Electronic States of HNCO, Cyanate Salts, and Organic Isocyanates. I. Luminescence Studies

The phosphorescence spectra and lifetimes of the cyanate salts of Na⁺, K⁺, Cd²⁺, Ag⁺, Hg²⁺, and Pb²⁺ have been measured. Similar measurements have been made on CH₃NCO, CH₃CH₂NCO, C₆H₅NCO, and HNCO. The results of these studies indicate that the lowest‐energy excited state of the cyanate ion and the covalent isocyanates is of ··· ${\mathrm{(\pi )}}^3{\mathrm{(\pi *)}}^1$ MO excitation type: This state is ${}^3{\Sigma }^{+}{\mathrm{(C}}_{\mathrm{\infty \upsilon }})$ in NCO⁻, ${}^3{\mathrm{A\prime (C}}_s)$ in HNCO and the alkyl isocyanates, and ${}^3{A}_1{\mathrm{(C}}_{\text{2υ}})$ in phenyl isocyanate. Fluorescence is observed only from the phenyl isocyanate and is assumed to initiate in an excited state which has dominant excitation amplitude on the phenyl moiety. Vibrational structure observed in the phosphorescence of NaOCN and C₆H₅NCO is analyzed, and similarities in the spectra are pointed out. A heavy‐atom spin–orbit coupling effect is observed in the cyanate salt series. Spin–orbit coupling calculations have been performed on NCO⁻, linear and bent. A phosphorescence lifetime of ∼ 0.12 sec, in good agreement with experiment, is predicted for the ${}^3{\Gamma }_{\mathrm{\pi \pi }}{\mathrm{*\hspace{0.17em}\to \hspace{0.17em}}}^1{\Gamma }_1$ transition.