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Analytical Study of Superaromaticity in Cycloarenes and Related Coronoid Hydrocarbons Jun-Ichi Aihara,*,†,‡ Masakazu Makino,‡ Toshimasa Ishida,§ and Jerry R

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Analytical Study of Superaromaticity in Cycloarenes and Related Coronoid Hydrocarbons Jun-Ichi Aihara,*,†,‡ Masakazu Makino,‡ Toshimasa Ishida,§ and Jerry R Article pubs.acs.org/JPCA Analytical Study of Superaromaticity in Cycloarenes and Related Coronoid Hydrocarbons Jun-ichi Aihara,*,†,‡ Masakazu Makino,‡ Toshimasa Ishida,§ and Jerry R. Dias∥ † Department of Chemistry, Faculty of Science, Shizuoka University, Oya, Shizuoka 422-8529, Japan ‡ Institute for Environmental Sciences, University of Shizuoka, Yada, Shizuoka 422-8526, Japan § Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-Nishibirakicho, Kyoto 606-8103, Japan ∥ Department of Chemistry, University of Missouri, Kansas City, Missouri 64110-2499, United States *S Supporting Information ABSTRACT: Recently synthesized septulene is a unique cycloarene molecule in that no macrocyclic conjugation circuits can be chosen from the π-system. This molecule has essentially no superaromatic stabilization energy (SSE) and can be viewed as an ideal nonsuperaromatic macrocycle. SSEs for kekulene and other cycloarenes are also very small. In these hydrocarbons, a macrocycle formed by fused benzene rings effectively suppresses not only the aromaticity inherent in macrocyclic (4n+2)-site conjugation circuits but also the antiaromaticity inherent in macrocyclic (4n±1)-site circuits. Comparative study of superaromaticity in multilayered coronoid hydro- carbons revealed that not only SSE but also the HOMO contribution to SSE is minimized in odd-layered coronoids. ■ INTRODUCTION superaromaticity of many different macrocycles, such as fi cycloarenes, cyclacenes, carbon nanotubes, and porphyr- Cycloarenes are de ned as comprising many annelated benzene 14,25−38 rings that form a macrocycle with inward-pointing C−H ins. For this purpose, we devised some analytical − ff bonds.12 Since before the synthesis of kekulene (1),3 5 the theories of superaromaticity applicable to many di erent π 14,28−31,37,38 ́ prototypical cycloarene, electronic structure of cycloarenes has macrocyclic -systems. Hajgato et al. noticed − been a target of many theoretical and computational oscillatory behaviors of the HOMO LUMO gap and the 6−20 π macrocyclic -circulation in D6h-symmetric coronoid hydro- studies. Kekulene had been viewed as a closed cycle of 19−21 angularly annelated benzene rings and also as a combination of carbons including D6h-symmetric cycloarenes. Here, a two interacting [4n+2]annulenes. The synthesis and character- coronoid hydrocarbon is a planar polycyclic benzenoid − 39−41 ization of kekulene (1)3 5 in 1978 answered a fundamental hydrocarbons with a central cavity. In this paper, we question about the nature of macrocyclic conjugation. Proton explore some novel aspects of superaromaticity in typical chemical shifts suggest that ring currents are induced primarily cycloarenes and related coronoid hydrocarbons using our own in individual benzene rings.21,22 Such a π-circulation pattern is theories of superaromaticity. Comparative study of super- not compatible with the annulene-within-an-annulene aromaticity in multilayered coronoid hydrocarbons is useful to model.21,22 Recently, Kumar et al. reported the synthesis and seeking for the proper location of cycloarenes in the coronoid properties of kekulene’s nonalternant cousin septulene (2) and family. found that its properties reinforce this conclusion.21 In fact, electronic and magnetic properties of 2 are remarkably similar ■ THEORETICAL BACKGROUNDS to those of 1. It seemed likely that both are composed of fi Our theories of aromaticity and superaromaticity were benzenoid and ole nic benzene rings. This picture is also ̈ supported by their nucleus independent chemical shift (NICS) constructed within the framework of simple Huckel molecular − values.16,22 24 orbital (HMO) theory. In these theories, circuits stand for all The next question to be answered may be whether possible cyclic or closed paths that can be chosen from a cyclic π cycloarenes, such as 1 and 2, are superaromatic or not. -system. Two types of circuits can be chosen from macrocyclic π 14 Superaromaticity, or macrocyclic aromaticity, represents extra -systems: local and macrocyclic circuits. Local circuits are thermodynamic stabilization due to macrocyclic conjugation. those enclosing one or more benzene rings but not enclosing Cioslowski et al. first attempted to estimate the degree of the central cavity, while macrocyclic circuits are those enclosing superaromaticity in 1 using their own homodesmotic reaction scheme.13 Subsequently, Jiao and Schleyer concluded from Received: February 16, 2013 their computational studies that 1 was only a superbenzene Revised: May 15, 2013 from an aesthetic point of view.16 We have been studying Published: May 15, 2013 © 2013 American Chemical Society 4688 dx.doi.org/10.1021/jp4016678 | J. Phys. Chem. A 2013, 117, 4688−4697 The