The quantum chemical solvation of indole: accounting for strong solute-solvent interactions using implicit/explicit models

In this paper, we investigate the efficacy of different quantum chemical solvent modelling methods of indole in both water and methylcyclohexane solutions. The goal is to show that one can yield good photophysical properties in strongly coupled solute-solvent systems using standard DFT methods. We use standard and linearly-corrected Polarisable Continuum Models (PCM), as well as explicit solvation models, and compare the different model parameters, including the choice of density functional, basis set, and number of explicit solvent molecules. We demonstrate that implicit models overestimate energies and oscillator strengths. In particular, for indole-water, no level inversion is observed, suggesting a dielectric medium on its own is insufficient. In contrast, energies are seen to converge fairly rapidly with respect to cluster size towards experimentally measured properties in the explicit models. We find that the use of B3LYP with a diffuse basis set can adequately represent the photophysics of the system with a cluster size of between 9-12 explicit water molecules. Sampling of configurations from a molecular dynamics simulation suggests that the single point results are suitably representative of the solvated ensemble. For indole-water, we show that solvent reorganisation plays a significant role in stabilisation of the excited state energies. It is hoped that the findings and observations of this study will aid in the choice of solvation model parameters in future studies.

The role of solvation models on the computed absorption and emission spectra: the case of fireflies oxyluciferin

Absorption and emission energies calculation covering both implicit and explicit solvation models using oxyluciferin as the case of study.

On the accuracy of the general, state-specific polarizable-continuum model for the description of correlated ground- and excited states in solution

Equilibrium and non-equilibrium formulations of the state-specific PCM are evaluated in combination with correlated ground- and excited-state densities provided by ADC/ISR approach of up to third order of perturbation theory.

An efficient computational scheme for electronic excitation spectra of molecules in solution using the symmetry-adapted cluster-configuration interaction method: the accuracy of excitation energies and intuitive charge-transfer indices

Solvent effects on electronic excitation spectra are considerable in many situations; therefore, we propose an efficient and reliable computational scheme that is based on the symmetry-adapted cluster-configuration interaction (SAC-CI) method and the polarizable continuum model (PCM) for describing electronic excitations in solution. The new scheme combines the recently proposed first-order PCM SAC-CI method with the PTE (perturbation theory at the energy level) PCM SAC scheme. This is essentially equivalent to the usual SAC and SAC-CI computations with using the PCM Hartree-Fock orbital and integrals, except for the additional correction terms that represent solute-solvent interactions. The test calculations demonstrate that the present method is a very good approximation of the more costly iterative PCM SAC-CI method for excitation energies of closed-shell molecules in their equilibrium geometry. This method provides very accurate values of electric dipole moments but is insufficient for describing the charge-transfer (CT) indices in polar solvent. The present method accurately reproduces the absorption spectra and their solvatochromism of push-pull type 2,2'-bithiophene molecules. Significant solvent and substituent effects on these molecules are intuitively visualized using the CT indices. The present method is the simplest and theoretically consistent extension of SAC-CI method for including PCM environment, and therefore, it is useful for theoretical and computational spectroscopy.

Calculation of standard electrode potential of half reaction for benzoquinone and hydroquinone

Geometric parameters, the vibrational frequencies and thermochemical values of benzoquinone and hydroquinone were computed using ab initio molecular orbital calculations (HF) and density function theory (B3LYP) methods with the 6-31G(d) basis set, respectively. The calculated frequencies for benzoquinone and hydroquinone were used for the assignment of the IR frequencies observed in the experimental IR spectrum. Cyclic voltammetry with a glassy carbon electrode of hydroquinone solutions in phosphate buffers at pH 7.0 showed that standard electrode potential of half reaction for benzoquinone and hydroquinone is 0.714V. Standard electrode potential of half reaction for benzoquinone and hydroquinone was calculated using the sum of electronic and thermal free energies, enthalpies of sublimation and energies of solvation for benzoquinone and hydroquinone.

