Reaction Free Energy Surface Research Articles

A Theory of Electron Transfer and Steady-State Optical Spectra of Chromophores with Varying Electronic Polarizability

Electron transfer (ET) reactions and optical transitions are considered in chromophores with both the dipole moment and the electronic polarizability varying with the transition. An exact solution for reaction free energy surfaces of ET along a reaction coordinate has been obtained in the Drude model for the solute and solvent polarizabilities. The ET surfaces manifest the following effects of a nonzero polarizability variation: they (i) are asymmetric, (ii) have different curvatures at their minima, and (iii) become infinite for reaction coordinates outside a one-sided fluctuation band. The physical origin of these effects is the renormalization of the solute dipole by the solvent reaction field depending on the nonequilibrium solvent configuration. The reorganization energies of ET are substantially different for forward and backward transitions with higher reorganization energy in the state with hider solute polarizability. The dependence of the ET rate on the equilibrium energy gap is quadratic near the rate maximum and is linear at large energy gaps. The energy gap curves are also flatter from the side of exothermic reactions and broader for the state with a higher polarizability. The bandshape analysis of optical transitions is extended to the case of a nonzero polarizability variation.

A theoretical study on the mechanism of charge transfer state formation of 4-(N,N-dimethylamino)benzonitrile in an aqueous solution

The mechanism of charge transfer (CT) state formation of excited state 4-(N,N-dimethylamino)benzonitrile (DMABN) in an aqueous solution has been studied theoretically. Ab initio configuration interaction (CI) calculations were carried out for the potential energy surfaces of ground and excited state DMABN. The potential surface of second excited S2 state was represented by a superposition of three diabatic states, one is of the ion pair type and the other two of the neutral ones, to facilitate the calculations in a polar solution. The intermolecular pair potentials between DMABN and H2O were developed with the aid of electron distributions in DMABN obtained from ab initio calculations. These potential functions were applied to determine the geometries of DMABN–H2O complex and the results were compared with the available experimental data. Monte Carlo simulation calculations were further performed for the aqueous solution of DMABN. The potentials of mean force for the torsional angle of dimethylamino group revealed that the S2 state potential profile is remarkably altered due to the solvation and the twisted intramolecular CT state becomes a stable point on the surface while this point corresponds to the top of potential barrier in the gas phase. The origin of broad emission band at a longer wavelength region observed in the experiments were discussed on the basis of present calculations. In order to elucidate the mechanism of CT state formation, the reaction free energy surfaces were constructed as the function of solvation coordinate and amino torsional angle. The results obtained here were that: (a) the shape of free energy curve of S2 state is far from a parabolic form along the solvation coordinate while the S1 state curve is nearly parabolic; and (b) the torsional coordinate is required to undergo a deformation to reach the transition state region of CT state formation reaction.

Theoretical studies of aqueous solution structure

Abstract

Evaluation of reaction free energy surfaces in aqueous solution: an integral equation approach

Evaluation of reaction free energy surfaces in aqueous solution: an integral equation approach

Reaction free energy surfaces in myosin-actin-ATP systems.

If we select for consideration any reaction M1 in equilibrium M2 in the myosin-ATPase cycle, the question arises as to the relations between the rate constants for (1) M1 equilibrium M2, (2) AM1 in equilibrium AM2 (A = actin), (3) A + M1 in equilibrium AM1, and (4) A + M2 equilibrium AM2, with actin and myosin either (a) in solution or (b) in the myofilament structure. It is shown here, by means of examples, that a single so-called potential of mean force, W, and structural free energy, Am, suffice to determine the reaction free energy surfaces for all of these transitions (W for the solution case, W + Am for the structured case). In fact, Am is the same for all reactions in the myosin-ATPase cycle. Of course, though indispensable as the starting point and adequate for qualitative understanding, the reaction free energy surface does not provide (without additional theory) the actual values of the rate constants or of the corresponding basic free energy changes in the myosin states involved. These rate constants and free energies are discussed, in a preliminary way, in two other papers.