Effect of cation-π interaction on NMR: A theoretical investigation on complexes of Li+, Na+, Be2+, and Mg2+ with aromatics

Density functional theory calculations were performed on 16 cation-π complexes formed by cations of Li+, Na+, Be2+, and Mg2+ and π systems of benzene, 1,3,5-trifluorobenzene (TFBZ), 1,3,5-trimethylbenzene and 1,3,5-trimethoxybenzene (TMOBZ). The calculations using the GIAO-based method revealed that cation-π interaction has remarkable effect on NMR chemical shifts. It was also found that, for all substituted aromatics, the cation-π interaction between the cations and the aromatic carbon atoms without substituent attached is stronger than that with substituent, and that TMOBZ with electron-giving substituent is the best cation-π acceptor, while TFBZ with electron-withdrawing substituent is the worst.