Homoconjugated (NH···N)-Hydrogen Bonds with Great Proton PolarizabilityFTIR 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 AMI analysis of the systems performed. Theproton potential in the NHN - - NHN 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 v o calculated with the AMI 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.