Solvent effects on host-guest complexation

Complex formation between 15-crown-5 and malononitrile was studied in twelve solvents by calorimetry. Thermodynamic parameters for the crown ether adduct were determined and used in a LFER analysis to ascertain the solvent effects on the complexation process. Enthalpy of solution data show that malononitrile is solvated by electron pair donation by the solvents and the crown ether is solvated by donating electron pairs to the solvents. The complex is more solvated than the monomers.     

Substituent effects on electrophilicity of flavins: an experimental and semi-empirical molecular orbital study

The reactivities of the triplet excited states of several flavins with different N(10)-phenyl substituents have been found to be very similar. There is a good correlation between semi-empirical molecular orbital calculations at the MINDO/AM1 level and observed redox potentials. However, the most interesting observation is that the substituent effects are very small; this suggests that the N(10)-phenyl substituents are nearly perpendicular to the plane of the main ring due to their steric interactions with the main isoalloxazine system. Computed calculations as well as the fact that similar absorption maxima are obtained for 3-methyl-10-phenylisoalloxazine and 3-methyl-10-methylisoalloxazine are in agreement with this prediction.     

Solvent effects on ozonolysis reaction intermediates

Solvent effects on relative stability, electronic and molecular structure of ozonolysis reaction intermediates are analyzed with the help of ab initio MP2/6-31+G^** calculations. A continuum model is employed to account for solute–solvent electrostatic interactions. The results show that there are large effects on the structure and relative stability of carbonyl oxide by substantially favoring its zwitterionic character. A complex formed by carbonyl oxide and formaldehyde is shown to be stable in the gas phase and in solution. This complex can be involved in solvent cage reactions leading to secondary ozonides. Thermodynamically, primary ozonide decomposition is favored by the solvent.     

Solvent effects on amphotericin B monolayers

Monolayers of the antifungal antibiotic amphotericin B undergo the liquid expanded/liquid condensed state transition if spread from chloroform/methanol solvent. The transition disappeared after a long spreading time. The presence of the transition may be due to the retention of solvent and/or the presence of metastable aggregates of amphotericin B.     

Solvent effects on hydrogen bond formation

Enthalpies of solution have been used to calculate transfer enthalpies for phenol, pyridine, and DMSO between the solvent cyclohexane and the solvents CCl₄, benzene, and CHCl₃. By use of model compounds, enthalpies due to interactions with phenol, pyridine, and DMSO have been determined. These enthalpies are used to calculate the effect of solvation relative to cyclohexane on hydrogen bonded complexes in CCl₄ and benzene solvents. Correlations with enthalpies due to interactions and frequency shifts for the hydroxyl stretch in these solvents have also been made.     

Solvent effects in copolymerization

Extension of the Mayo-Lewis Model of copolymerization concerning solvent effects in free-radical polymerization is discussed on the basis of the bootstrap and penultimate unit effects.     

Solvent effects on tautomerism equilibria in heterocycles

. High-level ab initio quantum mechanical methods have been used to analyze the tautomeric preferences in the gas phase and in aqueous solution of three important five-member heterocycles: 4-(5-)methylimidazole, 5-hydroxyisoxazole, and 3-hydroxypyrazole. Solvent effects have been introduced by means of self-consistent reaction field (SCRF) calculations at the ab initio level using our parametrized version of the polarizable continuum model developed by Miertus, Scrocco and Tomasi (MST), including geometry relaxation upon solvation. The extent to which the MST model, and SCRF methods in general, are suitable for the study of processes of this type is discussed.     

Solvent effects on the tautomerism of apigeninidin

Solvent effects are found to be responsible for the predominance in water solution of a highly unstable tautomer of apigeninidin in vacuo. We present free energy perturbation in molecular dynamics simulations and self-consistent reaction field calculations of the relative solvation of the anionic tautomers of apigeninidin.     

Solvent effects on mechanism of tetramethylene polymerization

The polymerization mechanism of tetramethylenes was reinvestigated under inclusion of solvent effects. The approach of a methanol molecule to a borderline diradical, a typical diradical, and a typical zwitterion was studied by a valence, charge, and dipole moment analysis of SINDO 1 calculations. Whereas the solvent molecule has no effect on the character of the zwitterion, the borderline diradical was found to switch to a zwitterion at the approach of the methanol molecule if the distance between the donor carbon and the methanol oxygen is below 2 Å. A similar switch of character was observed for the typical diradical at CO distances below 1.5 Å. From energy considerations it is concluded that borderline diradicals can follow a zwitterionic polymerization mechanism in polar solvents, whereas typical diradicals are much less likely to do so.     

Solvent effects in dense gases

The effects of perturber gases, partial pressures in the range 0–50 atm, on the absorption spectra of high-n Rydbergs, n ≥ 10, in methyl iodide and benzene have been investigated. The perturbers were rare gases and H₂. It has been shown that the spectroscopic energy shift Δ = Δ(ρ, n) is given by Δ = Δ°ρ, where ρ is the number density of the perturber, for n ≥ 10. The shift data can be interpreted using a Fermi model, and values of scattering lengths for perturbers can be extracted. These scattering lengths are internally consistent, independent of the absorber, and in excellent agreement with electron-swarm results. Given that the Fermi model is nonspecific and microscopic, scattering shifts and lengths for mixtures of perturber gases can be predicted. These predictions, having been verified by experiment for He/Ar mixtures, provide a means of “tailoring” any required scattering length. Finally, it is shown empirically that the determinative scattering parameter is the polarizability of the perturber, and a universal relationship a = ?0.26α + 1.18 between scattering length a, in Bohr radii, and polarizability α is found to exist.     

