Probing the Ionic Hydrogen Bond in <i>meta</i>-Nitrophenolate-Water Complex Anions by Cryogenic Photodissociation Spectroscopy

We present visible and infrared (IR) spectroscopy of mono hydrated meta nitrophenolate (mNP) anions. The experiments were conducted using cryogenic photodissociation spectroscopy and interpreted by quantum chemistry calculations. mNP represents a model NO photodonnor system for which solvation effects are important to study for possible biological applications. It turns out that the ionic hydrogen bond (IHB) between the water and phenolate groups is quite strong, as evidenced by a -850 cm^-1 red shift of the IHB stretch and a +1750 cm^-1 solvatochromism of the electronic excitation energy. While time-dependent density functional theory (TD-DFT) calculations provide a good match with the experimental vibronic spectrum, comparisons of the IR vibrational spectrum to harmonic and anharmonic calculations, including vibrational perturbation theory (VPT2) and discrete variable representation (DVR) approaches have raised some issues in theoretical treatments on anharmonicity for IHB. In particular, only discrete variable representation (DVR) method including a 4-modes representation (4MR) and a 6-dimensions (6D) potential energy surface (PES) can reproduce the experimental ν_IHB frequency but fails to recover all of the observed combination bands.