Spectra and hydrogen-bonding of characteristics of thiocyanic acid. Part 4.—Association with weak proton acceptors

Abstract

Free energy, enthalpy and entropy changes for the 1 : 1 association of iso-thiocyanic acid with a series of organic sulphides, selenides, nitriles, thiocyanates, aromatic hydrocarbons and with nitromethane in carbon tetrachloride solution at 25°C are reported. The thermodynamic data are compared with those for other proton donor complexes and are discussed in terms of electro-negativity of the proton acceptor atom, ring strain, steric and inductive effects, and the frequency shifts of the fundamental vibrations of HNCS on bonding.

Departures from the Badger-Bauer rule for complexes with the more polar bases are attributed to an electrostatic interaction between the base and the NCS moiety of the acid. A rough correspondence between the enthalpy and entropy of association for all the bases studied is found and the factors influencing this relationship are briefly discussed. In particular, a correction for the effect of variation in mass of the base on the translational entropy contribution is shown to improve the ΔH/ΔS correlation slightly.