The effects of solute-solvent electrostatic interactions on solvation dynamics in supercritical CO2

We present here the results of molecular-dynamics simulation of solvation dynamics in supercritical CO(2) at a temperature of about 1.05T(c), where T(c) is the critical temperature, and at a series of densities ranging from 0.4 to 2.0 of the critical density rho(c). We focus on electrostatic solvation dynamics, representing the electronic excitation of the chromophore as a change in its charge distribution from a quadrupolar-symmetry ground state to a dipolar excited state. Two perturbations are considered, corresponding to different magnitudes of solute excited-state dipoles, denoted as d5 and d8. The d8 solute is more attractive, leading to a larger enhancement in CO(2) clustering upon solute electronic excitation. This has a large impact on solvation dynamics, especially at densities below rho(c). At these densities, solvation dynamics is much slower for the d8 than for the d5 solute. For both solutes, solvation dynamics becomes faster at densities above rho(c) at which solvent clustering diminishes. We show that the slowest solvation time scale is associated with solvent clustering and we relate it to solute-solvent mutual translational diffusion and the extent of change in effective local density resulting from solute electronic excitation.     

The theory of electron transfer reactions: what may be missing?

Molecular dynamics simulations are presented for condensed-phase electron transfer (ET) systems where the electronic polarizability of both the solvent and the solute is incorporated. The solute polarizability is allowed to change with electronic transition. The results display notable deviation from the standard free energy parabolas of traditional ET theories. A new three-parameter ET model is applied, and the theory is shown to accurately model the free energy surfaces. This paper presents conclusive evidence that the traditional theory for the free energy barrier of ET reactions requires modification.     

Solvent structural relaxation dynamics in dipolar solvation studied by resonant pump polarizability response spectroscopy

Resonant pump polarizability response spectroscopy (RP-PORS) was used to study the isotropic and anisotropic solvent structural relaxation in solvation. RP-PORS is the optical heterodyne detected transient grating (OHD-TG) spectroscopy with an additional resonant pump pulse. A resonant pump excites the solute-solvent system and the subsequent relaxation of the solute-solvent system is monitored by the OHD-TG spectroscopy. This experimental method allows measuring the dispersive and absorptive parts of the signal as well as fully controlling the beam polarizations of incident pulses and signal. The experimental details of RP-PORS were described. By performing RP-PORS with Coumarin 153(C153) in CH(3)CN and CHCl(3), we have successfully measured the isotropic and anisotropic solvation polarizability spectra following electronic excitation of C153. The isotropic solvation polarizability responses result from the isotropic solvent structural relaxation of the solvent around the solute whereas the anisotropic solvation polarizability responses come from the anisotropic translational relaxation and orientational relaxation. The solvation polarizability responses were found to be solvent-specific. The intramolecular vibrations of CHCl(3) were also found to be coupled to the electronic excitation of C153.     

Solvation in supercritical water

Solvation in supercritical water under equilibrium and nonequilibrium conditions is studied via molecular dynamics simulations. The influence of solute charge distributions and solvent density on the solvation structures and dynamics is examined with a diatomic probe solute molecule. It is found that the solvation structure varies dramatically with the solute dipole moment, especially in low-density water, in accord with many previous studies on ion solvation. This electrostrictive effect has important consequences for solvation dynamics. In the case of a nonequilibrium solvent relaxation, if there are sufficiently many water molecules close to the solute at the outset of the relaxation, the solvent response measured as a dynamic Stokes shift is almost completely governed by inertial rotations of these water molecules. By contrast, in the opposite case of a low local solvent density near the solute, not only rotations but also translations of water molecules play an important role in solvent relaxation dynamics. The applicability of a linear response is found to be significantly restricted at low water densities.     

丙酮分子n→π*跃迁光谱的平均溶剂静电势/分子动力学方法研究

采用量子力学/分子动力学方法研究了具体溶剂分子结构对溶质光谱行为的静电影响. 通过拟合溶质所处外电场和引入溶剂分子极化率, 考虑了溶质溶剂分子之间的相互极化效应, 得到合理的溶质和溶剂分子的电荷分布. 经过严格推导发现, 在传统的显溶剂模型中, 平衡和非平衡溶剂化能表达式均未考虑溶剂分子永久偶极弹簧能的贡献. 因此, 在正确计算永久偶极弹簧能的基础上, 重新建立了溶剂化能的表达式和新的吸收/发射光谱移动公式. 采用修改后的ASEP/MD程序, 计算得到了与实验值比较吻合的丙酮在水溶液中n→π*跃迁的光谱移动值, 验证了新公式的合理性.     

