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

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 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.