Calculated thermodynamics of reactions involving NO+·X complexes (where X = H2O, N2 and CO2) Original Research Article

Ab initio calculations using Møller-Plesset perturbation theory to the second and fourth orders have been performed on the NO+·X complexes (X = H2O, N2 and CO2). Optimised geometries (giving rotational constants) and vibrational frequencies as well as computed total energies have been used to calculate enthalpies, entropies and Gibbs free energies for the complexing reactions and ligand-switching reactions between the three molecular complexes. The results obtained have been compared with experimental values, where available. Although ab initio molecular orbital calculations have previously been performed on these NO+ complexes, minimum energy structures for X = N2 and CO2 have only been determined using methods which did not include electron correlation and there has been no study in which all three complexes were examined using the same computational method. In this work, a relatively cost-effective computational approach has been developed which allows reliable thermodynamic quantities associated with all three complexes to be calculated. The following thermodynamic values are recommended for the association reaction NO+ + X → NO+·X: ΔH298 = −18.3, −4.6 and −8.8 kcal mol−1 and ΔS298 = −23.6, −19.6 and −20.2 cal mol−1 K−1, for X = H2O, N2 and CO2, respectively. Also, for the first time, standard enthalpy and entropy changes are presented for ligand-switching reactions involving these complexes. Of note is that for the association reaction with X = N2, ΔG is positive at 298 K, but at temperatures typical of the ionosphere, ΔG is negative. The implications of the computed quantities to ionospheric chemistry and ion mobility mass spectrometric studies are briefly discussed. The structures of the complexes are viewed in terms of Lewis acid/base behaviour.