Prediction of absolute rate constants for the reactions of NH{sub 2} with alkanes from ab initio G2M/TST calculations

Journal Article · Thu Apr 01 04:00:00 EST 1999 · Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory

DOI:https://doi.org/10.1021/jp9844922· OSTI ID:344622

Mebel, A M ^[1]; Lin, M C ^[2]

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1. Academia Sinica, Taipei (Taiwan, Province of China). Inst. of Atomic and Molecular Sciences
2. Emory Univ., Atlanta, GA (United States). Dept. of Chemistry

Systematic ab initio calculations of potential energy surfaces for the reactions of NH{sub 2} with various alkanes (CH{sub 4}, C{sub 2}H{sub 6}, C{sub 3}H{sub 8}, and i-C{sub 4}H{sub 10}) which involve abstraction of a hydrogen atom from primary, secondary, and tertiary C-H bonds have been performed using the G2M method. The calculated activation barrier for the NH{sub 2} + CH{sub 4} reaction, 15.2 kcal/mol, is higher than those for the H-abstraction from a primary C-H bond in C{sub 2}H{sub 6}, C{sub 3}H{sub 8}, and i-C{sub 4}H{sub 10}, 11--12 kcal/mol. The barrier height decreases to 8.4 and 8.3 kcal/mol for the abstraction from a secondary C-H bond in C{sub 3}H{sub 8} and a tertiary C-H bond in i-C{sub 4}H{sub 10}, respectively, in line with the weakening strength of the C-H bond and the increase of the reaction exothermicity. The G2M energies and the molecular and transition-state parameters are used to compute thermal reaction rate constants within the transition-state theory formalism with tunneling corrections. A good agreement of the theoretical rate constants with the experimental is found if the computed barriers are adjusted by 0.5--2 kcal/mol, which is within the accuracy of the G2M method. The H-abstraction from the tertiary C-H bond is shown to be faster than the other considered reactions at T {le} 2000 K, while the secondary H-abstraction is the second fastest reaction at T {le} 1600 K. The rate of the primary H-abstraction decreases with the increase of the alkane size, from ethane to propane and to isobutane. The calculated rate constants for the H-abstraction by NH{sub 2} from primary, secondary, and tertiary C-H bonds can serve as models for the reactions of the amino radical with various saturated hydrocarbons.

• Sponsoring Organization: USDOE, Washington, DC (United States)
• DOE Contract Number: FG02-97ER14784
• OSTI ID: 344622
• Journal Information: Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory, Journal Name: Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory Journal Issue: 13 Vol. 103; ISSN 1089-5639; ISSN JPCAFH
• Country of Publication: United States
• Language: English

Related Subjects

66 PHYSICS
ALKANES
CALCULATION METHODS
CHEMICAL ACTIVATION
CHEMICAL BONDS
CHEMICAL REACTIONS
NITROGEN HYDRIDES
PLANETARY ATMOSPHERES
POTENTIAL ENERGY
RADICALS
SATELLITE ATMOSPHERES