Computational Study of 13C NMR Chemical Shift Anisotropy Patterns

Article pubs.acs.org/JPCA

Computational Study of 13C NMR Chemical Shift Anisotropy Patterns in C20H10 and [C20H10]4−. Insights into Their Variation upon Planarization and Formation of Concentric Aromatic Species in the Smaller Isolated-Pentagon Structural Motif Alvaro Muñoz-Castro,*,†,‡ Wilson Caimanque-Aguilar,‡ and Cesar Morales-Verdejo§ †

Grupo de Química Inorgánica y Materiales Moleculares, Universidad Autonoma de Chile, El Llano Subercaseaux 2801, Santiago, Chile ‡ Doctorado en Fisicoquímica Molecular, Universidad Andres Bello, Av. Republica 275, 8370146 Santiago, Chile § Universidad Bernardo OHiggins, Laboratorio de Bionanotecnología, Departamento de Ciencias Quimicas y Biologicas, General Gana 1702, Santiago, Chile ABSTRACT: Corannulene, C20H10, exhibits a concave surface in the ground state that is able to experience a bowl-to-bowl inversion through a planar conformation. Such a structure is the smaller example resembling an isolated-pentagon motif, as a relevant fragment in fullerene chemistry. Here, we explored the differences between bowl and planar conformations involving both energetic and 13C NMR properties, for the neutral and tetraanionic species by using density functional theory (DFT) methods. This allows us to understand the variation of the chemical environment at the carbon atoms upon planarization of this representive motif. Our results reveal that the variation of the chemical shift comes about from the variation of different main components of the shielding tensor, according to the relative position of the carbon atoms in the structure (i.e., rim, hub, and protonated), which is more relevant for both hub and protonated sites, in contrast to the rim carbon remaining almost unshifted. Interestingly, the planar transition state exhibits a more favorable bonding situation than the bowl-shaped conformation; however, the higher strain is enough to overcome this extra stabilization. Upon reduction to the tetraanionic counterpart (C20H104−), a lesser strain in the planar conformation is observed, decreasing the inversion barrier. In addition, the formation of the concentric aromatic ring systems in C20H104−, results in a more axially symmetric chemical shift anisotropy (CSA) tensor for the hub carbons, accounting in a local manner, for the concentric aromatic behavior in such structure in contrast to the neutral parent. These observations can be useful to evaluate the aromatic behavior of teh isolated-pentagon rule (IPR) motif in fullerene cages.

■

INTRODUCTION Since the early characterization of Buckminsterfullerene, C60,1,2 several efforts have been devoted to the understanding of the mechanism of synthesis and the relationship between the polyhedral structure and fullerene stability.3−8 This discovery was the result of the Kroto’s research interest in microwave spectroscopy of outer-space9 carbon structures. The aesthetic spherical shape of C60 exhibits an icosahedral structure composed of pentagonal and hexagonal faces obeying the isolated-pentagon rule (IPR).10,11 The elucidation of the different fullerene cage structures stand on the IPR, leading to a well established structural motif involving one five-membered ring surrounded by five six-membered counterparts (Figure 1). In this concern, 13C NMR studies have facilitated the determination of molecular symmetries in both solution and the solid state.12−16 Owing to the high symmetry of C60, the overall structure is accounted for by one type of carbon atom displayed by the single resonance at room temperature from its 13 C NMR spectra (143.15 ppm), revealing the equivalence of the 60 sp2-carbon centers.17 At lower temperatures (77 K), the molecular tumbling is hindered to a large extent, unraveling the chemical shift anisotropy (CSA) pattern,15,18 which provides © XXXX American Chemical Society

Figure 1. Schematic representation of C60 and corannulene (C20H10), highlighting the IPR motif. Concentric aromaticity in the tetranionic moiety, with a 6π and 18π systems.

valuable information concerning the local structural and electronic properties, reflecting a nonaxial symmetry of the chemical environment at the probe nuclei.19 Corannulene, C20H10, can be viewed as a minimal IPR motif obtained by Barth and Lawton in 1966.20 The isolated pentagon at the hub of the structure, induces a curvature in the π-surface resulting in the smallest bowl-shaped fullerene Received: February 15, 2017 Revised: March 20, 2017 Published: March 20, 2017 A

DOI: 10.1021/acs.jpca.7b01477 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

σTMS = 192.34. This approach shows good agreement with experimental data as depicted in previous works on fused ring systems.48 For the tetraanionic corannulene, the shielding calculations were done by using COSMO,40,41 with tetrahydrofuran (THF) as solvent, where the reference value for tetramethylsilane varies to σTMS = 193.33.

