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

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 ¹³C 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 CS group of 2a exhibited a significant blue shift in CHCl₃. X-ray crystallographic analysis of 1−4 clearly showed that the CS group of 2a and the CO 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 CS group is sandwiched between two pyridinium rings. These experimental results strongly suggested the existence of attractive (CS)···Py⁺ and (CO)···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 (CS)···Py⁺ and (CO)···Py⁺ interactions would be classified as a cation−π interaction.