Spectrochemistry of solutions. Part 24.—Li, Na, K and Bu n4N thiocyanates in methanol: infrared spectroscopic evidence for ion pairing and hydrogen bonding

Abstract

Solutions of LiNCS, NaNCS, KNCS and (Buⁿ₄N)NCS in methanol, at various concentrations and four temperatures, have been studied in the C—N stretching region by infrared spectroscopy. The spectra have been analysed to show the presence of a minimum of six species in solution, which are identified as the thiocyanate ion with 0, 1, 2 or 3 hydrogen bonds. It is proposed that the thiocyanate ion has donor sites for hydrogen bonding on both the nitrogen atom and on the π electron cloud. Some fine structure is observed in the spectra which indicates that there is also hydrogen bonding from the sulfur atom. In solutions of lithium thiocyanate an additional band was observed at ca. 2101 cm^–1, which is attributed to a contact ion pair in which the nitrogen atom is attached to Li⁺ and the sulfur atom is hydrogen bonded, or vice versa. Equilibrium constants for this ion-pair and for hydrogen bonding have been determined from the absorption intensity at ca. 2101 cm^–1. There is evidence for the existence of solvent-shared ion pairs in solutions of LiNCS, NaNCS and KNCS.