Showing papers on "Solvent effects published in 1982"

THE Py SCALE OF SOLVENT POLARITIES. SOLVENT EFFECTS ON THE VIBRONIC FINE STRUCTURE OF PYRENE FLUORESCENCE and EMPIRICAL CORRELATIONS WITH ET and Y VALUES

Abstract: — Pyrene fluorescence spectra have been run in 62 solvents of widely differing solvent polarity. As has been noted previously, the intensity ratio of the first (the 0–0 band) and third bands in vibronic fine structure of these spectra are very sensitive to solvent polarity. These I1/I3 values, however, are not sensitive to hydrogen bonding aspects of solvent-solute interactions. Correlations are reported with Winstein's Y values and with Dimrotb's ET values. On this basis the I1/I3 values for pyrene fluorescence are suggested as the basis for a new empirical scale of solvent polarity, called the Py scale, which offers certain conveniences over other scales of solvent polarity.

Dynamical polar solvent effects on solution reactions: A simple continuum model

Abstract: The influence of polar solvent dynamics in solution reactions is investigated for a simple model. In this model, a charge is subject to a chemical free energy barrier, the successful crossing of which constitutes reaction. The retarding influence of the time dependent solvent polarization is described at the continuum Debye relaxation level. The reaction rate constant k is determined as a function of the barrier curvature, the charge‐solvent interaction strength, and the solvent polarization relaxation time. The reduction of k with increasingly slower solvent relaxation is found to depend sensitively on the magnitude of charge‐solvent interactions. When the latter are weak, the behavior of k differs qualitatively from a standard Kramers prediction. The analogous reaction problem for a dipole is discussed briefly. Directions for a more realistic treatment of polar solvent effects on reaction rates are described.

Tautomerization of formamide, 2-pyridone, and 4-pyridone: an ab initio study

Abstract: The tautomerism of formamide, 2-pyridone, and 4-pyridone has been investigated by ab initio calculations using minimal, extended, and polarization basis sets. When the effects of geometry optimization, polarization functions, correlation energy (estimated by second-order Merller-Plesset perturbation theory), and zero-point vibration energy are combined, the following theoretical estimates are obtained: formimidic acid is 12 kcal/mol less stable than formamide, 2-pyridone is 0.3 kcal/mol more stable than 2-hydroxypyridine and 4-hydroxypyridine is 2.4 kcal/mol more stable than 4-pyridone. Only the 2-pyridone tautomerism has been observed directly in the gas phase, and theory is in good agreement with all three experimental values (0.3 f 0.3,O.l & 0.6,0.6 f 0.1 kcal/mol). In the case of 4-pyridone, the theoretical value may be closer to the actual tautomerization energy than the 7 kcal/mol in favor of hydroxypyridine obtained from indirect experiments. For the heterocycles, relative geometries of tautomers optimized with a minimal basis or semiempirical methods are as satisfactory as structural changes obtained at extended basis set levels. Relative tautomerization energies are reproduced well with the minimal or extended bases, while absolute tautomerization energies require consideration of polarization functions, correlation energy, and zero-point vibration. Tautomerism such as displayed by pyridone/hydroxypyridine plays a role in many areas of chemistry and biochemistry: e.g., the rationalization of structures, properties, and reactivities in heterocyclic chemistry;'*2 concepts and probes of aromaticity;3 measures of intrinsic stabilities vs. solvent mechanisms of enzymatic catalysis and receptor interactions;6 and possibly even mutations during DNA replicati~n.~.~ Investigations of tautom- erism of 2-pyridone date from 1907.8 Most studies since then have dealt with the equilibrium in liquid media,'v9 where the pyridone tautomer is preferred by a factor of 1000. X-ray crystallography shows that pyridone is also favored in the solid.'*I2 The dominance of the pyridone tautomer in solution, neat liquid, and solids has been shown to be the result of strong solvent effects, ion binding, and self- association^.'^^^^^^^'^ In contrast, recent IR and UV measurements have established that the two tautomers are nearly equal in energy when unassociated in the vap~r.~,'~J~ Similar gas-phase tautomerizations have since been investigated for a number of lactam/lactim pairs by using IR,I9 UV,zo pho- toelectron,21*22 ion cyclotron resonan~e,~~-~~ and mass spectros- copy.26*27 All of these gas-phase equilibria show marked dif- ferences from solution data.'*2,9,'3-'7*z8 Numerous theoretical studies with almost every available method have attempted to reproduce the tautomerization energy for pyridone/hydroxypyridine and similar heterocyclic systems2- Simulations of hydrogen bonding and solvent interactions re- produce qualitatively the shift in the equilibrium toward pyridone in condensed However, quantitative agreement with the tautomerization energy in the vapor has been difficult to obtain. Geometry optimization, basis-set flexibility, correlation energy, and zero-point vibration have been recognized as important contributors to these and related45-55 isomerization reactions. In this paper, we report an extensive series of ab initio computations on formamide, 2-pyridone, 4-pyridone, and their tautomers that take these factors into account. Method Ab initio calculations were carried out with the GAUSSIANBO series of programss6 by using minimal (STO-3G)," extended(3-21G and 6- 31G),5*~~~ and polarization (6-31G*)" basis sets. The extended basis sets are of the split-valence. type, and the polarization basis set is an extended basis augmented by a shell of six Cartesian d-type Gaussians on each non-hydrogen atom. Energies were calculated in the Hartree-Fock (HF) approximation, and correlation effects were estimated via second-order

