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Central European Journal of Chemistry

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Received 2 July 2013; Accepted 11 September 2013

 Abstract: 7KHDURPDWLFFKDUDFWHURIWKUHHGLIIHUHQWĠRZHUVRIJHQHUDOIRUPXOD> n:(pi,pj)n/2@QDPHO\>  4@>  4@DQG>  4@KDVEHHQ HYDOXDWHGE\PHDQVRIJHRPHWULF +20$LQGH[ HQHUJHWLF KHDWVRIIRUPDWLRQ DQGPDJQHWLFFULWHULD 1,&6LQGH[H[DOWDWLRQRIPDJQHWLF VXVFHSWLELOLW\ $OVRWKHUHDFWLYLW\GHVFULSWRUVZLWKLQWKH')7DSSURDFKļDEVROXWHKDUGQHVVHOHFWURSKLOLFLW\)XNXLIXQFWLRQVļKDYHEHHQ FRPSXWHG$OOWKHGLIIHUHQWPHWKRGVXVHGIRUHVWLPDWLQJWKHDURPDWLFLW\OHGWRDXQLWDU\FRQFOXVLRQ7ZRIXOOHUHQHVWUXFWXUHVSDWFKHGE\ WKHPRVWVWDEOH6XPDQHQHĠRZHUKDYHEHHQGHVLJQHGDQGHYDOXDWHGE\PHDQVRIDELQLWLRFRPSXWDWLRQV Keywords: )XOOHUHQHVľ&LUFXOHQHVľ$URPDWLFLW\ľ1,&6ľ,VRGHVPLFUHDFWLRQV © Versita Sp. z o.o.

1. Introduction designing new materials with adjustable properties has led to an increased interest for the study of precursors of The chemistry of polycyclic aromatic nanomaterials [7@6SHFL¿FSURSHUWLHVRIIXOOHUHQHEDVHG (PAHs) has been widely investigated in the last decades, QDQRPDWHULDOVDUHPDLQO\GXHWRWKHʌHOHFWURQV\VWHPV two main directions being emphasized: (1) PAHs as so that investigating the aromatic character of environmental pollutants (evaluating their toxic and and their precursors may offer a good insight into the carcinogenic effects) [1-3] and (2) PAHs as precursors for nanomaterial properties. nanomaterials like fullerenes, nanotubes or graphenes Such precursors of fullerenes are considered to be [4-6]. the circulenes, compounds with the general formula

It is nowadays a common fact that nanotechnologies [n:(pi,pj)n/2], where n is the folding of the core polygon and

and nanomaterials have gained importance and pi and pj are the polygonal petals [8]. The present study applications in almost every industry. The demand for aims to evaluate the aromatic character of circulenes

* E-mail: [email protected] 90 53RSHWDO

Figure 1. Circulenes 1a [8:(5,7)4] (left); 1b [8:68](middle) and 1c [8:(5,6)4] (right).

with 8-membered ring core and petals consisting of where EP, ER are the total energies of products and

5- to 7-rings by means of the widely used geometric, reactants, ZPEP, ZPER are the zero point corrections magnetic, and energetic criteria [9,10]. Thus, the bond- while TCP, TCR represent the thermal corrections. length equalization, the magnetic behavior and the Suppose the isodesmic reactions correspond to the stability –as measures of aromaticity– will be described. reaction: Last but not least, descriptors like total hardness [11], electrophilicity [11] or Fukui functions [12] (within the Circulene + PAH1ĺ3$+2 + PAH3 + PAH4 DFT approach) will be used in describing the chemical

