Molecular dynamics simulations as probes of the decomposition kinetics of atmospheric molecular complexes: A case study of nitrate chemical ionization

The formation and breakup of non-covalently-bound nitrate ion-analyte clusters is an important process in newly developed nitrate ion CIMS (Chemical Ionization Mass Spectrometry) instruments designed to detect trace molecules implicated in atmospheric aerosol and new-particle formation.  Here, we show some results from using mainly classical molecular dynamics methods with empirical force fields to model these systems. These cheap methods allow us to approach the problem as a statistical one, by easily running 100s-1000s of separate simulations.

We study three different scenarios: 1) Cluster decomposition in vacuo, 2) Cluster decomposition in presence of N₂, 3) Acceleration of the charged cluster by an electric field, leading to collisions with N₂ and eventual decomposition. For scenarios 1 and 2, we find that the distribution of survival times has a very long tail, and can be effectively modelled as a stretched exponential, or a sum of two of them. Analysis of the survival time distribution at different initial temperatures can be used to predict the mean lifetime of the clusters at 300 K. We aim to use these data to aid the interpretation of CIMS experiments in our group[1].

Under the influence of electric field, average lifetimes vary with gas pressure and field strength [see figure]. As in similar studies of small water-ion clusters, we note that collisions between the cluster and gas can be energetic enough to cause decomposition directly in high field and low pressure[2].  However, at low field and atmospheric pressure (similar to conditions in the CIMS inlet) our results show that cluster decomposition is unlikely to occur.

[1] N. Hyttinen et al., J. Phys. Chem. A, 122, 269 (2018); S. Iyer et al., JPCA, 120, 576 (2016); A. Kumar et al., JACS 146, 15562 (2024); O. Garmash et al., Environ. Sci.: Atmos., DOI: 10.1039/d4ea00087k (2024)
[2] C.D. Daub and N.M. Cann, Anal. Chem., 83, 22393 (2011).