Benzene potential curves

Another type of interaction is the association of radical ions with the parent compounds. Recently (118), a theoretical study was reported on the interaction of butadiene ions with butadiene. Assuming a sandwich structure for the complex, the potential curve based on an extended Hiickel calculation for two approaching butadienes (B + B) revealed only repulsion, as expected, while the curves for B + and B + B" interactions exhibit shallow minima (.068 and. 048 eV) at an interplanar distance of about 3.4 A. From CNDO/2 calculations, adopting the parameter set of Wiberg (161), the dimer cation radical, BJ, appears to be. 132 eV more stable than the separate B and B species, whereas the separate B and B species are favored by. 116eV over the dimer anion radical, BJ. This finding is consistent with experimental results formation of the dimer cation radical was proved in a convincing manner (162) while the attempts to detect the dimer anion radical have been unsuccessful. With other hydrocarbons, the reported formation of benzene dimer anion radical (163) represents an exceptional case, while the dimeric cation radical was observed... 

Shaik and co-workers have predicted, using valence bond correlation, that in f) /, symmetry is unstable, and that it easily distorts from this symmetry <87JA363,88IC2219>. This result is supported, using (j-n separability criteria, by Jug and Koester <90JA6772>. This study predicts that, in Ng, the Tj-electrons have a much higher tendency to localize than in benzene. Consequently the potential curve for is quite flat, and little energy is needed to distort it. Thus, N5 is only metastable on the potential surface, and easily dissociates into SNj. 

According to the experimental data, when increasing the rate of potential sweep from 0.01 to 0.04 V/seo, hindering of elect-rooxidizing process appears at more positive potential values. At rates of potential sweep being equal or more than 2.5 V/sec, c u rent slump on the benzene oxidation curves disappears at all... 

Some higher-quality, CCSD(T /CBS benchmark data became available around this time for potential energy curves of a small number of prototype van der Waals dimers, and it was used to assess the accuracy of CHARMM, AMBER, OPLS-AA, and MM3 potentials for non-covalent tt interactions (Sherrill et al., 2009a). Specifically, potential curves for benzene CH4, benzene H2S, and the sandwich, T-shaped, and parallel-displaced configurations of the benzene dimer were examined. While all of the tested force fields were qualitatively correct, none of them provided a close match to more than one or two of the benchmark quantum potential curves. 

Figure 3.1 Interaction energies (kcal mor ) for the parallel-displaced benzene dimer with a fixed vertical separation of 3.6 A. The OPT-FF results were obtained by optimizing the unique atomic chaise and the Lennard-Jones parameters to minimize the errors for all benzene dimer potential curves considered (sandwich, T-shaped, and three vertical separations for parallel-displaced) in Sherrill et al. (2009a).

FIG. 19 Dependence of the half-wave potentials for Fc (curve 1) and ZnPor (curve 2) oxidation in benzene on CIO7 concentration in the aqueous phase. In these measurements, half-wave potentials were extracted from reversible steady-state voltammograms obtained at a 25 pm diameter Pt UME. The benzene phase contained 0.25 M tetra-w-hexylammonium perchlorate (THAP) and either 5 mM Fc or 1 mM ZnPor. All potentials were measured with respect to an Ag/AgCl reference electrode in the aqueous phase. (Reprinted from Ref. 48. Copyright 1996 American Chemical Society.)... 

Figure 3.42 Potential-energy curves for benzene (17), comparing the total energy (-Etotai, solid curve) with the energy of the idealized single Lewis structure dotted curve) along a D3h distortion coordinate AR = R2,3 — R i that lowers the D6h symmetry to cyclohexatriene form.

The form of potential energy curve deduced by Olah from kinetic evidence on the nitration of benzene, and some alkyl- and halo-benzenes, by nitronium ions derived from NOJ BIV is shown in Fig. 18. In this diagram, position D is associated with a localized structure analogous to that of Fig. 16 and 19b. 

FIG. 31. (a) Current voltage curve of the PbOi anode in electrolytes without and saturated with benzene and (b) potential dependence of current efficiency of benzene oxidation. 

The frequency exaltation of the Kekule mode is mirrored by the structural manifestations in the twin states, discussed with reference to Figures 16 and 17. Thus, the repulsive jr-curve in the ground state softens the potential and thereby enables the ground-state molecule to distort along the Kekule mode when angular strain is exerted. In contrast, the attractive jr-curve in the twin excited state stiffens the potential and restores the local Deh symmetry of the benzene nucleus. The two physical effects are in perfect harmony and find a natural reflection in the VB model. 

The resulting SAPT(DFT) potential energy curves turn out to be very accurate in the wide range of intermolecular separations. For the benzene dimer225,228 the results are very close to those of the much more expensive CCSD(T) treatment. For systems of the size of the benzene dimer and for the triple-zeta quality basis sets, a SAPT(DFT) calculation actually takes less time than a conventional supermolecular DFT calculation. Due to the favorable computational scaling the SAPT(DFT) approach is applicable to much larger molecules than any method used thus far for a reliable calculation of dispersion-dominated interaction potential. 

Sinnokrot, M.O. and Sherrill C.D., Highly Accurate Coupled Cluster Potential Energy Curves for the Benzene Dimer Sandwich, T-Shaped, and Parallel-Displaced Configurations. J. Phys. Chem. 

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