Bond strengths of ethylene and acetylene

Article abstract of DOI:10.1021/ja00171a013

Negative ion photoelectron spectroscopy and gas-phase proton transfer kinetics were employed to determine the CH bond dissociation energies of acetylene, ethylene, and vinyl radical: D₀(HCC-H) = 131.3 ± 0.7 kcal mol^-1, D₀(CH₂CH-H) = 109.7 ± 0.8 kcal mol^-1, and D₀(CH₂C-H) = 81.0 ± 3.5 kcal mol^-1. The strengths of each of the other CH and CC bonds in acetylene and ethylene and their fragments were derived. The energy required to isomerize acetylene to vinylidene was also determined: HC≡CH → H₂C=C: ΔH_isom,0 = 47.4 ± 4.0 kcal mol^-1. As part of this study, proton transfer kinetics in a flowing afterglow/selected-ion flow tube apparatus were used to refine the acidities of ethylene, acetylene, and vinyl. The gas-phase acidity of acetylene was tied to the precisely known values for hydrogen fluoride, ΔG_acid,298(HF) = 365.6 ± 0.2 kcal mol^-1, and water, ΔG_acid,298(H₂O) = 383.9 ± 0.3 kcal mol^-1, yielding ΔG_acid,298(HCC-H) = 369.8 ± 0.6 kcal mol^-1. The gas-phase acidity equilibria of acetylene with isopropyl alcohol and tert-butyl alcohol were also measured. Combined with relative acidities from the literature, these measurements yielded improved acidities for the alcohols, ΔG_acid,298((CH₃)₂CHO-H) = 370.1 ± 0.6 kcal mol^-1, ΔG_acid,298((CH₃)₃CO-H) = 369.3 ± 0.6 kcal mol^-1, ΔG_acid,298(C₂H₅O-H) = 372.0 ± 0.6 kcal mol^-1, and ΔG_acid,298(CH₃O-H) = 375.1 ± 0.6 kcal mol^-1. The gas-phase acidity of ethylene was measured relative to ammonia, ΔG_acid,298(NH₃) = 396.5 ± 0.4 kcal mol^-1, giving ΔG_acid,298(C₂H₄) = 401.0 ± 0.5 kcal mol^-1. The gas-phase acidity of vinyl radical was bracketed, 375.1 ± 0.6 kcal mol^-1 ≤ ΔG_acid,298(CH₂C-H) ≤ 380.4 ± 0.3 kcal mol^-1. The electron affinities of ethynyl, vinyl, and vinylidene radicals were determined by photoelectron spectroscopy: EA(HCC) = 2.969 ± 0.010 eV, EA(CH₂CH) = 0.667 ± 0.024 eV, and EA(CH₂C) = 0.490 ± 0.006 eV.