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 ···(π)3(π*)1; 3Σ+ 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 ···(π)3(π*)1;1Σ− ← ···(π)4;1Σ+ 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 ···(π)3(π*)1;1Δ ← ···(π)4;1Σ+. 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.