Journal of Physical Chemistry A Article β π the cavity. All species are assumed to be in a singlet electronic where 0 is the standard integral for a CC -bond and i is the state.42 square root of −1. This superaromaticity-free reference is The choice of reference structures is of central importance to nothing other than the reference structure used to calculate the the evaluation of aromaticity and superaromaticity. Our bond resonance energy (BRE).45,46 BRE for each π-bond references are graph-theoretically exact ones. Topological represents the extra stabilization energy due to the circuits that resonance energy (TRE) is calculated relative to the energy pass through the π-bond. of the aromaticity-free reference defined by the polynomial Note that all macrocyclic circuits in 3 pass through either the obtained by deleting the contribution of all circuits from the C C bond or the C C bond, where such a pair of π-bonds − c d e f coefficients in the HMO characteristic polynomial.14,43 45 must belong to the same ring. The superaromaticity-free Percentage TRE (%TRE) is defined as 100 times the TRE, reference for 3 can then be constructed by modifying the divided by the total π-binding energy of the polyene resonance integrals for these bonds in the following reference.14,45 This quantity is useful when one wants to manner:28,34,37 compare the degrees of aromaticity in different π-systems. β ==ββiiand ββ ==− β Superaromatic stabilization energy (SSE) is calculated relative c,d e,f 0 d,c f,e 0 (2) to the energy of the superaromaticity-free (i.e., nonsuperar- Both methods for constructing the superaromaticity-free omatic) reference, which is defined by the polynomial obtained reference bring about exactly the same superaromaticity-free by deleting the contribution of all macrocyclic circuits from the reference energy and necessarily the same SSE. coefficients in the HMO characteristic polynomial.28,37 Two Likewise, the superaromaticity-free reference for 8 in Figure methods have been developed for constructing such super- 2 can be constructed by modifying the resonance integrals for aromaticity-free references. 28,34,37 three bonds in the following manner: The first method is applicable to macrocycles with one or − more π-bonds through which all macrocyclic circuits pass.30 33 β ===ββii βand β ===− ββ β π a,b c,d e,f 0 b,a d,c f,e 0 For example, the -system 3 in Figure 1 has 66 local circuits (3) and 2048 macrocyclic circuits; all macrocyclic circuits pass through the CaCb bond. The superaromaticity-free reference for All macrocyclic circuits in 8 pass through one or three of the 3 can be constructed simply by modifying the resonance CaCb,CcCd, and CeCf bonds. Since kekulene (1) and septulene integrals for this π-bond in the manner (2) have no π-bond shared by all macrocyclic circuits, the first method cannot be applied to them. The second method is π βa,b==−iiβββ 0and b,a 0 (1) applicable to all possible macrocyclic -systems. For the details of this method, see ref 28. ■ RESULTS AND DISCUSSION We first study global aromaticity and superaromaticity of kekulene (1), septulene (2), and related species (3−15)in Figures 1 and 2. If a polycyclic benzenoid hydrocarbon (PBH) with a central cavity is cut out of a graphene sheet, it may be − called a coronoid hydrocarbon.39 41 The central cavity has been called an antidot.20 Coronoids 7−15 have been objects of − recent theoretical studies.18 20 Coronoids studied are classified into single-layered (1, 7, 9, and 12), double-layered (8, 10, and 13), triple-layered (11 and 14), and quadruple-layered (15) coronoids.39 Exactly speaking, 2 is not a coronoid hydrocarbon but a semibenzenoid analogue with a central cavity. TREs and SSEs for these species are listed in Tables 1 and 2. In fact, most of the coronoids studied are too large to calculate TREs. Global Aromaticity of Kekulene and Septulene. Kekulene (1) and septulene (2) are only two cycloarenes of their kind so far prepared. Both 1 and 2 are aromatic with positive %TREs of 2.340 and 2.336, respectively. As can be seen from Table 3, they are slightly less aromatic than most PBH molecules45 but are more aromatic than higher members of the polyacene series.47,48 Structural formulas of PBHs cited in this table are given in the Supporting Information, Figure S1. The difference between the total π-binding energy and the SSE may be called the superaromaticity-free π-binding energy of the π- system. Quite interestingly, the superaromaticity-free π-binding energy per unit structure for 1 is very close to that for 2; both values are ca. 11.43 445 |β|. Note that the numbers of unit structures are six for 1 and seven for 2. We previously showed that the distribution of π-bonds with large BREs within a PBH π-system is closely associated with the locations of aromatic or benzenoid sextets in the Clar Figure 1. Kekuléstructures of cycloarenes and related species studied. structure.45 BREs in units of |β| for nonidentical π-bonds in 1 4689 dx.doi.org/10.1021/jp4016678 | J. Phys. Chem. A 2013, 117, 4688−4697 The Journal of Physical Chemistry A Article Figure 2.
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