Comparison of theoretical methods for the determination of the Li+ Affinities of neutral and anionic first- and second-row bases

Abstract

The structures of the Li+ complexes with the bases NH3, H2O, HF, PH3, H2S, HCl, and HCN and with the corresponding deprotonated anions have been optimized using second-order Møller–Plesset perturbation theory with the 6-31+G(d,p) basis set. Single-point calculations on all species were performed using the 6-31+G(2d,2p) basis set using the following correlation methods: many-body (Møller–Plesset) perturbation theory at second (MP2), third (MP3), and fourth (MP4) order; the linearized coupled-cluster method (LCCM); the averaged coupled pair functional (ACPF); configuration interaction with all single and double excitations (CISD); and CISD with the Davidson and Pople corrections. The computed lithium ion affinities of the neutral bases vary between 15 and 40 kcal/mol, while the values for the anionic bases are between 145 and 190 kcal/mol. The effect of correlation on the lithium ion affinities varies between about –5 and +2 kcal/mol. For each neutral base, the lithium ion affinities computed with the various correlated methods agree with each other to between 0.1 and 1 kcal/mol. For the anions, the various correlated methods agree with each other to between 0.5 and 3 kcal/mol. Agreement with the available experimental data is within the estimated uncertainties in the latter. Structural differences between corresponding first- and second-row complexes reflect differences in the strength of the electrostatic interaction due to differences in the charge distribution.