UHDFWLYLW\RIWKHFLUFXOHQHV7KHWKUHHDOOFDUERQÀRZHUV 7KHQWKHKHDWRIIRUPDWLRQǻHf for a given circulene herein investigated are depicted in Fig. 1. can be written as: The geometric index HOMA and NICS(0) index and magnetic susceptibilities have been computed; in 'Hff( Circulene ) ' H () PAH 2 addition, two different isodesmic reactions (for each 'Hf () PAH341 ' Hf () PAH ' Hf () PAH  Ereaction 1a to 1c) have been used for calculating the enthalpies of formation. Electronegativity [11], total The experimental heats of formation of the PAHs (e.g. hardness [11], electrophilicity index [11] and Fukui , , cyclooctatetraene, indene, functions [12] (for an electrophilic attack) have been SKHQDQWKUHQHD]XOHQHDFHQDSKWK\OHQHDQGÀXRUHQH  computed by using DFT methods. were taken from a validated reference [14]. In order to compute the exaltation of magnetic VXVFHSWLELOLW\RIWKHÀRZHUV1a to 1cWKHFKDQJHV ǻȁ  2. Computational details in the magnetic susceptibility for all the six isodesmic schemes have been computed using the equation: Geometry optimization of the circulenes 1a to 1c and the '/ FF  cyclic compounds appearing in the isodesmic reaction PR schemes (benzene, naphthalene, cyclooctatetraene, ZKHUH ȤP ȤR represent the magnetic susceptibilities indene, , , , computed at B3LYP/6-311G(d,p) level of theory. ÀXRUHQHDQGFRURQHQH KDVEHHQSHUIRUPHGDW+) According to the above formulated isodesmic 311 G(d,p) level of theory. No imaginary frequencies reaction, the exaltation of magnetic susceptibility of the were obtained. NICS(0) and the reactivity descriptors corresponding circulene can be written as: computation has been performed at B3LYP/6-311G(d,p) level of theory. In order to compute the enthalpies of formation of WKHÀRZHUV1a to 1c, the reaction energy for all the six The magnetic susceptibilities of the needed isodesmic schemes was computed using the equation compounds (namely: benzene, naphthalene, derived [13] from the Hess law: cyclooctatetraene, indene, phenanthrene, azulene, DFHQDSKWK\OHQH ÀXRUHQH DQG FRURQHQH  ZHUH WDNHQ  Ereaction ¦¦ EEPR from references [15-17]. All the computations were performed with Gaussian 09 package [18].  ¦ZPEP  ¦ ZPE R  ¦¦ TC PR TC

91 )XOOHUHQHVSDWFKHGE\ĠRZHUVZLWKRFWDJRQDOFRUH

1a (1) [8:(5,7) ] + 40/6 4 + 4 + 2

(2) + 28/3 [8:(5,7)4] + 8

1b

+ 8 (1) [8:(6,6)4] + 56/6

+ 4 (2) [8:(6,6)4] + 32/6

1c

(1) [8:(5,6) ] + 56/6 + 4 + 4 4 +

(2) [8:(5,6) ] + 32/6 + 4 4 +

Figure 2. ,VRGHVPLFUHDFWLRQVIRUWKHHQWKDOS\FDOFXODWLRQLQĠRZHUV1a to 1c.

the circulene 1c [8:(5,6) ], the one showing a planar 3. Results and discussions 4 structure. The same conclusion was drawn from the 3.1. Energetic criterion of aromaticity: enthalpy single point calculations on the optimized structures at of formation HF/6-311G(d,p)) level of theory (Table 2). The stability of a polycyclic , which also gives a good estimation of its aromatic character, 3.2. Magnetic criterion: NICS(0) index and the can be investigated on the basis of some computed exaltation of magnetic susceptibility

thermodynamic values, namely the enthalpy of NICS(0) index (or variants like NICS(0)ʌ]]) [23] is widely formation. Comparison between the calculated used as a local descriptor of aromaticity. NICS(0) heats of formation of the circulenes 1a to 1c and the computations have been performed for the 8-membered experimental available data for , a polycyclic central ring of the circulenes, as well as for the “marginal” aromatic hydrocarbon with similar structure, may lead to petal-rings. valuable conclusions regarding the stability of the three In the previous study [20] we performed NICS PHQWLRQHGÀRZHUVDQGWKHLUSRVVLEOHV\QWKHVLV computations for evaluating the local aromatic character In this regard, we propose two isodesmic reaction of circulenes with 6-membered core. For comparisons, schemes for each of these 1a to 1c circulenes (Fig. 2). results for the petal rings are presented in parentheses The averaged heat of formation will be considered for (italics). Regardless the size of the core, the trend of further discussion (Table 1). NICS(0) index appears to be the same. Highest aromatic If we compare the results with the experimental character (i.e., larger negative values) was found in heat of formation of coronene (36.4 kcal mol-1) [14,19], 6-atom petals of circulenes 1b and 1c; in case of 1a, only D VLJQL¿FDQW GLIIHUHQFH RI VWDELOLW\ DSSHDUV i.e., the 5-atom petals exist, that show a low aromatic character 8-circulene 1b is far more unstable). The most stable (see the azulene (5,7) molecule). Interestingly, a negative compound, according to these calculations, should be values of NICS index was obtained for the 8-core (i.e.,

