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.