A molecular dynamics study of chirality transfer: The impact of a chiral solute on an achiral solvent

A solvation shell may adapt to the presence of a chiral solute by becoming chiral. The extent of this chirality transfer and its dependence on the solute and solvent characteristics are explored in this article. Molecular dynamics simulations of solvated chiral analytes form the basis of the analysis. The chirality induced in the solvent is assessed based on a series of related chirality indexes originally proposed by Osipov [M. A. Osipov et al., Mol. Phys. 84, 1193 (1995)]. Two solvents are considered: Ethanol and benzyl alcohol. Ethanol provides insight into chirality transfer when the solvent interacts with the solute primarily by a hydrogen bond. Several ethanol models have been considered starting with a nonpolarizable model, progressing to a fluctuating charge model, and finally, to a fully polarizable model. This progression provides some insights into the importance of solvent polarizability in the transfer of chirality. Benzyl alcohol, by virtue of the aromatic ring, increases the number of potential solvent-solute interactions. Thus, with these two solvents, the issue of compatibility between the solvent and solute is also considered. The solvation of three chiral solutes is examined: Styrene oxide, acenaphthenol, and n-(1-(4-bromophenyl)ethyl)pivalamide (PAMD). All three solutes have the possibility of hydrogen bonding with the solvent, the last two may also form ring-ring interactions, and the last also has multiple hydrogen bonding sites. For PAMD, the impact of conformational averaging is examined by comparing the chirality transfer about rigid and flexible solutes.     

Solvation dynamics in acetonitrile: a study incorporating solute electronic response and nuclear relaxation

The solvent reorganization process after electronic excitation of a polar solute in a polar solvent such as acetonitrile is related mainly to the time evolution of the solute-solvent electrostatic interaction. Modern laser-based techniques have sufficient time resolution to follow this decay in real time, providing information to be confirmed and interpreted by theories and models. We present here a study aimed at the investigation of the different steps involved in the process taking place after a vertical S(0) --> S(1) excitation of a large size chromophore, coumarin 153 (C153), in acetonitrile, from both the solute and the solvent points of view. To do this, we use accurate quantum mechanical calculations for the solute properties within the polarizable continuum model (PCM) and classical molecular dynamics (MD) simulations, both equilibrium and nonequilibrium, for C153 in the presence of the solvent. The geometry of the solute is allowed to change in order to study the role of internal motions in the time-dependent solvation process. The solvent response function has been obtained from the simulation data and compared to experiment, while the comparison between equilibrium and nonequilibrium MD results for the solvation response confirms the validity of the linear response approximation in the C153-acetonitrile system. The MD trajectories have also been used to monitor the structure of the solvation shell and to determine its change in response to the change in the solute partial charges.     

Solvation of coumarin 153 in supercritical fluoroform

We present a study of local density augmentation around an attractive solute (i.e., giving rise to more attractive interaction with the solvent than solvent-solvent interactions) in supercritical fluoroform. This work is based on molecular dynamics simulations of coumarin 153 in supercritical fluoroform at densities both above and below the critical density, ranging from dilute gas-like to liquid-like, at a reduced temperature (T/T(c)) of 1.03. We focused on studying the structure of the solvation shell and the variation of the solute electronic absorption and emission shifts with density. Quantum calculations at the density functional theory (DFT) level were run on the solute in the ground state, and time-dependent DFT calculations were performed in the solute excited state in order to determine the solute-solvent potential parameters. The results obtained for the Stokes shift are in agreement with the experimental measurements. To evaluate local density augmentation from simulations, we used two different definitions, one based on the solvation number and the other derived from solvatochromic shifts. In the former case, the agreement with experimental results is good, while, in the latter case, better agreement is achieved by perturbatively including the induced-dipole contribution to the solvation energy.     

