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Lack of kinetic hydrogen isotope effect in acetylene pyrolysis
Author(s) -
Durán R. P.,
Amorebieta V. T.,
Colussi A. J.
Publication year - 1989
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550210909
Subject(s) - chemistry , acetylene , kinetic isotope effect , carbene , singlet state , reaction rate constant , hydrogen , kinetic energy , atmospheric temperature range , mass spectrometry , photochemistry , computational chemistry , thermodynamics , kinetics , deuterium , excited state , organic chemistry , catalysis , atomic physics , physics , quantum mechanics , chromatography
Second order rate constants for C 2 H 2 or C 2 D 2 polymerizations into vinylacetylene and higher C n H n products have been measured in a static reactor by dynamic mass spectrometry between 770–980 K. They are nearly identical within experimental error (±50%). It is shown that these results are consistent with the participation of thermally equilibrated vinylidene H 2 C C: as a reactive intermediate:since this assumption only introduces a modest reverse equilibrium isotope effect (K iH /K iD ca. 0.48 in this range) into overall rate constants. At the same time they seem to discriminate in general against alternative mechanisms in which the required H‐atom transfers take place in rate determining steps. Present evidence, in conjunction with an updated analysis of relevant issues such as experimental and theoretical vs. termochemical estimates of the heat of formation of H 2 CC:, the nature of the transition states of singlet vinylidene addition reactions and the likelihood of discrete biradical intermediates in C 2 H 2 dimerization, seems to lend further support to the notion that acetylene behaves as a singlet carbene at high temperatures.