Solvent effects on the lifetimes of photogenerated biradicals

The lifetimes of biradicals in the Norrish type II reaction are shown to be sensitive to the presence of heavy atoms and upon solvent hydrogen bonding properties.     

Solvent effects on the acid-catalyzed hydrolysis of tetramethylurea

Tetramethylurea was hydrolyzed in aqueous hydrochloric acid at temperatures of 80, 90, and 100°C. All reactions were carried out in an excess of both tetramethylurea and water, with first-order dependence on acid concentration being observed. As the concentration of water was increased, the rate of hydrolysis decreased until equimolar amounts of water and tetramethylurea were present. Subsequent addition of water had no effect on the observed rate.     

Solvent effects on piezochromism of transition metal complexes

We report solvent effects on the piezochromic behaviour of wavelengths of maximum absorption for metal-to-ligand charge-transfer (m.l.c.t.) bands of several molybdenum–diimine–carbonyl and titanium or vanadium–cyclopentadienyl or ethylmaltolate–thiocyanate complexes. These and previously published data are collated in an attempt to establish a pattern for solvent effects on piezochromism of m.l.c.t. bands. Strategies for the design of compounds and complexes for use in establishing a clear pattern for such effects are outlined.     

Solvent effects on glycine II. Water-assisted tautomerization

The water-assisted tautomerization of glycine has been investigated at the B3LYP/6-31+G** level using supermolecules containing up to six water molecules as well as considering a 1:1 glycine-water complex embedded in a continuum. The conformations of the tautomers in this mechanism do not display an intramolecular H bond, instead the functional groups are bridged by a water molecule. The replacement of the intramolecular H bond by the bridging water reduces the polarity of the N-H bond in the zwitterion and increases that of the O-H bond in the neutral, stabilizing the zwitterion. Both the charge transfer effects and electrostatic interactions stabilize the nonintramolecularly H-bonded zwitterion conformer over the intramolecularly hydrogen bonded one. The nonintramolecularly H-bonded neutral is favored only by charge transfer effects. Although there is no strong evidence whether the intramolecularly hydrogen bonded or non hydrogen bonded structures are favored in the bulk solution represented as a dielectric continuum, it is likely that the latter species are more stable. The free energy of activation of the water-assisted mechanism is higher than the intramolecular proton transfer channel. However, when the presumably higher conformational energy of the zwitterion reacting in the intramolecular mechanism is taken into account, both mechanisms are observed to compete. The various conformers of the neutral glycine may form via multiple proton transfer reactions through several water molecules instead of a conformational rearrangement.     

Solvent effects on solution enthalpies of adamantyl derivatives

Solution enthalpies of 1-bromoadamantane, 1-adamantanol, and 2-adamantanone in a large set of protic and aprotic solvents are reported at 298.15 K. Solvent effects on the solution processes of these solutes are analyzed in terms of a modified TAKA equation, involving δ_cav h ^s as the cavity term. The nature and magnitude of the major interactions which influence these processes are assessed and discussed in terms of the solutes’ characteristics. New insights on the solution processes under scrutiny are presented.     

Solvent polarity effects on carbene/ether-O-ylide equilibria

p-Nitrophenylchlorocarbene (PNPCC) reacted reversibly with tetrahydrofuran (THF), tetrahydropyran (THP), 1,3-dioxane (1,3-D), and 1,4-dioxane (1,4-D) to form O-ylides 8, 9, 10, and 11, respectively. The O-ylides were visualized by their characteristic UV-vis spectroscopic signatures. Equilibrium constants (K) were determined spectroscopically, and studies of K as a function of pentane/1,2-dichloroethane (DCE) solvent blends illustrated the dependence of K on solvent polarity. Electronic structure calculations based on density functional theory provided carbene/ether O-ylide structures and energetics, as well as electronic spectroscopic parameters for use in the determination of K. Comparisons of the computed and experimental data were generally satisfactory.     

Solvent effects on the solubilities of reference electrolytes

Solubilities of bis(biphenyl)chromium(I) tetraphenylborate were measured in 25, solubilities of tetraphenylarsonium tetraphenylborate in nine and solubilities of bis(biphenyl)chromium(0) in seven nonaqueous solvents. Linear dependences of the pK_s-values for these compounds as well as the pK_s values for tetraphenylstibonium tetraphenylborate, tetraphenylmethane and tetraphenylgermane were observed. The relations between the pK_s-values of the above mentioned compounds and solvent parameters as well as solvent structure are discussed.     

Solvent effects on triplet state quenching by tetramethylpiperidcme-N-oxide

Measurements of the triplet state quenching of a series of aromatic hydrocarbons by tetramethylpiperidine-N-oxide in two solvents, acetonitrile and perfluoro-n-hexane, are reported. In both solvents the previously established trend of quenching rate constants with the triplet energy of the aromatic hydrocarbon being quenched was observed. No correlation of quenching efficiency with the charge transfer properties of the aromatic hydrocarbon—free radical collision complex was found.