A (U)MP2(full) and (U)CCSD(T) theoretical investigation on the substituent effect on the cation-π interactions between Na+ and LCCL (L = H, CH3, OH, F, Cl, CO, NN, CN−, NC− and OH−)

The substituent effect on the cation-π interactions are investigated between Na+ and LCCL (L = H, CH3, OH, F, Cl, CO, NN, CN−, NC− and OH−) using (U)MP2(full) and (U)CCSD(T) methods at 6-311+G(2d) (or 6-311++G(2d,p)) and aug-cc-pVTZ levels. A CC triple-bond contraction is observed in (HOCCOH)2−…Na+. The electron-donating substituent increases electron density of the CC multiple-bond by means of the p-π, π-π conjugative or σ-π hyperconjugative effects, resulting in the increased cation-π interaction compared with that of the unsubstituted HCCH…Na+, while the electron-withdrawing substituent decreases the cation-π interaction. Furthermore, the stronger the electron-donating ability of the substituent is the higher binding energy the complex holds. Moreover, the more lost density from the electron-donating CH3, OH, NN or OH− substituent in complex than in monomer is shifted to the π-orbital of the CC bond upon complexation. The analyses of the “truncated” model, natural bond orbital (NBO), atoms in molecules (AIM) and electron density shifts reveal the nature of the substituent effects and explain the origin of the CC bond contraction in (HOCCOH)2−…Na+.