Pressure-dependent kinetics of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg" display="inline" id="d1e233"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mtext>NH</mml:mtext> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow

This study presents a theoretical investigation of the reaction between ammonia and the hydroxyl radical, combining high-level electronic structure calculations with advanced chemical kinetics methods. Specifically, the potential energy surface was explored at the CCSD(T)/CBS//MPWB1K/6-31+G(d,p) level of theory. Furthermore, high-pressure rate constants were calculated using the canonical unified statistical (CUS) theory, which incorporates variational effects and tunneling corrections within the small-curvature (SCT) approximation. The computational results are in good agreement with available experimental data over a wide temperature range (20–2500 K). In addition, pressure effects were evaluated using system-specific quantum Rice–Ramsperger–Kassel (SS-QRRK) theory, which revealed pronounced falloff behavior at low pressures, reproducing well the experimental findings obtained at very low temperatures ( ≈ 22 K). • The reaction mechanism was characterized at the CCSD(T)/CBS//MPWB1K/6-31+G(d,p) level. • CUS theory was used to compute high-pressure limit thermal rate constants. • SS-QRRK calculations reveal strong pressure falloff at cryogenic temperatures. • Theoretical predictions reproduce the experimental rate constant at 22 K.