Computational Study of the One‐ and Two‐Dimensional Infrared Spectra of a Proton‐Transfer Mode in a Hydrogen‐Bonded Complex Dissolved in a Polar Nanocluster

Abstract

The signatures of nanosolvation on the one- and two-dimensional (1D and 2D) IR spectra of a proton-transfer mode in a hydrogen-bonded complex dissolved in polar solvent molecule nanoclusters of varying size are elucidated by using mixed quantum-classical molecular dynamics simulations. For this particular system, increasing the number of solvent molecules successively from N=7 to N=9 initiates the transition of the system from a cluster state to a bulk-like state. Both the 1D and 2D IR spectra reflect this transition through pronounced changes in their peak intensities and numbers, but the time-resolved 2D IR spectra also manifest spectral features that uniquely identify the onset of the cluster-to-bulk transition. In particular, it is observed that in the 1D IR spectra, the relative intensities of the peaks change such that the number of peaks decreases from three to two as the size of the cluster increases from N=7 to N=9. In the 2D IR spectra, off-diagonal peaks are observed in the N=7 and N=8 cases at zero waiting time, but not in the N=9 case. It is known that there are no off-diagonal peaks in the 2D IR spectrum of the bulk version of this system at zero waiting time, so the disappearance of these peaks is a unique signature of the onset of bulk-like behavior. Through an examination of the trajectories of various properties of the complex and solvent, it is possible to relate the emergence of these off-diagonal peaks to an interplay between the vibrations of the complex and the solvent polarization dynamics.

Signatures of Nanoconfinement on the Linear and Nonlinear Vibrational Spectroscopy of a Model Hydrogen-Bonded Complex Dissolved in a Polar Solvent

“…In order to elucidate the signatures of nanoconfinement on ground-state proton-transfer reactions in nonlinear vibrational spectra, we compute both the one-dimensional IR (1D-IR) and IR pump–probe spectra of a proton-transfer mode in a model H-bonded phenol–amine complex dissolved in pools of methyl chloride molecules confined in various nanosized spherical hydrophobic cavities. Calculations of the proton-transfer rate constant have been carried out for this complex in bulk methyl chloride − and in small unconfined methyl chloride clusters. , In addition, the signatures of solvation and proton-transfer dynamics in these systems have been investigated in simulated 1D-IR, 2D-IR, and IR pump–probe spectra. − It should be noted that a similar model has been studied previously by Thompson and co-workers, ,, but, in contrast to our study, their hydrogen-bonding potential governing the proton-transfer dynamics was different and the O–N bridge length in the complex was constrained. In ref , Monte Carlo simulations were used to investigate the distribution of the complex’s position in the cavity and it was found that the proton-transfer reaction involves diffusion of the complex from the cavity wall to the interior of the cavity and vice versa.…”

Assessment of approximate solutions of the quantum–classical Liouville equation for dynamics simulations of quantum subsystems embedded in classical environments

“…These results are qualitatively compared to the results of adiabatic dynamics simulations in Refs [73,74], which are known to give a realistic picture of the dynamics.…”

“…It should be noted that the PT kinetics [73] and 1D-IR/2D-IR spectroscopy [74] of this model were previously investigated. Within our mixed quantum -classical approach, the proton constitutes the quantum subsystem and the remaining atoms constitute the classical environment.…”

Mixed Quantum-Classical Liouville Equation Treatment of Electronic Spectroscopy of Condensed Systems: Harmonic and Anharmonic Electron–Phonon Coupling

“…While Geva et al 37−39 and Hanna 40,41 have applied MQCLE to calculate vibrational spectra in MQC environment, Hanna 42,43 calculated Optical response function to probe electron transfer dynamics using the basis mapping technique developed by Nassimi and Kapral. 44 However, the path to the end results, approach, purpose, rigor, applications, and results differ from those reported by Toutounji.…”

“…The response function approach has been widely utilized in calculating spectroscopic signals to probe systems of interest. , For example, Staib and Borgis employed quantum-classical dynamics to compute IR spectra of OH stretch mode spectra of hydrogen-bonded methanol complexes dissolved in carbon tetrachloride are computed; however, MQCLE was never used in their simulation and no electronic spectra were reported. While Geva et al − and Hanna , have applied MQCLE to calculate vibrational spectra in MQC environment, Hanna , calculated Optical response function to probe electron transfer dynamics using the basis mapping technique developed by Nassimi and Kapral . However, the path to the end results, approach, purpose, rigor, applications, and results differ from those reported by Toutounji. − ,− The optical response function developed by Hanna , to probe electron transfer dynamics seems to neglect frequency change (excited state vibrational force constant change) and anharmonicity upon electronic excitation, and they do not seem to offer in-depth spectroscopic analysis, especially with respect to electronic dephasing, electron–phonon coupling, shape, and symmetry of spectral profiles, all of which mark differences compared to MQCD-based report reviewed herein.…”