYNAPHTHALENE, 1,6-DIMETHYL-1,6-DIAZACYCLODECANE, AND 1,6-DIAZABICYCLO[4.4.4]TETRADECANE

Ab initio quantum mechanical calculations were used in studying the or igin of the exceptionally high basicities of four diamines (13-16) wit h pK(al) values ranging from 12.1 to 25. The computational approach in volved the calculation of the gas-phase proton affinities of the molec ules studied at the MP2/6-31G//HF/6-31G* level and the solvation ener gies with the polarizable continuum model at the HF/6-31G level. The calculated gas-phase and aqueous-phase proton affinities of a structur ally diverse series of amines were compared with the corresponding exp erimental gas-phase proton affinities and pK(al) values. The calculate d values were found to be in reasonable agreement with the experimenta l ones. The high basicities of the studied diamines were found to orig inate from the nitrogen lone-pair repulsion, solvation effects, and st rong intramolecular hydrogen bonds. Each of these factors were found t o be able to increase the pK(al) values of the high-basicity diamines by 2-6 pK(a) units. The relative contributions of the factors varied b etween the compounds. The nitrogen lone-pair repulsion was estimated t o be the most important factor in increasing the pK(al) values. In add ition, barriers for proton transfers between the nitrogens of selected diamines were calculated, and comparison was made between the barrier heights and the geometries of the diamines.