92 53RSHWDO

F\FORRFWDWHWUDHQH RIWKHÀRZHU1a and alternating NICS pronounced aromatic character, among the studied values (-7.788/-3.863) for the 6-membered petals of compounds. Also, circulene 1c has a negative value of circulene 1b. The results presented in Table 2 show that ȁEXWDERXWWLPHVORZHUWKDQWKHRQHFRUUHVSRQGLQJ for a saddle-shaped circulene, like the 8-coronene [8:68] to the circulene 1b. (1b) the outer aromatic rings (even if they are all 6-rings) are no longer equivalent (and, by this reason, we can 3.3. Geometric criterion: HOMA index write for 8-coronene the formula [8:(6,6)4]), in contrast to HOMA (harmonic oscillator model of aromaticity) the 6-coronene [6:66]). [22] index was computed for all the outer 5-, 6- and In order to evaluate the global aromatic character 7-membered rings of the circulenes 1a to 1c. The of the compounds 1a to 1c, by means of the magnetic results are presented in Table 6; it can be seen, the properties, we performed computations of the exaltation outer 6-membered rings of compound 1c have the most of their magnetic susceptibility. Negative values of pronounced local aromatic character. Also, alternating the exaltation of the magnetic susceptibility prove the values for the 6-atom petals of the saddle-shaped aromaticity of a molecule, while the opposite means an circulene [8:68] 1b (also shown in case of NICS(0) anti-aromatic character. values – see Table 3) are observed. Two isodesmic reaction schemes were chosen for each circulene in order to compute the magnetic 3.4. Reactivity descriptors susceptibility exaltation (Fig. 3 and Table 4). Besides their use as tools in evaluating the reactivity The positive value of the exaltation of the magnetic and regioselectivity of chemical reactions, reactivity susceptibility of circulene [8:(5,7)4] (1a) (see Table 5) GHVFULSWRUVOLNHDEVROXWHKDUGQHVV Ș HOHFWURSKLOLFLW\ Ȧ  suggests a non-aromatic character. Circulene [8:68] and Fukui functions have also been applied to evaluate (1b) has the “most negative” value (close to the one of the aromatic character of molecular compounds [21]. coronene, -102.9 ppm cgs) and it should have the most The absolute hardness Ș  UHSUHVHQWV KDOI RI WKH HOMO-LUMO gap; a harder molecule is associated with Table 1. Enthalpies of formation of compounds 1a to 1c (HF/6-311G(d,p) level of theory). DQLQFUHDVHGVWDELOLW\VRPROHFXOHVZLWKODUJHUȘYDOXHV are believed to be more stable, thus showing a possible -1 Circulene Isodesmic reaction ǻHf (kcal mol ) aromatic character. Also, a lower electrophilicity Ȧ value can be taken as a proof of aromaticity. Regarding 1 390.5 1a the local reactivity descriptors, the Fukui functions 2 344.9 computed for an electrophilic attack are good indicators 1 334.9 of reactivity of each C atom in the studied circulenes, 1b 2 343.4 thus a hierarchy of the most electrophilic sites can be 1 219.0 established. 1c 7KH DERYH GHVFULSWRUV RI UHDFWLYLW\ DUH GH¿QHG DV 2 213.2 follows:

11 Table 2. Total energy, total energy per carbon atom and HOMO- Absolute hardness : LUMO HL gap of circulenes 1a to 1c (HF/6-311G(d,p)). HH K | LUMO HOMO

Circulene Etot (a.u.) Etot/C (a.u.) HL gap (eV) 2 Eelectrophilicity index11: 1a [8:(5,7)4] -1221.154 -38.161 7.15 2 1b [8:6 ] -1221.295 -38.165 8.03 P 8 Z 2K 1c [8:(5,6)4] -1069.903 -38.211 8.77 Fukui functions12:

Table 3. NICS(0) values, computed in circulenes 1a to 1c (B3LYP/6-31G(d) level of theory).

