Correlation of Oxidation and Ionization Potentials for Azoalkanes Werner M. Nau,* ,† Waldemar Adam, § Dieter Klapstein, ‡ Coskun Sahin, § and Herbert Walter § Institut fu ¨ r Physikalische Chemie der Universita ¨ t Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland, Institut fu ¨ r Organische Chemie der Universita ¨ t Wu ¨ rzburg, Am Hubland, D-97074 Wu ¨ rzburg, Germany, and Department of Chemistry, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada Received March 31, 1997 X Oxidation and ionization potentials of azoalkanes have been measured and combined with the available literature data to afford a data set of ten cyclic, bicyclic, and polycyclic derivatives with a wide structural variation. A linear correlation (r ) 0.939) between the peak oxidation potentials (E p ) and the vertical ionization potentials (IP v ) of the azoalkanes 1-10 applies (E p ) 0.95(IP v ) - 6.4). The approximately unit slope is interpreted in terms of relatively constant differential solvation and cationic relaxation energies for the various azoalkanes. Density functional calculations (B3LYP/ 6-31G*) for bicyclic azoalkanes confirm that the cationic relaxation energies are relatively insensitive to molecular strain and rigidity; the latter are known to dictate their ionization potentials. The theoretical data indicate further that the preferred modes of geometry reorganization in the azoalkane radical cations are shortening of the NdN, lengthening of the C-N bonds, and widening of the C-NdN, but no torsion about the C-NdN-C dihedral angle. The experimental and theoretical data for bicyclic azoalkanes are compared with those for the corresponding bicyclic peroxide analogues. Introduction There is a substantial interest in correlating redox potentials in solution with experimental electron affini- ties 1,2 or ionization potentials 3-12 in the gas phase, or with theoretical heats of ionization and frontier orbital (HOMO, LUMO) energies. 12-19 Although the solution and gas- phase measurements represent complementary tech- niques to assess redox properties, they are subject to various experimental limitations. This may render dif- ficult to determine either the gas phase values, in particular the electron affinities, 1,2 or more commonly the solution values. Hence, in the absence of experimental data, a correlation between the solution and gas phase parameters is desirable to estimate unknown redox properties in one or the other phase. For example, most often the experimental gas phase ionization potentials for a particular class of compounds are available but only few solution oxidation potentials are at hand. The former have usually been obtained through photoelectron spec- troscopy (PES), traditionally motivated by the elucidation of general relationships between electronic properties and molecular structure, while the latter are frequently required to evaluate the mechanism of electron transfer in a redox reaction. Azoalkanes constitute a typical class of substrates for which such a dilemma between the solution oxidation and gas phase ionization potentials obtains. Through inten- sive PES investigations in the 1970s, which persist to date, more than 100 ionization potentials for azoalkanes have been reported, 11,20-33 but since the one-electron † Universita ¨ t Basel. § Universita ¨t Wu ¨ rzburg. ‡ St. Francis Xavier University. X Abstract published in Advance ACS Abstracts, July 1, 1997. (1) Briegleb, G. Angew. Chem. 1964, 76, 326. (2) Janousek, B. K.; Brauman, J. I. Gas Phase Ion Chem. 1979, 2, 531. (3) Pysh, E. S.; Yang, N. C. J. Am. Chem. Soc. 1963, 85, 2124. (4) Miller, L. L.; Nordblom, G. D.; Mayeda, E. A. J. Org. Chem. 1972, 37, 916. (5) Parker, V. D. J. Am. Chem. Soc. 1976, 98, 98. (6) Gassman, P. G.; Yamaguchi, R. J. Am. Chem. Soc. 1979, 101, 1308. (7) Klingler, R. J.; Kochi, J. K. J. Am. Chem. Soc. 1980, 102, 4790. (8) Howell, J. O.; Goncalves, J. M.; Amatore, C.; Klasinc, L.; Wightman, R. M.; Kochi, J. K. J. Am. Chem. Soc. 1984, 116, 3968. (9) Nelsen, S. F.; Teasley, M. F.; Bloodworth, A. J.; Eggelte, H. J. J. Org. Chem. 1985, 50, 3299. (10) Nelsen, S. F.; Blackstock, S. C.; Petillo, P. A.; Agmon, I.; Kaftory, M. J. Am. Chem. Soc. 1987, 109, 5724. (11) Nelsen, S. F.; Petillo, P. A.; Chang, H.; Frigo, T. B.; Dougherty, D. A.; Kaftory, M. J. Org. Chem. 1991, 56, 613. (12) Eberson, L. Adv. Phys. Org. Chem. 1982, 18, 79. (13) Streitwieser, A. Molecular Orbital Theory for Organic Chemists; Wiley: New York, 1961. (14) Neikam, W. C.; Desmond, M. M. J. Am. Chem. Soc. 1964, 86, 4811. (15) Gleicher, G. J.; Gleicher, M. K. J. Phys. Chem. 1967, 71, 3693. (16) Dewar, M. J. S.; Trinajstic, N. Tetrahedron 1969, 25, 4529. (17) Dewar, M. J. S.; Hashmall, J. A.; Trinajstic, N. J. Am. Chem. Soc. 1970, 92, 5555. (18) Gerson, F.; Ohya-Nishiguchi, H.; Wydler, C. Angew. Chem. 1976, 88, 617. (19) Gassman, P. G.; Mullins, M. J.; Richtsmeier, S.; Dixon, D. A. J. Am. Chem. Soc. 1979, 101, 5793. (20) Boyd, R. J.; Bu ¨ nzli, J. C.; Snyder, J. P.; Heyman, M. L. J. Am. Chem. Soc. 1973, 95, 6479. (21) Brogli, F.; Eberbach, W.; Haselbach, E.; Heilbronner, E.; Hornung, V.; Lemal, D. M. Helv. Chim. Acta 1973, 56, 1933. (22) Schmidt, H.; Schweig, A.; Trost, B. M.; Neubold, H. B.; Scudder, P. H. J. Am. Chem. Soc. 1974, 96, 622. (23) Houk, K. N.; Chang, Y.-M.; Engel, P. S. J. Am. Chem. Soc. 1975, 97, 1824. (24) Boyd, R. J.; Bu ¨ nzli, J.-C. G.; Snyder, J. P. J. Am. Chem. Soc. 1976, 98, 2398. (25) Domelsmith, L. N.; Houk, K. N.; Timberlake, J. W.; Szilagyi, S. Chem. Phys. Lett. 1977, 48, 471. (26) Gilbert, K. E. J. Org. Chem. 1977, 42, 609. (27) Mirbach, M. J.; Liu, K.-C.; Mirbach, M. F.; Cherry, W. R.; Turro, N. J.; Engel, P. S. J. Am. Chem. Soc. 1978, 100, 5122. (28) Albert, B.; Berning, W.; Burschka, C.; Hu ¨ nig, S.; Martin, H.- D.; Prokschy, F. Chem. Ber. 1981, 114, 423. (29) Gleiter, R.; Scha ¨ fer, W.; Wamhoff, H. J. Org. Chem. 1985, 50, 4375. (30) Engel, P. S.; Gerth, D. B.; Keys, D. E.; Scholz, J. N.; Houk, K. N.; Rozeboom, M. D.; Eaton, T. A.; Glass, R. S.; Broeker, J. L. Tetrahedron 1988, 44, 6811. The lowest ionization bands for 2,3- diazabicyclo[2.2.2]oct-2-ene (5) and its 1,4-dimethyl derivative (6) reported in this work display vibrational fine structure. The Franck- Condon maxima of these bands, which are given as IPv in Table 1, were graphically interpolated and found to lie at 8.32 and 8.20 eV. (31) Adam, W.; Fragale, G.; Klapstein, D.; Nau, W. M.; Wirz, J. J. Am. Chem. Soc. 1995, 117, 12578. (32) Brand, U.; Hu ¨ nig, S.; Martin, H.-D.; Mayer, B. Liebigs Ann. 1996, 1401. 5128 J. Org. Chem. 1997, 62, 5128-5132 S0022-3263(97)00574-4 CCC: $14.00 © 1997 American Chemical Society