Dynamic Solvent Effects Research Articles

The effect of solvent dynamics on the low frequency collective motions of DNA in solution and unoriented films

Infrared spectroscopy is used to probe the dynamics of in vitro samples of DNA prepared as solutions and as solid unoriented films. The lowest frequency DNA mode identified in the far-infrared spectra of the DNA samples is found to shift in frequency when the solvent influence in the hydration shell is altered. The lowest frequency mode also has characteristics that are similar to beta-relaxations identified in other glass forming polymers.

ESR and ENDOR investigations of the degenerate electron exchange reactions of various viologens in solution. Solvent dynamical effects

The temperature dependences of the rates of the degenerate electron transfer of various viologens (1,1′-di(hydrocarbyl)-4,4′-bipyridinium salts) are measured in seven different solvents by means of electron spin resonance (ESR) line broadening. Rates vary between 1.7·108 and 1.1·109 M−1s−1 at room temperature and clearly show a solvent dynamical effect, which is inferred from the dependence of the rate constants on the longitudinal relaxation time of the solvent. Activation energies ranging from 5.3 to 24.4 kJ mol−1 are found. For the first time, hyperfine coupling constants are reported for the radical cations of the hydroxyethyl viologen and the amino viologen based on both continuous-wave ESR and electron-nuclear double resonance spectroscopy. Furthermore, the temperature and the solvent dependence of the hyperfine coupling constants of the methyl viologen radical cation are reported.

Near-contact adiabatic suppression of electron transfer in the inverted region

A few theories of activated electron transfer in inverted Marcus region are used for bridging the non-adiabatic, solvent controlled and deeply adiabatic transfer. The simple analytical interpolation between dynamic and stochastic theories provides a continuous description of the electron transfer rate at any non-adiabatic coupling between the diabatic states. When coupling increases with shortening of inter-particle distance the pre-exponent of the Arrhenius transfer rate first increases being quadratic in coupling, then levels off approaching the “dynamic solvent effect” (DSE) region and finally is cut off exponentially due to adiabaticity of the transfer. These changes affect significantly the spatial dependence of the transfer rate near the contact provided the coupling there is strong. The rate tends to zero at contact distance being strongly suppressed nearby adiabatically. It is much smaller then the perturbation (golden rule) and even DSE results. The latter is actually unattainable anywhere if contact tunneling is really strong. The transfer rate is a bell-shaped curve adiabatic and non-adiabatic on the opposite sides and sensitive to the friction (DSE damping) only in between, near the maximum.

Theoretical Study of the 1,3-Hydrogen Shift of Triazene in Water

The 1,3-hydrogen shift of triazene in aqueous solution was studied with a combination of QM/MM methods. First, the different species involved were characterized and the activation free-energies calculated with ASEP/MD, a method that makes use of the mean field approximation. Then the reaction dynamics was simulated with a QM/MM/MD method. A very strong influence of the solvent was observed, both specific, with the participation of a water molecule, and from the rest of the solvent. The effect of solvation on the geometry and electron distribution of triazene is important: N-N bond lengths tend to be more similar and the molecule acquires a planar structure. For the transition state structure, a substantial degree of ionic nature was found. Dynamic solvent effects were also analyzed.

Dynamic Nature of the Transition State for the SN1 Reaction Mechanism of Diphenylmethyl Acetates

Picosecond absorption spectroscopy is employed in the study of the reaction dynamics for the contact ion pairs produced upon the photolysis of a series of substituted diphenylmethyl acetates in the solvents acetonitrile, dimethyl sulfoxide, and 2,2,2-trifluoroethanol. From the temperature dependence of the rate constants, the activation parameters associated with covalent bond formation and diffusional separation to the solvent-separated ion pair are obtained. The activation parameters for bond formation are examined within the context of the Hynes theory for solvent dynamical effects on the passage through the transition state; deviations from the transition-state theory are found to be large. Factors that control nucleophilicity are discussed. Finally the validity of applying the Marcus equation to the SN1 reaction mechanism is addressed.

Variation of the Resonant Transfer Rate When Passing from Nonadiabatic to Adiabatic Electron Transfer

Two competing theories are used for bridging the gap between the nonadiabatic and the deeply adiabatic electron transfer between symmetric parabolic wells. For the high friction limit, a simple analytic interpolation is proposed as a reasonable alternative to them, well-fitted to the results of numerical simulations. It provides a continuous description of the electron transfer rate in the whole range of variation of the nonadiabatic coupling between the diabatic states. For lower friction, the original theories are used for the same goal. With an increase in coupling, the cusped barrier transforms into the parabolic one. Correspondingly, the pre-exponent of the Arrhenius transfer rate first increases with coupling, then levels off approaching the "dynamic solvent effect" plateau but finally reduces reaching the limit of the adiabatic Kramers theory for the parabolic barrier. These changes proceeding with a reduction in the particle separation affect significantly the spatial dependence of the total transfer rate. When approaching the contact distance, the exact rate becomes smaller than in the theory of dynamical solvent effects and much smaller than predicted by perturbation theory (golden rule), conventionally used in photochemistry and electrochemistry.

