STRUCTURES, BINDING-ENERGIES, AND SPECTRA OF ISOENERGETIC WATER HEXAMER CLUSTERS - EXTENSIVE AB-INITIO STUDIES

We investigated five lowest energy structures of the water hexamer (ri ng, book, bag, cage, and prism) using extensive ab initio calculations . High levels of theory using various basis sets were employed. On the basis of Moller-Plesset second order perturbation (MP2) calculations using a large basis set [9s6p4d2 f1g/6s4p2d] + diffuse(2sp/s), the low est energy structure with zero point energy (ZPE) correction is the ca ge conformer, followed by the book (within 0.1 kcal/mol) and the prism (within 0.2 kcal/mol). The spectra of the five conformers have been i nvestigated. The predicted rotational constants and dipole moments of the cage conformer are in good agreement with the experiment [Liu et n l., Nature 381, 501 (1996)] as compared to other structures. This prov es that the experiment surely found the cage structure, which was firs t reported by one of the authors [Kim et nl., Chem. Phys. Lett. 131, 4 51 (1986)]. However, the five structures would still be nearly isoener getic within 0.7 kcal/mol at 0 K. Above similar to 40 K, the free ener gy of the book is slightly lower than the cage, which might imply that the book structure would be detected. Upon deuteration, the cage stru cture is the lowest energy conformer, followed by two competing struct ures of the book and prism whose energies are only 0.2 kcal/mol higher at 0 K; above similar to 55 K the book would be more populated than t he cage. (C) 1998 American Institute of Physics.