Theoretical Phase Resolved Ammonia-Ammonium Nitrogen Equilibrium Isotope Exchange Fractionations: Applications for Tracking Atmospheric Ammonia Gas-to-Particle Conversion

Nitrogen (N) equilibrium isotope fractionation ((15)alpha) involving gaseous, dissolved, and solid phases of ammonia (NH3) and ammonium (NH4+) (e.g., NH3(g)-NH3(aq)-NH4(aq)+-NH4(s)+) represents a fundamental chemical process that has important implications for understanding the environmental dynamics involving NHx (NH3 + NH4+). However, recent literature disagrees with early experimental results from Urey and co-workers, suggesting the need for an update on theoretical estimates. Here, we have calculated theoretical (15)alpha values for NH4(g)+/NH3(g), NH3(aq)/NH3(g), NH4(aq)+/NH3(g), NH4(aq)+/NH3(aq), and NH4(s)+/NH3(g) using HF/6-31G(d) and B3LYP/6-31G(d) levels of theory. Overall, our theoretical calculated values matched experimental data reported by Urey and co-workers, with best agreement obtained at the HF/ 6-31G(d) level of theory with solvent effect accounted for using water cluster calculations. Our calculated results have important implications for tracing NH3 gas-to-particle phase conversions that may have distinctive isotopic separation factors (Delta(15)delta(NH4+/NH3) = delta(15) N(NH4+) - delta(15) N(NH3)) between N isotopic compositions (delta(15) N) of NH4+ and NH3 depending on its conversion mechanism. While further experimental work is necessary to validate our predicted isotope effects over the considered temperature range, this work demonstrates the potential of N isotopic measurements of phase-resolved NHx to better understand its dynamics in the atmosphere.