Cation-π Interactions of a Thiocarbonyl Group and a Carbonyl Group with a Pyridinium Nucleus

摘要

Attractive interactions between a thiocarbonyl group and a pyridinium nucleus, and between a carbonyl group and a pyridinium nucleus have been proven by ~1H and ~(13)C NMR studies, UV—vis spectral analyses, and X-ray crystallographic analyses of nicotinic amides 1 and 3, and pyridinium salts 2 and 4. Comparison of the Δδ values, which are the differences in the chemical shifts with reference compounds 5 or 6, showed that the absolute Δδ values of 2 and 4 are much larger than those of 1 and 3. In the UV—vis spectra, the n→ π~* absorption of the C=S group of 2a exhibited a significant blue shift in CHCl_3. X-ray crystallographic analysis of 1 -4 clearly showed that the C=S group of 2a and the C=O group of 4 are very close to the pyridinium moiety compared to the case of 1 and 3. In addition, the X-ray crystal packing structure of 2a showed the C=S group is sandwiched between two pyridinium rings. These experimental results strongly suggested the existence of attractive (C=S)··y~+ and (C=O)···Py~+ interactions in solution and in crystal. The optimized geometries of 1 and 2 calculated at the HF/6-311G~(**) level are in good agreement with their X-ray geometries. MP2/6-311G~(**) calculations for the model systems of pyridinium salts 2 and 4 predicted that the electrostatic and induction energies are the major source of the attractive interactions. Since the larger contribution of electrostatic and induction interactions are characteristic features of cation-π interactions, the (C=S)···Py~+ and (C=O)···Py~+ interactions would be classified as a cation-π interaction.