A theoretical study of five water/ammonia/formaldehyde cyclic trimers: Influence of cooperative effects

Ab initio computations at the MP2 level on five dimers and five cyclic trim ers, drawn from water, ammonia, and formaldehyde are presented. Trimers hav e been drawn to present cyclic X-H---Y patterns. Particular attentions have been devoted in analyzing the energetic contributions resulting from coope rative effects in the trimer binding energies (BEs) and in analyzing the tr ends of several parameters from monomers to dimers and from dimers to trime rs [in particular, the trends of the R(X-H) bond lengths, of the R(X---Y) d istances, of the delta nu(XH) shifts in the nu(XH) stretch vibrational freq uencies, and of the electronic density rho(c) value at the XH---Y axis crit ical point when it exists]. The results have exhibited that cooperative eff ects represent from 10% to 16% of the trimer BEs and that they reinforce, f rom dimers to trimers, the trends observed for the above parameters from mo nomers to dimers. In particular, for "typical'' X-H---Y HB (i.e., where X a nd Y atoms correspond to oxygen or nitrogen atoms), R(X-H) bond lengths are increased within 0.01 Angstrom from monomers to dimers and from dimers to trimers, R(X---Y) distances shortened within 0.18 Angstrom, rho(c) values i ncreased by about 17% and nu(XH) red-shifted from 18 to 164 cm(-1) from dim ers to trimers. As contrasted to those HBs the R(X-H) and delta nu(XH) para meters corresponding to C-H---Y interaction (with Y=O or N) follows an oppo site trend from monomers to dimers and from dimers to trimers (i.e., they a re respectively smoothly shortened and blue-shifted). All of these results therefore exhibit the great incidence of cooperative effects on the propert ies of X-H---Y interactions (corresponding to typical HBs or not), which ar e of importance to understand the properties of biochemical systems. (C) 19 99 American Institute of Physics. [S0021-9606(99)30215-4].