Lower-Energy Electronic States of HNCS, NCS−, and Thiocyanate Salts

The phosphorescence emission spectra and lifetimes of the thiocyanate salts of K⁺, Ba²⁺, Zn²⁺, Cd²⁺, Pb²⁺, and Ag⁺ have been measured. The converse absorption process has also been detected and studied. It has been established, as a result of these studies, that the lowest‐energy excited state of the thiocyanate ion is of ${\mathrm{···(\pi )}}^3{\mathrm{(\pi *)}}^1{;}^3{\Sigma }^{+}$ nature in the linear NCS⁻ ion. The 240‐mμ transition of NCS⁻, previously assigned as a charge‐transfer‐to‐solvent (ctts) transition, has been detected in all thiocyanate salt solutions and in gaseous HNCS. This absorptive transition has been reassigned as a ${\mathrm{···(\pi )}}^3{\mathrm{(\pi *)}}^1{;}^1{\Sigma }^{-}{\mathrm{\hspace{0.17em}\leftarrow \hspace{0.17em}···(\pi )}}^4{;}^1{\Sigma }^{+}$ transition of the isolated ion. This transition possesses some intramolecular charge‐transfer nature and should be expected to exhibit the solvent effects previously adduced as evidence for its supposed ctts nature. A third transition has been detected at 197 mμ in HNCS gas and is tentatively assigned to the transition ${\mathrm{···(\pi )}}^3{\mathrm{(\pi *)}}^1{;}^1{\mathrm{\Delta \hspace{0.17em}\leftarrow \hspace{0.17em}···(\pi )}}^4{;}^1{\Sigma }^{+}$ . A number of other topics are also broached. These are isomerism in thiocyanate salts of K⁺, Zn²⁺, Cd²⁺, and Ba²⁺; polymerization in thiocyanate salts of Ag⁺ and Pb²⁺; spin–orbit coupling in the SCN⁻ ion and the effects of associated metal ion thereon; and energy levels of the SCN⁻ ion. All conclusions drawn are based on experiment and/or computation and/or analogy with the known characteristics of some of the excited states of the isoelectronic CS₂ and CO₂ molecules.