Hydrogen bonding in some adducts of oxygen bases with acids. Part VI. Infrared and dielectric studies of adducts with isothiocyanic acid

Abstract

The evolution of the i.r. absorption intensity of the NH stretching band and the band width, and the origin of additional protonic bands, have been studied in adducts of isothiocyanic acid with a series of proton acceptors covering a range of hydrogen bond strengths from very weak to those with proton transfer. Equations correlating the integrated intensity and the band width with Δν are given. In the spectra of adducts with strong acceptors (sulphoxides, phosphine oxides, etc.) extra protonic bands were observed similar to those in adducts of carboxylic acids. Isothiocyanic acid allows a better establishment of the relation between these bands and the original νNH which is important in view of the recent hypothesis about the origin of these bands. Some far-i.r. bands and overtones were recorded. Dielectric studies confirm the existence of both the 1 : 1 and 1 : 2 acid–base ratio in some adducts as found by i.r. work. The polarisation of the hydrogen bond of some adducts is derived, yielding Δµ up to 3·64 D.