The electron as a protecting group. 3. Generation of acenaphthyne radical anion and the determination of the heat of formation of a strained cycloalkyne

Acenaphthyne dicarboxylate (12) was transferred into the gas phase from sol ution via electrospray ionization and subsequently was sequentially fragmen ted in a Fourier transform mass spectrometer to afford acenaphthyne radical anion (9). Structural confirmation of 9 was achieved by converting it to a cenaphthenone enalate (13) and demonstrating that this species is identical to the ion produced upon deprotonation of acenaphthenone (5). The reactivi ty ef 9 was explored. and since an electron can serve as a protecting group , we were able to measure the heat of hydrogenation (98 +/- 4 kcal mol(-1)) and the heat of formation (160 +/- 4 kcal mol(-1)) of acenaphthyne (1) via the application of a thermodynamic cycle. Its strain energy (68 kcal mol(- 1)) and acenaphthylene's (10H) first and second C-H bond dissociation energ ies (117 +/- 4 and 84 +/- 2 kcal mol(-1)) also were obtained. Bb initio and density functional theory calculations were carried out on the species of interest to explore their geometries and energetics. Our results were inter preted in comparison to cyclopentyne, and its predicted heat of formation ( 98 kcal mol(-1)) and strain energy (59 kcal mol(-1)) are reported.