Gas phase cation-π complexation of cyclic and acyclic organosilicon compounds: electrospray mass spectrometry analysis and theoretical investigation by ab initio molecular orbital calculations †

Abstract

The gas phase cation-π complexation of oligo[(dimethylsilylene)phenylene]s with alkali metal cations was observed by electrospray mass spectrometry for the first time. The cation-π complexes were derived not only from cyclic oligomers but also from acyclic derivatives in spite of the large conformational flexibility. The substituent effect of silyl groups on cation-π interaction was investigated by ab initio calculations of the model compounds. The calculation showed that not the silyl (SiH3) group but the trimethylsilyl (SiMe3) group substantially increased the attraction between the cation and the π system, especially due to the large gain in induction energy. Theoretical calculations for trimethylsilylbenzene and a model compound of silamacrocycles suggested that significant conformational changes occurred due to the complexation. The observed Cs+ selectivity of the silamacrocycle in the electrospray mass spectrometry was explained by the calculated binding energies of the cation-π complexes, if the solvation of the cations was considered. The results from mass spectrometry can be understood in terms of competition between cation-π complexation and solvation by species in the solvent matrix.