Electron Delocalization in the Radical Cation of 1,3,6,8-Tetraazatricyclo[4.4.1.1 3,8 ]dodecane, a 4-Nitrogen-7-Electron System Jurriaan M. Zwier, ² Albert M. Brouwer,* ,² Tama ´ s Keszthelyi, ‡,| Gurusamy Balakrishnan, ‡,# Jesper F. Offersgaard, ∇,‡ Robert Wilbrandt, X,‡ Fre ´ de ´ rique Barbosa, § Urs Buser, § Je ´ ro ˆ me Amaudrut, § Georg Gescheidt,* ,§ Stephen F. Nelsen, ⊥ and Charles D. Little ⊥ Contribution from the Institute of Molecular Chemistry, UniVersity of Amsterdam, Nieuwe Achtergracht 129, NL-1018 WS Amsterdam, The Netherlands, Condensed Matter Physics and Chemistry Department, Risø National Laboratory, 4000 Roskilde, Denmark, Institute of Physical Chemistry, UniVersity Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland, and Department of Chemistry, UniVersity of Wisconsin, 1101 UniVersity AVenue, Madison, Wisconsin 53706-1396 Received September 4, 2001 Abstract: The radical cation of 1,3,6,8-tetraazatricyclo [4.4.1.1 3,8 ]dodecane (TTD) has been studied using magnetic resonance and optical spectroscopic methods and computational techniques. With the help of deuterated isotopomers, assignments of EPR and resonance Raman spectra could be unequivocally established. The results demonstrate that the radical cation has D2d symmetry, and instantaneous electron delocalization over the four equivalent nitrogen atoms occurs. This extensive delocalization in a completely saturated system is a unique feature of the TTD radical cation. The spectroscopy of TTD, in contrast to that of simpler diamines such as 1,4-diaza[2.2.2]bicyclooctane, simultaneously reveals the consequences of orbital interactions through space and through bonds. The relationship between nitrogen pyramidalization and hyperfine coupling constants in nitrogen-centered radical cations with a number of different bonding arrangements is reviewed. 1. Introduction McKinney and Geske discovered the remarkable kinetic stability of the radical cation of 1,4-diazabicyclo[2.2.2]octane (DABCO, 1, Chart 1) and discussed its EPR spectrum, which shows 2 equivalent nitrogens and 12 equivalent hydrogens, in terms of rapidly equilibrating single nitrogen-centered radical cations. 1 Hoffmann’s elegant theoretical formulation of the concepts of orbital interactions through bond versus through space led to the realization that 1 •+ is a through-σ-bond delocalized species in which the nitrogen lone pair orbitals interact through the three aligned C-C σ-bonds. 2,3 Heilbronner and Muszkat demonstrated on the basis of the vibronic structure in the photoelectron spectrum (PES) of 1 that its highest occupied molecular orbital (HOMO) has the a 2 ′′ symmetry that is required by this description. 4 Subsequently, in the 1970s, several studies were conducted that were aimed at finding other examples of radical cations stabilized by through-bond interac- tions. Most radical cations of alkylamines decompose rapidly in room-temperature solutions, which leads to irreversible waves in cyclic voltammetry (CV) experiments. 5 DABCO, on the other hand, shows a re-reduction wave in the CV experiment, and an ESR signal of the radical cation can be readily observed at room * To whom correspondence should be addressed. E-mail for A.M.B.: fred@science.uva.nl. E-mail for G.G.: Georg.Gescheidt@unibas.ch. ² University of Amsterdam. ‡ Risø National Laboratory. § University Basel. ⊥ University of Wisconsin. | Present address: Surface Spectroscopy Group, Chemical Research Center, P.O. Box 17, H-1525 Budapest, Hungary. # Present address: Department of Chemistry, Princeton University, Princeton, NJ 08544-1009. ∇ Present address: Delta Light and Optics, Hjortekærsvej 99, DK-2800 Lyngby, Denmark. X Present address: Bornholms Amtsgymnasium, Søborgstræde 2, 3700 Rønne, Denmark. (1) McKinney, T. M.; Geske, D. H. J. Am. Chem. Soc. 1965, 87, 3013-3014. (2) Hoffmann, R.; Imamura, A.; Hehre, W. J. J. Am. Chem. Soc. 1968, 90, 1499. (3) Hoffmann, R. Acc. Chem. Res. 1971, 4, 1. (4) Heilbronner, E.; Muszkat, K. A. J. Am. Chem. Soc. 1970, 92, 3818-3821. (5) Lindsay Smith, J. R.; Masheder, D. J. Chem. Soc., Perkin Trans. 2 1977, 1732. Chart 1 Published on Web 12/13/2001 10.1021/ja016999n CCC: $22.00 © 2002 American Chemical Society J. AM. CHEM. SOC. 9 VOL. 124, NO. 1, 2002 159