Charge Separation in Ground-State 1,2,4,5-Tetra-Substituted Benzene Derivatives Yehuda Haas* and Shmuel Zilberg* Contribution from the Department of Physical Chemistry and the Farkas Center for Light Induced Processes, The Hebrew UniVersity of Jerusalem, Jerusalem, Israel Received February 28, 2004; E-mail: yehuda@chem.ch.huji.ac.il Abstract: The conditions required for a formal biradical to exist in a zwitterionic form in the ground state are discussed following the recent experimental observation 1 of zwitterionic structure in the ground state of a quinoid molecule (di-tert-butyl derivative of 2,5-diamino-1,4-benzoquinonediimine, I). A unique characteristic of molecules of this class is the fact that they may be considered as being formed by the union of two radicals, each having an odd number of π electrons. In the case of I, one fragment carries the two amino group having 7 π electrons; it acts as the electron donor. The other fragment carries the two oxygen atoms (carrying 5 π electrons) and acts as an electron acceptor. A model that predicts the properties of these systems is presented, based on previous work on non-Kekule hydrocarbons 2,3 and on the electron donating and attracting properties of the donor and acceptor groups, respectively. The zwitterion is formed by an electron transfer leading to two subunits carrying 6 π electrons each and may become more stable than the triplet biradical even in the gas phase (i.e., in the absence of an external field) if the ionization potential of the donor is small (of the order of 3-4 eV). In some cases solvation in a polar solvent is required to make the zwitterionic form the lowest energy species on the ground-state surface. The ‘spacer’ between the donor and acceptor groups (which need not be necessarily derived from an aromatic structure) can be varied and influences the overall dipole moment that is calculated in some cases to be quite large (over 20 D in the gas phase). I. Introduction The recent experimental observation 1 of zwitterionic structure in a quinoid molecule (di-tert-butyl derivative of 2,5-diamino- 1,4-benzoquinonediimine, I, see Figure 1 for structure), is an unprecedented synthetic accomplishment that attracted consider- able theoretical interest. 4-6 In this paper, we show that the properties of these molecules can be understood in terms of a model used by Borden and Davidson to predict the properties of non-Kekule ´ hydrocarbons. 2,3 Several quantum chemical calculations on the model molecule Ia were shown to reproduce the salient properties of this molecule, especially the large dipole moment. Sawicka et al. 4 used second-order Moeller-Plesset theory with a large basis set to show that the zwitterionic isomer of the quinoid species is indeed the lowest energy isomer in the gas phase. All canonical tautomers were found to lie at higher energies, and to be separated from the zwitterion isomer by a relatively high barrier. Le et al. 5 using HF, CASSCF, and DFT methods showed that the molecule can be considered as consisting of two separately subunits in which the π electrons are delocalized: a positively charged N-C-C(H)-C-N subunit, connected by two single C-C bonds to a negatively charged O-C-C(H)-C-O subunit. The division into two separated subunits containing 6 π electrons each was confirmed by Braunstein et al. 6 using DFT calculations; the calculated dipole moment of I was found to be 10.0 D, in very good agreement with the measured value (9.7 D in dichloromethane). The authors pointed out the similarity to other heterocyclic molecules having a zwitterionic ground state. 7,8 The DFT calculation showed that the lowest triplet state was considerably (1) Siri, O.; Braunstein, P. Chem. Commun. 2002, 208. (2) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99, 4587. (3) Borden, W. T.; Mol. Cryst. Liq. Cryst. 1993, 232, 195. (4) Sawicka, A.; Skurski, P.; Simons, J. Chem. Phys. Lett. 2002, 362, 527. (5) Le, H. T.; Nam, P. C.; Dao, Y. L.; Veszpremi, T.; Nguyen, M. T. Mol. Phys. 2003, 101, 2347; Delaere, D.; Nam, P. C.; Nguyen, M. T. Chem. Phys. Lett. 2003, 382, 349. (6) Braunstein, P.; Siri, O.; Taquet, J.-P.; Rohmer, M.-M.; Benard, M.; Welter, R. J. Am. Chem. Soc. 2003, 125, 12 246. Figure 1. Structures of molecules discussed in the paper. Only the sigma structure of the ring is shown, as the detailed pi structure varies between different molecules. The detailed structure is further discussed in Sections II, VI, and VII. Published on Web 06/30/2004 10.1021/ja048872e CCC: $27.50 © 2004 American Chemical Society J. AM. CHEM. SOC. 2004, 126, 8991-8998 9 8991