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 α) involving gaseous, dissolved, and solid phases of ammonia (NH ₃ ) and ammonium (NH ₄ ⁺ ) (e.g., NH _3(g) -NH _3(aq) -NH ₄ ⁺ _(aq) -NH ₄ ⁺ _(s) ) represents a fundamental chemical process that has important implications for understanding the environmental dynamics involving NH _x (NH ₃ + NH ₄ ⁺ ). 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 α values for NH ₄ ⁺ _(g) /NH _3(g) , NH _3(aq) /NH _3(g) , NH ₄ ⁺ _(aq) /NH _3(g) , NH ₄ ⁺ _(aq) /NH _3(aq) , and NH ₄ ⁺ _(s) /NH _3(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 NH ₃ gas-to-particle phase conversions that may have distinctive isotopic separation factors (Î" ¹⁵ Î _NH4+/NH3 = Î ¹⁵ N(NH ₄ ⁺ ) - Î ¹⁵ N(NH ₃ )) between N isotopic compositions (Î ¹⁵ N) of NH ₄ ⁺ and NH ₃ 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 NH _x to better understand its dynamics in the atmosphere.