Theoretical study of the shifts of electronic spectra from solute-solvent interaction

Previous theories of spectral solvent shifts are briefly discussed. The basic dipole approximation is analyzed both theoretically and in relation to experimental information. On account of the restricted validity of this approximation — especially for molecules consisting of several polar groups — another, electrostatic model, originating from the Born charging, is investigated. This model is applied to semi-empirical PPP-calculations of spectral solvent shifts of some quinones. Among other things, this model predicts that not only n → π⁸ transitions may be blue shifted but also certain π → π* transitions. When the electrostatic effect is small, other terms may be responsible for the solvent shift, e.g. polarizability. The cavity effect is not expected to be important in the present context.     

The effects of solute-solvent interactions on the electronic visible spectra of bis(β-diketonato)oxovanadium(IV) complexes

Summary The solvatochromic behaviour of bis(-diketonato)oxovanadium(IV) complexes is quantitatively correlated by an approach that models specific and non-specific solute-solvent interactions. The applicability of the Selbin-Gutmann relation is discussed. The solvent-induced spectral shifts in VO(acac)₂ are dominated by specific interactions of the donor-acceptor type, whereas for VO(tfa)₂, the nonspecific solute-solvent interactions make the dominant contribution.

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Der Einfluß von Wechselwirkungen zwischen Lösungsmittel und gelöstem Stoff auf die VIS-Spektren von Bis(-diketonato)oxovanadium(IV)-Komplexen

Zusammenfassung Das solvatochrome Verhalten von Bis(-diketonato)oxovanadium(IV)-Komplexen wird mit einem Modell, das spezifische und nichtspezifische Wechselwirkungen in Lösung berücksichtigt, quantitativ beschrieben. Es wird die Anwendbarkeit der Selbin-Gutmann-Relation diskutiert. Die Solvens-induzierten Verschiebungen in den VIS-Spektren von VO(acac)₂ werden von den spezifischen Wechselwirkungen vom Donor-Acceptor-Typ bestimmt, währenddessen für VO(tfa)₂ die nichtspezifischen Wechselwirkungen zwischen Substrat und Lösungsmittel den dominierenden Beitrag liefern.

     

Study of solute-solvent interactions

A microscopic model is proposed for the calculation of solute-solvent interaction energies. Calculated energies for gaseous solute-nitrobenzene systems are compared with experimental dissolution energies.     

Charge-transfer interactions with ferrocyanide ion: electronic absorption spectra

The interaction of ferrocyanide ion with a series of heterocyclic cations in aqueous medium has been investigated by electronic absorption spectroscopy. New electronic transitions due to charge-transfer have been identified. The equilibrium constants and enthalpies indicate that the interaction is appreciable.     

Molecular rotation as a tool for exploring specific solute-solvent interactions.

Solute-solvent interactions play an important role in determining the physicochemical properties of liquids and solutions. As a consequence, understanding these interactions has been one of the long-standing problems in physical chemistry. This Minireview describes our approach towards attaining this goal, which is to investigate rotational relaxation of a pair of closely related, medium-sized nondipolar solutes in a set of appropriately chosen solvents. Our studies indicate that solute-solvent hydrogen bonding significantly hinders solute rotation. We have also examined the role of solvent size both in the absence and presence of specific interactions and it has been observed that the size of the solvent has a bearing on solute rotation especially in the absence of specific interactions. Our results point to the fact that only strong solute-solvent hydrogen bonds have the ability to impede the rotation of the solute molecule because, in such a scenario, hydrogen-bonding dynamics and rotational dynamics transpire on comparable time scales. This aspect has been substantiated by measuring the reorientation times of the chosen solutes in solvents such as ethanol and trifluoroethanol, which have distinct hydrogen-bond donating and accepting abilities, and correlating them with solute-solvent interaction strengths. As an alternative treatment, it has been shown that specific interactions between the solute and the solvent can be modeled as dielectric friction with the extended charge distribution model. This approach is not unrealistic considering the fact that specific as well as non-specific interactions are electrostatic by nature and the differences between them are subtle.     

