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Title: The strange kinetics of the C{sub 2}H{sub 6} + CN reaction explained.

Abstract

In this paper, we employ state of the art quantum chemical and transition state theory methods in making a priori kinetic predictions for the abstraction reaction of CN with ethane. This reaction, which has been studied experimentally over an exceptionally broad range of temperature (25-1140 K), exhibits an unusually strong minimum in the rate constant near 200 K. The present theoretical predictions, which are based on a careful consideration of the two distinct transition state regimes, quantitatively reproduce the measured rate constant over the full range of temperature, with no adjustable parameters. At low temperatures, the rate-determining step for such radical-molecule reactions involves the formation of a weakly bound van der Waals complex. At higher temperatures, the passage over a sub-threshold saddle point on the potential energy surface, related to the formation and dissolution of chemical bonds, becomes the rate-determining step. The calculations illustrate the changing importance of the two transition states with increasing temperature and also clearly demonstrate the need for including accurate treatments of both transition states. The present two transition state model is an extension of that employed in our previous work on the C2H4 + OH. It incorporates direct ab initio evaluations of the potential inmore » classical phase space integral based calculations of the fully coupled anharmonic transition state partition functions for both transition states. Comparisons with more standard rigid-rotor harmonic oscillator representations for the 'inner' transition state illustrate the importance of variational, anharmonic, and nonrigid effects. The effects of tunneling through the 'inner' saddle point and of dynamical correlations between the two transition states are also discussed. A study of the kinetic isotope effect provides a further test for the present two transition state model.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
914898
Report Number(s):
ANL/CHM/JA-58141
Journal ID: ISSN 1089-5639; JPCAFH; TRN: US200817%%2
DOE Contract Number:
DE-AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: J. Phys. Chem. A; Journal Volume: 111; Journal Issue: 19 ; May 17, 2007
Country of Publication:
United States
Language:
ENGLISH
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; CHEMICAL REACTION KINETICS; ETHANE; CYANIDES; MATHEMATICAL MODELS

Citation Formats

Georgievskii, Y., Klippenstein, S. J., Chemistry, and SNL. The strange kinetics of the C{sub 2}H{sub 6} + CN reaction explained.. United States: N. p., 2007. Web. doi:10.1021/jp068430k.
Georgievskii, Y., Klippenstein, S. J., Chemistry, & SNL. The strange kinetics of the C{sub 2}H{sub 6} + CN reaction explained.. United States. doi:10.1021/jp068430k.
Georgievskii, Y., Klippenstein, S. J., Chemistry, and SNL. Thu . "The strange kinetics of the C{sub 2}H{sub 6} + CN reaction explained.". United States. doi:10.1021/jp068430k.
@article{osti_914898,
title = {The strange kinetics of the C{sub 2}H{sub 6} + CN reaction explained.},
author = {Georgievskii, Y. and Klippenstein, S. J. and Chemistry and SNL},