Flower Core Petals (5-atoms) Petals (6-atoms) Petals (7-atoms)

[8:(5,7)4] -2.330 -4.093 (-3.136) - 1.842 (0.393) -7.788/-3.863 [8:6 ] 9.465 - - 8 (-10.406)

[8:(5,6)4] 8.172 0.686 (3.189) -6.314 (-10.080) -

93 )XOOHUHQHVSDWFKHGE\ĠRZHUVZLWKRFWDJRQDOFRUH

1a (1) [8:(5,7) ] + 40/6 4 + 4 + 2

(2) + 28/3 [8:(5,7)4] + 8

1b

(1) 3 [8:(6,6)4] + 4 3 + 4

+ 4 (2) [8:(6,6)4] + 32/6

1c

(1) [8:(5,6) ] + 56/6 + 4 + 4 4 +

(2) [8:(5,6) ] + 32/6 + 4 4 +

Figure 3. ,VRGHVPLFUHDFWLRQVIRUFDOFXODWLQJWKHPDJQHWLFVXVFHSWLELOLW\H[DOWDWLRQLQĠRZHUV1a to 1c.

Table 4. Computed values of the isotropic magnetic susceptibilities (6-311G(d,p)) and experimental exaltations of the magnetic susceptibility (for the in the six isodesmic reactions – Fig. 3).

Compound Isotropic magnetic susceptibility Exaltation of magnetic susceptibility (cgs-ppm)(6-311G(d,p)) (experimental value) (ȁ, cgs-ppm)

1a [8:(5,7)4] -210.006 -

1b [8:68] -293.639 -

1c [8:(5,6)4] -239.779 - cyclooctatetraene -58.050 0.9 [15] azulene -99.468 -29.6 [15] acenaphtylene -114.167 -32.5 [16] coronene -260.205 -102.9 [15] phenanthrene -138.898 -46.2 [15] indene -85.144 -19.1 [15] naphthalene -101.227 -30.5 [15] ĠXRUHQH -122.366 -25.7 [15]

Exaltation of the magnetic susceptibility of the circulenes Table 5. DD2 to , computed from the above hypothetical reaction 1a 1c fck ¦ P schemes (Fig. 3). Pk

Circulene Reaction scheme ȁ (ppm cgs) ZKHUHĮ +202RUELWDOZLWKQHJOHFWLRQRIWKHRYHUODS

1 9.1 integral) 'DWD IRU WKH ÀRZHUV 1a to 1c are given in 1a 2 12.8 Table 7. 1 -81.5 The results in Table 7 are in good agreement with the 1b 2 -90.0 geometric, magnetic and energetic criteria above used 1 -29.5 1c for the evaluation of aromaticity in the three circulenes 2 -20.3 1a to 1c. The highest value of the absolute hardness, a

94 53RSHWDO

Table 6. HOMA values, computed for the circulenes 1a to 1c (HF/6-311G(d,p) level of theory).

Circulene Core (8-atoms) Petals (5-atoms) Petals (6-atoms) Petals (7-atoms)

1a [8:(5,7)4] -0.811 -0.983 - -0.414

1b [8:68] -0.432 - 0.703/0.335 -

1c [8:(5,6)4] -0.524 -0.817 0.960 -

Table 7. Absolute hardness (K) and electrophilicity (Z), computed at B3LYP/6-311G(d,p) level of theory.

Circulene Ș (eV) Ȧ (eV)

1a 1.07 5.60 1b 1.62 4.08 1c 1.93 3.38

Figure 6. Fukui function (for an electrophilic attack, f+) in circulene 1c (B3LYP/6-311G(d,p).

measure of the stability, has been obtained for circulene 1c, showing the most pronounced aromatic character among the investigated compounds. Conversely, WKH PRVW HOHFWURSKLOLF ÀRZHU LV 1a, which is related to its lower aromatic character among the studied ÀRZHUV The Fukui functions (for an electrophilic attack) have Figure 4. Fukui function (for an electrophilic attack, f+) in circulene been computed for each C atom on the contour of the 1a (B3LYP/6-311G(d,p). circulenes (Figs. 4 to 6). The results should represent an estimation of the place where the electrophilic attack is most likely to occur. It results that: - in circulene 1a, the three outer C atoms of the 7-membered cycles show different reactivity, suggesting a kind of circular polarization of pi-electrons;  LQ ÀRZHU 1b, the 6-atom petals show alternating properties, as concluded from the values of local aromaticity indices (HOMA and NICS(0)); - circulene 1c, the only planar structure, has two equivalents C atoms on each benzene unit, with a uniform distribution around the molecule. The results of the different criteria used for quantifying the aromaticity of the three circulenes led to the conclusion that, the most “aromatic” one is the

