Explicit versus Implicit Solvent Modeling of Raman Optical Activity Spectra

Abstract

Raman and Raman optical activity (ROA) spectra of molecules reflect not only molecular structure and conformation but also the dynamics and interactions with the solvent. For polar, biologically relevant molecules in aqueous environment, this often complicates the band assignment and interpretation of the spectra. In the present study, implicit dielectric and explicit solvent models are compared with respect to the influence of the choice of solvent model on the spectral shape. Lactamide and 2-aminopropanol were selected as model compounds, and the Raman and ROA spectra were measured for both enantiomers. Geometries of explicitly solvated clusters were derived from quantum-mechanical calculations, classical (MD), and Car-Parrinello (CPMD) molecular dynamics. The results indicate that although the dielectric model reasonably well reproduces the main spectral features, more faithful intensity profiles, including the inhomogeneous band broadening, are obtained from the explicit MD and CPMD clusters. Additionally, the CPMD clusters are capable of reproducing most spectral features better than the classical dynamics, provided the simulation time is long enough to allow for a complete sampling of the conformational space. The hydrogen-bonded water molecules of the first hydration shell significantly influence the spectral intensities, whereas the effect of loosely attached or distant solvent molecules is minor. In order to average the signal, however, a relatively large number of MD geometries need to be considered, as was also exemplified by simulations of the ROA spectrum of the achiral molecule glycine. An explicit solvent modeling of sizable systems thus requires extensive computations, which became possible only recently due to the development of efficient analytical computational techniques.

Where does the Raman optical activity of [Rh(en)3]3+ come from? Insight from a combined experimental and theoretical approach

“…32,33 The calculations were performed with the B3LYP hybrid exchange-correlation functional [34][35][36] because of its good performance for ROA calculations 25,26,[37][38][39][40] . In order to have a good description of the dispersion interactions, we employed the D2 dispersion correction of Grimme.…”

“…The study combines the measurement of Raman and ROA spectra and their analyses based on quantum chemical calculations. Because the solutesolvent interactions are likely to significantly influence the recorded spectra 25,26 , solvents effects were included in the calculations. In a first approach, an implicit solvation model was used.…”

Raman spectra from ab initio molecular dynamics and its application to liquid S-methyloxirane

“…Another possibility is the combination of continuum solvation models with micro-solvation including explicit solvent molecules. Although this approach improves the agreement between computational and experimental data, [5][6][7][8] it does not account for dynamic effects. This problem can, to some extend, be overcome by taking molecular structures from snapshots of molecular dynamics (MD) runs.…”

“…This problem can, to some extend, be overcome by taking molecular structures from snapshots of molecular dynamics (MD) runs. 6,[9][10][11][12] Subsequent Raman spectra calculations for each of these molecular structures and averaging over the resulting spectra 6,11,12 lead to a better agreement with experimental data, but -although still relying on the double-harmonic approximation -with a significantly higher computational effort compared to a single static harmonic calculation. Employing such a procedure, snapshots from ab initio MD (AIMD) 13,14 simulations have been found to be superior to the ones generated employing classical MD.…”

“…Employing such a procedure, snapshots from ab initio MD (AIMD) 13,14 simulations have been found to be superior to the ones generated employing classical MD. 6 A completely different approach solely based on AIMD is the computation of Raman spectra via time correlation functions of the polarizability tensor. This is not only a convenient way to simulate conformational dynamics and solvent effects at finite temperatures, the employment of periodic boundary conditions makes the calculation of Raman spectra for condensed matter possible.…”