abstractNote = {In this paper, we employ state of the art quantum chemical and transition state theory methods in making a priori kinetic predictions for the abstraction reaction of CN with ethane. This reaction, which has been studied experimentally over an exceptionally broad range of temperature (25-1140 K), exhibits an unusually strong minimum in the rate constant near 200 K. The present theoretical predictions, which are based on a careful consideration of the two distinct transition state regimes, quantitatively reproduce the measured rate constant over the full range of temperature, with no adjustable parameters. At low temperatures, the rate-determining step for such radical-molecule reactions involves the formation of a weakly bound van der Waals complex. At higher temperatures, the passage over a sub-threshold saddle point on the potential energy surface, related to the formation and dissolution of chemical bonds, becomes the rate-determining step. The calculations illustrate the changing importance of the two transition states with increasing temperature and also clearly demonstrate the need for including accurate treatments of both transition states. The present two transition state model is an extension of that employed in our previous work on the C2H4 + OH. It incorporates direct ab initio evaluations of the potential in classical phase space integral based calculations of the fully coupled anharmonic transition state partition functions for both transition states. Comparisons with more standard rigid-rotor harmonic oscillator representations for the 'inner' transition state illustrate the importance of variational, anharmonic, and nonrigid effects. The effects of tunneling through the 'inner' saddle point and of dynamical correlations between the two transition states are also discussed. A study of the kinetic isotope effect provides a further test for the present two transition state model.},
doi = {10.1021/jp068430k},
journal = {J. Phys. Chem. A},
number = 19 ; May 17, 2007,
volume = 111,
place = {United States},
year = {Thu May 17 00:00:00 EDT 2007},
month = {Thu May 17 00:00:00 EDT 2007}
}
  • W{sub 2}(OR){sub 6} (M {triple bond}M) compounds and ethylene (1 atm, 22{degree}C) react in alkane and aromatic hydrocarbon solvents to give W{sub 2}(OR){sub 6}({mu}-CCH{sub 2}CH{sub 2}CH{sub 2}) compounds and ethane, where R = i-Pr, c-C{sub 5}H{sub 9}, c-C{sub 6}H{sub 11}, and CH{sub 2}-t-Bu. Under comparable conditions, W{sub 2}(O-t-Bu){sub 6} and ethylene fail to react. In the formation of W{sub 2}(OR){sub 6}({mu}-CCH{sub 2}CH{sub 2}CH{sub 2}) compounds, the intermediates W{sub 2}(OCH{sub 2}-t-Bu){sub 6}({eta}{sup 2}-C{sub 2}H{sub 4}){sub 2} and W{sub 2}(OR){sub 6}(CH{sub 2}){sub 4}({eta}{sup 2}-C{sub 2}H{sub 4}), where R = C-C{sub 5}H{sub 9}, i-Pr, and CH{sub 2}-t-Bu, have been characterized. For R =more » i-Pr and CH{sub 2}-t-Bu, the intermediates are shown to be formed reversibly from W{sub 2}(OR){sub 6} and ethylene. The compound W{sub 2}(O-i-Pr){sub 6}(CH{sub 2}){sub 4}({eta}{sup 2}-C{sub 2}H{sub 4}) has been fully characterized by an X-ray study and found to contain a metallacyclopentane ring and a W-{eta}{sup 2}-C{sub 2}H{sub 4} moiety, one at each metal center. The pyridine adduct W{sub 2}(O-i-Pr){sub 6}({mu}-CCH{sub 2}CH{sub 2}ch{sub 2})(py) has been fully characterized and shown to contain a novel 1,6-dimetallabicyclo(3.1.0)hex-1(5)-ene organometallic core. All compounds have been characterized by {sup 13}C and {sup 1}H NMR studies. Various aspects of the reaction pathway have been probed by the use of isotopically labeled ethylenes, and a proposed general scheme is compared to previous studies of ethylene activation at mononuclear metal centers and carbonyl dinuclear and cluster compounds.« less
  • The reactions of the cis-dioxorhenium(VII)-catecholate complex [(CH{sub 3}CH{sub 2}){sub 4}N][ReO{sub 2}(O{sub 2}C{sub 6}H{sub 4}){sub 2}] (1) with either monosubstituted organohydrazines (C{sub 6}H{sub 5}NHNH{sub 2}; 4-BrC{sub 6}H{sub 4}NHNH{sub 2}) or 1,1 disubstituted organohydrazines (Ph{sub 2-}NNH{sub 2}) yield the cis-bis(diazenido) core complexes [(CH{sub 3}CH{sub 2}){sub 4}N][Re(NNR){sub 2}(O{sub 2}C{sub 6}H{sub 4}){sub 2}] (5, R = C{sub 6}H{sub 5}; 6, R = 4-BrC{sub 6}H{sub 4}) and the cis-bis(hydrazido) core species [(CH{sub 3}CH{sub 2}){sub 4}N][Re(NNPh{sub 2}){sub 2}(O{sub 