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

Abstract

We present the simulated photoelectron spectrum (PES) for cyanide-water CN(H(2)O)(-) based on quasiclassical trajectory molecular dynamics (QCT-MD). Using density functional theory to generate trajectories and to calculate vertical detachment energies, we obtain simulated spectra that are in qualitative agreement with experiment. We obtain a theoretical 12 → 300 K temperature red shift of 0.1 eV as compared to an experimental redshift of 0.25 eV. The calculated linewidths of 0.3 eV are in excellent agreement with experiment. Our trajectories show that the temperature red shift as being dominated by dynamics within the basin of the N-bound minimum, however, at 300 K we predict conversion into the basin of the C-bound minimum, equilibrating at a 80:20 ratio of N- vs C-bound mixture. We discuss the potential advantages of QCT-MD over anharmonic Franck-Condon analysis such as natural incorporation of anharmonicity (as necessary for weakly bound systems), and reduced computational scaling, but also drawbacks such as neglect of final-state (e.g., Duschinsky) effects.

Vibrational spectroscopy of the hexahydrated sulfate dianion revisited: role of isomers and anharmonicities

“…Head-Gordon, Xantheas and co-workers performed the first coupled cluster calculations, also in combination with anharmonic vibrational corrections and estimates of the basis set effects up to the complete basis set limit using MP2, on this system. 13,14 Interestingly, these calculations predict yet another structure, a 6.6.6 structure containing two three-membered water rings, to be the global minimum, but after zero-point energy corrections are taken into account the C 3 isomer (6.9.3) is predicted to be the most stable one at 0 K for both isotopologues. However, the authors also note that ''the errors in the different theoretical treatments might still be too large to make a clear distinction between very closely spaced isomers possible''.…”

“…5,6 The structure of the hexahydrated sulfate dianion has been the subject of ongoing discussions over the last two decades. [7][8][9][10][11][12][13][14][15] Two low-energy structures exist (see Fig. 1), which differ in the way the six water molecules function as hydrogenbond acceptors (A) and donors (D).…”

“…The C 3 isomer contains two types of water molecules, three DD and three ADD ones, and is referred to as the 6.9.3 isomer, since it contains six water molecules that form nine anion-water and three water-water hydrogen bonds. 13 In contrast, the six DD water molecules in the T d isomer 6.12.0 are symmetry equivalent, forming twelve anion-water and no water-water hydrogen bonds. Here, we study the vibrational spectroscopy of the hexahydrated sulfate dianions, SO 4 2À (H 2 O) 6 , and its fully deuterated isotopologue, SO 4 2À (D 2 O) 6 , in order to unambiguously resolve, which isomers contribute to its gas phase IR spectrum at cryogenic temperatures and above.…”

“…14 Since anharmonic as well as entropic effects are important for a reliable description of such systems a molecular dynamics description of this system may prove more appropriate. 13,17 In this publication we consider many of the previously made suggestions and report messenger-tagged spectra of cold, hexahydrated sulfate dianions. We explicitly explore anharmonic effects by studying the effect of (i) isotope-substitution, (ii) the number of messenger tags and (iii) temperature on the vibrational spectra.…”

Advances in molecular quantum chemistry contained in the Q-Chem 4 program package

“…Some (though not all!) of the limitations of classical trajectories may be overcome through the use of quasi-classical trajectories [252,253], which is now available in Q-CHEM [254]. An old but useful idea, quasi-classical trajectories are initialised with kinetic energy corresponding approximately to the appropriate quantum distributions based on normal mode analysis.…”