Potential barrier intramolecular

Wang L-S, Ding C-F, Wang X-B and Nicholas J B 1998 Probing the potential barriers in intramolecular electrostatic interactions in free doubly charged anions Phys. Rev.Lett. at press... 

The knowledge of the two-minima energy surface is sufficient theoretically to determine the microscopic and static rate of reaction of a charge transfer in relation to a geometric variation of the molecule. In practice, the experimental study of the charge-transfer reactions in solution leads to a macroscopic reaction rate that characterizes the dynamics of the intramolecular motion of the solute molecule within the environment of the solvent molecules. Stochastic chemical reaction models restricted to the one-dimensional case are commonly used to establish the dynamical description. Therefore, it is of importance to recall (1) the fundamental properties of the stochastic processes under the Markov assumption that found the analysis of the unimolecular reaction dynamics and the Langevin-Fokker-Planck method, (2) the conditions of validity of the well-known Kramers results and their extension to the non-Markovian effects, and (3) the situation of a reaction in the absence of a potential barrier. 

In Table II are reported the values of v0, and rR obtained for different temperatures as well as the experimental and calculated wavenumber v of the peak of the stationary spectrum. Figure 2.21, where the solid lines represent calculated decays, shows that the experimental results can well be accounted for by the expressions (2.37) and (2.38). These results indicate that the relaxation of the electronic energy of the TICT state of DMABN due to interaction with the polar medium can well be described by a single exponential law not only for the n-butyl chloride solution but also for the solutions in alcohols. This relaxation process, leading to final states having an electronic energy markedly lower than that of the unrelaxed charge-transfer states, is responsible for the presence of an intramolecular potential barrier for the reverse reaction to the locally excited B state the barrier is made evident by the... 

As a result of intramolecular PET between the structural units of such bridging molecules one can obtain within 10 9 s spatially separated charges, localized at different ends of the molecule. Due to the rigidity and large size of the molecules used and also to a higher potential barrier for back electron tunneling, the reverse intramolecular recombination proceeds relatively slowly. For the best triad molecules the characteristic time of the intramolecular charge recombination... 

The chemical reaction is assumed to proceed when the molecule passes irreversibly from a region of configuration space identified as the reactant to a region identified as the product through a saddle point in the multidimensional potential surface. It is assumed that this saddle point is characterized by a local maximum of the potential along one degree of freedom (the reaction coordinate), while the other intramolecular degrees of freedom maintain stable oscillations about their local minima. This saddle point constitutes the potential barrier to the reaction. 

The role of the intramolecular bond potential (fig. 4) k midal, and two parameters characterize it, 1- the potential barrier hdght e to dissodate the molecule, 2- the dissodation enm gy De the Best one is shown to be correlated wiA Ae Areshold detonatkm, the smmtivityofAe material, Aesecoad wiA the vekxity, mce Ae value of f has been fixed. It is interesting to note tiiat e and De are two indepradent... 

The unusual feature about this reaction is that it provides the first authentic example of quant mi-mechanical tunneling in an intramolecular H-transfer, that is, the H atom is transferred through a potential barrier, rather than over the top of the barrier as would be required by classical mechanics. Both igU and J.2R can be observed by ESR spectroscopy and has been measured... 

The thermodynamic chain rigidity along with the potential barriers of inter-and intramolecular interactions control the temperature of the -transition. For a large number of flexible-chain linear polymers, the regular relationships for Qfi s (0.3 + 0.05) Ec i,S + Qo and, according to the Arrhenius equation, for Tf = Qp(kJ mole )/(0.25-0.019 Igv) were demonstrated using DSC [112, 114]. 

Introduction of a second adsorbed ammonia molecule in the neighboring position generates a weak potential barrier in this rotation (broken line in Fig. 7b), which decreases with increase of the distance between the two adsorbed species. The most influenced intramolecular vibration after the adsorption is the umbrella, connected with molecular inversion of the three N-H bonds in gas-phase ammonia. Stronger attraction of the nitrogen atom to the siuface and repulsion of the hydrogens converts the symmetric double-well inversion potential of the gas molecule into a distorted single well (Fig. 8). This change in the potential ciuve modifies the distances between the vibrational levels for this mode. 

L.-S. Wang, C.-F. Ding, X.-B. Wang, and J. B. Nicholas, Phys. Rev. Lett., 81, 2667-2670 (1998). Probing the Potential Barriers and Intramolecular Electrostatic Interactions in Free Doubly Charged Anions. 

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