Infrared Signatures of a Water Molecule Attached to Triatomic Domains of Molecular Anions: Evolution of the H-bonding Configuration with Domain Length

We present mid-IR argon predissociation spectra for a series of complexes, M - ·H 2 O (M = CS 2 -, OCS -, SO 2 -, CH 3 NO 2 -, CH 3 CO 2 -, and NO 2 - ), chosen to explore how changes in the triatomic binding site affect the H-bonding configuration of the attached water molecule. With the exception of NO 2 -, the calculated global minima on the potential surfaces of all of the complexes occur in a configuration where both OH groups are attached to the anion. The observed spectra, on the other hand, fall into three distinct categories. Simple spectra characteristic of the double ionic H-bonding arrangement are observed for the monohydrates of SO 2 -, OCS -, and CS 2 -, whereas the CH 3 NO 2 - ·H 2 O and CH 3 CO 2 - ·H 2 O spectra are complicated, displaying a progression of closely spaced bands with a broad, bell-shaped envelope beginning several hundred wavenumbers below the calculated fundamentals. Although the spectrum of the NO 2 - ·H 2 O complex is the most red-shifted, it is again simple, reflecting the expected asymmetric (single ionic H-bonded) motif. These data indicate that the transition from single to double ionic H-bonding occurs at a critical domain length of about 2.2 Å. We explore the potential surfaces governing the interconversion between the two forms with density functional calculations and construct vibrationally adiabatic potential surfaces to assess the cause of the spectral complexity displayed by the methylated anion hydrates.