Theoretical Investigation of the Neutral Precursor of (H₂O)₆⁻

In a recent ingenious experiment Diken et al. (Diken, E. G.; Robertson, W. H.; Johnson, M. A. J. Phys. Chem. A 2004, 108, 64) obtained the vibrational spectrum in the OH stretch region of the neutral (H 2 O) 6 precursor to the (H 2 O) 6 - ion. Comparison of the measured spectrum with the calculated harmonic vibrational spectra of various low-energy isomers of (H 2 O) 6 led these authors to conclude that the observed spectrum is due to the book isomer, although agreement between theory and experiment is only qualitative. In the present study a hybrid MP2/QCISD method is used to overcome a shortcoming of earlier theoretical calculations of the harmonic spectra of low-energy (H 2 O) 6 isomers, and the coupling of the fundamentals with overtones and combination states is accounted for by means of a cubic force-field approximation. The results of these calculations provide further support for the assignment of the book form of (H 2 O) 6 as the precursor of the dominant observed (H 2 O) 6 - isomer.