Gas/wall collision efficiencies in very low pressure pyrolysis experiments

Abstract

A readily applicable empirical formula is obtained for the collisional efficiency for energy transfer between a highly vibrationally excited reactant and a seasoned (usually quartz) wall, in terms of the molecular weight, potential well depth and dipole moment of the reactant. This expression is used to examine corrections due to nonunit wall collision efficiency in the high-pressure rate parameters obtained from very low-pressure pyrolysis experiments. It is found that these corrections are up to ca. 2 5 kJ/mol in the high-pressure activation energy and a factor of ca. 2 in the high-pressure frequency factor, for molecules with molecular weight less than ca. 100 and where experiments are carried out at temperatures exceeding 1000 K.

Very Low Pressure Pyrolysis of Hexamethyldisilane. Kinetic Determination of the Bond Dissociation Enthalpy DH.degree.(Me3Si-SiMe3) and Implications for the Enthalpy of Formation of the Trimethylsilyl Radical

“…Values of log(A3/s"1) = 16.0 and £3," = 272 kJ mol"1 are obtained on the assumption that the reverse process of CH3 reaction with Me2Si resembles a radical combination, with no activation barrier and an approximate collisional rate constant (using the usual methods of thermochemical kinetics33). An RRKM calculation details of which are shown in Table 3 (along with ß = 0.437 at 1000 K, taken to be the same as that estimated for MesSiH using the empirical formula of Dick et al 39), indicates that at 1000 K, k}/k-¡" is ca. 0.01.…”

A Shock Tube Study of Benzylamine Decomposition: Overall Rate Coefficient and Heat of Formation of the Benzyl Radical

“…In contrast to the shock tube data analysis, where energy transfer is with gas-gas collisions, the efficiency of a gaswall collision used in the VLPP, β w , was required for the RRKM calculations. For some molecules, the β w values were obtained experimentally, but data for benzylamine were not available, so we estimated the range of β w values on the basis of a study by Dick et al, 16 who pointed out that gas-wall collision under VLPP conditions had collision efficiencies of β w ) 0.5 ( 0.1. In the VLPP study 4 performed prior to the work of Dick et al, 16 a unit gas-wall collision efficiency (β w ) 1) was used.…”

“…For some molecules, the β w values were obtained experimentally, but data for benzylamine were not available, so we estimated the range of β w values on the basis of a study by Dick et al, 16 who pointed out that gas-wall collision under VLPP conditions had collision efficiencies of β w ) 0.5 ( 0.1. In the VLPP study 4 performed prior to the work of Dick et al, 16 a unit gas-wall collision efficiency (β w ) 1) was used. From the RRKM calculations with the given input parameters, we obtained combinations of A ∞ and E 0 fitting the VLPP as well as the shock tube data.…”

Pyrolysis of styrene: kinetics and mechanism of the equilibrium styrene .tautm. benzene + acetylene

“…We estimated ß values using the expression ß = 23.5 exp(-4.11 X 10~37), based on King and Gilbert's recipe, which involves the normal boiling temperature of styrene. 19 Assignments of molecular parameters for styrene and the transition state of reaction 1 are given in the Appendix. We assumed negligible geometrical changes and adiabatic external rotations for this relatively tight complex.…”