IR Spectra of the Water Hexamer: Theory, with Inclusion of the Monomer Bend Overtone, and Experiment Are in Agreement

Signature IR spectra of isomers of the water hexamer in the spectral range 3000-3800 cm(-1) have been reported by experimentalists, but crucial theoretical interpretation has still not been definitive. Using ab initio potential and dipole moment surfaces and a fully coupled quantum treatment of the intramolecular modes, the ring and book are assigned to spectra obtained in the He nanodroplet and Ar tagging experiments, respectively. The overtone of the intramolecular bend at ca. 3200 cm(-1) is a new calculated feature that completes an important missing piece in previous experimental and theoretical comparisons and leads to a consistent assignment of these two experimental spectra. Calculated IR spectra for the lowest energy forms of the water heptamer and octomer are also presented and compared to experiment. In all the calculated spectra, the bend overtone is demonstrated to be a noticeable feature, and this is one important conclusion from the work. Also, the danger in using scaled double-harmonic spectra to assign spectra is demonstrated.