Resolving the HONO formation mechanism in the ionosphere via ab initio molecular dynamic simulations

Rongxing He, Lei Li, Jie Zhong, Chongqin Zhu, Joseph S. Francisco, Xiao Cheng Zeng

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


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 chainlike structure through which all of the lowest-energy isomersmust 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.

Original languageEnglish (US)
Pages (from-to)4629-4633
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number17
StatePublished - Apr 26 2016


  • Clusters
  • HONO
  • Ionosphere
  • Mechanism
  • Water

ASJC Scopus subject areas

  • General

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