Photodissociation of NCN3 in the Vacuum-Ultraviolet Production of CN B2Σ and NCN A 3Π

The photodissociation of NCN₃ in the vacuum ultraviolet has yielded CN $B^2\Sigma$ and NCN $A^3\Pi$ . No NCN ¹Π was detected. The fluorescence intensity of the CN violet emission, $I_f{\mathrm{\hspace{0.17em}/\hspace{0.17em}I}}_0$ , was measured as a function of incident wavelength, $\lambda$ . The $I_f{\mathrm{\hspace{0.17em}/\hspace{0.17em}I}}_0-\mathrm{vs}\mathrm{-\lambda }$ curve shows structure, indicating that the process $${\mathrm{NCN}}_3\mathrm{\hspace{0.17em}\to \hspace{0.17em}}{\displaystyle {lim}^{\mathrm{hv}}}{\mathrm{CN\; B}}^2{\mathrm{\Sigma \hspace{0.17em}+\hspace{0.17em}N}}_3,$$ is predissociative. The yield at 1216 Å is estimated to be approximately 2.5% The threshold wavelength of incident photons to yield CN $B^2\Sigma$ is 1685 ± 20 Å. The production of NCN $A^3\Pi$ is attributed to the formation of N₂ $A^3\Sigma$ , $${\mathrm{NCN}}_3\to {\displaystyle {lim}^{\mathrm{h\nu }}}\mathrm{NCN}{X}^3\mathrm{\Sigma \hspace{0.17em}+\hspace{0.17em}}{\mathrm{N}}_2{A}^3\mathrm{\mu ,}$$ with the threshold wavelength of 1915 ± 30 Å followed by the sensitized reaction, ${\mathrm{N}}_2{A}^3{\mathrm{\Sigma \hspace{0.17em}+\hspace{0.17em}NCN}}_3\mathrm{\hspace{0.17em}\to \hspace{0.17em}}\mathrm{NCN}{A}^3{\text{Π\hspace{0.17em}+\hspace{0.17em}2N}}_2$ . Bond dissociation energies obtained from threshold energies are $\mathrm{D(}\mathrm{NC}–{\mathrm{N}}_3\text{)\hspace{0.17em}=\hspace{0.17em}4.2\hspace{0.17em}±\hspace{0.17em}0.1}\mathrm{eV}\text{(=\hspace{0.17em}96\hspace{0.17em}±\hspace{0.17em}2}{\mathrm{kcal\; mole}}^{\text{−1}})$ and $\mathrm{D(}\mathrm{NCN}–{\mathrm{N}}_2\text{)\hspace{0.17em}=\hspace{0.17em}0.3\hspace{0.17em}±\hspace{0.17em}0.1}\mathrm{eV}\text{(=\hspace{0.17em}7\hspace{0.17em}±\hspace{0.17em}2}{\mathrm{kcal\; mole}}^{\text{−1}})$ where the error limit indicates an experimental uncertainty of the threshold energy measurements. Estimated heats of formation are ${\mathrm{\Delta H}}_f\mathrm{°(}{\mathrm{NCN}}_3\text{)\hspace{0.17em}=\hspace{0.17em}4.7\hspace{0.17em}±\hspace{0.17em}0.2}\mathrm{eV}\text{(=\hspace{0.17em}108\hspace{0.17em}±\hspace{0.17em}5}{\mathrm{kcal\; mole}}^{\text{−1}}$ and ${\mathrm{\Delta H}}_f\mathrm{°(}\mathrm{NCN}\text{)\hspace{0.17em}=\hspace{0.17em}5.0\hspace{0.17em}±\hspace{0.17em}0.2}\mathrm{eV}\text{(=\hspace{0.17em}115\hspace{0.17em}±\hspace{0.17em}5}{\mathrm{kcal\; mole}}^{\text{−1}})$ from which $\mathrm{D(}\mathrm{N}–\mathrm{CN})$ of 4.3 ± 0.2 eV $\text{(=\hspace{0.17em}99\hspace{0.17em}±\hspace{0.17em}5}{\mathrm{kcal\; mole}}^{\text{−1}})$ is obtained. The absorption coefficient of NCN₃ in the region 1200 to 2000 Å has been measured. A comparison is made of bond energies of several azide compounds.