title: Resolving the HONO formation mechanism in the ionosphere via ab initio molecular dynamic simulations.
abstract: Solar emission produces copious nitrosonium ions (NO(+)) in the D layer of the ionosphere, 60 to 90 km above the Earth's surface. NO(+) is believed to transfer its charge to water clusters in that region, leading to the formation of gaseous nitrous acid (HONO) and protonated water cluster. The dynamics of this reaction at the ionospheric temperature (200-220 K) and the associated mechanistic details are largely unknown. Using ab initio molecular dynamics (AIMD) simulations and transition-state search, key structures of the water hydrates-tetrahydrate NO(+)(H2O)4 and pentahydrate NO(+)(H2O)5-are identified and shown to be responsible for HONO formation in the ionosphere. The critical tetrahydrate NO(+)(H2O)4 exhibits a chain-like structure through which all of the lowest-energy isomers must go. However, most lowest-energy isomers of pentahydrate NO(+)(H2O)5 can be converted to the HONO-containing product, encountering very low barriers, via a chain-like or a three-armed, star-like structure. Although these structures are not the global minima, at 220 K, most lowest-energy NO(+)(H2O)4 and NO(+)(H2O)5 isomers tend to channel through these highly populated isomers toward HONO formation.
authors.affiliation: Key Laboratory of Luminescence and Real-Time Analytical Chemistry, Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China; Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588.
authors.forename: Rongxing
authors.initials: R
authors.lastname: He
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588.
authors.forename: Lei
authors.initials: L
authors.lastname: Li
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588.
authors.forename: Jie
authors.initials: J
authors.lastname: Zhong
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588.
authors.forename: Chongqin
authors.initials: C
authors.lastname: Zhu
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588 jfrancisco3@unl.edu xzeng1@unl.edu.
authors.forename: Joseph S
authors.initials: JS
authors.lastname: Francisco
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588 jfrancisco3@unl.edu xzeng1@unl.edu.
authors.forename: Xiao Cheng
authors.initials: XC
authors.lastname: Zeng
keywords: HONO; clusters; ionosphere; mechanism; water