Theoretical and experimental investigations of diradicals, pyramidalized alkenes, and bent alkynes

Abstract

Radical anions have been generated in the gas-phase as potential precursors of diradicals, pyramidalized alkenes, and bent Akynes. The radical anions were studied by gas-phase ion experiments and by density functional theory (DFT) and ab initio calculations. The reaction of cyclopentanone with O·- produced three isomers---the radical anions of cyclopentanone-2,5-diyl (3·-), cyclopentanone-2,4-diyl, and 2-carbenacyclopentanone. The proton affinity (PA) and electron affinity (EA) of 3·- were determined by bracketing experiments to be, respectively, 361.6 +/- 4.8 kcal/mol and ca. 0.5 eV. Although the experimental PA of 3·- is consistent with the calculated values, both DFT and ab initio calculations predict an EA that is 1.0 eV higher than the experimental value. The apparent conflict between the calculated and experimental EA values is resolved by proposing that adiabatic electron transfer leads, not to cyclopentanone-2,5-diyl, but to 1,4-pentadien-3-one. Preliminary negative ion photoelectron spectroscopy experiments support this hypothesis. The radical anions of 1,5-dehydroquadricyclane (6 ·-), norbornyne (16·- ), and norbornenyne (17·-) were generated from the Squires reaction of, respectively, 1,5-bis(trimethylsilyl)quadricyclane, 2,3-bis(trimethylsilyl)norbomene, and 2,3bis(trimethylsilyl)norbornadiene. The PAs and EAs of these ions were measured by bracketing experiments and the experimental values were compared with those predicted by calculations. The PA and EA of 6·- were found to be, respectively, 385.6 +/- 5.1 kcal/mol and ca. 0.6 eV, in good agreement with the predicted values, and lead to an experimental heat of hydrogenation (DeltaHH2) of 91.0 +/- 5.1 kcal/mol for 1,5-dehydroquadricyclane. The PA and EA of 17 ·- were measured to be, respectively, 377.5 +/- 3.0 kcal/mol and 0.81 +/- 0.29 eV, again in good agreement with calculated values, leading to DeltaHH2 = 93.9 +/- 11.6 kcal/mol for norbornenyne. The PA and EA of 16·- were determined to be, respectively, 372.9 +/- 3.0 kcal/mol and 0.68 +/- 0.1 eV. The experimental EA of 16·- is in good agreement with the predicted values; but the experimental PA is ca. 10 kcal/mol lower than the values predicted by ab initio and DFT calculations. The discrepancy between the measured and predicted PAs is resolved by proposing that adiabatic proton transfer occurs through rearrangement of 16·- to cyclopentylacetylide radical anion.