Kinetic and Mechanistic Study of the Reactions of NO 3 Radicals with Unsaturated Aldehydes: 2-Butenal, 2-Methyl-2-butenal, and 3-Methyl-2-butenal.

The kinetics and mechanisms of the gas-phase reactions of NO 3 radical with two branched unsaturated aldehydes, 2-methyl-2-butenal (also called 2-methyl-crotonaldehyde) and 3-methyl-2-butenal (or 3-methyl-crotonaldehyde), have been investigated by experimental and theoretical approaches. Kinetic data were also provided, for comparison, for 2-butenal (or crotonaldehyde). Experiments were performed in a simulation chamber at 295 ± 3 K and atmospheric pressure. Rate constants were determined using both absolute and relative rate methods. Experimental results were found to be in good agreement leading to the following rate constants (in cm 3 molecule -1 s -1 ): k (2-butenal + NO 3 ) = (4.6 ± 1.3) × 10 -15 ; k (2-methyl-2-butenal + NO 3 ) = (14.0 ± 2.8) × 10 -15 ; and k (3-methyl-2-butenal + NO 3 ) = (19.1 ± 4.1) × 10 -15 . Theoretical calculations were also performed using the DFT-BH&HLYP/6-311++G(d,p) method and lead to rate constants in agreement with experiments and allow us to explore mechanisms for abstraction and addition pathways. Impact on atmospheric chemistry is discussed.