Dynamical theory of activated processes in globular proteins

Abstract: A method is described for calculating the reaction rate in globular proteins of activated processes such as ligand binding or enzymatic catalysis. The method is based on the determination of the probability that the system is in the transition state and of the magnitude of the reactive flux for transition-state systems. An "umbrella sampling" simulation procedure is outlined for evaluating the transition-state probability. The reactive flux is obtained from an approach described previously for calculating the dynamics of transition-state trajectories. An application to the rotational isomerization of an aromatic ring in the bovine pancreatic trypsin inhibitor is presented. The results demonstrate the feasibility of calculating rate constants for reactions in proteins and point to the importance of solvent effects for reactions that occur near the protein surface.

Picosecond dynamics of twisted internal charge transfer phenomena. The role of the solvent

Abstract: To probe the role of the solvent in intramolecular charge transfer processes, and in particular, the origin of the well‐known dual fluorescence phenomena of p‐dimethylamino benzonitrile (DMABM), picosecond laser studies in mixed polar/nonpolar solutions were undertaken. The anomalous long wavelength emission is attributed to a complex formed between excited DMABN and butanol with a rate constant of (9.7±1.5)×108 M−1 s−1. The dominant stabilization of the twisted intramolecular charge transfer state is therefore concluded to be due to a short range specific interaction with a polar solvent molecule. A secondary solvent effect arises from a further stabilization of the complex by long range polarization interactions with solvent molecules. Evidence on the existence of ground state complexes between DMABN and butanol are also presented. Excitation of these ground state complexes leads to the rapid formation of the excited state complexes in 30 ps, which we have interpreted to be the time required for the com...

High‐pressure nmr study of dynamical effects on conformational isomerization of cyclohexane

Abstract: The effect of temperature and pressure on the conformational inversion of cyclohexane in solution has been investigated with use of /sup 1/H FT NMR spectroscopy. The solvents used in this study are methylcyclohexane-d/sub 14/, carbon disulfide, and acetone-d/sub 6/. The activation parameters as obtained from the temperature dependence of the rate constant are independent of solvent. However, the coalescence temperature in the methylcyclohexane-d/sub 14/ solvent is 1.5/sup 0/C higher than in the other two solvents. It is observed that the ring inversion in cyclohexane is accelerated when pressure is increased, and this pressure dependence of the rate constant is nonlinear. In the lower viscosity solvents, acetone-d/sub 6/ and carbon disulfide, the rate constant shows a larger pressure dependence than in the more viscous methylcyclohexane-d/sub 14/ solvent. The experimental data are interpreted with use of results of the stochastic models for isomerization reactions. In these models it is proposed that there are dynamical effects on isomerization because the reaction coordinate is coupled to the surrounding medium. We find that the observed activation volume is strongly pressure and solvent dependent as is the collisional contribution to the activation volume. Since the collision frequency reflecting the coupling of the reaction coordinate to themore » solvent mediums is proportional to solvent viscosity, the observed activation volume and the transmission coefficient kappa correlate well with the solvent viscosity. In addition, the experimental data indicate a non-monotonic transition between the inertial (weak coupling) and diffusive (strong coupling) regimes of isomerization reactions. Our results represent the first experimental proof of the predictions of stochastic models for isomerization reactions in condensed phases. 9 figures, 1 table.« less

Solvent Effect on the Nonradiative Deactivation of the Excited State of Tris(2,2′-bipyridyl)ruthenium(II) Ion

Abstract: Emission quantum yields and lifetimes of the [Ru(bpy)3]2+ ion were measured in various solvents at 25 °C. Nonradiative rate constants (knr) were calculated from these data. A plot of knr vs. the Gutmann’s acceptor number was found to be linear. This result can be explained in terms of the contributions of charge transfer to solvent (CTTS).