Effects of solute-solvent proton exchange on polypeptide chain dynamics: a constant-pH molecular dynamics study

A method for performing implicit-solvent molecular dynamics simulations at constant pH was applied to a pentapeptide acetyl-Ala-Asp-Ala-Lys-Ala-amide at pH 4. As a reference, molecular dynamics simulations were done for the same peptide with two variants of its fixed protonation patterns expected to dominate at pH 4, i.e., with a protonated and a deprotonated side chain of the Asp residue and the protonated Lys residue in both cases. The dynamic trajectories of the peptide were used to discuss the problem of the significance of the solute-solvent proton exchange phenomena for the dynamics and structural distributions of the polypeptide chain. The Asp-Lys distance was used as a probe of the overall molecular structure of the investigated pentapeptide. To characterize the dynamics, distributions of the "waiting" times for a transition from a "short" distance conformation to a "long" distance conformation were constructed, based on the generated molecular dynamics trajectories. We show that the relaxation time for the transitions, derived from the constant-pH simulations, is very close to the relaxation time characterizing a permanently protonated molecule, although the average protonation probability of the short-distance conformation is close to zero. However, the distribution of the Asp-Lys distances obtained from constant-pH simulations cannot be reproduced as a linear combination of the distributions resulting from the simulations with fixed protonation states.     

Formal-mathematical analysis of electronic paramagnetic resonance spectra with a hyperfine structure

Conclusion We intend to apply the methods exposed above in calculations on actual spectra. The author acknowledges Ya. S. Lebedev for a discussion of the results and S. P. Koneva and A. M. Kogan for doing the computations.Translated from Zhurnal Strukturnoi Khimii, Vol. 6, No. 1, pp. 39–46, January–February, 1965     

Specific solute-solute and solute-solvent interactions in organic solutions of indole

The temperature dependence of the UV absorption spectra of indole and N-methylindole in propyl ether and in 3-methylpentane was investigated. The results indicate the existence of hydrogen bond interactions between indole and the ether, and also the existence of self-associations of indole through bonding in the non-polar solvent.     

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.     

Mass transport in Triton X series nonionic surfactant solutions: a new approach to solute-solvent interactions

Densities, viscosities and tracer diffusion coefficients for solutions of the nonionic surfactants Triton X-45, X-114, X-100 and X-102 in water (except for Triton X-45) and methanol, and for Triton X-100 in three water-methanol mixtures have been measured at 298.15 K and 308.15 K. The activation energy for viscous flow and the contributions to it from solute and solvent have been calculated. Comparison of the Gibbs energies of these systems shows the roles of polyoxyethylene chains of Triton X molecules in the interactions between solute and solvent, and also the effects of solvents on the inter-actions for different solutes. The viscosityB coefficients are positive for all of the surfactant solutions, and the temperature coefficients ofB are negative. In the non-micellar solutions in methanol, values ofB are small and the temperature coefficient ofB is nearly zero. In water-methanol mixtures the critical micelle concentration becomes larger as the methanol content increases up to 40% w/w; micelles are not formed at higher methanol concentrations. TheB coefficient decreases with increasing proportion of methanol in the solvent, and the temperature coefficient ofB changes from a fairly large positive value at low methanol contents to a small negative value at 80% w/w methanol.     

The effects of solute-solvent electrostatic interactions on solvatochromic shifts in supercritical CO2

Solvent clustering around attractive solutes is an important feature of supercritical solvation. We examine here the effects of the local density enhancement on solvatochromic shifts in electronic absorption and emission spectra in supercritical CO2. We use molecular dynamics (MD) simulation to study the spectral line shifts for model diatomic solutes that become more polar upon electronic excitation. The electronic transition is modeled as either a change from a quadrupolar to a dipolar solute charge distribution or as an increase in the magnitude of the solute dipole. Our main focus is on the density dependence of the line shifts at 320 K, which corresponds to about 1.05 times the solvent critical temperature, Tc, but results for higher temperatures are also obtained in order to determine the behavior of the line shifts in the absence of local density enhancement. We find that the extent of local density enhancement at 1.05Tc is strongly correlated with solute-solvent electrostatic attraction and that the density dependence of the emission line shifts resembles the behavior of the effective local densities, rho(eff), obtained from the first-shell coordination numbers. The differences that are seen are shown to be due to solute-solvent orientational correlations which provide an additional source of enhancement for electrostatic solvation energies and spectral line shifts.     