fragment, displaying a bowl depth of 0.87 Å.21 This molecule serves as a prototype for nonplanar π-aromatic systems, which have experienced a renewed interest22−26 leading to a large scale synthesis route.27 Moreover, corannulene is prone to experience a bowl-to-bowl inversion in a well-defined process involving a planar intermediate structure.28 The difference in energy between the nonplanar structure and the planar transition state accounts for the energy barrier in reverting the curvature of the π-surface.29 The required energy for the bowl inversion has been estimated by variable temperature NMR studies that lie in the range 10.2−11.5 kcal·mol−1 for different functionalized corannulenes.30 In C20H10, the barrier has been estimated to amount to 11.5 kcal mol−1,31 which can be modified by the formation of host− guest complex with ExBox4+, as shown by Siegel and coworkers.29 Corannulene has been depicted as a hub-antiaromatic and rim-aromatic C20H10, which can be reduced to its tetraanionic parent.32,33 In such a form the structure exhibits an aromatic 5C/6e central ring within an aromatic 15C/18e rim, resulting in the annulene-within-an-annulene model (Figure 1), proposed by Scott et al.32 The C20H104− structure is considerably less curved than the neutral counterpart, denoting the decrease in the geometrical strain, and thus, a lower inversion barrier can be expected. In the overall bowl-to-bowl inversion process, corannulene goes from a C5v conformation through a planar D5h transition state, leading to the reverted ground state structure. In this concern, we set to explore the differences between such conformations involving both energetic and 13C NMR properties for the neutral and tetraanionic species by using DFT methods. We describe the orientation and magnitude of the chemical shift anisotropy (CSA) in representative carbon atoms to extend our understanding of the expected NMR patterns throughout the bowl-to-bowl process in a prominent IPR structural motif, and also for the formation of fully aromatic species.

■

RESULTS AND DISCUSSION The structure of corannulene resembles the IPR motif in fullerenes, which is characterized by a bowl-shaped conformation. The relaxed structure lies in the C5v point group and exhibits a bowl depth of 0.89 Å, which compares well with the 0.87 Å determined from experimental X-ray measurements.21 The three different types of sp2 atoms are classified as hub, rim, and protonated carbon. The pyramidalization angle (θp) of the atoms at the central C5 ring (hub) amounts to 8.1° and decreases to 1.8° at the rim atoms, similarly to previous reports. The planar conformation (D5h) is located about 9.52 kcal·mol−1 above the global minimum structure, as a first-order saddle point on the potential energy surface (PES), which dictates the bowl-to-bowl inversion. This value is similar to the experimentally reported value ranging from 11.5 to 10.8 kcal· mol−1 as per 1H NMR31 and other computational studies.29 To gain a deeper understanding of the factors governing the destabilization of the planar conformation in comparison to the bowl-shaped conformation, we describe in terms of different chemically meaningful terms in the framework of the Ziegler− Rauk energy decomposition scheme.49 In this framework, the overall formation energy (ΔEint) can be decomposed in several meaningfully terms, given by ΔE int = ΔE Pauli + ΔEelstat + ΔEorb

Starting from the individual atoms from their electronic ground states (i.e., 2S1/2 and 3P0, for H and C, respectively), ΔEint can be decomposed in the destabilizing ΔEPauli term, which accounts for the steric hindrance, in the stabilizing ΔEelstat term, which refers to the electrostatic character of the interaction, and in the ΔEorb term accounting for the stabilizing covalent character.35 The repulsive term (ΔEPauli) due to the superposition of undisturbed wave functions of the isolated fragments obeying the Pauli principle, responsible for the steric repulsion. The classical electrostatic energy between the superimposed fragments with their frozen charge distribution leads to the ΔEElstat term. And last, the resulting charge transfer between the individual atoms owing to the formation of the overall molecular electronic structure is accounted for by the ΔEOrb term. In the C5v ↔ D5h interconversion, the energy differences of each term indicates an orbital stabilization of −74.17 kcal·mol−1 (ΔEorb) which is indicative of a more favorable bonding in the planar conformation. This accounts for the more effective σand π-bonding formation in the overall neutral structure when the carbon atoms are disposed in a planar conformation. The electrostatic interaction (ΔEelstat) varies to a small extent leading to an extra stabilization of −10.25 kcal·mol−1 in the D5hC20H10 structure. However, despite the more favorable orbital and electrostatic character of the energy in the planar conformation, the increase in the Pauli repulsion term (ΔEPauli) of 93.94 kcal·mol−1 is enough to overcome the aforementioned stabilizing terms, resulting in a transition state that is 9.52 kcal· mol−1 less stable than the bowl-shaped conformation. Thus, the increase in strain destabilizes the planar conformation, despite the more favorable bonding. This results from the steric

