Gas-phase rate coefficients for the reactions of nitrate radicals with (Z)-pent-2-ene, (E)-pent-2-ene, (Z)-hex-2-ene, (E)-hex-2-ene, (Z)-hex-3-ene, (E)-hex-3-ene and (E)-3-methylpent-2-ene at room temperature

Abstract

Rate coefficients for reactions of nitrate radicals (NO₃) with (Z)-pent-2-ene, (E)-pent-2-ene, (Z)-hex-2-ene, (E)-hex-2-ene, (Z)-hex-3-ene, (E)-hex-3-ene and (E)-3-methylpent-2-ene were determined to be (6.55 ± 0.78) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, (3.78 ± 0.45) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, (5.30 ± 0.73) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, (3.83 ± 0.47) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, (4.37 ± 0.49) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, (3.61 ± 0.40) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ and (8.9 ± 1.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, respectively. We performed kinetic experiments at room temperature and atmospheric pressure using a relative-rate technique with GC–FID analysis. The experimental results demonstrate a surprisingly large cis–trans (Z–E) effect, particularly in the case of the pent-2-enes, where the ratio of rate coefficients is ca. 1.7. Rate coefficients are discussed in terms of electronic and steric influences, and our results give some insight into the effects of chain length and position of the double bond on the reaction of NO₃ with unsaturated hydrocarbons. Atmospheric lifetimes were calculated with respect to important oxidants in the troposphere for the alkenes studied, and NO₃-initiated oxidation is found to be the dominant degradation route for (Z)-pent-2-ene, (Z)-hex-3-ene and (E)-3-methylpent-2-ene.