Theoretical kinetic investigation of thermal decomposition of nitropropane

Abstract

The thermal decomposition of nitropropane (CH3CH2CH2NO2) has been investigated at the CBS-QB3 level of theory. The pyrolysis of CH3CH2CH2NO2 mainly includes the simple bond ruptures mechanism, hydrogen abstraction processes, isomerization and secondary reactions. As a result, for the simple bond ruptures mechanism, the formation of $${\text{CH}}_{3} {\text{CH}}_{2} {\text{CH}}_{2}^{\cdot} +\,^{\cdot}{\text{NO}}_{2}$$ products is dominant with the energy barrier of 49.77 kcal mol−1. The process of H atom on the β–CH2 abstracted by one O atom of NO2 moiety in CH3CH2CH2NO2(CH3CH2CH2NO2 → CH3CH=CH2 + HONO) needs to overcome lower energy barrier than that of the rate-determining step (one of H atom on the α-CH2 and γ-CH3 abstracted of reaction) of the other hydrogen abstraction reactions. Therefore, we predict that the corresponding alkenes and HONO are the main products in the hydrogen abstraction reaction of nitroparaffin. Besides, the channel of the CH3CH2CHO + HNO formations (CH3CH2C(α)H2NO2 → CH3CH2C(α)H2ONO → CH3CH2CHO + HNO), occurring through the H atom of C(α) abstracted by the N atom of NO2 moiety after the isomerization reaction from CH3CH2CH2NO2 to CH3CH2CH2ONO, is favorable in the isomerization secondary reactions. Rate constants and branching ratios are estimated by means of the conventional transition state theory with zero curvature tunneling over the temperature range of 400–1500 K. The calculation shows that the overall rate constant in the temperature of 400–1500 K is mainly dependent on the competitive channels of formations of CH3CH=CH2 + HONO and $${\text{CH}}_{3} {\text{CH}}_{2} {\text{CH}}_{2}^{\cdot} +\,^{\cdot}{\text{NO}}_{2}$$ The three-parameter expression for the total rate constant is fitted to be k total = 1.74 × 10−13 T 8.20exp(17038.7/T) (s−1) between 400 and 1500 K.