A theoretical study of the ligand-exchange reactions of Na~+ · X complexes (X = O, O_2, N_2, CO_2 and H_2O): implications for the upper atmosphere

摘要

High-level ab initio calculations are used to calculate the thermodynamics and kinetics of a number of ligand-exchange reactions involving Na~+ · X complexes (X = O, O_2, N_2, CO_2 and H_2O). The exchange reaction Na~+ · N_2 + O <-> Na~+ · O + N_2 is examined in detail using RRKM theory, where the microcanonical rate coefficients are determined from inverse Laplace transformation of the Langevin collision frequencies. This shows that ligand-switching dominates over recombination (to form O · Na~+ · N_2) up to pressures above 1000 Torr. A model of the ion-molecule chemistry of sodium in the mesosphere/lower thermosphere region is then constructed by constructed by calculating the rate coefficients of these ligand-switching reactions using Langevin theory, constrained by detailed balancing. The model predicts vertical profiles of Na~+, Na~+ · CO_2 and Na~+ · H_2O in very good agreement with the limited set of rocket-borne mass spectrometer measurements. The lifetime of Na~+ against neutralisation to Na is shown to decrease exponentially from about 1 day at 110 km to about 1 min at 90 km, which supports the theory that sporadic sodium layers form from Na~+ in descending sporadic E layers.