Vibrational Lifetimes of Cyanide Ion in Aqueous Solution from Molecular Dynamics Simulations: Intermolecular vs Intramolecular Accepting Modes

Abstract

The lifetimes of the first vibrational state of (12)C(14)N(-) and (13)C(15)N(-) dissolved in H2O or D2O were calculated. The calculations were based on the Landau-Teller formula that puts the vibrational lifetimes in terms of the autocorrelation function of the force exerted on the C-N stretch by the remaining degrees of freedom. The force autocorrelation functions were calculated from classical molecular dynamics simulations of the four cyanide/water isotopomer combinations ((12)C(14)N(-)/H2O, (12)C(14)N(-)/D2O, (13)C(15)N(-)/H2O, (13)C(15)N(-)/D2O). The cyanide ion was described by a polarizable force field, and the water was described by either the rigid SPC/E model or the flexible SPC/Fw model, in order to compare two different types of accepting modes, namely, (1) intermolecular (translational and rotational) solvent accepting modes (rigid SPC/E water) and (2) intramolecular (vibrational) solvent accepting modes (flexible SPC/Fw water). Since quantum effects are expected to increase in size with increasing frequency mismatch between relaxing and accepting modes, different quantum correction factors were employed depending on the identity of the accepting modes, more specifically, the harmonic/Schofield quantum correction factor in the case of intermolecular accepting modes and the standard quantum correction factor in the case of intramolecular accepting modes. The lifetimes with either the rigid SPC/E or flexible SPC/Fw water models were found to be in good quantitative agreement with the experimentally measured values for all isotopomer combinations. Our results suggest that taking into account quantum effects on the vibrational energy relaxation of cyanide in aqueous solution can make the intermolecular pathway at least as likely as the intramolecular pathway.

Vibrational energy flow in the villin headpiece subdomain: Master equation simulations

“…We have also added a damping rate of 0.2 ps −1 to each residue to account for coupling to the solvent. Damping rates for solute modes in water vary widely; [64][65][66][67] we use a 5 ps lifetime as representative for a protein in water, in rough agreement with results of experimental and computational studies of energy flow from peptides and proteins into water. 68,69 We observe in Fig.…”

Vibrational Energy Relaxation of Thiocyanate Ions in Liquid-to-Supercritical Light and Heavy Water. A Fermi’s Golden Rule Analysis

“…In fact, the solvent forces acting on a cyanide anion do not reveal any signs of a bending−librational continuum as the infrared spectrum. 43 Finally, Ziegler and co-workers also explored the role of the solvent density of states in the context of energy relaxation of nitrous oxide as a structural probe for interfacial aqueous and hydrophobic sites in phospholipid bilayers. 44 From fs MIR spectra, a certain interfacial N 2 O population was identified whose vibrational lifetime was found to be hydrationdependent.…”

Femtosecond 2DIR spectroscopy of the nitrile stretching vibration of thiocyanate anions in liquid-to-supercritical heavy water. Spectral diffusion and libration-induced hydrogen-bond dynamics

“…1,2 A special emphasis is put on unraveling the dynamics of such interactions in hydrogen-bonded systems like aqueous solutions because of their quintessential importance in a wealth of chemical and biomolecular transformations. 3,4 Anionic pseudohalide solutes, like cyanide, [5][6][7][8][9][10] azide, 7,[11][12][13][14][15][16][17][18][19] cyanate, 20,21 and thiocyanate, [22][23][24][25][26][27][28][29][30] have proven to be particularly sensitive and useful probes 21,31 for studying dynamical solute-solvent interactions involving hydrogen bonds (H-bonds). These ions exhibit stretching oscillators with very strong transition moments that can easily be excited and probed with state-of-the-art nonlinear femtosecond infrared (IR) spectroscopy.…”

“…Furthermore, the anions are small enough to facilitate high-level computational studies of the dynamical solute-solvent interactions at the molecular level. 6,10,[32][33][34][35][36] Femtosecond IR spectroscopy can reveal two distinct types of vibrational dynamics; namely energy relaxation and spectral diffusion. In its simplest kind, IR pump-probe spectroscopy is normally used to disclose the time scales and the pathways for the relaxation of the excess energy deposited by an ultrafast, and vibrationally resonant, excitation pulse into a target vibration of the solute.…”