■

COMPUTATIONAL DETAILS Geometry optimizations and subsequent calculations were performed at the density functional theory (DFT) level employing the ADF code.34,35 We used the all-electron tripleζ Slater basis set augmented with double polarization functions (STO-TZ2P) and the nonlocal Becke−Perdew (BP86) functional within the generalized gradient approximation (GGA).36−38 London dispersion correction to DFT was taken into account by the pairwise method of Grimmer (DFT-D3),39 in addition to the BP86 functional, BP86-D3. For the anionic C20H104− species, the solvent and counterions were taken into account by the conductor-like screening model (COSMO),40,41 with tetrahydrofuran (THF) as solvent, to improve the performance of DFT in anionic systems. Vibrational analyses were carried out to determine the global minima structure for the bowl-shaped conformation, and the transition state nature of the planar structure, as a first-order saddle point in the potential energy surface (one negative frequency).The nuclear magnetic shielding tensors were calculated with the NMR module of ADF employing gaugeincluding atomic orbitals (GIAO)42−45 with the exchange expression proposed by Handy and Cohen 46 and the correlation expression proposed by Perdew, Burke, and Ernzerhof47 (OPBE), and all-electron STO-TZ2P basis set. Reference value for a carbon at tetrametylsylane amounts to B


Article

The Journal of Physical Chemistry A hindrance introduced when fused six-membered rings are brought together in a 5-fold moiety, contrary to coronene, where a 6-fold array is able to retain a planar structure. For the tetraanionic counterpart, the relaxed structure exhibits a related bowl-shaped conformation, where the bowl depth decreases to 0.44 Å. This value is related to the finding of the [Li5(C20H104−)2]3− sandwich, denoting the effect of lithium atoms in the resulting structure.50,51 Interestingly, the planar conformation is now located 2.69 kcal·mol−1 above of the ground state structure, suggesting a more favorable C5v ↔ D5h interconversion. In this case, the variation of the energetic terms is, to a small extent, an indication of a similar situation between both conformations. A minimal decrease in the destabilizing Pauli repulsion term (−2.14 kcal·mol−1) with a slightly decrease in the stabilizing terms, namely, ΔEorb (1.39 kcal·mol−1) and ΔEelstat (3.44 kcal·mol−1) is observed in the D5h conformation. Thus, both conformations are rather similar in view of the energetic terms, denoting a small decrease in the structural strain accompanied with a slightly diminished bonding situation in the planar intermediate. In addition, the role of the dispersion term from the Grimme correction is small because it exhibits a scarce variation between the bowl-shaped and planar conformation amounting to less than 1.0 kcal/mol in both neutral and anionic cases. The 13C NMR spectra obtained by Grant and co-workers18 are indicative of three different types of carbon atoms located at 136, 131, and 126 ppm accounting for the hub, rim, and protonated carbon positions (Figure 2), respectively. The peaks

Table 1. Calculated CSA Parameters Obtained at the TZ2P/ OPBE Level of Theory for C20H10, Involving the Asymmetry Terma 11 hub rim prot

hub rim prot C60

σ δ σ δ σ δ

−23.9 216.2 −8.9 201.2 −19.8 212.1

σ δ σ δ σ δ σ δ

−11.5 203.9 −10.6 202.9 −53.0 212.4 −24.7 217.0

22 C5v 18.5 173.8 7.2 185.1 54.9 137.4 D5h 25.5 166.9 10.8 181.5 52.0 140.4 2.3 190.0

33

iso

η

179.0 13.3 188.9 3.4 162.8 29.5

57.9 134.4 62.4 129.9 66.0 126.3

0.35

165.7 26.7 197.7 −5.4 164.3 28.1 156.3 36.0

59.9 132.5 66.0 126.4 65.4 126.9 44.6 147.7

0.13 0.77

0.35 0.16 0.73 0.25

a

Chemical shift values (ppm) are relative to tetramethylsilane, as given by δij = σTMS − σij. σTMS = 192.34.

istic pattern in the possible nonplanar to planar transition in isolated-pentagon structural motifs in fullerene cages, which can be evaluated from the high field shifts rim and hub carbons. In the tetraanionic species, the calculated peaks appear at 103.4, 121.6, and 87.7 ppm, for hub, rim, and protonated carbons, respectively (Table 2). Such values are in line with the Table 2. Calculated CSA Parameters Obtained at the TZ2P/ OPBE/COSMO Level of Theory for C20H104−, Involving the Asymmetry Terma 11 Hub Rim Prot

Hub

Figure 2. Calculated 13 C NMR chemical shift relative to tetramethylsylane obtained at the TZ2P/OPBE level of theory. For the anionic species, TZ2P/OPBE/COSMO was employed; see text. The black circle denotes the calculated value for C60. Color code: hub, green; rim, yellow; protonated, orange.

