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.