Density-functional theory studies on standard electrode potentials of half reaction for l-adrenaline and adrenalinequinone

DFT (B3LY/6-31G(d) and B3PW91/6-31G(d)) calculations are performed for L-adrenaline and adrenalinequinone. The calculated IR spectrum of L-adrenaline is used for the assignment of IR frequencies that are observed in the experimental IR spectrum. Cyclic voltammetry with a platinum electrode of L-adrenaline solutions in phosphate buffers at pH 1 shows that standard electrode potential of half reaction for L-adrenaline and adrenalinequinone is 0.803 V. Standard electrode potential of half reaction for L-adrenaline and adrenalinequinone is calculated using the energies of solvation and the sum of electronic and thermal free energies of L-adrenaline and adrenalinequinone.

Electronic excitation energies of molecules in solution within continuum solvation models: Investigating the discrepancy between state-specific and linear-response methods

In a recent article [R. Cammi, S. Corni, B. Mennucci, and J. Tomasi, J. Chem. Phys. 122, 104513 (2005)], we demonstrated that the state-specific (SS) and the linear-response (LR) approaches, two different ways to calculate solute excitation energies in the framework of quantum-mechanical continuum models of solvation, give different excitation energy expressions. In particular, they differ in the terms related to the electronic response of the solvent. In the present work, we further investigate this difference by comparing the excitation energy expressions of SS and LR with those obtained through a simple model for solute-solvent systems that bypasses one of the basic assumptions of continuum solvation models, i.e., the use of a single Hartree product of a solute and a solvent wave function to describe the total solute-solvent wave function. In particular, we consider the total solute-solvent wave function as a linear combination of the four products of two solute states and two solvent electronic states. To maximize the comparability with quantum-mechanical continuum model the resulting excitation energy expression is recast in terms of response functions of the solvent and quantities proper for the solvated molecule. The comparison of the presented expressions with the LR and SS ones enlightens the physical meaning of the terms included or neglected by these approaches and shows that SS agrees with the results of the four-level model, while LR includes a term classified as dispersion in previous treatments and neglects another related to electrostatic. A discussion on the possible origin of the LR flaw is finally given.

Electronic excitation energies of molecules in solution: State specific and linear response methods for nonequilibrium continuum solvation models

We present a formal comparison between the two different approaches to the calculation of electronic excitation energies of molecules in solution within the continuum solvation model framework, taking also into account nonequilibrium effects. These two approaches, one based on the explicit evaluation of the excited state wave function of the solute and the other based on the linear response theory, are here proven to give formally different expressions for the excitation energies even when exact eigenstates are considered. Calculations performed for some illustrative examples show that this formal difference has sensible effects on absolute solvatochromic shifts (i.e., with respect to gas phase) while it has small effects on relative (i.e., nonpolar to polar solvent) solvatochromic shifts.

A Sequential Molecular Mechanics/Quantum Mechanics Study of the Electronic Spectra of Amides

We report gas-phase electronic spectra of formamide, N-methyformamide, acetamide, and N-methylacetamide at 300 K calculated using a combination of classical molecular dynamics and time-dependent density functional theory (TDDFT). In comparison to excitation energies computed using the global minima structures, the valence npi* and pi(nb)pi* states show a significant red-shift of 0.1-0.35 eV, while smaller shifts are found for the n3s and pi(nb)3s Rydberg states. In this work, we have identified the physical origin of these shifts arising from variations of the molecular structure. We present simple relationships between key geometrical parameters and spectral shifts. Consequently, electronic spectra can be generated directly from ground-state structures, without additional quantum chemical calculations. The electronic spectrum of formamide in aqueous solution is computed using TDDFT using an explicit solvent model. This provides a quantitative determination of the condensed-phase spectrum. In general, this study shows that temperature effects can change the predicted excitation energies significantly and demonstrates how electronic spectra at elevated temperatures can be computed in a computationally efficient way.