Solvent effects on chemical reactivity

Abstract: Typical examples for different types of solvent effects on chemical reactivity are given, e. g. solvent effects on reaction rates, on the position of chemical equilibria, on competitive reaction mechanisms, on dichotomic reaction paths, on chemoselectivity, and on stereoselectivity. According to their solvent sensitivity, most organic reactions can be classified into dipolar transition-state reactions, isopolar transition-state reactions, and free-radical transition-state reactions, typical examples of which are given. Attempts to describe the solvation capability by virtue of empirical parameters of solvent polarity are mentioned. Particular attention is merited by the ET(30)-parameter, an empirical parameter derived from a negatively solvatochromic pyridinium-N-phenolate betaine dye as reference compound. Some examples of the application of this ET(30)-scale to solventsensitive chemical reactions are given.

Solvent effects on the nitrogen and β-hydrogen hyperfine splitting constants of aminoxyl radicals obtained in spin trapping experiments

Abstract: The nitrogen and β-hydrogen hyperfine splitting constants (hfsc) for phenyl, 4-nitrophenyl, 4-pyridyl, benzoyl, and trichloromethyl spin adducts of α-phenyl tert-butyl nitrone (PBN) as well as for the tert-butoxyl adduct of 5,5-dimethylpyrroline-N-oxide (DMPO) have been obtained as a function of solvent (30 solvents). A useful linear relationship between the β-H hfsc and the N-hfsc of each aminoxyl is found except for the benzoyl adduct of PBN. Some speculations regarding the structural significance of these correlations is presented.

Solvent effects on the tautomeric equilibrium of 2,4-pentanedione

Abstract: The influence of solvent on the equilibrium position of the tautomeric forms of 2,4-pentanedione was studied by calorimetric and nmr spectroscopic techniques. For solvents such as CCl4 and cyclohex...

Theoretical studies on the conformation of saccharides. IV. Solvent effect on the stability of β‐maltose conformers

Abstract: The conformational equilibria of four β-maltose conformers have been studied theoretically in 12 solvents. The stability of the conformers in dilute solution has been compared by using the continuum reaction field method. The solvation energy consists of electrostatic, dispersion, and cavity terms, which have been determined from the calculated properties of β-maltose and physicochemical properties of solvents. The calculated population of four conformers significantly depends on the solvent (e.g., in dioxane, M1:M2:M3:M4 = 53.3: 20.3:17.7:8.7; in dimethyl sulfoxide, 40.1:21.8:22.8:15.3; and in water, 25.7:17.5:26.3:30.5) and was found to correlate with experimentally observed data. The results obtained indicate that the conformation adopted by β-maltose in the crystalline form is not the one preferred in solution. The roles of the individual contributions to the solvation energy have been analyzed. Based on the determined abundance of conformers, averaged residual optical activity and vicinal carbon–proton coupling constants have been calculated and discussed from the point of view of the solution behavior of β-maltose.

Theory of solvent effects on the hyperfine splitting constants in ESR spectra of free radicals

Abstract: An expression for the effects of solvation and hydrogen bonding on the hyperfine splitting constants of a free radical has been derived by obtaining ..pi..-electron spin densities of the radical in solution by perturbation theory. When no hydrogen bonding occurs between the radical and a solvent molecule, the splitting constant is approximately proportional to the Block and Walker parameter of theta(epsilon/sub r/) identical with 3 epsilon/sub r/ (ln epsilon/sub r/)/(epsilon/sub r/ ln epsilon/sub r/ - epsilon/sub r/ + 1) - 6/(ln epsilon/sub r/) - 2, where epsilon/sub r/ is the relative permittivity of the solvent. The expression is successfully applied to the di-tert-butyl nitroxide radical, the 1-methyl-4-(methoxycarbonyl)pyridinyl radical, and other free radicals. The effects of hydrogen bonding are discussed.