Ultrafast excited-state charge transfer at a conical intersection: effects of an environment

Excited-state processes at conical intersections (CIs) involving charge transfer phenomena can depend sensitively on the influence of a polar and polarizable environment. To describe the chromophore-environment interaction for such situations, we propose a theoretical formulation in terms of a diabatic free energy model depending on molecular coordinates plus a solvent coordinate. We focus on the S 1 – S 0 CI in protonated Schiff bases, representing a model for retinal isomerization, and comment upon static and dynamic solvation effects.

二分子反応過程における動的溶媒効果：2-(2,4-dinitrobenzyl)pyridineのプロトン互変異性

二分子反応過程における動的溶媒効果：2-(2,4-dinitrobenzyl)pyridineのプロトン互変異性

Structural studies of { 6Li} 2-lithiopyrrolidines using NMR spectroscopy

A selection of N-substituted 2-lithiopyrrolidines were prepared and their structures were investigated by 6Li and 13C NMR spectroscopy. Evidence is presented for aggregation and dynamic solvation effects, depending on the nature of the N-substituent and substituents on the pyrrolidine ring. Studies were performed with N-Boc (coordinating carbonyl group), N-methoxyethyl (coordinating methoxy group) and N-alkyl (no coordinating group) heterocycles to represent three different classes of organolithiums: dipole-stabilized, unstabilized and chelated, and unstabilized.

Solvent Dependence of the Spectra and Kinetics of Excited-State Charge Transfer in Three (Alkylamino)benzonitriles

Steady-state absorption and emission spectra and emission decay kinetics are reported for 4-aminobenzonitrile (ABN), 4-(1-azetidinyl)benzonitrile (P4C), 4-(1-pyrrolidinyl)benzonitrile (P5C), and 4-(1-piperidinyl)benzonitrile (P6C) in 24 room temperature solvents. In solvents of modest to high polarity, P4C, P5C, and P6C exhibit dual fluorescence and emission decays characteristic of the transformation from an initially prepared (LE) state to a more polar charge transfer (CT) state, whereas ABN does not undergo this reaction. The frequencies of the steady-state absorption and emission spectra of all of these solutes can be rationalized using a dielectric continuum description of the solvent and considering only the minima on the reactive surfaces, which are assumed to involve both an intramolecular (twisting) and a solvation coordinate. Characteristics of the gas-phase solutes deduced from this analysis are in good agreement with electronic structure calculations and indicate that differences in their spectra mainly reflect differences in the relative energies of the gas-phase LE and CT states. The relative yields of LE and CT emission are not described as satisfactorily by this model, and reasons for this failure are discussed. The kinetics of the LE --> CT reaction vary considerably with solute and solvent. In many solvents, the emission decays of P4C are reasonably described by a simple two-state kinetic scheme with time-independent rate constants. In P5C and P6C multiexponential decays are observed that reflect time-dependent shifts of the component spectra as well as time-dependent reaction rates. A simplified analysis of these complex dynamics provides estimates for both the free energy change Delta(r)G and (average) LE --> CT rate constant k(f) for a wide range of solute and solvent combinations. The driving force for reaction (-Delta(r)G) follows the order P6C > P5C > P4C and increases with increasing solvent polarity. The reaction rates are correlated to Delta(r)G and follow the opposite trend. The relationships observed between k(f) and Delta(r)G suggest that static solvent effects, i.e., barrier height changes, are the primary determinants of the solvent dependence in P4C. Correlations between barrier-corrected rates and solvation times suggest that dynamical solvent effects contribute substantially to the solvent dependence of the rates in P5C, and especially P6C.

Tautomerism and polarizability in uracil: coupled cluster and density-functional theory study

Geometries, relative stabilities, static, dynamic and vibrational polarizabilities of the six uracil tautomers were calculated by coupled cluster (CCSD, CCSD(T)) and DFT B97-1 and PBE1PBE functionals in vacuo and in solution. The dioxo structure is invariably the neutral ground state of uracil, with the lowest energy keto-enol form about 11 kcal/mol less stable. The polarizability behaviour of the uracil tautomers has been characterized by computing static and dynamic values, vibrational contributions and solvent effects. Our best static gas-phase electronic CCSD(T)/aug-cc-pVDZ dipole polarizability of uracil is reported to be 70.22 a.u. Dynamic (ω=0.10 a.u.) and vibrational contributions are significant, amounting to ca. 10% and 20%, respectively, of the static 〈αe〉 value. Solvent effects, investigated with the polarizable continuum model, show that both the static electronic and vibrational polarizabilities increase noticeably on passing from gas phase to solution, the αv/αe ratio being higher in solution than in vacuo. The throughout excellent agreement between CC and DFT results is worth noting.