2}C36H{sub 4}){sub 2}(O{sub 2}C36H{sub 4}){sub 2}] (7). Elution of 5 in a 3:1 mixture of toluene/methanol on a column of silica gel resulted in cation exchange to givemore » Na[Re(NNPh){sub 2}-(O{sub 2}C{sub 6}H{sub 4}){sub 2}]{center_dot}CH{sub 3}CN (8) as a one-dimensional polymer ([Na(CH{sub 3}CN)]{sup +}[Re(NNPh){sub 2}(O{sub 2}C{sub 6}H{sub 4}){sub 2}]{sup {minus}}){sub 2}. Crystal data for C{sub 32}H{sub 38}N{sub 5}O{sub 4}Re (5): P2{sub 1}/c,a = 14.458(3) {angstrom}, b = 10.436(2) {angstrom}, c = 21.767(4) {angstrom}, {beta} = 107.04(3){degrees}, V = 3140(2) {angstrom}{sup 3}, Z = 4, D {sub calc} = 1.572 g cm{sup {minus}3}; structure solution and refinement based on 3256 reflections with I{sub o} {ge} 3{sigma}(I{sub o}) converged at R = 0.053. Crystal data for C{sub 44}H{sub 48}N{sub 5}O{sub 4}Re (7): P1, a = 11.660(2) {angstrom}, b = 11.864(2) {angstrom}, c = 15.400(2) {angstrom}, {alpha} = 107.12(3){degrees}, {beta} = 94.99(3){degrees}, {gamma} = 97.61(3){degrees}, V = 2000(1) {angstrom}{sup 3}, Z = 2, D{sub calc} = 1.490 g cm{sup {minus}3}; 3702 reflections, R = 0.0534. Crystal data for C{sub 26}H{sub 18}N{sub 5}NaO{sub 4}Re (8): P2/n, a = 5.785(1) {angstrom}, b = 9.670(2) {angstrom}, c = 23.142(5) {angstrom}. {beta} = 90.91(30)degrees, V = 1294.4(7) {angstrom}{sup 3}, Z = 2, D{sub calc} = 1.737 g cm{sup {minus}3}; 1517 reflections, R = 0.049.« less
  • The reactions of ammonium perrhenate, HCl(aq), and the phosphinothiol ligands (2-HSC{sub 6}H{sub 4})P(C{sub 6}H{sub 4}X){sub 2}, where X = H or SH [abbreviated P-(SH){sub x} (x = 1, 3)], in alcohol have led to the isolation of a series of rhenium complexes containing the [M(P{minus}S{sub x}){sub 2}] core, represented by [Re[P(C{sub 6}H{sub 4}S){sub 3}] [P(C{sub 6}H{sub 4}S){sub 2}(C{sub 6}H{sub 4}SH)]] (1), [ReOCl-[OP(C{sub 6}H{sub 5}){sub 2}(C{sub 6}H{sub 4}S)][P(C{sub 6}H{sub 5}){sub 2}(C{sub 6}H{sub 4}S)]] (3), and [ReOCl[OP(C{sub 6}H{sub 5}){sub 2}(2-SC{sub 6}H{sub 3}-3-SiMe{sub 3})]{sub 2}] (5). The reaction of 1 with NEt{sub 3}results in the formation of [HNEt{sub 3}][Re[P(C{sub 6}H{sub 4}S){sub 3}]{sub 2}]more » (2) by deprotonation of a thiol ligand. The reaction of ammonium perrhenate with P-(SH) also led to the isolation of the binuclear species [ReOCl[OP(C{sub 6}H{sub 5}){sub 2}(2-SC{sub 6}H{sub 3}-3-SiMe{sub 3})]{sub 2}] (4). Crystal data are given.« less
  • The chemical reaction dynamics to form cyanobenzene C[sub 6]H[sub 5]CN(X hthinsp;[sup 1]A[sub 1]), and perdeutero cyanobenzene C[sub 6]D[sub 5]CN(X hthinsp;[sup 1]A[sub 1]) via the neutral[endash]neutral reaction of the cyano radical CN(X hthinsp;[sup 2][Sigma][sup +]), with benzene C[sub 6]H[sub 6](X hthinsp;[sup 1]A[sub 1g]) and perdeutero benzene C[sub 6]D[sub 6](X hthinsp;[sup 1]A[sub 1g]), were investigated in crossed molecular beam experiments at collision energies between 19.5 and 34.4 kJ hthinsp;mol[sup [minus]1]. The laboratory angular distributions and time-of-flight spectra of the products were recorded at mass to charge ratios m/e=103[endash]98 and 108[endash]98, respectively. Forward-convolution fitting of our experimental data together with electronic structure calculationsmore » (B3LYP/6[minus]311+G[sup [asterisk][asterisk]]) indicate that the reaction is without entrance barrier and governed by an initial attack of the CN radical on the carbon side to the aromatic [pi] electron density of the benzene molecule to form a C[sub s] symmetric C[sub 6]H[sub 6]CN(C[sub 6]D[sub 6]CN) complex. At all collision energies, the center-of-mass angular distributions are forward[endash]backward symmetric and peak at [pi]/2. This shape documents that the decomposing intermediate has a lifetime longer than its rotational period. The H/D atom is emitted almost perpendicular to the C[sub 6]H[sub 5]CN plane, giving preferentially sideways