Rim Prot

σ δ σ δ σ δ

32.2 161.1 −11.0 204.3 35.7 157.7

σ δ σ δ σ δ

43.2 150.1 3.0 190.4 53.0 140.3

22 C5v 39.2 154.2 29.5 163.8 118.7 74.6 D5h 44.2 149.2 28.0 165.4 116.7 76.6

33

iso

η

198.5 −5.2 196.7 −3.3 162.4 30.9

90.0 103.4 71.7 121.6 105.6 87.7

0.06

207.3 −14.0 205.6 −12.3 163.5 29.9

98.2 95.1 78.8 114.5 111.1 82.2

0.32 1.46

0.01 0.20 1.22

a

Chemical shift values (ppm) are relative to tetramethylsilane via the COSMO model, as given by δij = σTMS − σij. σTMS = 193.33.

at 136 and 131 ppm are related to quaternary carbons, which correlates with the depicted positions. The calculated 13C NMR for C5v-C20H10 agrees well with the reported data (Table 1), which is shielded in comparison to that for C60 (exp 143.2 ppm;15,17 calc 147.7 ppm). The three different carbon types exhibit a calculated isotropic chemical shift (δiso) of 134.4 ppm for the hub carbons, 129.9 ppm for the rim atoms, and 126.3 ppm for the peripheral carbon atoms (Figure 2). When the structure shifts to the D5h-C20H10 conformation, the expected 13 C NMR peaks are located at 132.5, 126.4, and 126.9 ppm, respectively, depicting a shielding of the quaternary atoms (i.e., hub and rim carbons) and a slight deshielding for the protonated carbons. This observation can serve as a character-

95, 112, and 87 ppm peaks determined from the lithiated salt in THF-d8 by Scott and co-workers.33 All the carbon atoms are shifted toward more shielding regions, as expected in the increase of charge in the carbon backbone and the presence of the concentric aromatic ring in the annulene-within-anannulene structure. The protonated carbon is the most upfield shifted atom by Δδ = −38.6 ppm, followed by the hub in about Δδ = −31.1 ppm. In contrast, the rim carbon is shifted to a smaller extent, Δδ = −8.3 ppm. In the respective planar transition state geometry, the expected 13C NMR peaks are C


