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

Abstract

The photodissociation of NCN3 in the vacuum ultraviolet has yielded CN B2Σ and NCN A 3Π. No NCN 1Π was detected. The fluorescence intensity of the CN violet emission, If / I0, was measured as a function of incident wavelength, λ. The If / I0−vs−λ curve shows structure, indicating that the process NCN3 → lim hvCN B2Σ + N3, is predissociative. The yield at 1216 Å is estimated to be approximately 2.5% The threshold wavelength of incident photons to yield CN B2Σ is 1685 ± 20 Å. The production of NCN A 3Π is attributed to the formation of N2 A 3Σ, NCN3→ lim hνNCN X3Σ + N2 A 3μ, with the threshold wavelength of 1915 ± 30 Å followed by the sensitized reaction, N2 A 3Σ + NCN3 → NCN A 3Π + 2N2. Bond dissociation energies obtained from threshold energies are D(NC–N3) = 4.2 ± 0.1 eV (= 96 ± 2 kcal mole−1) and D(NCN–N2) = 0.3 ± 0.1 eV (= 7 ± 2 kcal mole−1) where the error limit indicates an experimental uncertainty of the threshold energy measurements. Estimated heats of formation are ΔHf°(NCN3) = 4.7 ± 0.2 eV (= 108 ± 5 kcal mole−1 and ΔHf°(NCN) = 5.0 ± 0.2 eV (= 115 ± 5 kcal mole−1) from which D(N–CN) of 4.3 ± 0.2 eV (= 99 ± 5 kcal mole−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.