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

Abstract

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, ${\mathrm{\Delta \; H\; f}}_0\mathrm{°(}{\mathrm{C}}_2\mathrm{HCN}\text{) ≥ 85 ± 1\hspace{0.17em}}{\mathrm{kcal\; mol}}^{\text{−1}}\text{(355 ± 4\hspace{0.17em}}{\mathrm{kJ\; mol}}^{\text{−1}})$ . ${\mathrm{\Delta \; H\; f}}_0\mathrm{°(}{\mathrm{C}}_2\mathrm{H}\text{) = 127 ± 1\hspace{0.17em}}{\mathrm{kcal\; mol}}^{\text{−1}}\text{(531 ± 4\hspace{0.17em}}{\mathrm{kJ\; mol}}^{\text{−1}})$ and D₀ (C₂H–H) = 5.38 ± 0.05 eV. ${\mathrm{\Delta \; H\; f}}_0\mathrm{°(}{\mathrm{C}}_2\mathrm{H})$ obtained is in good agreement with the recent value obtained directly from a study of the high temperature reactions of graphite with hydrocarbons. ${\mathrm{\Delta \; H\; f}}_0\mathrm{°(}{\mathrm{CH}}_2\mathrm{CN}\text{) ≥ 14 ± 1\hspace{0.17em}}{\mathrm{kcal\; mol}}^{\text{−1}}\text{(59 ± 4\hspace{0.17em}}{\mathrm{kJ\; mol}}^{\text{−1}})$ 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.