Synthesis and Hydrolysis of Atmospherically Relevant Monoterpene-Derived Organic Nitrates.

Abstract

The partition of gas-phase organic nitrates (ONs) to aerosols and subsequent hydrolysis are regarded as important loss mechanisms for ON species. However, the hydrolysis mechanisms and the major factors controlling the hydrolysis lifetime are not fully understood. In this work, we synthesized seven monoterpene-derived ONs and systematically investigated their hydrolysis in bulk solutions at different pH values. The hydrolysis lifetimes ranged from 12.9 min to 8.5 h for allylic primary ON and tertiary ONs, but secondary ONs were stable at neutral pH. The alkyl substitution numbers, functional groups, and carbon skeletons were three important factors controlling hydrolysis rates. Tertiary and secondary ONs were found to hydrolyze via the acid-catalyzed unimolecular (SN1) mechanism, while a competition of SN1 and bimolecular (SN2) mechanisms accounted for the hydrolysis of primary ONs. The consistency of experimental and theoretical hydrolysis rates calculated by density functional theory further supported the proposed mechanisms. Reversible reactions including hydrolysis and nitration were first reported to explain the hydrolysis of ONs, highlighting the possibility that particulate nitric acid can participate in nitration to generate new nitrogen-containing compounds. These findings demonstrate that ON hydrolysis is a complex reaction that proceeds via different mechanisms and is controlled by various parameters.