Kinetics and thermochemistry of R + HBr {r_reversible} RH + Br reactions: Determinations of the heat of formation of C{sub 2}H{sub 5}, i-C{sub 3}H{sub 7}, sec-C{sub 4}H{sub 9}, and t-C{sub 4}H{sub 9}
- Oxford Univ. (United Kingdom)
- Catholic Univ., Washington, DC (United States); and others
The reactions of alkyl radicals (R = CH{sub 3}, C{sub 2}H{sub 5}, i-C{sub 3}H{sub 7}, and t-C{sub 4}H{sub 9}) with HBr have been studied by excimer laser flash photolysis coupled with photoionization mass spectrometry. R + HBr rate constants are approximately a factor of 2 higher than previously reported. The source of this disparity is explained. The kinetics of reverse reactions, Br + RH (R = C{sub 2}H{sub 6}, C{sub 3}H{sub 8}, n-C{sub 4}H{sub 10}, i-C{sub 4}H{sub 10}), have also been investigated using laser flash photolysis/resonance fluorescence methods. These results, combined with previously obtained kinetic information, were used in second- and third-law thermochemical calculations to obtain accurate determinations of the heats of formation of the C{sub 2}-C{sub 4} alkyl radicals involved. Second- and third-law determinations agreed extremely closely (differences were under 1.3 kJ mol{sup {minus}1}). The heats of formation of the radicals thus obtained are in excellent agreement with values obtained from studies of dissociation/association equilibria, within 2.6 kJ mol{sup {minus}1}. Recommended alkyl-radical heats of formation (with uncertainties) at 298 K are provided that are based on an assessment of all the results of the current study and a review of other recent determination (kJ mol{sup {minus}1}): C{sub 2}H{sub 5}, 121.0 {+-} 1.5; i-C{sub 3}H{sub 7}, 90.0 {+-} 1.7; sec-C{sub 4}H{sub 9}, 67.5 {+-}2.2; t-C{sub 4}H{sub 9}, 51.3 {+-} 1.8. Accurate determination of carbon-hydrogen bond enthalpies (298 K) are provided that are based on these heats of formation (kJ mol{sup {minus}1}): primary C-H in C{sub 2}H{sub 6} (422.8 {+-} 1.5); secondary C-H in C{sub 3}H{sub 8} (412.7 {+-} 1.7) and n-C{sub 4}H{sub 10} (411.1 {+-} 2.2); tertiary C-H in i-C{sub 4}H{sub 10} (403.5 {+-} 1.8). 44 refs., 4 figs., 8 tabs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 554938
- Journal Information:
- Journal of Physical Chemistry, Journal Name: Journal of Physical Chemistry Journal Issue: 24 Vol. 96; ISSN JPCHAX; ISSN 0022-3654
- Country of Publication:
- United States
- Language:
- English
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