title: O-H stretch modes of dodecahedral water clusters: a statistical ab initio study.
abstract: Infrared frequencies calculations were carried out for 20 (H2O)20 water clusters obeying the 5(12) dodecahedral geometry, optimized at the B3LYP/6-311++G** level. Their combined spectra contained 800 O-H stretch modes, ranging from 2181 to 3867 cm(-1) (unscaled), which were treated and studied as a database. Of these, 752 modes (94%) could be assigned to a single dominant O-H stretch. These 752 were classified into five subdatabases depending on the local H-bond type of the dominant stretch. The frequency (nu) was correlated with the O-H distance (b(OH)), with H-bond length (R(OO)) where applicable, and with other variables. The parameter b(OH) alone accounted for 96-99% of the variance in nu for stretches in H-bonds. The correlation with R(OO) is substantially weaker. Normal modes were classified as "high ratio" or "low ratio" depending upon the mode's distribution of kinetic energy among the O-H bonds. High-ratio modes (389 modes, or 49% of our sample) are modeled well as a single oscillator undergoing small perturbations by weak coupling from other oscillators. Low-ratio modes involve strong coupling with at least one other O-H stretch for which b(OH2) is close to b(OH). The IR intensities of modes vary widely but can be explained in terms of a single equation giving dipole moment derivatives as a function of b(OH). For the lowest-energy (H2O)20 clusters, their IR stretch spectra contained eight distinguishable absorption bands. An explanation for eight bands in terms of the theory of polyhedral water clusters is offered.
authors.affiliation: Harvard Medical School, McLean Hospital, Centre Building, Room 11, 115 Mill Street, Belmont, Massachusetts 02478, USA. David.Anick@rcn.com
authors.forename: David J
authors.initials: DJ
authors.lastname: Anick
mesh_terms: D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D008956:Models, Chemical; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry*; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; D006859:Hydrogen; D010100:Oxygen