Photodecarboxylation of N-Phthaloyl-α-amino Acids

Abstract: N-Phthaloylglycine (1a) was irradiated with a high pressure mercury lamp to give N-methylphthalimide (2a) in an excellent yield. N-Phthaloyl derivatives 1b-k of other α-amino acids also afforded the decarboxylated products 2b-k. In the cases of N-phthaloylserine (1l) and di-phthaloylcystine (1m), N-vinylphthalimide (3) was isolated as a major product. On the other hand, N-phthaloylmethionine (11a) and methyl N-phthaloylmethionate (11b) were treated in the same manner to give the aza-thiacycloheptanol derivatives 12a-b having a new ring system. Solvent effects and possible pathways of these reactions were examined.

Matrix and solvent effects on the room-temperature phosphorescence of nitrogen heterocycles

Abstract: Several experimental parameters were studied to enhance the room-temperature phosphorescence of nitrogen heterocycles adsorbed on polyacrylic acid-salt mixtures and filter paper. The room-temperature phosphorescence was very sensitive to the salt content of the solid surface and the solvent used to adsorb the phosphor onto the surface. Sodium chloride and methanol were particularly important for inducing strong room-temperature phosphorescence. The results obtained partially explain some of the interactions responsible for room-temperature phosphorescence from nitrogen heterocycles. 2 figures, 5 tables.

Linear solvation energy relationships. Part 19. Correlation of the free energies of solution of 41 solutes in select solvents with Hildebrand's solubility parameter, δH, and with the solvatochromic parameter, π*

Abstract: The effect of a series of select solvents on the standard free energies of solution of 34 volatile solutes and on the standard free energies of transfer of 7 involatile solutes has been correlated with δH, with π*, and with a linear combination of δH and π*. For nonpolar solutes, only the regression with δHis significant, but for polar solutes the multiple regression with δH, and π* is generally much more satisfactory than either single regression. It is suggested that the term in δH represents the solvent-solvent interaction that has to be overcome in order to create a suitably sized cavity in the solvent. Indeed, the value of the coefficient in δHis directly proportional to the size of the (nonpolar) solute. It is also suggested that the term in π* represents the mainly dipolar–dipolar solute–solvent interaction; thus the coefficient in, π* is related, though not linearly, to the solute dipole moment or charge separation. Use of the combination of δH and, π* thus enables the solvent effect on the free energies of a very wide range of solutes to be correlated and interpreted in a conceptually simple manner.

Mechanistic and kinetic aspects of the photoinduced OCH3 substitution in 3,5-dinitroanisole. A probe for solvent effects in thermal reactions

Abstract: Nucleophiles (N) such as triethylamine (TEA) and OH– react with 3,5-dinitroanisole (3,5-DINA) in its lowest triplet state in aqueous or in mixed aqueous solutions and cause the formation of 3,5-dinitrophenolate (3,5-DNP). The quantum yield (Φ) of 3,5-DNP and the kinetics of the conversion have been investigated as a function of the concentration of N. The kinetics have been studied spectroscopically in the period between 50 ns and 120 s after flash excitation. The absorption spectra at three intermediate stages {X}, {Y} and {Z} are given and the transient species present at each stage are identified. The hydrogen-bonded triplet state is involved in three parallel reactions with N and yields an exciplex (E) and two σ-complexes with either a C(2)—N or a C(4)—N bond. The two σ-complexes return to 3,5-DINA. It is argued that binding in E arises from overlap of lone-pair orbitals on OCH3 and on N and that E is the immediate precursor of a thermally more stable exciplex (Xu) and of the anion 3,5-DINA–. Clustering of H2O molecules around the H-bonded NO2 groups in E lowers the activation energy required for its dissociation into 3,5-DINA– and an ˙OH radical. This explains why an asymptotic value Φ= 0.37 is reached at high concentrations of NaOH in aqueous solution, while Φ can become as large as 0.8 in CH3CH + H2O (1:1 by volume). Exciplex Xu is attacked on C(1) by N and H2O. In the case of OH– these two reactions produce a complex σ{C(1)—OH–}, which is subsequently converted into 3,5-DNP. When TEA replaces NaOH in the solutions, there are two paths to 3,5-DNP. One starts with the reaction of the triplet-state molecule with OH– present and is essentially the same path encountered in the case of NaOH. The second path starts with the reaction of the triplet-state molecule with TEA, which yields an exciplex E(TEA). At low concentration of TEA this route is of relatively little importance, presumably because E(TEA) dissociates efficiently into the anion of 3,5-DINA and TEA+.