Experimental Studies of Allene, Methylacetylene, and the Propargyl Radical: Bond Dissociation Energies, Gas-Phase Acidities, and Ion–Molecule Chemistry

Electron affinities and ΔH_acid are combined in a thermochemical cycle to arrive at bond dissociation energies for allene, methylacetylene, and the propargyl radical: D₀(CH₂=C=CH—H) = 88.7 ± 3 kcal mol^–1, D₀(H–CH₂C≡CH) = 90.3 ± 3 kcal mol^–1, D₀(CH₃C≡C–H) = 130.2 ± 3 kcal mol^–1, and D₀(CH₂=C=Ċ–H) = 100 ± 5 kcal mol^–1. Electron affinity measurements were determined using negative ion photoelectron spectroscopy and yielded the following for the propargyl, 1-propynyl, and propadienylidene radicals: EA(CH₂=C=ĊH) = 0.918 ± 0.008 eV, EA(CH₃C≡Ċ) = 2.718 ± 0.008 eV, and EA(CH₂=C=Ċ) = 1.794 ± 0.008 eV. Gas-phase acidity measurements were made using proton transfer kinetics in a flowing afterglow/selected-ion flow tube and yielded the following for allene, methylacetylene, and the propargyl radical: ΔG_acid(CH₂=C=CH–H) = 372.8 ± 3 kcal mol^–1, ΔG_acid(H–CH₂C≡ĊH) = 374.7 ± 3 kcal mol^–1, ΔG_acid(CH₃C≡C–H) = 373.4 ± 2 kcal mol^–1, and ΔG_acid(CH₂=C=ĊH) = 364 ± 5 kcal mol^–1. AG_acid was converted to ΔH_acid by employing ΔS_acid: ΔH_acid(CH₂=C=CH–H) = 381.1 ± 3 kcal mol^–1, ΔH_acid(H-CH₂C≡CH) = 382.7 ± 3 kcal mol^–1, ΔH_acid(CH₃C=C–H) = 381.1 ± 3 kcal mol^–1, and ΔH_acid(CH₂=C=ĊH) = 372 ± 5 kcal mol^–1. Evidence is provided for the isomerization of the allenyl anion (CH₂=C=CH^–) to the 1-propynyl anion (CH₃C≡C^–) in the proton transfer reactions of CH₂=C=CH^– with CH₃OH and CH₃CH₂OH. This complexity limits the precision of experimental measurements. This study explores the intricacies of determining gas phase acidity values by proton transfer reactions for systems in which isomerization can occur.