scattering. This experimental finding can be rationalized in light of the electronic structure calculations depicting a H[endash]C[endash]C angle of 101.2[degree] in the exit transition state. The latter is found to be tight and located about 32.8 kJ hthinsp;mol[sup [minus]1] above the products. Our experimentally determined reaction exothermicity of 80[endash]95 kJ hthinsp;mol[sup [minus]1] is in good agreement with the theoretically calculated one of 94.6 kJ hthinsp;mol[sup [minus]1]. Neither the C[sub 6]H[sub 6]CN adduct nor the stable iso cyanobenzene isomer C[sub 6]H[sub 5]NC were found to contribute to the scattering signal. The experimental identification of cyanobenzene gives a strong background for the title reaction to be included with more confidence in reaction networks modeling the chemistry in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as Saturn[close quote]s moon Titan. This reaction might further present a barrierless route to the formation of heteropolycyclic aromatic hydrocarbons via cyanobenzene in these extraterrestrial environments as well as hydrocarbon rich flames. [copyright] [ital 1999 American Institute of Physics.] thinsp« less
  • The chemical reaction dynamics to form cyanobenzene C{sub 6}H{sub 5}CN(X&hthinsp;{sup 1}A{sub 1}), and perdeutero cyanobenzene C{sub 6}D{sub 5}CN(X&hthinsp;{sup 1}A{sub 1}) via the neutral{endash}neutral reaction of the cyano radical CN(X&hthinsp;{sup 2}{Sigma}{sup +}), with benzene C{sub 6}H{sub 6}(X&hthinsp;{sup 1}A{sub 1g}) and perdeutero benzene C{sub 6}D{sub 6}(X&hthinsp;{sup 1}A{sub 1g}), were investigated in crossed molecular beam experiments at collision energies between 19.5 and 34.4 kJ&hthinsp;mol{sup {minus}1}. The laboratory angular distributions and time-of-flight spectra of the products were recorded at mass to charge ratios m/e=103{endash}98 and 108{endash}98, respectively. Forward-convolution fitting of our experimental data together with electronic structure calculations (B3LYP/6{minus}311+G{sup {asterisk}{asterisk}}) indicate that the reactionmore » is without entrance barrier and governed by an initial attack of the CN radical on the carbon side to the aromatic {pi} electron density of the benzene molecule to form a C{sub s} symmetric C{sub 6}H{sub 6}CN(C{sub 6}D{sub 6}CN) complex. At all collision energies, the center-of-mass angular distributions are forward{endash}backward symmetric and peak at {pi}/2. This shape documents that the decomposing intermediate has a lifetime longer than its rotational period. The H/D atom is emitted almost perpendicular to the C{sub 6}H{sub 5}CN plane, giving preferentially sideways scattering. This experimental finding can be rationalized in light of the electronic structure calculations depicting a H{endash}C{endash}C angle of 101.2{degree} in the exit transition state. The latter is found to be tight and located about 32.8 kJ&hthinsp;mol{sup {minus}1} above the products. Our experimentally determined reaction exothermicity of 80{endash}95 kJ&hthinsp;mol{sup {minus}1} is in good agreement with the theoretically calculated one of 94.6 kJ&hthinsp;mol{sup {minus}1}. Neither the C{sub 6}H{sub 6}CN adduct nor the stable iso cyanobenzene isomer C{sub 6}H{sub 5}NC were found to contribute to the scattering signal. The experimental identification of cyanobenzene gives a strong background for the title reaction to be included with more confidence in reaction networks modeling the chemistry in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as Saturn{close_quote}s moon Titan. This reaction might further present a barrierless route to the formation of heteropolycyclic aromatic hydrocarbons via cyanobenzene in these extraterrestrial environments as well as hydrocarbon rich flames. {copyright} {ital 1999 American Institute of Physics.} thinsp« less