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Structure and Energy Spectra of Molecules Containing Anti-Aromatic Ring Systems

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Structure and Energy Spectra of Molecules Containing Anti-Aromatic Ring Systems Structure and Energy Spectra of Molecules Containing Anti-Aromatic Ring Systems. IV. Aromaticity and Anti-Aromaticity in Electronic Ground and Excited States Fritz Dietz*,a, Henryk Vogel3, Anna Schleitzer3, Nikolai Tyutyulkovab, Mordecai Rabinovitzc 3 Universität Leipzig, Institut für Physikalische und Theoretische Chemie, Augustusplatz 10/11, D-04109 Leipzig, Germany b University of Sofia, Faculty of Chemistry, BG-1126 Sofia, Bulgaria c University of Jerusalem, Department of Organic Chemistry, Jerusalem 91904, Israel Z. Naturforsch. 52 b, 1072-1076 (1997); received May 13, 1997 Energy Spectra, Anti-Aromatic Ring Systems, Excited States Using various criteria to characterize the terms aromaticity and anti-aromaticity it is shown that the non-benzoid aromatic azulene and the benzoid aromatic naphthalene should have anti-aromatic properties in the first excited singlet state. Introduction [8 ] that the excitation of molecules which are built up from odd-membered cyclic conjugated ring sys­ The concept of aromaticity of monocyclic pla­ tems by annelation or by connection through dou­ nar conjugated (4 n +2) 7r-electron systems and of ble bonds results in a charge transfer from the for­ anti-aromaticity of the corresponding 4 n 7r-electron mal negatively charged (4 n +2 ) 7r-electron frag­ systems has so far been restricted to the ground state ment to the formal positively charged (4 n +2) 7r- [1], From this arises the question: is it possible to fragment. This electron transfer upon excitation cre­ extend (generalize) the terms aromaticity and anti- ates species in the excited state whose electronic aromaticity to electronic excited states? The aim of structure can be described by anti-aromatic 4 n 7r- the present study is to answer this question. fragments, e.g. Recently, it was shown [2] that the antiaro- matic cyclobutadiene and cyclopentadienyl cation, respectively, show aromatic character in some low- lying excited singlet and triplet states. The aromatic “benzene either loses or reduces the aromatic char­ acter upon electronic excitation” [ 2 ]. t> - 0 The transition states of pericyclic reactions in ground and excited states were termed to be aro­ CHD- 0 - © lv - 0 - 0 ) matic or anti-aromatic [3-7]. Special rules for the characterization of the transition state of pericyclic Another example is the s-indacene which shows thermal and photochemical reactions as aromatic all the features of an anti-aromatic compound (small or anti-aromatic depending on the number of atoms excitation energy, bond alternation) [9]. Relatively and on the charge of even- and odd-membered cyclic large excitation energies for the Si -^S 2 and S2—>S3 conjugated ring systems have been proposed [5, 6 ]. transitions and a (quantum-chemically calculated) There are some indications that the aromatic char­ maximum equalization of the bond lengths in the acter of some compounds in the ground state is excited singlet states are typical criteria for aromatic changed to a more or less anti-aromatic character properties [ 10]. upon electronic excitation of the molecules. Re­ Various criteria have been used to characterize the sults of quantum-chemical calculations have shown term aromaticity (and anti-aromaticity). Generally, these criteria are connected with typical physical and chemical properties: Reprint requests to Prof. Dr. F. Dietz. (i) geometrical structure, 0939-5075/97/0900-1072 $ 06.00 © 1997 Verlag der Zeitschrift für Naturforschung. All rights reserved. K Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung This work has been digitalized and published in 2013 by Verlag Zeitschrift in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der für Naturforschung in cooperation with the Max Planck Society for the Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Advancement of Science under a Creative Commons Attribution Creative Commons Namensnennung 4.0 Lizenz. 4.0 International License. F. Dietz et al. • Structure and Energy Spectra of Molecules Containing Anti-Aromatic Ring Systems 1073 (ii) molecular spectra of a suitable (iso-7r-electronic) reference compound (NMR spectra, energy spectra), which is not cyclically delocalized”. (iii) magnetic properties, The aim of this paper is to characterize the first (iv) chemical reactivity. excited singlet state of azulene 1 and naphthalene 2 as aromatic or anti-aromatic using these criteria. Recently, Schleyer [11] has proposed the follow­ ing definition: “Compounds which exhibit signifi­ cantly exalted diamagnetic susceptibility are aro­ matic. Cyclic delocalization may also result in equalization of the bond length, abnormal chemical 1 2 shifts and magnetic anisotropies, as well as chem­ ical and physical properties which reflect energetic Results and Discussion stabilization”. The opposite features should be valid for anti-aromatic character. Geometry criterion. Azulene is a typical non- Besides these criteria other typical features can benzoide aromatic compound which can be thought be