Browsing by Subject "Ab initio calculations"

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  • Eskola, Arkke J.; Pekkanen, Timo T.; Joshi, Satya P.; Timonen, Raimo S.; Klippenstein, Stephen J. (2019)
    The reaction of O-2 with butyl radicals is a key early step in the oxidation of n-butane, which is a prototypical alkane fuel with combustion properties that mimic those of many larger alkanes. Current combustion mechanisms employ kinetic descriptions for such radical oxidations that are based on fairly limited information. The present work employs a combination of experiment and theory to probe the kinetics of O-2 reacting with both 1- and 2-butyl radicals. The experiments employ laser photolysis to generate butyl radicals and thereby initiate the reaction kinetics. Photoionization mass spectrometric observations of the time-dependent butyl radical concentration yield rate coefficients for the overall reaction. The experiments cover temperatures ranging from 200 to 500 K and He bath gas pressures ranging from 0.3 to 6 Torr. Ab initio transition state theory (TST) based master equation calculations are used to predict the kinetics over a broad range of conditions. The calculations consider both the barrierless R + O-2 entrance channel, treated with direct CASPT2 variable reaction coordinate TST, and the decomposition of the RO2 complex to HO2 + alkenes, treated with CCSD(T)/CBS based TST. Theory and experiment are in good agreement, with maximum discrepancies of about 30%, suggesting the appropriateness of the theory based predictions for conditions of greater relevance to combustion. The kinetic description arising from this work should be of considerable utility to combustion modeling of n-butane, as well as of other related saturated hydrocarbons. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
  • Sundholm, Dage; Rabaa, Hassan; Chiheb, Mohammed; Balch, Alan L. (2019)
    Calculations have been performed at the MP2 and DFT levels for investigating the reasons for the difficulties in synthesizing bis(isocyanide)gold(I) halide complexes. Three-coordinated gold(I) complexes of the type (R3P)(2)(AuX)-X-I (1) can be synthesized, whereas the analogous isocyanide complexes (RNC)(2)(AuX)-X-I (2) are not experimentally known. The molecular structures of (R3P)(2)(AuX)-X-I (X = Cl, Br, and I) and (RNC)(2)(AuX)-X-I with X = halide, cyanide, nitrite, methylthiolate, and thiocyanate are compared and structural differences are discussed. Calculations of molecular properties elucidate which factors determine the strength of the gold-ligand interactions in (RNC)(2)(AuX)-X-I. The linear bonding mode of RNC favors a T-shaped geometry instead of the planar Y-shaped trigonal structure of (R3P)(2)(AuX)-X-I complexes that have been synthesized. An increased polarity of the Au-X bond in 2 leads to destabilization of the Y-shaped structure. Chalcogen-containing ligands or cyanide appear to be good X-ligand candidates for synthesis of (RNC)(2)(AuX)-X-I complexes.