Dynamic solvent effects on the fading of a merocyanine formed from a spironaphthoxazine. A case of a strong coupling between chemical and medium coordinate

The pressure dependence of the rate constant for the fading of a merocyanine formed from a spironaphthoxazine was measured in three pairs of nonviscous/viscous solvents of similar polarity, such as ethanol/2-methylpentane-2,4-diol, methyl acetate/glycerol triacetate, and methylcyclohexane/2,4-dicyclohexyl-2-methylpentane. The results unequivocally demonstrated a necessity to treat the medium and the chemical coordinate as two independent coordinates. These coordinates were found strongly coupled in the present reaction, especially in protic solvents. The results also revealed that hydrogen-bond solvation to the oxygen atom in the initial state and its desolvation during the activation step play important roles in both reaction mechanisms and dynamics.

Gianfranco Cainelli, Paola Galletti, Silvia Pieraccini, Arianna Quintavalla, Daria Giacomini, Gian Piero Spada, "Chiral aldehydes in hydrocarbons: Diastereoselective nucleophilic addition, NMR, and CD spectroscopy reveal dynamic solvation effects,"Chirality(2004) 16(1) 50–56

AbstractThe original article to which this Erratum refers was published in Chirality (2004) 16(1) 50–56Correction to the article: “Chiral Aldehydes in Hydrocarbons: Diastereoselective Nucleophilic Addition, NMR, and CD Spectroscopy Reveal Dynamic Solvation Effects” by Gianfranco Cainelli, Paola Galletti, Silvia Pieraccini, Arianna Quintavalla, Daria Giacomini, and Gian Piero Spada, Chirality 16:50–56, 2004.

Chiral aldehydes in hydrocarbons: diastereoselective nucleophilic addition, NMR, and CD spectroscopy reveal dynamic solvation effects.

Temperature-dependent studies on the diastereoselective nucleophilic addition of n- BuLi to alpha-chiral aldehydes as (S)-O-(t-butyl-dimethylsilyl)lactal, (S)-O-(t-butyl-dimethylsilyl) mandelic aldehyde, and (R)-2-phenylpropanal in n-decane and n-dodecane reveal dynamic solvation phenomena with the presence of inversion temperatures (T(inv)) in the Eyring plots of ln (anti/syn) vs. 1/ T. These dynamic solvent effects were disclosed by temperature-dependent studies of the (13)C NMR, CD, and UV spectra of the starting aldehydes in solution of n-decane and n-dodecane. The concomitant presence of three peculiar temperatures T(CD), T(UV), and T(NMR), whose values are identical and match T(inv), clearly confirms our earlier interpretation of the solvent-dependent nature of T(inv). The inversion temperature, as well as T(CD), T(UV), and T(NMR) represents the interconversion temperature of two different solvation clusters which act as two different supramolecules with different stereoselectivities.

Multiconfigurational self-consistent field linear response for the polarizable continuum model: Theory and application to ground and excited-state polarizabilities of para-nitroaniline in solution

This paper presents the linear response (LR) function for a multiconfigurational self-consistent field (MCSCF) molecular wave function for the integral equation formalism version of the polarizable continuum model (PCM). Both equilibrium and nonequilibrium PCM solvation schemes are described: The nonequilibrium scheme is applied to the calculation of excited state wave functions (Franck–Condon states) and/or of dynamic response properties. An important characteristic of the LR-PCM-MCSCF theory is the explicit inclusion of the effects of solvent dynamics, and this allows us to treat a large variety of time-dependent phenomena. Here, in particular, the theory is applied to the study of the solvent effect on transition energies and on static and dynamic polarizabilities of para-nitroaniline (pNA). The study of the polarizability dispersion of pNA is performed for the ground state and for low-lying electronic excited states including the charge transfer state. We compare our results with available experimental and theoretical data.