8-Sumanene 1c, [8:(5,6)4]. Following this result, two Figure 5. Fukui function (for an electrophilic attack, f+) in circulene different structures, bearing 6-Sumanene and 1b (B3LYP/6-311G(d,p). 8-Sumanene patches, have been designed (Fig. 7). The

95 )XOOHUHQHVSDWFKHGE\ĠRZHUVZLWKRFWDJRQDOFRUH

-1 Table 8. Total energy per C atom, Etot/C (au), HOMO-LUMO gap, HLGap (eV), Heat of formation, Hf (kJ mol ), NICS and HOMA values

(HF/6-31G(d,p)) for cages C52 and C120.

5-ring 6-ring (petal) 6-ring (core) 8-ring (core)

C52

Etot/C -37.864 NICS(+1) -0.275 NICS(+1) -6.059 NICS(+1) -2.803

HLGap 5.317 NICS(0) -2.292 NICS(0) -15.704 NICS(0)) -11.372

Hf 2095.9 NICS(-1) -17.783 NICS(-1) -28.663 NICS(-1) -28.281

- - HOMA -0.093 HOMA 0.333 HOMA 0.187

C120

Etot/C -37.871 NICS(+1) -0.711 NICS(+1) -3.589 NICS(+1) -3.565 NICS(+1) 6.196

HLGap 6.251 NICS(0) -0.242 NICS(0) -7.635 NICS(0) -7.563 NICS(0) 5.164

Hf 2181.8 NICS(-1) -12.058 NICS(-1) -16.646 NICS(-1) -16.746 NICS(-1) -3.498

- - HOMA -0.553 HOMA 0.537 HOMA 0.821 HOMA -1.478

Figure 7. Fullerenes tessellated by 6-Sumanene: C52([6:(5,6)3]4) (left) and 8-Sumanene C120 ([8:(5,6)3]6);[6:(5,6)3]8)) (right).

values of NICS (0; -1 (inside the cage); +1 (out of the [6:(5,6)3] +2 3 + 6/52C cage)) and HOMA indices, for the 5-, 6-, and 8-rings of 52 the fullerenes in Fig. 7 are listed in Table 8.

[8:(5,6)4] + 16/6 4 + 1/15C In case of C120, the NICS(0) values for the 120 8-Sumanene patch show the same trend as in the planar nd 8-Sumanene. The values NICS(-1), characterizing The values of Hf (Table 8, 2 column) are in good the inside cage electron density, show more aromatic agreement with the ones of the single point computations

character (i.e., larger negative values) in comparison (Etot& WKHUHDUHQRVLJQL¿FDQWGLIIHUHQFHVEHWZHHQWKH to the outside cage describing NICS(+1), as expected. computed stability of the two structures. However, the

Comparing the 6-Sumanene patch in the two cages in HOMO-LUMO gap value is in favor of C120 (6.251 for

Fig. 7, one can see a more aromatic character of petals C120 vs. 5.317 for C52). vs. the core, according to NICS(0) values. The HOMA values follow in general the trend of NICS values, excepting the 6-Sumanene patch in C120, where the 4. Conclusions 6-ring core was found more aromatic (i.e., more positive value, 0.821) than the 6-ring petal (0.537). The stability and aromatic character of three different Two isodesmic reactions –in order to estimate the circulenes has been investigated; in this regard, NICS

stability of the C52 and C120 cages- have been proposed. and HOMA indices, heats of formation and exaltation

96 53RSHWDO

of the magnetic susceptibility have been computed. and HOMA values are also kept in case of fullerenes Also, local and global reactivity descriptors have been bearing 8-Sumanene patches. used for estimating the behavior of the compounds 1a to 1c in the case of an electrophilic attack. The results of the geometric, magnetic and energetic criteria led Acknowledgements to the same conclusion: circulene 1c (i.e., the planar

8-Sumanene [8:(5,6)4]) is proved to have the most The paper is supported by the Grants: Romanian pronounced aromatic character, while the 1a compound Ministry of Education, CNCS – UEFISCDI, No. PN-II-RU-

(saddle-shaped, [8:(5,7)4]) is the most unstable and is PD-2012-3-0102 and PN-II-ID-PCE-2011-3-0346/2011; characterized by a weak aromaticity. The trend of NICS Computational Grant No. 133 PCSS, PosnaĔ, Poland.

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