Ab initio investigations on neutral clusters of ammonia: (NH3)(n) (n=2-6)

Hydrogen-bonded neutral clusters of ammonia, (NH3)(n) (n = 2-6), have been theoretically investigated employing the 6-31++G(d,p) land wherever necessa ry, higher) basis sets at the Hartree-Fock (HF) level as well as with secon d-order Moller-Plesset (MP) perturbation theory. While the ammonia trimer a nd tetramer exhibit perfect molecular symmetries and are non-polar, the pen tamer and the hexamer both optimize with slight deviations from perfect sym metries and are seen to possess marginal, but nonzero dipole moments, in co ntradiction to a conjecture made in 1979. The (NH3)(n) 'linear' clusters ar e seen to be chemically softer than the corresponding 'cyclic' ones. (C) 20 01 Published by Elsevier Science B.V.