Modelling approaches for atmospheric ion-dipole collisions: all-atom trajectory simulations and central field methods

Collisions between ions and dipolar molecules can facilitate the formation of atmospheric aerosol particles and play an important role in their detection in chemical ionization mass spectrometers. Conventionally, analytical models or simple parametrizations have been used to calculate rate coefficients of ion-dipole collisions in the gas phase. Such models, however, neglect the atomistic structure and charge distribution of the collision partners.

         To determine the accuracy and applicability of these theoretical approaches, we calculated collision cross sections and rate coefficients from all-atom molecular dynamics collision trajectory simulations, sampling a relevant range of impact parameters and relative velocities, and from a central field model using an effective attractive interaction fitted to the long-range potential of mean force between the collision partners. We considered collisions between various atmospherically relevant molecular ions and dipoles, as well as charged and neutral dipolar clusters.

         We find excellent agreement between the collision cross sections and rate coefficients obtained from the molecular dynamics trajectory simulations and the central field model. Therefore, we conclude that the effective interactions between the collision partners are highly isotropic, and the central field model is able to capture the relevant physicochemical properties of the system.

         Comparing the molecular dynamics trajectory simulations with the often-used parametrization by Su and Chesnavich (1982), we find that the latter can predict the collision rate coefficient quite well for systems with a molecular dipole, but the agreement worsens for systems with a dipolar cluster.

         Based on our results, we propose the combination of potential of mean force calculation and central field model as a viable and elegant alternative to brute force sampling of individual collision trajectories over a large range of impact parameters and relative velocities.

         We are currently using the combination of potential of mean force calculation and central field model, as well as the atomistic trajectory simulations, to understand the relatively large increase in rate coefficient observed in chemical ionization mass spectrometers when sulfuric acid is charged with acetate, as compared to nitrate (Fomete, 2022).

 

Su, T. and Chesnavich, W. J.: J. Chem. Phys. 76, 5183–5185, 1982.

Fomete. S. et al.: J. Phys. Chem. A 126, 44, 8240–8248, 2022.