DYNAMIC SOLVENT EFFECT ON THE ISOMERIZATION OF 2-ALKENYLANTHRACENE

Dynamic solvent effect on the isomerization of two 2-alkenylanthracene derivatives in the lowest excited singlet (S1) state was investigated. The isomerization rates were determined in supercritical fluids (SCF) and in a series of compressed liquid n-alkane solvents. Kramer's turnover behavior was observed. The maximum rate is substantially smaller than expected from transition state theory. Torsional potentials between anthracene ring and alkene plane for the isomerization reaction coordinate of 2-alkenylanthracene were calculated by ab initio method as a function of dihedral angle. It was confirmed by the calculation that the isomerization followed by experiment is from s-trans to s-cis conformers.

Ultrafast charge separation and radiationless relaxation processes from higher excited electronic states of directly linked porphyrin-acceptor dyads.

We have investigated photoinduced electron transfer (ET) and related processes from the higher excited electronic state (S2) of Zn-porphyrin-imide acceptor directly linked supramolecular systems (ZP-I) designed especially for the critical studies of the energy gap law (EGL) of the charge separation (CS) from the S2 state, effects of solvent dynamics and intramolecular vibrations on this CS, and competition or cooperation between this CS and S2-->S1 conversion, etc. In this study, we have confirmed the modification of the EGL for the CS from S2 induced by the change of solvent polarity by comparing the EGL in toluene solution with that in THF, i.e. the EGL in toluene extends over a wider range of the energy gap for CS in the inverted region and becomes somewhat similar to the case of the weak coupling limit of an intramolecular radiationless transition. Moreover, we have compared the rate constants (lambda p) of the S1 state formation by the S2 excitation with the decay rate constants (lambda 1) of the S2 state in the ZP-I series and have also examined solvent polarity effects on these rate constants comparing THF and toluene solutions. Our studies have revealed that S1 formation by S2 excitation occurs mainly due to the CS in S2 followed by charge recombination (CR) producing S1, and these processes are affected by the modification of EGL owing to the solvent polarity, resulting in the smaller lambda p in toluene.

Dynamic solvent effects on the thermal cyclization of a hexadienone formed from a diphenylnaphthopyran: An example of a system with distinctly separate medium and chemical contributions to the overall reaction coordinatePreliminary results were presented at the 18th International Conference on High Pressure Science and Technology, July 23–27, 2001, Beijing, China and published as a conference paper.1

Pressure effects on the rate of thermal fading of a colored hexadienone formed from 3,3-diphenyl-3H-naphtho[2,1-b]pyran were measured at various temperatures in three highly viscous solvents, i.e., 2,4-dicylohexyl-2-methylpentane, glycerol triacetate, and 2-methylpentane-2,4-diol, as well as in their chemically similar nonviscous counterparts, such as methylcyclohexane, methyl acetate, and ethanol. Both experimental data and quantum-mechanical calculations are in agreement with formation of a cyclic activated complex. In the viscous solvents, high viscosities resulted in retardations of solvent thermal fluctuations which, in turn, slowed down the cyclization. The analysis of experimental results unequivocally demonstrated that the dynamic solvent effects in the present systems have to be described by a two-dimensional model that includes separately defined chemical and medium coordinates. Predictions of an alternative one-dimensional model with a diffusion-controlled reaction coordinate are not supported by the experimental data.

Dynamic Solvation Effects on the endo/exo Selectivity of the Diels—Alder Reaction.

Abstract The endo/exo stereoselectivity of the Diels–Alder cycloaddition between cyclopentadiene and 3-crotonoyl-2-oxazolidinone catalysed by Et2AlCl is temperature- and solvent-dependent. Eyring plots of ln(endo/exo) versus 1/T show different inversion temperatures (Tinv) depending on the reaction solvent. Using toluene-d8 we found a break temperature (TNMR) in a variable temperature 13C NMR experiment of the starting oxazolidinone, and this TNMR corresponded to the Tinv. In several cases the differential activation entropy manages the observed stereoselectivity.

Theoretical Study of Ultrafast Photoinduced Electron Transfer Processes in Mixed-Valence Systems

The recently proposed self-consistent hybrid method is applied to study photoinduced electron transfer (ET) reactions in mixed-valence compounds (NH3)5RuIIINCRuII(CN)5- and (NH3)5RuIIINCFeII(CN)5- in solution. To describe these ET processes in the condensed phase, both intramolecular modes (inner sphere) and the solvent environment (outer sphere) are taken into account using a model that is based on the analysis of experimental optical line shapes. The dynamics of the back ET process after photoexcitation is studied in detail. In particular we investigate the effects of intramolecular vibrational modes and solvent dynamics, as well as the influence of important physical parameters such as the electronic coupling and the temperature. In qualitative agreement with results of time-resolved optical experiments, the simulations predict an ultrafast decay of the population of the charge-transfer state. Oscillatory features are observed superimposed on the population decay, and their relations to electronic and ...