Effect solvent

1 Solvent Effect The method most widely utilized for computing optical activities is B3LYP/aug-cc-pVDZ, as advocated by the Stephens and Gaus- 

In the rod-selective solvent system, the aggregated shape of a conjugated rod affecting the photophysical properties was well addressed by Chen and Jenekhe (Jenekhe and 

PF-based block copolymers consisted of a terfluorene rod segment and thermoresponsive PNlPAAm coil block (Xiao et al, 2007) exhibited a reversible phase transition from a coiled structure to a collapsed globular structure at the LCST. Additionally, the energy transfer efficiency of the block copolymer doped with tetraphenylporphine tetrasidfonic acid decreased as a result of the globule-to-coU transition from PNlPAAm segments above the LCST. The result indicated that these copolymers have potential applications to be used as responsive fluorescent probes in facile detection of dye-labeled biopolymers. 

The aromatic Claisen rearrangement proceeds at around 200 °C. Although the reaction is the first order in diphenyl ether as the solvent, it was observed that the reaction rate increases with the production of phenol derivatives [26]. Independently, addition of a phenolic compound results in the enhancement of the reaction rate. These observations indicate that a phenol derivative formed since the reaction product works as an autocatalyst. 

Although solvent effect on the reaction rate had already been shown by White [15a] and Goering [15b] in 1958, later (in 1970) White reported the rate constants of the rearrangement of allyl p-tolyl ether at 170 °C using 17 solvents of different polarity [27]. The selected results are shown below. In protic solvent, the reaction is faster than that in aprotic solvent and the reaction in p-chlorophenol proceeds 300 times faster than that under neat conditions. 

As mentioned in the solvent effect section, protic solvents accelerate the Claisen rearrangement. In particular the reaction rate enhances by increasing the acidity of the protic solvent used and trifluoroacetic acid (pKj=0.2) is one of the most effective catalysts. It was reported that in trifluoroacetic acid the Claisen rearrangement of allyl phenyl ether proceeds at room temperature to afford the ortho-rearrangement product 3 and 2-methylcoumaran 25 along with a small amount of phenol and unreacted starting ether [28, 29], 

The physical effects are always present they are in fact the effects of a condensed state environment, and are the more complicated to study. The main difficulty arises from the dynamical response of the solvent in the presence of a molecular system which is transforming under a chemical reaction. Actually, no one has so far developed a general method that can treat in detail this difficult aspect, but the literature is dense with contributions which try to study the solvent dynamics (see for example Reference [9]). 

In the present chapter, the solvent will be considered always at equilibrium with the reacting system. 

One of the first attempts to introduce the solvent effect in a VB analysis for the comprehension of a chemical reaction in solution has been given by Warshel and Weiss [10]. These authors introduced the Empirical Valence Bond method (EVB) for the modeling of proton transfer processes in enzymatic reactions in aqueous environment. 

The EVB method is a semiempirical method based on the construction of the wavefunction by solving a secular problem in which the Hamiltonian matrix elements are written in terms of empirical parameters. In order to obtain these parameters, the reaction is first studied in the gas phase with the most convenient method, ab initio when possible, and 

in 1990, Kim and Heynes [11] investigated the role of solvent polarization in fast electron transfer processes and pointed out that, when the solvent is instantaneously equilibrated to the quantum charge distribution of the solute, the Hamiltonian itself is a functional of the wave-function, giving a non-linear Schrodinger equation. The resulting solvent contribution to the Hamiltonian matrix on the diabatic basis thus cannot be simply described as in the former EVB method. 

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PEP theory has also been applied to modelling the free energy profiles of reactions in solution. An important example is the solvent effect on the SN2 reaction... 

Zhu J and Rasaiah J C 1989 Solvent effects in weak electrolytes II. Dipolar hard sphere solvent an the sticky electrolyte model with L = a J. Chem. Phys. 91 505... 

In the sections below a brief overview of static solvent influences is given in A3.6.2, while in A3.6.3 the focus is on the effect of transport phenomena on reaction rates, i.e. diflfiision control and the influence of friction on intramolecular motion. In A3.6.4 some special topics are addressed that involve the superposition of static and transport contributions as well as some aspects of dynamic solvent effects that seem relevant to understanding the solvent influence on reaction rate coefficients observed in homologous solvent series and compressed solution. More comprehensive accounts of dynamics of condensed-phase reactions can be found in chapter A3.8. chapter A3.13. chapter B3.3. chapter C3.1. chapter C3.2 and chapter C3.5. 

