An ab initio study of the interaction in dimethylamine dimer and trimer

Clusters consisting of two and three dimethylamine molecules were studied by using the HF, DFT/B3LYP, and MP2 ab initio methods in conjunction with the 6-31+G* and aug-cc-pvdz/cc-pvdz basis sets. Three different minima were located for the dimer, two of which form a N-H . . .N hydrogen bond and present similar interaction energies. The most stable structure of the dimer possesses C-s symmetry and an interaction energy of - 15.6 kJ/mol. The least stable minimum has an interaction energy of -7.9 kJ/mol and exhibits no N-H . . .N hydrogen bonds (the interaction is established via the methyl hydrogen atoms). In all the structures, electron correlation exhibits a significant contribution (more than 40% of the overall energy). Only cyclic structures were considered for the trimer, the most stable of which possesses an interaction energy of -43.9 kJ/mol. The dipole moments for the dimer differ by up to 30% from the vector addition of the molecular dipoles (in the trimer minima, this difference amounts to 40%); also, N-H distances are lengthened by effect of the interaction (by up to 0.004 Angstrom, in the dimer and 0.009 Angstrom in the trimer), which suggests the presence of cooperative phenomena. Nonadditive terms contribute about 12% of the overall interaction energy, the contribution being primarily of the inductive type. Calculations predict significant red shifts in the vibrational frequency of the N-H group when it takes part in the formation of a hydrogen bond. Similarly, the N-H wagging frequency undergoes a blue shift with hydrogen bonding. (C) 2000 American Institute of Physics. [S0021-9606(00)31145-X].