EFFECT OF ANHARMONICITIES ON THE THERMODYNAMIC PROPERTIES OF THE WATER DIMER

A study of the effect of anharmonicities and large amplitude vibration s on the thermodynamic properties of the water dimer is presented. Dif ferent vibrational models have been constructed using ab initio data o btained at the MP2(Full)/6-311++G(2d,2p) level. In particular, we pres ent the first complete analysis of the rotation of the hydrogen donor monomer around the O ... O axis. The potential barrier was found to be 221 cm(-1). A variational calculation of the torsional energy levels yields a fundamental frequency of 105 cm(-1) The O ... O stretching mo de is described using a Morse function. The fundamental frequency and the dimerization energy are calculated to be 153 cm(-1) and 5.15 kcal/ mol, respectively, in agreement with the experimental results. For the dimerization reaction we have calculated Delta S, Delta H, and the eq uilibrium constant, K-p. The results show that inclusion of anharmonic ity into the vibration modes favors the lower experimental limit for D elta S and the upper limit for Delta H. In addition, the anharmonic co rrections reduce the difference between calculated and experimental K- p. This difference decreases with temperature. A high-temperature limi t of 3.47 x 10(-5)atm(-1) was found for K-p.