Dissociative molecules, resonance constants

Bauer G (2000) Reactive oxygen and nitrogen species efficient, selective and interactive signals during intercellular induction of apoptosis. Anticancer Res 20 4115-4140 Beckwith AU, Davies AG, Davison IGE, Maccoll A, Mruzek MH (1989) The mechanisms of the rearrangements of allylic hydroperoxides 5a-hydroperoxy-3p-hydrocholest-6-ene and 7a-hydro-peroxy-3(1-hydroxycholest-5-ene. J Chem Soc Perkin Trans 2 815-824 Behar D, Czapski G, Rabani J, Dorfman LM, Schwarz HA (1970) The acid dissociation constant and decay kinetics of the perhydroxyl radical. J Phys Chem 74 3209-3213 Benjan EV, Font-Sanchis E, Scaiano JC (2001) Lactone-derived carbon-centered radicals formation and reactivity with oxygen. Org Lett 3 4059-4062 Bennett JE, Summers R (1974) Product studies of the mutual termination reactions of sec- alkylper-oxy radicals Evidence for non-cyclic termination. Can J Chem 52 1377-1379 Bennett JE, Brown DM, Mile B (1970) Studies by electron spin resonance of the reactions of alkyl-peroxy radicals, part 2. Equilibrium between alkylperoxy radicals and tetroxide molecules. Trans Faraday Soc 66 397-405... 

The dependence of the dissociation constant kj) and the recombination constants kjt in the reaction (a) on the optical polarization of the atoms A was predicted by Bernheim [65] and Kastler [225] and later demonstrated experimentally [7, 363]. The dependence can be understood from the obvious fact that only hydrogen-like atoms with opposite electronic spins may recombine and form a molecule A2(X1E+). Hence we have kR = kft(l — S2), where S is the degree of electron spin polarization of the atoms. A convenient indicator of dimer formation is provided by the kinetics of the laser-induced molecular fluorescence after switching on magnetic resonance which destroys the polarization of the atoms, as performed by Huber and Weber [201] for a Na — Na2 mixture. The se-... 

Figure 1 (a) Illustration of the unimolecular dissociation of a reactant molecule ABC into products A and BC. p( ), Rts and Nts are the density of reactant states, the intermolecular distance of the transition state (TS) and the number of open channels at the transition state, respectively. The vertical axis on the left-hand side shows a spectrum dominated by sharp resonances, (b) Schematic representation of the energy dependence of the micro-canonical rate constant k E) (solid line). The dots represent the state-specific quantum mechanical rates kn-... 

In a thermal ensemble, the temperature and pressure dependent uni-molecular rate constant, A uni(w, T), is directly related to the individual dissociation rates of resonances in a rotating molecule with angular momentum J and its projection K according to... 

The main obstacle for calculating temperature and pressure dependent rate constants according to Eq. (76) or variants of it is the need for state-resolved dissociation rates for high rotational states. To perform exact quantum mechanical calculations for J = 40, for example, is not possible at present time even for a triatomic molecule, especially when it consists of three heavy atoms like O3. Until now, except for very few studies — HCO and HOCl, for example, discussed in Sect. 5 — most studies of resonance widths have been performed for J = 0. However, even at temperatures well below room temperature the molecules with J — 0 form only a small fraction of the ensemble. The common way of evaluating the resonance... 

Theoretical studies (Beswick and Shapiro, 1982 Chu and Datta, 1982 Hutson et al., 1983 Ashton et al., 1983 Schatz et al., 1988) have shown that many van der Waals molecules, such as He—12 and Ar—HCl, dissociate via isolated resonances with state-specific rate constants. The wave functions for the resonances are found to be assignable, so that the unimolecular decomposition is mode specific. However, for the van der Waals molecules ArCl2 (Halberstadt et al., 1992), Arlj (Gray, 1992a) and those formed by rare-gas atoms attached to aromatic molecules (Semmes et al., 1990) there is substantial coupling between zero-order states in forming the resonance states. 

There are seven possible tautomeric forms for cytosine (XLIX, Lllla-f). That the actual predominant tautomeric structure is (XLIX) has been adequately shown from ultraviolet spectral comparisons with N- and 0-methyl derivatives in aqueous solution [214-216], from proton magnetic resonance studies in dimethyl sulphoxide and other solvents [214, 217], from infrared spectra in the solid state [218], and from X-ray crystallography [219]. Cytosine has pX values of 4-45 and 12-2 for the dissociation of the cation and base, respectively [215] therefore, in neutral medium, the molecule is uncharged. The corresponding 1- and 3-methylcytosines have pK values of 4-55 [215] and 7-49 [214] respectively for the dissociation of the cations, from which the equilibrium constant for the tautomeric forms (XLIX) and (Lllla) of cytosine may be calculated as c. 10 in favour of (XLIX) [214] pro tonation of cytosine occurs at N3 [214],... 

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