For analysing equilibrium solvent effects on reaction rates it is connnon to use the thennodynamic fomuilation of TST and to relate observed solvent-mduced changes in the rate coefficient to variations in Gibbs free-energy differences between solvated reactant and transition states with respect to some reference state. Starting from the simple one-dimensional expression for the TST rate coefficient of a unimolecular reaction a— r... 

If reliable quantum mechanical calcnlations of reactant and transition state stnictures in vacnnm are feasible, treating electrostatic solvent effects on the basis of SRCF-PCM rising cavity shapes derived from methods... 

Reichardt C 1988 Solvents and Solvent Effects in Organic Ohemistry (Weinheim VCH)... 

Abraham M H 1974 Solvent effects on transition states and reaction rates Prog. Rhys. Org. Ohem. 11 1-87... 

Miertus S, Scrocco E and Tomasi J 1981 Electrostatic interactions of a solute with a continuum. A direct utilization of ab initio molecular potentials for the provision of solvent effects Ohem. Rhys. 55 117-25... 

Cramer C J and Truhlar D G 1996 Continuum solvation models Solvent Effects and Ohemical Reactivity ed O Tapia and J Bertran (Dordrecht Kluwer) pp 1-80... 

Caste]on H and Wiberg K B 1999 Solvent effects on methyl transfer reactions. 1. The Menshutkin reaction J. Am. Ohem. Soc. 121 2139-46... 

Ladanyi B M and Hynes J T 1986 Transition state solvent effects on atom transfer rates in solution J. Am. Ohem. Soc. 108 585-93... 

Van der Zwan G and Hynes J T 1982 Dynamical polar solvent effects on solution reactions A simple continuum model J. Chem. Phys. 76 2993-3001... 

Tapia O and Bertran J (eds) 1996 Solvent effects and chemical reactivity Understanding Chemical Reactivity vo 17 (Dordrecht Kluwer)... 

A3.8.4 QUANTUM ACTIVATED RATE PROCESSES AND SOLVENT EFFECTS... 

A3.8.5 SOLVENT EFFECTS IN QUANTUM CHARGE TRANSFER PROCESSES... 

Cacace M G, Landau E M and Ramsden J J 1997 The Hofmeister series salt and solvent effects on interfacial phenomena Q. Rev. Biophys. 30 241-78... 

Barbara P F, Walker G C and Smith T P 1992 Vibrational modes and the dynamic solvent effect in electron and proton transfer Science 256 975-81... 

G. Ramachandran and T. Schlick. Solvent effects on supercoiled DNA dynamics explored by Langevin dynamics simulations. Phys. Rev. E, 51 6188-6203, 1995. 

The intensities are plotted vs. v, the final vibrational quantum number of the transition. The CSP results (which for this property are almost identical with CI-CSP) are compared with experimental results for h in a low-temperature Ar matrix. The agreement is excellent. Also shown is the comparison with gas-phase, isolated I. The solvent effect on the Raman intensities is clearly very large and qualitative. These show that CSP calculations for short timescales can be extremely useful, although for later times the method breaks down, and CTCSP should be used. 

The method for calculating effective polarizabilitie.s wa.s developed primarily to obtain values that reflect the stabilizing effect of polarizability on introduction of a charge into a molecule. That this goal was reached was proven by a variety of correlations of data on chemical reactivity in the gas phase with effective polarizability values. We have intentionally chosen reactions in the gas phase as these show the predominant effect of polarizability, uncorrupted by solvent effects. 

Although there are examples of enzymes which maintain their catalytic activity even when ciystallized, they normally work in their natural (i.e., aqueous) environment. This is the reason why the majority of the simulations are carried out applying a technique that accounts for solvent effects. But what is the effect of a solvent ... 

The GB equation is suitable for the description of solvent effects in molecular mechanics and dynamics [16], as well as in quantum mechanical calculations (17,18]. An excellent review of implicit solvation models, with more than 900 references, is given by Cramer and Truhlar [19]. 

Solvation can have a profound effect on the results of a chern ical calculation, Th is is especially true wh en tti e solute an d solven t are polar or when they can participate in hydrogen honding. The solvent effect is expressed in several ways, including these ... 

Caution For ion ic reaction s in solution, solven t effects can play a sign ificari I roic. fhesc, of course, arc neglected in calculation s on a single molecule. You can obtain an indication of solvent effects from sem i-eni pirical calculations by carefully adding water molecules to th e solute m olectile. 

Yun-Yu S, W Lu and W F van Gunsteren 1988. On the Approximation of Solvent Effects on Conformation and Dynamics of Cyclosporin A by Stochastic Dynamics Simulation Teclmiqi Molecular Simulation 1 369-383. 

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