Specific intramolecular interactions C-H•••N in heteroaryl vinyl ethers and heteroaryl vinyl sulfides studied by 1H, 13C, and 15N NMR spectroscopies and by ab initio calculations on molecular structures as well as on nuclear shieldings

H-1, C-13, and N-15 NMR spectra have been measured for a series of isostructural aryl (heteroaryl) vinyl sulfides and compared with analogues of vinyl ethers. It was found that "intramolecular interaction" of the alpha-hydrogen of the vinyl group with endocyclic nitrogen takes place in 2-vinylthiopyridines. This is in common with the findings for 2-vinyloxypyridine and the intramolecular interaction is shown to have a profile characteristic of hydrogen bonding. Also, the NMR spectral data show that this type of interaction is more intense (similar to 1.5 times) in 2-vinyloxypyridine than in 2-vinylthiopyridine. This is probably due to the greater electronegative effect of oxygen compared to that of sulfur. To discuss these findings further in detail, ab initio calculations have been made, and the conformational energy map and the NMR chemical shifts have been discussed. The ab initio calculations, which were carried out on the D95++(d,p) basis set at the B3LYP level, confirm the planar structure of 2-vinyloxy- and 2-vinylthiopyridines, while other analogues of these ethers and sulfides were essentially nonplanar. Gauge-Independent Atomic Orbital (GIAO) calculations of the shielding constants, based on the same basis set, support the extraordinary low-field shift of the a-hydrogen in the vinyl group when it is situated close to endocyclic nitrogen. All these calculations support our proposal that the vinyl proton is involved in intramolecular hydrogen bonding in 2-vinyloxy- and 2-vinylthiopyridines.