Effects of inductive substituents on electronic spectra aza analogs of fluoranthene

The conditions under which the Hückel molecular orbital (HMO) treatment of inductive substituent effects can be expected to be reliable are discussed from the viewpoint of the Pariser-Parr-Pople (PPP) model. The HMO theory fails to predict correctly the direction of the shift of the first intense band in the ultraviolet spectrum of fluoranthene (1–1 transition) on 3-aza substitution. Calculations on the five azafluoranthenes using the simple version of the PPP method suggest that this is due to strong mixing of hydrocarbon states by the effect of the substituent and predict significant changes in energies, intensities, and polarizations of fluoranthene electronic transitions on aza substitution. Transition moment directions derived from dichroic absorption of 3-azafluoranthene in stretched polyethylene sheets are in agreement with PPP predictions. Arguments are given against a previous assignment of the first transition in 1-azafluoranthene as n- ^*.

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Zusammenfassung Aus der Sicht des Pariser-Parr-Pople-Modells (PPP) werden die Bedingungen diskutiert, unter denen die HMO-Behandlung induktiver Substituenteneffekte als zuverlässig erscheint. Die HMO-Theorie ist nicht in der Lage, die Verschiebung der ersten intensiven Bande im UV-Spektrum des Fluoranthens (1–1-Übergang) bei 3-Aza-Substitution richtig vorauszusagen. Berechnungen an den 5 Azafluoranthenen nach der einfachen PPP-Methode lassen vermuten, daß diese Tatsache durch eine starke Vermischung von Kohlenwasserstoffzuständen — hervorgerufen durch die Wirkung des Substituenten — bedingt ist. Die PPP-Methode sagt ferner beträchtliche Änderungen in den Energien, Intensitäten und Polarisationen der Elektronenübergänge des Fluoranthens voraus, wenn man im Fluoranthen eine Aza-Substitution vornimmt. Die Richtungen des Übergangsmoments, die aus der dichroitischen Absorption des 3-Azafluoranthens in langgestreckten Polyäthylenschichten abgeleitet werden können, stimmen mit den Voraussagen von PPP-Berechnungen überein. Es werden Argumente vorgebracht, die gegen eine Kennzeichnung des ersten Übergangs in 1-Azafluoranthen als n- ^*-Übergang sprechen.

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Résumé On discute à l'aide du modèle de Pariser-Parr-Pople (PPP) les conditions de validité du traitement par la méthode de Hückel (HMO) des effets de substituants inductifs. La méthode de Hückel ne peut prédire correctement la direction du déplacement de la première bande intense du spectre UV du fluoranthène (dite transition «1–1») dans les substitutions aza en 3. Les calculs sur 5 azafluoranthènes à l'aide de la méthode PPP ordinaire suggèrent que cet échec est du à un fort mélange des états de l'hydrocarbure sous l'effet du substituant, et prédit des changements importants dans les énergies, les intensités et les polarisations des transitions du fluoranthène par les substitutions aza. Les directions des moments de transition déduits de l'absorption dichroïque du 3 aza fluoranthène dans des feuilles de polyéthylène étiré sont en accord avec le calcul PPP. On apporte des objections à la caracterisation antérieure de la 1 ère bande du 2 azafluoranthène comme une bande n- ^*.

