Prediction of Absolute Rate Constants for the Reactions of NH2 with Alkanes from ab Initio G2M/TST Calculations

Abstract

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 bondmore » 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.« less