Photon impact studies of C₂HCN and CH₃CN in the vacuum ultraviolet; heats of formation of C₂H and CH₃CN

A photodissociation process to produce CN B²Σ from C₂HCN and CH₃CN has been studied as a function of incident wavelength. Threshold photon energies required for the production of CN B²Σ from C₂HCN and CH₃CN are 9.41 ± 0.04 and 8.52 ± 0.03 eV, respectively, from which D₀ (C₂H–CN) ≤ 6.21 ± 0.04 eV and D ₀(CH₃–CN) ≤ 5.32 ± 0.03 eV are obtained. The photoionization yield curves have been measured for the C₂HCN⁺ and C₂H⁺ ions. Threshold photon energies obtained for the production of CN B²Σ, C₂HCN⁺, and C₂H⁺ from C₂HCN lead to the following thermochemical values; I.P.(C₂HCN) = 11.64 ± 0.01 eV, I.P.(C₂H) = 11.96 ± 0.05 eV, $Δ H f0°(C2HCN) ≥ 85 ± 1 kcal mol−1 (355 ± 4 kJ mol−1)$⁠. $Δ H f0°(C2H) = 127 ± 1 kcal mol−1 (531 ± 4 kJ mol−1)$ and D₀ (C₂H–H) = 5.38 ± 0.05 eV. $Δ H f0°(C2H)$ obtained is in good agreement with the recent value obtained directly from a study of the high temperature reactions of graphite with hydrocarbons. $Δ H f0°(CH2CN) ≥ 14 ± 1 kcal mol−1 (59 ± 4 kJ mol−1)$ derived from D₀ (CH₃–CN) agrees within the stated error limit with the value obtained recently by bomb calcorimetry. The fluorescence efficiency vs incident wavelength curves for C₂HCN and CH₃CN show several peaks corresponding to Rydberg states indicating that the process is predissociative. The absorption coefficient of C₂HCN has been measured in the vacuum ultraviolet. The photoionization yield curve for C₂HCN⁺ shows at least two Rydberg series converging to vibrationally excited C₂HCN⁺ ions.