Ab initio study of interactions in methylamine clusters. The significance of cooperative effects

Methylamine clusters consisting of up to four molecules were studied by emp loying Hartree-Fock, density functional theory, and Moller-Plesset calculat ions with the 6-31+G* basis set. The dimer was found to exhibit two minima with similar interaction energies (-13 kJ/mol) and involving a hydrogen bon d. The dipole moment for the dimer differs by up to 20% from the vector add ition of the dipole moments for the individual molecules by effect of the i nteraction; also, the N-H bond distance in the group involved in the hydrog en bond is lengthened by up to 0.006 A as a result. The minima identified f or the trimer and tetramer possess cyclic structures that differ in the pos ition of the methyl groups with respect to the plane containing the hydroge n bonds. The contribution of nonadditivity to the interaction in these stru ctures is quite significant (12%-18% of the overall interaction energy). N- H distances in the donor molecule are longer than in the dimer and increase with increasing cluster size. Likewise, the hydrogen bonding energy increa ses with cluster size. These results expose the significance of cooperative phenomena in the interactions between methylamine molecules. The computati ons predict slight shifts in the C-N stretching frequencies, which are cons istent with their experimental counterparts. The N-H stretching and the NH2 wagging modes undergo large shifts, with a magnitude that increases as the size of the cluster grows. (C) 2000 American Institute of Physics. [S0021- 9606(00)30703-6].