     

IR-spectroscopic investigations on solute-solvent interactions Part 1: Solvation of triethylphosphine oxide in aprotic and CH-acidic solvents

A systematic study has been carried out on the solvent and concentration dependence of the P=O stretching vibrationv(PO) of the model compoundEt ₃PO in various aprotic and several CH-acidic solvents. Evidence has been found for the existence of dipole-dipole complexes in concentrated solutions ofEt ₃PO in solvents of low acceptor numbers. In dilute solutions, however, the P=O stretching band generally occurs as a single symmetrical peak which shows that only one kind of solvate species is present in solution. In CH-acidic solventsEt ₃PO is present in the form of hydrogen bonded complexes. Informations about the structure of these complexes have been obtained by means of spectrophotometric titrations. It has been shown that the wavenumbersv°(PO) obtained by extrapolation ofv(PO) values to zero concentration are linearly related to the acceptor numbers of the solvents, previously derived from³¹P-chemical shift measurements. The existence of this linear relationships proves that both parameters are linearly related to the strength of the intermolecular interactions and are virtually free from unspecific contributions such as magnetic anisotropy or vibrational coupling effects. The results of the present study show that the P=O stretching vibration ofEt ₃PO represents an ideal probe for the investigation of solvent effects and solvation mechanisms.

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Infrarotspektroskopische Untersuchungen über Substrat-Lösungsmittel-Wechselwirkungen. Teil 1: Solvatation von Triethylphosphinoxid in aprotischen und CH-aciden Lösungsmitteln

Zusammenfassung Es wurde die Konzentrations- und Lösungsmittelabhängigkeit der P=O-Valenzschwingungv(PO) der ModellverbindungEt ₃PO in verschiedenen aprotischen und CH-aciden Lösungsmitteln systematisch untersucht. In hinreichend verdünnten Lösungen tritt die P=O-Valenzschwingung als einzelne symmetrische Bande auf, was beweist, daß nur eine einzige Solvatform vorliegt. In Lösungsmitteln niedriger Akzeptorzahl konnte bei höheren Konzentrationen die Existenz von Dipol-Dipol-Komplexen nachgewiesen werden. In CH-aciden Lösungsmitteln liegtEt ₃PO in Form von Wasserstoffbrückenkomplexen vor. Aussagen über die Struktur dieser Komplexe konnten mit Hilfe spektrophotometrischer Titrationen erhalten werden. Die durch Extrapolation auf Konzentration null erhaltenen Wellenzahlenv° (PO) hängen linear von den aus den³¹P-chemischen Verschiebungen vonEt ₃PO bereits früher bestimmten Akzeptorzahlen der Lösungsmittel ab. Die Existenz dieser linearen Beziehung beweist, daß beide Parameter frei von störenden Einflußfaktoren (magnetische Anisotropie- und Schwingungskopp-lungseffekte) sind und tatsächlich linear mit der Stärke der intermolekularen Wechselwirkung variieren. Die Ergebnisse der vorliegenden Studie zeigen, daß die P=O-Valenzschwingung vonEt ₃PO eine hervorragende Sonde zur Untersuchung von Lösungsmitteleffekten und Solvatationsmechanismen darstellt.

     

Theoretical representation of solvation in biochemical systems: From discrete solute-solvent interactions to bulk solvation

The interaction between solutes and simple solvents in dilute solutions is analyzed in a systematic way. Different theoretical methods to describe specific solute-solvent interactions and general solvation effects are presented. Finally, the importance of solvent-induced changes in solute properties is discussed through the use of mixed quantum mechanics/classical mechanics strategies. © 1996 John Wiley & Sons, Inc.     

C6H and C6D: electronic spectra and Renner-Teller analysis

Rotationally resolved spectra of the B(2)Π - X(2)Π 0(0)(0) electronic origin bands and 11(1)(1) μ(2)Σ-μ(2)Σ vibronic hot band transitions of both C(6)H and C(6)D have been recorded in direct absorption by cavity ring-down spectroscopy through a supersonically expanding planar plasma. For both origin and hot bands accurate spectroscopic parameters are derived from a precise rotational analysis. The origin band measurements extend earlier work and the 11(1)(1) μ(2)Σ-μ(2)Σ vibronic hot bands are discussed here for the first time. The Renner-Teller effect for the lowest bending mode ν(11) is analyzed, yielding the Renner parameters ε(11), vibrational frequencies ω(11), and the true spin-orbit coupling constants A(SO) for both (2)Π electronic states. From the Renner-Teller analysis and spectral intensity measurements as a function of plasma jet temperature, the excitation energy of the lowest-lying 11(1) μ(2)Σ vibronic state of C(6)H is determined to be (11.0 ± 0.8) cm(-1).     