Absorption and emission lineshapes and solvation dynamics of NO in supercritical Ar

We perform a theoretical study of electronic spectroscopy of dilute NO in supercritical Ar fluid. Absorption and emission lineshapes for the A(2)Sigma(+)<--X(2)Pi Rydberg transition of NO in argon have been previously measured and simulated, which yielded results for the NO/Ar ground- and excited-state pair potentials [Larregaray et al., Chem. Phys. 308, 13 (2005)]. Using these potentials, we have performed molecular dynamics simulations and theoretical statistical mechanical calculations of absorption and emission lineshapes and nonequilibrium solvation correlation functions for a wide range of solvent densities and temperatures. Theory was shown to be in good agreement with simulation. Linear response treatment of solvation dynamics was shown to break down at near-critical temperature due to dramatic change in the solute-solvent microstructure upon solute excitation to the Rydberg state and the concomitant increase of the solute size.     

Quantum simulation of solution phase intramolecular electron transfer rates in betaine-30

Mixed quantum-classical atomistic simulations have been carried out to investigate the mechanistic details of excited state intramolecular electron transfer in a betaine-30 molecule in acetonitrile. The key electronic degrees of freedom of the solute molecule are treated quantum mechanically using the semiempirical Pariser-Parr-Pople Hamiltonian, including the solvent influence on electronic structure. The intramolecular vibrational modes are also treated explicitly at a quantum level, with the remaining elements treated classically using empirical potentials. The electron-transfer rate, corresponding to S1 --> S0 relaxation, is evaluated via time-dependent perturbation theory with the explicit inclusion of the dynamics of solvation and intramolecular conformation. The calculations reveal that, while solvation dynamics is critical to the rate, the intramolecular torsional dynamics also plays an important role. The importance of the use of multiple high-frequency quantum modes is also discussed.     

Resolving the emission times of solute and solvent four-wave mixing signals by spectral interferometry

Electric field-resolved transient grating measurements are used to distinguish the four-wave mixing signal emission from a resonant solute and a non-resonant solvent. The two components of the solution (i.e., solute and solvent) emit signal fields at different times with respect to the arrival of the probe pulse to the sample. This gives rise to a recurrence in the temporal profile of the total signal field. We show that the origin of this interference is the difference in relaxation time scales of the holographic gratings associated with the solute and solvent. The grating of the resonant solute relaxes on the time scale of a few picoseconds due to depopulation of its excited electronic state, whereas the electronic polarizability response of the solvent relaxes on the femtosecond time scale. This separability of responses is a general phenomenon that is particularly useful for studying weakly absorbing solute dynamics in polarizable solvents.     

Ultrafast solvent response upon a change of the solute size in non-polar supercritical fluids

Non-polar solvation dynamics has been investigated using steady-state absorption and emission spectroscopy of the NO A ²Σ⁺(3sσ) Rydberg state in fluid Ar over a wide range of densities spanning the supercritical regime. Equilibrium molecular dynamics simulations were implemented to derive a new isotropic NO A(3sσ)–Ar pair potential which was further used to investigate the role of local density enhancements on the solvation process by non-equilibrium molecular dynamics simulations. These density inhomogeneities were found to have no influence on the solvation dynamics. Furthermore, the latter was shown to take place in a strongly non-linear regime, especially at low temperatures. This process results from the dramatic change of solute–solvent short range interaction associated with the large solute size change upon excitation to the Rydberg state.     

Effects of solute structure on local solvation and solvent interactions: results from UV/vis spectroscopy and molecular dynamics simulations

Solvation of heterocyclic amines in CO(2)-expanded methanol (MeOH) has been explored with UV/vis spectroscopy and molecular dynamics (MD) simulations. A synergistic study of experiments and simulations allows exploration of solute and solvent effects on solvation and the molecular interactions that affect absorption. MeOH-nitrogen hydrogen bonds hinder the n-pi* transition; however, CO(2) addition causes a blue shift relative to MeOH because of Lewis acid/base interactions with nitrogen. Effects of solute structure are considered, and very different absorption spectra are obtained as nitrogen positions change. MD simulations provide detailed solvent clustering behavior around the solute molecules and show that the local solvent environment and ultimately the spectra are sensitive to the solute structure. This work demonstrates the importance of atomic-level information in determining the structure-property relationships between solute structure, local salvation, and solvatochromism.     

