Solvation by benzene: molecular dynamics simulation of orientational motion, translational diffusion, and site–site radial distributions of the solutes di- and trichloromethane (chloroform)

Abstract

By a standard, proven molecular dynamics procedure we generated individual equilibrium systems of dichloromethane and trichloromethane (chloroform) in benzene, each at equimolar concentration, as well as those of the neat components. We analysed the orientational dynamics of the C-H bond direction of chloroform in benzene-solvated and neat chloroform by the tumbling motion of its l ¼ 1 and l ¼ 2 Legendre polynomial auto-correlations between 250 and 438 K at iso-density conditions specific to the solution and neat system, respectively. We find that solvent benzene exerts a hindering effect on the orientational motion of chloroform over its neat state under an activation energy of 3.8 kJ mol À1 , although the density of the solution is less than that of the neat solute. The hindering effect diminishes with increasing kinetic energy to eventually disappear near the critical region. Probing benzene as solute within an equimolar solution with chloroform does not show a correspondingly increased hindering of the orientational motion of the (in-plane) C 0 2 axes of benzene. The short-time rotational regimes at ambient conditions were examined by simulating the auto-correlation functions of the three components of angular velocity, the results implying that the coherence in the correlation of the primary rotational variable is rather short-lived. The long-time translational-diffusional characteristics were examined by the mean-square distance of chloroform travelled during 100 ps, establishing that chloroform diffuses slightly faster in equimolar solution with benzene than in its neat state-most likely on account of more favourable free volume conditions in the solution. From various site-site radial distribution functions between solute and solvent species and from appropriate atom-atom distances between nearest-neighbour molecules, we found that chloroform aggregates, to a minimum, with two nearest-neighbour benzene shells in a three-member configuration. We conjecture that the hindering effect by benzene on the chloroform solute is caused by its temporary entrapment, on time scales of the order of 0.2 ps, in cages from momentary perpendicular and parallel configurations of nearest-and near-neighbour benzene molecules. We find no convincing evidence for the existence of a stable, symmetric-top 1:1 chloroform-benzene complex. Solute species dichloromethane in benzene exhibits similar, but less pronounced, solution dynamics.

Enhanced vibrational solvatochromism and spectral diffusion by electron rich substituents on small molecule silanes

“…In this context, molecular dynamics (MD) simulations can complement experimental measurements to provide detailed, structural insights into the microscopic mechanism of solvation. 14,[37][38][39][40][41][42][43][44][45][46] In this work, we report a combined experimental and computational investigation of vibrational solvatochromism and spectral diffusion in small silane molecules in various solvents using Fourier transform infrared (FTIR) and 2D-IR spectroscopies. To elucidate substituent effects on solvation dynamics, trimethoxysilane (TriMOS) and triphenylsilane (TriPS) were chosen in our study since the same vibrational mode can be monitored in the presence of different electron withdrawing ligands, giving rise to different solvatochromic responses.…”

Solvation Dynamics of Vaska’s Complex by 2D-IR Spectroscopy

“…Despite their relevance to reaction dynamics, the fast restructuring events that occur in the solvent are often poorly characterized due, in large part, to the challenges associated with the measurements and the interpretation of the resulting data. Important insight into the microscopic processes in neat solvents has been gleaned from molecular dynamics (MD) simulations. − In turn, these simulations have been validated and refined by experimental observables from higher-order spectroscopies such as optical Kerr effect, − fluorescence Stokes shift, , RaPTORS, and photon echo spectroscopies. ,− Two-dimensional IR (2D-IR) spectroscopy has emerged as a powerful experimental approach to characterizing the dynamics in the vicinity of a solute molecule. − In a 2D-IR experiment, an IR-active chromophore or functional group is viewed as a reporter of the local solvation environment. This is often modeled by the vibrational Stark effect, whereby the time-dependent electrostatics of the surrounding molecules induce frequency shifts with the characteristic time scales of the solvation dynamics. − By preparing a coherent collection of vibrations in the sample and allowing those ensembles to experience their surrounding environments for varying lengths of time before recovering the surviving oscillators in the form of a rephased vibrational echo signal, the associated molecular dynamics are obtained as vibrational dephasing and spectral diffusion from amidst the heavily inhomogeneously broadened environments that are common in the condensed phase.…”

Direct Measurement of the Angular Pair Correlation Coefficients in Molecular Liquids Using NMR. Benchmarking Force Fields for Atomistic Simulations

“…31 In the 2000s, interest in this complex resurfaced and efforts to prove or disprove the presence of this complex in the liquid state using MD simulations began. Rothschild 32,33 concluded that the structure proposed by earlier researchers like Reeves and Schneider (with the c axis of chloroform pointing toward the center of the ring plane of benzene) has a "near-vanishing" probability to exist in solution.…”