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, 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.