The electronic spectra of the sandwich stacked PFBT: a theoretical study

Stacked models that include 9,9'-bis(6'-N,N,N-trimethylammonium)hexyl]fluorene-co-alt-4,7-(2,1,3-benzothiadiazole)dibromide (F(BT)F) monomer sandwiched between two stacked 2,1,3-benzothiadiazole (BT) units were explored using theoretical approaches. Molecular structures and the optical characteristics of the investigated species were investigated at the M06-2X/6-311G(d,p)//TD-M06-2X/6-311G(d,p) level of theory. In all models, the electronic excitation to the lowest singlet ππ* excited state (S1(ππ*)) is governed by the highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO → LUMO) transitions. The obtained results suggest that stacking interaction might have only minor effects on the transition energy for both absorption and emission processes. Instead, the reduction in the excitation energy of the stacked complexes should be attributed to the dipole-dipole interaction. The larger the interaction energy of the stacked models, the bigger the observed differences between absorption-emission energies. The presence of the solvation medium with small dielectric constant may increase the absorption-emission energy differences. It is expected that the largest absorption-emission shift can be observed in the benzene solution.     

The electronic spectra of unsaturated hydrocarbons a VESCF-CI treatment

A modified Pariser-Parr method, which allows for the inductive effect of alkyl groups by a VESCF method, has been applied to the calculation of the electronic spectra of a number of hydrocarbons. All singly- and doubly-excited configurations are included in the configuration interaction treatment, except for large molecules for which the configuration interaction matrices are truncated to an order of 110. The red shifts which result from methylation of the polyenes are well calculated. For azulene, the shift to either the red or the blue, depending on the location of the substituent, is also well calculated. The largest deviation between the calculated and experimental transition energies for all of the compounds studied was 1.1 eV, while the standard deviation of all observed transitions for planar compounds was 0.24 eV. The ionization potentials and the singlet-triplet transitions of these compounds were also studied.

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Zusammenfassung Die Spektren einer Reihe von Kohlenwasserstoffen wurden nach einer modifizierten Pariser-Parr-Methode, bei der der induktive Effekt von Alkylgruppen mittels eines VESCF-Verfahrens berücksichtigt wird, berechnet, wobei alle ein- und zweifach angeregten Konfigurationen eingeschlossen wurden außer bei sehr großen Molekülen, wo deren Zahl auf 110 begrenzt wurde. Dabei ergeben sich in schöner Weise die Rotverschiebungen, die bei Methylierung von Polyenen beobachtet werden, desgleichen die Rot- und Violettverschiebungen je nach Ort des Substituenten bei Azulen. Die größten Abweichungen zwischen berechneten und beobachteten Übergangsenergien ist 1,1 eV, die Standardabweichung bei planaren Verbindungen 0,24 eV. Ferner wurden Ionisationspotentiale sowie Sigulett-Triplett-Übergänge der behandelten Verbindungen untersucht.

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Résumé Une méthode Pariser-Parr modifieé, qui tient compte de l'effet inductif des groupes alkyles dans le cadre VESCF, a été employeé pour calculer les spectres électroniques de quelques hydrocarbures. Toutes les configurations mono- et di-excitées sont comprises dans le traitement d'interaction de configurations, excepté pour les grosses molécules; pour ces dernières, l'ordre des matrices d'interaction de configuration a été réduit à 110. Les déplacements versle rouge, après methylation de polyènes, sont bien calculés. Pour l'azulène, les déplacements vers le rouge ou le bleu, suivant le lieu de substitution, sont également bien calculés. La plus grande déviation entre les énergies de transition calculée et expérimentale était de 1.1 eV pour tous les produits étudiés, alors que la déviation standard de toutes les transitions observeés était de 0,24 eV dans les composés planaires. Le potential d'ionisation et les transitions singulet-triplet ont aussi été étudiés.