Knowledge Bank

The distortions from equilibrium and changes in the infrared spectra of a single water molecule when it is complexed with a halide ion (F$-$, Cl$-$, and Br$-$) have been of great interest for the past several years.}}, 7688 (2005).}$,$}}, 571 (2005).} In these systems the bonded hydrogen stretch ($\nu 2$), the so-called ``proton-transfer'' mode, as well as the in-plane ($\nu 4$) and the out-of-plane ($\nu 6$) bending modes have displayed unexpected intensity patterns in the experimental infrared spectra.}}, 4943 (2006).} An example of such anomalies can be seen in the $\nu 2$ mode of F$-$(H$2$O). This vibrational mode exhibits significant intensity in the fundamental transition, over ten times that of any other fundamental, and its frequency is shifted by nearly 1500 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ to the red of the free OH stretch. Analysis of the first and second overtones in $\nu 2$ indicates that anharmonicity is not as significant as one might expect from such a large redshift of the fundamental.\newline\newline We investigate the underlying motions associated with the $\nu 2$, $\nu 4$, and $\nu 6$ modes by calculating the potential energy and dipole moment surfaces at the MP2 level of theory using aug-cc-pVTZ basis sets. The X$-$(H$2$O) vibrational energies and wave functions are determined using one- and two-dimensional}}, 1982 (1992).} variational calculations. Our theoretical results reproduce the experimental findings extremely well and provide insight into the physics behind these intriguing spectral features.