used to distinguish between aromatic and anti- to be built up from an aromatic cyclopentadienide aromatic character. In an earlier paper [10] we have anion and an aromatic cycloheptatrienyl cation. proposed criteria to characterize the anti-aromatic character of molecules: 1. Geometry criterion. While aromatic monocy­ cles {e.g. benzene, cyclopropenide anion, tropylium 1a 1b 1c 1d cation) show a maximum bond length equalization, the geometric configurations of anti-aromatic struc­ The molecular structure of azulene was discussed tures are characterized by an alternation of the bond controversially in the literature [13]. All the ob­ distances and anomalously long bond distancesR served properties of the molecule correspond to the (in typical casesR > 1.5 A). structurelc (Id) of C2V symmetry with equalized C-C bonds, and not to the valence-tautomeric struc­ 2. Energy criterion. Aromatic (4n +2) 7r-electron tures la, b of Cs symmetry with alternation of the systems are characterized by a high excitation en­ bond lengths. The structure lc (Id) was recently ergy and a red shift of the longest-wavelength ab­ confirmed by high-levelab initio calculations (with sorption if the conjugated 7r-electron system is ex­basis sets up to TZP+f quality) [13]. The results tended. Contrary to this behaviour, the relatively(see Fig. 1) are in good agreement with the ex­ stable Jahn-Teller forms of anti-aromatic species perimentally determined (X-ray analysis assuming have small exitation energies (wide infra). If the C2v symmetry) structure [14]. The structure shows anti-aromatic 4n 7r-electron system is perturbed (ex­a maximum equalization of the C-C bond lengths tended), the HOMO-LUMO gap is increased and a typical for molecules with aromatic character. blue shift of the longest-wavelength absorption re­ sults. Upon excitation of azulene to the first excited singlet state it can be expected that the structure 3. Charge distribution criterion. For more com­ is distorted. One criterion of the anti-aromaticity plex compounds with an odd-membered antiaro-of the 4/7 (7r-electron containing) annulenes is the matic structural element {e.g. a cyclopentadienyl unusually strong alternation of the bond lengths [10] cation fragment), the sum of the7r-net charges (Q) as a result of a second order Jahn-Teller effect [16], of the atoms of the anti-aromatic monocyclic frag­From the energy spectra of the aromatic molecules ment can be compared with Q of the unsubstituted {e.g. naphthalene or azulene) it follows that for these anti-aromatic monocycle. molecules a Jahn-Teller effect can also be expected An additional criterion is the validity of Breslow'sin their first excited singlet state. The energy of the stability criterion [12]. Breslow [12] has defined electron-vibronic interaction of a molecule in an an anti-aromatic compound as “a cyclic conjugatedelectronic statep (second order perturbation theory) system ... if its 7r-electron energy is higher than that is given by the expression [17] 1074 F. Pietz et al. • Structure and Energy Spectra of Molecules Containing Anti-Aromatic Ring Systems du 'P: du SQ. a £ '2, = E ( 1) En - Er <i¥p where Eq and Wq are the energy and the wave £ Qs= 0.05 Z Q7= -0.08 function of the electronic stateq , respectively, and 3U / dQß are the coupling operators. From eq. (1) it H-------- can be seen that the energy of the electron-vibronic (J. = 0.44D coupling AE(2) and therefore also the geometry in S, the electronic state p are strongly influenced by the energy difference between the states p and q, i.e. the Fig. 2. Sums of the 7r-net charges in the five- and seven- excitation energy (denominator in eq.( 1)). The rel­ membered rings and calculated dipole moment of azulene atively small value of the energy differenceS\— in ground and first excited states (G AMESS-UK, 6-31G* basis set). (we consider the Si state of the molecules) of aro­ matic compounds(e.g. azulene and naphthalene) determines a large value of AE(2). This means that a strong electron-vibronic coupling in the first ex­ cited singlet state leads to a significant change of 1.424 1.415 (1.409) (1.417) the geometric configuration with an alternation of the bond lengths of the molecule as in the case of the anti-aromatic 7r-systems in their ground state. So S, The geometry of azulene in the first excited sin­ glet state (Si) was optimized using the QCFF/PI Fig. 3. Experimental [19] and optimized (values in brack­ (CISD) and the CASSCF (3-21G basis set) pro­ ets, ab initio, 6-31G* basis set) bond lengths (in A) in So and ab initio MCSCF optimized bond distances in Si [20] cedures [15], respectively. The bond lengths ob­ of naphthalene. tained by Negri and Zgierski[ 15] are within 0.01 A to those of Bearparket al. [ 18] optimized at CAS 10 Considering the So and Sj geometries and the level. An “experimental” S| geometry of azulenesums of the 7r-net charges in the five- and seven- [15] was evaluated by a correction of theab initio membered rings the structures of azulene in the HF (6-31G basis set) ground state geometry using ground and first excited singlet states can be rep­ CIS/6-31G calculated normal coordinates and ex­resented by the schematic formulae given in Fig.
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