Quantitative Kinetics of Iodide Reactions: Method Development and Insights into Higher-Order Coupled-Cluster...

Iodide reaction kinetics are indispensable for understanding halogen-driven atmospheric oxidation, ozone depletion, and new particle formation, but are very limited and even unknown. We propose a theoretical protocol to obtain these kinetics, focusing on iodide reactions containing different types of reactions: HOI + OH (R1), HI + OH (R2), ICl + Cl (R3), ICl + Br (R4), and IO + O3 (R5). We develop composite energy methods GMM(P)-PP and GMM(P).L1-PP to approach the full configuration interaction limit, and the efficient GMM(Q).FNO-PP composite method to obtain relative energies for larger systems. We calculate reaction kinetics using dual-level multistructural transition-state theory with small-curvature tunneling. The calculated rate constants are in good agreement with experimental data for R1 at 320 K and for R2, R3, and R5 at 298 K, but a notable discrepancy is observed for R4. We find that post-CCSD(T) coupled-cluster effects reduce activation-enthalpy differences between two transition states from 2.6 to <1 kcal mol-1 due to their remarkably different multireference features, making previously overlooked pathways competitive in R1. Furthermore, the present findings reveal that post-CCSD(T) contributions lower the activation enthalpy of R5 by 3.28 kcal mol-1, leading to a ∼250-fold increase in the rate constant at 298 K. Additionally, we find that tunneling and anharmonicity further boost rate constants by several orders of magnitude at low temperatures in R1. The present findings not only establish a theoretical framework for obtaining iodine-containing reaction kinetics but also reveal a previously unrecognized limitation of widely used "gold-standard" CCSD(T) electronic-structure methods.