Observation of a remarkable temperature effect in the hydrogen bonding structure and dynamics of the CN^-(H₂O) cluster

The CN^-(H₂O) cluster represents a model diatomic monohydrate with multiple solvation sites. We report joint experimental and theoretical studies of its structure and dynamics using temperature-controlled photoelectron spectroscopy (PES) and ab initio electronic structure calculations. The observed PES spectra of CN^-(H₂O) display a remarkable temperature effect, namely that the T = 12 K spectrum shows an unexpectedly large blue shift of 0.25 eV in the electron binding energy relative to the room temperature (RT) spectrum. Extensive theoretical analysis of the potential energy function (PEF) of the cluster at the CCSD(T) level of theory reveals the existence of two nearly isoenergetic isomers corresponding to H ₂O forming a H-bond with either the C or the N atom, respectively. This results in four topologically distinct minima, i.e., CN^-(H _aOH_b), CN^-(H_bOH_a), NC^-(H_aOH_b), and NC^-(H _bOH_a). There are two main pathways connecting these minima: (i) CN^- tumbling relative to water and (ii) water rocking relative to CN^-. The relative magnitude of the barriers associated with these two motions reverses between low (pathway i is preferred) and high (pathway ii is preferred) temperatures. As a result, at T = 12 K the cluster adopts a structure that is close to the minimum energy CN^-(H ₂O) configuration, while at RT it can effectively access regions of the PEF close to the transition state for pathway ii, explaining the surprisingly large spectral shift between the 12 K and RT PES spectra.