Determination of Bond Dissociation Energies in Hydrogen Cyanide. Cyanogen and Cyanogen Halides by the Photodissociation Method

The photodissociation process yielding $CN B 2Σ+$ from various cyanogen compounds has been studied in the vacuum ultraviolet. Threshold energies of incident photon to produce the $CN B 2Σ+$ are measured by monitoring the fluorescence due to the transition $CN B 2Σ+ − X 2Σ+$⁠. The upper bounds of bond energies can be obtained from these thresholds and the electronic energy of $CN B 2Σ+$⁠. The heat of formation of CN, $ΔHf0° (CN)$⁠, is computed using these bond energies together with various $ΔHf0°$⁠. $ΔHf0° (CN)$ of 101 ± 1 kcal mole⁻¹ (422 kJ mole⁻¹) is obtained from the lowest two values measured. This value is within an estimated error in excellent agreement with the photoionization value recently obtained by Dibeler and Liston. The bond energies based on $ΔHf0°(CN) = 101 kcal mole−1$ for various cyanogen compounds are $D(H–CN) = 120, D(Cl–CN) = 97, D(Br–CN) = 83, D(I–CN) = 73, D(NC–CN) = 128, D(C–N) = 184$ all in kilocalories per mole with an over‐all estimated error of ± 1 kcal mole⁻¹. Because of the weak fluorescence intensity, no reliable bond energy was obtained for CH₃CN. Other values obtained are $D(F–CN) ≤ 111 kcal mole−1$ and $ΔHf0°(FCN) ≥ 7.4 kcal mole−1$⁠. A correlation of the dissociation process with the absorption spectrum is briefly discussed. A comparison is made of bond energies obtained by the photodissociation, photoionization, and electron‐impact methods. Limitations of the photodissociation method to determine bond energies are discussed.