Homoconjugated (NH•••N)- hydrogen bonds with great proton polarizability-FTIR and NMR studies

Nine NH acids with pK(a) values between 0 and 8.57 were studied by FTIR spectroscopy in the middle and far-infrared region. In addition, NMR spectra were taken and a semiempirical AM1 analysis of the systems performed. The proton potential in the NH (...) N- reversible arrow N- (...) HN hydrogen bonds is almost the same. The proton polarizability of these hydrogen bonds is almost independent of the pK(a) of the studied N acids. There is no relation between the position of the hydrogen bond vibration in the far-infrared region and the pK(a) of the N acids. Only with the pyrrol, the pyrazol, and the 4,(5)-methylimidazol systems these hydrogen bonds are linear. With all other systems these hydrogen bonds are more or less strongly bent. The position of the hydrogen bond vibrations nu(sigma) calculated with the AM1 procedure agrees well with the experimentally obtained values. The chemical shift of the hydrogen-bonded proton increases in proportion to the pK(a) values of the N acids.