Energetics and structure of the 1- and 2-adamantyl radicals and their corresponding carbonium ions by photoelectron spectroscopy

Abstract

The first photoelectron bands of the 1- and 2-adamantyl radicals, formed by flash vacuum photolysis of 1- and 2-adamantylmethyl nitrite, have been obtained. The adiabatic (IP_a) and vertical (IP_v) ionization potentials of the 1-adamantyl radical are 6.21 ± 0.03 and 6.36 ± 0.05 eV, respectively. IP_a and IP_v for the 2-adamantyl radical are 6.73 ± 0.03 and 6.99 ± 0.05 eV, respectively. The difference in hydride affinities between the 1-adamantyl and tert-butyl cations (Sharma, R. B.; Sen Sharma, D. K.; Hiraoka, K.; Kebarle, P. J. Am. Chem. Soc. 1985, 107, 3747) combined with the difference in IP_a between the tert-butyl and 1-adamantyl radicals (0.49 ± 0.06 eV) yield a value of 99 kcal/mol for the tertiary C-H bond energy in adamantane, 3.7 ± 1.2 kcal/mol greater than the tertiary C-H bond energy in isobutane (assumed to be 95 kcal/mol). The effects of the geometrical constraints imposed by the adamantyl cage on the homolytic and heterolytic C-H bond cleavage energies are discussed for the 1- and 2-adamantyl cases. The width of the Franck-Condon envelope obtained is related to the geometry changes that occur upon ionization. The surprisingly broad envelope observed for the planar 2-adamantyl radical indicates that the Franck-Condon envelope for the 1-admantyl radical should not be interpreted as exclusively due to changes at the bridgehead position. Thermal decomposition products of the 1- and 2-adamantyl radicals are observed, and the pathways for thermal decompositions of the radicals are discussed. To confirm expected trends in ionization potentials and band shapes of tertiary radicals, the first photoelectron band of the 2-methyl-2-butyl radical has been obtained. The IP_a of the 2-metyl-2-butyl radical is 6.65 ± 0.04 eV with IPV = 6.91 ± 0.05 eV.