Radical potential energy surfaces

Cao, J.R. George, M. Holmes, J.L. Fragmentation of 1- and 3-Methoxypropene Ions. Another Part of the [C4HsO] Cation Radical Potential Energy Surface. J. Am. Chem. Soc. Mass Spectrom. 1992, 3,99-107. 

An intriguing competition arises in the context of cation radical cycloadditions (as in the context of Diels-Alder cycloadditions) which involve at least one conjugated diene component. Since both cyclobutanation and Diels-Alder addition are extremely facile reactions on the cation radical potential energy surface, it would not be surprising to find a mixture of cyclobutane (CB) and Diels-Alder (DA) addition to the diene component in such cases. Even in the cyclodimerization of 1,3-cyclohexadiene, syn and anti cyclobutadimers are observed as 1 % of the total dimeric product. Incidentally, the DA dimers have been shown not to arise indirectly via the CB dimers in this case [58]. The cross addition of tw 5-anethole to 1,3-cyclohexadiene also proceeds directly and essentially exclusively to the Diels-Alder adducts [endo > exo). Similarly, additions to 1,3-cyclopentadiene yield essentially only Diels-Alder adducts. However, additions to acyclic dienes, which typically exist predominantly in the s-trans conformation which is inherently unsuitable for Diels Alder cycloaddition, can yield either exclusively CB adducts, a mixture of CB and DA adducts or essentially exclusively DA adducts (Scheme 26) [59]. 

Keywords Atmospheric chemistry Water reactions Bromine atoms OH radicals Potential energy surfaces Ab initio computations... 

Figure 8.15 HBN radical potential energy surfaces and selected energy levels of 2 and II symmetry. Adiabatic (red, black) and diabatic (violet, magenta) PES. 2D cut at linear geometries, crossing seam shown as a green line ID cuts along BN stretching coordinate for / BH and 6 fixed at 2.1A and 150°, respectively. The component of the A" symmetry of the 11 state shown as a blue line. Assignment based on plots and expansion coefficients of vibrational part of wavefunctions. AU values in cm levels showing resonances are marked.

A very good correspondence is found between the nature of the unimolecular lifetime distribution and the fraction of trajectories that are quasiperiodic. Surface VA, which has the most intrinsically non-RRKM lifetime distribution, also contains the largest fraction of quasiperiodic trajectories. The fraction of quasiperiodic trajectories is negligibly small for the surfaces with intrinsically RRKM lifetime distributions. A summary of our findings is given in Table 4. The A and B surfaces are the ones with the largest number of quasiperiodic trajectories, and these surface types are most similar to the ethyl radical potential energy surface. 

C. S. Sloane and W. L. Ease, Ethyl radical potential energy surface, Faraday Disc. Chem. Soc. 62 210 (1977). 

Detailed results will be presented for the potential surfaces pertinent to these four reactions. In particular we have computed the minimum-energy pathways. A less complete description of the formyl radical potential energy surface has appeared previously. ... 

Figure 10. Three-dimensional potential-energy surface for the H + C2H3 C2H4 addition reaction. The lower left plot is taken in the symmetry plane of the vinyl radical. The other plots are taken in parallel planes at distances of O.S. O a.u. from the symmetry plane (1 a.u. = 0.52918 A). Solid contours are positive, dashed contours are negative, and the zero-energy contour (defined to be the energy of the reactant asymptote) is shown with a heavy sohd fine. The contour increment is 1 kcalmoU. Reproduced from [57] by pentrission of the PCCP Owner Societies.

Free radicals are short-lived, highly-reactive transient species that have one or more unpaired electrons. Free radicals are common in a wide range of reactive chemical environments, such as combustion, plasmas, atmosphere, and interstellar environment, and they play important roles in these chemistries. For example, complex atmospheric and combustion chemistries are composed of, and governed by, many elementary processes involving free radicals. Studies of these elementary processes are pivotal to assessing reaction mechanisms in atmospheric and combustion chemistry, and to probing potential energy surfaces (PESs) and chemical reactivity. 

Fig. 24. Schematic C iv potential energy surfaces for the methoxy radical as a function...

Fig. 25. Schematic C%v potential energy surfaces for the CH3S radical as a function of C—S bond length. (From Hsu et a/.,163 Cui et a/.,161 and Bise et a/.164)...

Fig. 28. Schematic of potential energy surfaces of the vinoxy radical system. All energies are in eV, include zero-point energy, and are relative to CH2CHO (X2A//). Calculated energies are compared with experimentally-determined values in parentheses. Transition states 1—5 are labelled, along with the rate constant definitions from RRKM calculations. The solid potential curves to the left of vinoxy retain Cs symmetry. The avoided crossing (dotted lines) which forms TS5 arises when Cs symmetry is broken by out-of-plane motion. (From Osborn et al.67)...

HCCS radical, Renner-Teller effect, tetraatomic molecules, II electronic states, 633-640 H2D molecule, non-adiabatic coupling, two-state molecular system, 107-109 HD2 molecule, permutational symmetry isotopomers, 713-717 potential energy surfaces, 692-694 Heaviside function ... 

FIGURE 3.21. Gas-phase potential energy surfaces for the 4-cyanochlorobenzene anion radical as a function of the C-Cl bond length (r) and the bending angle (0). R, reactant system TS, transition state. Adapted from Figure 4 of reference 32, with permission from the American Chemical Society. 

Barbaralene [85] undergoes a rapid Cope rearrangement with a doublewell potential. The radical cation was studied using CIDNP by Roth (1987) after one-electron oxidation of [85] by y or X-irradiation. On the time-scale of the CIDNP experiment ( 10 8s), a single-minimum potential energy surface was found, i.e. bishomoaromatic structure [156] was suggested. 

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