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J Phys Chem A. 2006 Apr 20;110(15):5135-43.

O-H stretch modes of dodecahedral water clusters: a statistical ab initio study.

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  • 1Harvard Medical School, McLean Hospital, Centre Building, Room 11, 115 Mill Street, Belmont, Massachusetts 02478, USA.


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.

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