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>Collisional Transition Probabilities for Vibrational Deactivation of Chemically Activatedsechyphen;Butyl Radicals. The Rare Gases
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Collisional Transition Probabilities for Vibrational Deactivation of Chemically Activatedsechyphen;Butyl Radicals. The Rare Gases
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机译：Collisional Transition Probabilities for Vibrational Deactivation of Chemically Activatedsechyphen;Butyl Radicals. The Rare Gases
Vibrationally excitedsechyphen;butyl radicals, having a narrow range of energies above 40 kcal moleminus;1, were produced in a heat bath ofcishyphen;butenehyphen;2hyphen;rarehyphen;gas mixtures by H addition tocishyphen;butenehyphen;2. He, Ne, Ar, and Kr were used. Two competing reactions of the chemically activated radicals, decomposition by Csngbnd;C rupture with critical energy of 33 kcal moleminus;1and collisional stabilization, were studied as a function of pressure. Apparent rate constants for decompositionkawere obtained at three temperatures over a range of bath pressures by analysis of the decomposition and stabilization products. Strong and weak multistep collisional deactivation processes, corresponding to transfer of all or only part of the initial excess vibration energy (ge;7 kcal moleminus;1) of the radicals, were considered. Multistep models of various step sizes (Dgr;E) give a family of calculated curves ofkaas a function ofp, each of which turns up from a quasihyphen;constant value at higher pressures to higher values at low pressures. Comparison with the lowhyphen;pressure experimental ``turnup'' yielded information on Dgr;E, independent of the assumed collision cross section. Comparison of relative rates in the higherhyphen;pressure region gave minimum estimates of Dgr;Eor, alternatively, conventional ``efficiency'' factors, bgr;min; these conclusions are somewhat dependent on the assumed cross section.Values of Dgr;Efrom the lowerhyphen;pressure turnup data bunch at 2.5ndash;3.5 kcal moleminus;1on an assumed stepladder model of transition probabilities. Such a model corresponds only formally to harmonic oscillator restrictions, and encompasses the characteristics of models in which small steps are less probable than large ones. Values of lang;Dgr;Erang;expfor an exponential weighting of transition probabilities group at 1.2ndash;1.8 kcal moleminus;1; this distribution describes the characteristics of models in which small steps are more probable than large ones and seems to be the preferred description of the rarehyphen;gas behavior here. He and Ne appear to transfer less energy than Kr and Ar. Dgr;Eforcishyphen;butene itself was confirmed to be ge;9 kcal, as derived in a previous study of deactivation ofsechyphen;butylhyphen;d1radicals by Harrington, Rabinovitch, and Hoare; and by contrast, the stepladder model is more appropriate here.From the higherhyphen;pressure data, values of bgr;minwhich varied from 0.16 for He at 100deg;C to 0.62 for Kr at minus;78deg;C were found. The efficiencies increase with increasing atomic weight of the rare gas and with decreasing bath temperature.Energy transfer between rarehyphen;gas atoms and butyl radicals is thus surprisingly efficient, with relatively large amounts of energy transferred per collision (lang;Dgr;Erang;kT). The findings provide further detailed evidence concerning the inapplicability of the Landauhyphen;Teller theory and related models to polyatomic molecules at high vibrational energies (cf. Herzfeld and Litovitz). The possible role of rotational degrees of freedom is considered.
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