Incorporating the excluded solvent volume and surface charges for computing solvation free energy

Gauss's law or Poisson's equation is conventionally used to calculate solvation free energy. However, the near‐solute dielectric polarization from Gauss's law or Poisson's equation differs from that obtained from molecular dynamics (MD) simulations. To mimic the near‐solute dielectric polarization from MD simulations, the first‐shell water was treated as two layers of surface charges, the densities of which are proportional to the electric field at the solvent molecule that is modeled as a hard sphere. The intermediate water was treated as a bulk solvent. An equation describing the solvation free energy of ions using this solvent scheme was derived using the TIP3P water model. © 2013 Wiley Periodicals, Inc.     

Interior and interfacial aqueous solvation of benzene dicarboxylate dianions and their methylated analogues: A combined molecular dynamics and photoelectron spectroscopy study

Aqueous solvation of benzene dicarboxylate dianions (BCD(2-)) was studied by means of photoelectron spectroscopy and molecular dynamics simulations. Photoelectron spectra of hydrated o- and p-BCD(2-) with up to 25 water molecules were obtained. An even-odd effect was observed for the p-BCD(2-) system as a result of the alternate solvation of the two negative charges. However, the high polarizability of the benzene ring makes the two carboxylate groups interact with each other in p-BCD(2-), suppressing the strength of this even-odd effect compared with the linear dicarboxylate dianions linked by an aliphatic chain. No even-odd effect was observed for the o-BCD(2-) system, because each solvent molecule can interact with the two carboxylate groups at the same time due to their proximity. For large solvated clusters, the spectral features of the solute decreased while the solvent features became dominant, suggesting that both o- and p-BCD(2-) are situated in the center of the solvated clusters. Molecular dynamics simulations with both nonpolarizable and polarizable force fields confirmed that all three isomers (o-, m-, and p-BCD(2-)) solvate in the aqueous bulk. However, upon methylation the hydrophobic forces overwhelm electrostatic interactions and, as a result, the calculations predict that the tetramethyl-o-BCD(2-) is located at the water surface with the carboxylate groups anchored in the liquid and the methylated benzene ring tilted away from the aqueous phase.     

Computational Analysis of Solute–Solvent Coupling Magnitude in the Z/E Isomerization Reaction of Nitroazobenzene and Benzylideneanilines

The dynamic solvent effect often arises in solution reactions, where coupling between chemical reaction and solvent fluctuation plays a decisive role in the reaction kinetics. In this study, the Z/E isomerization reaction of nitoroazobenzene and benzylideneanilines in the ground state was computationally studied by molecular dynamics simulations. The non-equilibrium solvation effect was analyzed using two approaches: (1) metadynamics Gibbs energy surface exploration and (2) solvation Gibbs energy evaluation using a frozen solvation droplet model. The solute–solvent coupling parameter (C_coupled) was estimated by the ratio of the solvent fluctuation Gibbs energy over the corresponding isomerization activation Gibbs energy. The results were discussed in comparison with the ones estimated by means of the analytical models based on a reaction–diffusion equation with a sink term. The second approach using a frozen solvation droplet reached qualitative agreement with the analytical models, while the first metadynamics approach failed. This is because the second approach explicitly considers the non-equilibrium solvation in the droplet, which consists of a solute at the reactant geometry immersed in the pre-organized solvents fitted with the solute at the transition state geometry.     

FACTS: Fast analytical continuum treatment of solvation

An efficient method for calculating the free energy of solvation of a (macro)molecule embedded in a continuum solvent is presented. It is based on the fully analytical evaluation of the volume and spatial symmetry of the solvent that is displaced from around a solute atom by its neighboring atoms. The two measures of solvent displacement are combined in empirical equations to approximate the atomic (or self) electrostatic solvation energy and the solvent accessible surface area. The former directly yields the effective Born radius, which is used in the generalized Born (GB) formula to calculate the solvent-screened electrostatic interaction energy. A comparison with finite-difference Poisson data shows that atomic solvation energies, pair interaction energies, and their sums are evaluated with a precision comparable to the most accurate GB implementations. Furthermore, solvation energies of a large set of protein conformations have an error of only 1.5%. The solvent accessible surface area is used to approximate the nonpolar contribution to solvation. The empirical approach, called FACTS (Fast Analytical Continuum Treatment of Solvation), is only four times slower than using the vacuum energy in molecular dynamics simulations of proteins. Notably, the folded state of structured peptides and proteins is stable at room temperature in 100-ns molecular dynamics simulations using FACTS and the CHARMM force field.