Observation of a Remarkable Temperature Effect in the Hydrogen Bonding Structure and Dynamics of the CN⁻(H₂O) Cluster

Abstract

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 H2O forming a H-bond with either the C or the N atom, respectively. This results in four topologically distinct minima, i.e., CN(-)(H(a)OH(b)), CN(-)(H(b)OH(a)), NC(-)(H(a)OH(b)), and NC(-)(H(b)OH(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(-)(H2O) 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.

Simulated Photoelectron Spectra of the Cyanide-Water Anion via Quasiclassical Molecular Dynamics

“…8(b)) show less good agreement with experiment. This supports earlier claims [18] that the photoelectron spectrum is mainly determined by min1. However, with a min1→min2 conversion rate of 20% (see Fig.…”

“…In summary, we obtain a theoretical 12 K→300 K temperature red shift of 0.1 eV as compared to an experimental red shift of 0.25 eV, while the calculated linewidths of 0.3 eV are in excellent agreement with experiment. Our trajectories support a previous model [18] that explains the temperature red shift as being dominated by dynamics within the basin of the N-bound minimum. However, our calculations lead to a refined picture: At 300 K we predict conversion into the basin of the Cbound minimum, equilibrating at a 80:20 N-vs. C-bound mixture.…”

“…5 of Ref. [18]). At low temperature the system resides in the (almost doubly degenerate) vibrational ground state, which has probability maxima at the equilibrium structure (4.54 eV detachment energy).…”

“…In order to obtain a full comparison of theory and experiment on the cyanide-water anion, we performed quasiclassical molecular dynamics simulations to generate theoretical spectra at 12 K and 300 K. As discussed in the literature [18], vibrational effects may be very important in the sampling of the transition state regions and thus the observation of a red shift. We therefore employ the quasiclassical approximation, which puts vibrational energy into each normal mode when setting up the initial velocities (as opposed to a purely classical MD, which uses a thermal Boltzmann distribution for the initial kinetic energy).…”

Exploring the Rich Energy Landscape of Sulfate–Water Clusters SO42– (H2O)n=3–7: An Electronic Structure Approach

“…We note, however, that it is known that some clusters require the inclusion of connected triples. 51 Furthermore, our reference values were obtained only at the aug-cc-pVTZ basis set level; it is therefore possible that larger basis sets would be required to ensure basis set convergence of the observed properties. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Table 2: Comparison of CC-VSCF/MP2/TZP calculations from Ref.…”