In order to explore the contribution of the cation−π interaction in the photocyclodimerization of styrylpyridines, the effects of the acid concentration and the substituent on the aryl ring on the product distribution were investigated. The structures of the product dimers were clarified by X-ray crystallographic analysis. In addition, the X-ray packing structures of trans-4-styrylpyridine (1) and its HCl salt were compared. On increasing the acid amount, the yield of the syn-HT dimer significantly increased, whereas that of the cis-isomer decreased. The substituent on the aromatic ring had a significant effect on the product distribution. Irradiation of the substrate styrylpyridine bearing a CF3 group resulted in much lower selectivity, whereas the photolysis of the substrate having a MeO group resulted in a syn-HT dimer in 95% selectivity. Comparison of the X-ray packing structures of 1a and 1a·HCl clarified the significant differences between them. The molecules of 1a·HCl are packed alternately in antiparallel alignment in a face-to-face manner, the distance of which is 3.295 Å, strongly suggesting the existence of an intermolecular cation−π interaction. On the other hand, no such interaction was observed in 1a. These results lead to a conclusion that pyridinium−π interactions govern the orientation of the trans-styrylpyridinium in solution, the irradiation of which would result in the selective formation of the syn-HT dimer.

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Hydrogen bonding in acetone clusters was investigated using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory calculations in the carbon and oxygen K-edge regions. The partial-ion-yield (PIY) curves of the cluster ions were measured as the NEXAFS spectra of acetone clusters. In the carbon K-edge region, the first resonance peak, which was assigned to the C(CO) 1s-->pi( *)(C=O) resonance transition, showed no substantial change in the PIY curves of the acetone clusters, while the C(CH3) 1s-->3ppi(CH(3)) excitation feature was found to be strongly suppressed. The selective suppression of the C(CH3) 1s-->3ppi(CH(3)) resonance transition can be explained by the change in the character of the 3ppi(CH(3)) orbital due to the C=O...H-C type of hydrogen-bonding interaction. On the other hand, the NEXAFS spectra of the acetone molecule and clusters were almost identical in the oxygen K-edge region, except for a small shift in the pi( *)(C=O) resonance of 0.13 eV, because the character of the pi( *)(C=O) orbital remained, regardless of the C=O...H-C hydrogen bonding interaction.