Article


the CSA tensor.18 For the rim and protonated carbons, such an angle rises to 24.8° and 24.3°, respectively. By comparison of the neutral and tetranionic species, the calculated tensor is more tilted in relation to the Z-axis in the latter, increasing such an angle from 9.9° to 14.2° despite the less curved structure of C20H104−. For the rim carbon, the tilt angle is reduced from 24.7° to 20.7°. Lastly, in the protonated carbon the α-angles remain similar (24.3° and 23.9°, respectively). This points out that the formation of the concentric aromatic ring, or annulene-within-an-annulene structure in the tetranionic counterpart, induces a pronounced variation of the CSA tensor orientation in the carbons standing the IPR motif, i.e., both hub and rim carbons. The σ22 and σ11 components are mainly contained in the carbon-carbon backbone (Figure 4). The σ22 component is oriented along the C−C for the hub and rim carbons, and through the C−H bond in the protonated carbons atoms. The most deshielded component (σ11) is oriented between the bonds.18 The estimated variation of δiso of −2.0 ppm in the C5v↔ D5h interconversion for the neutral specie involves a modification in all the CSA components. Whereas σ11 and σ22 are more shielded (−12.4 and −6.9 ppm, respectively), σ33 is deshielded. Thus, the structural rearrangement in the transition state modifies the “in-plane” components (i.e., σ11 and σ22) with lower influence over the component oriented along the bonds (σ22). The resulting deshielding in the σ33 component is attributed to the deshielding cone from the antiaromatic hub ring. As a result of the reduction of corannulene to C20H104−, the main variation in the hub carbon (Δδiso= −32.1) involves all the principal components where σ11 shifts in −56.1 ppm, from −23.9 to 32.2 ppm, σ22 in −20.2 ppm (from 18.5 to 39.2 ppm) (Figure 4), and σ33 in −19.5 ppm (from 179.0 to 198.5 ppm). Interestingly, at the rim carbons, the variation of δiso of −9.3 ppm, is mostly driven by the modification of one axial component oriented along the C−C bond, i.e., σ22, of −22.3 ppm and from σ33 (−7.8 ppm), and to a lesser extent from σ11 (2.1 ppm). This is in contrast to the observed variation of the protonated carbons of Δδiso = −39.6, which appear in the sizable variation of the axial components (σ11 and σ22), whereas σ33 remains almost unchanged (Δ = 0.4 ppm). Thus, upon reduction, the variation of the chemical environment around the different carbons atoms, as accounted for by the shielding shift observed from 13C NMR spectrometry, is driven by different modifications of the CSA components in relation to their positions in the curved carbon backbone. The difference between the σ22 and σ11 components is reduced when the sp2-carbon is involved in an aromatic backbone. This leads to a more axial symmetry of the 13C NMR CSA tensor, as observed for us in the theoretical estimation of between the nonaromatic C60 and the hypothetical spherical aromatic C6010+ structure.19 A useful parameter to account for this difference is given by the asymmetry parameter (η) concerning the axial components (η = (δ22 − δ11)/(δ33 − δiso)), with an axial-symmetric tensor when η = 0. For C60 the η for 13 C NMR CSA amount to 0.25, which decreases notably to 0.05 in C6010+ as a more aromatic character appears in the latter sp2-cage.19 In the neutral C5v-C20H10, such a parameter amounts to η = 0.35 owing to the antiaromatic character of the central C5 ring (hub). The rim carbon exhibits a value of η = 0.13, denoting its participation in an aromatic backbone. For the protonated

located at 95.1, 114.5, and 82.2 ppm, respectively, denoting a smaller shielding shift for the protonated carbon. To exploit the information of the CSA, we provide a graphical representation of the absolute shielding (σij, i, j = 1, 2, 3) accounting for the orientation, magnitude and sign of the local response in relation to its own principal axis system (PAS)45,52 (Figures 3 and 4). Note that such relevant

Figure 3. Orientation and magnitude of the CSA tensor for a representative carbon at hub, rim, and protonated positions, obtained at the TZ2P/OPBE level of theory. (For C20H104−, the COSMO model was also included.) Arrows denote the orientation of the σ33 component.

Figure 4. Arrow representation of σ22 (red circles) and σ11 (green circles) components of the 13C NMR CSA tensor at each representative carbon atom. Values were obtained at the TZ2P/ OPBE level of theory.

parameters are reduced to a single value when the isotropic representation is employed. Here, the principal components of the shielding tensor at the nuclei are designated according to σ11 < σ22 < σ33, which allows a clear description of the shielding tensor characteristics. Moreover, the σij eigenvalues can be directly related to δ, from the relation: δij = σTMS − σij,45 allowing a direct comparison to the experimental data. The main shielding component (σ33) is oriented perpendicularly to the curved surface (Figure 3), which is similar to the case of C60 and related PAH structures where the π-surface seems to dominate the orientation of this component.18 Along the bowl-to-bowl inversion such a component is reoriented in function to the curvature of the structure. In relation to the C5symmetry axis (or Z-axis), the σ33 component of rim and protonated carbons is more tilted in comparison to that for the hub in the neutral D5h-C20H10 structure. The orientation of the CSA tensor is similar to that obtained from experimental solid state NMR and theoretical computations by Grant and coworkers.18 The angle between the Z-axis and the orientation of the σ33 component (or δ33) of the hub carbon amounts to α = 9.9° (exp α = 13°),18 showing no direct relation to the pyramidalization angle (θphub = 8.3°). Such an angle is estimated experimentally to amount to α = 13° involving the respective uncertainties of the determined principal values of D



■

carbon, such a parameter is not comparable for both the hub and rim carbons, owing to carbon’s different environments given by C−C and C−H bonds. For benzene η amounts to 0.81, supporting this observation. At the transition state, η remains almost unchanged, denoting the larger influence of the electronic structure in the shape of the CSA tensor, over the conformational change. The same trend is observed for the rim carbons. For the tetraanionic counterpart a larger variation of the term is observed in the hub carbon, which exhibits a more axial symmetry (η = 0.06), which is related to the expected in the spherical aromatic C6010+, thus reaching an aromatic IPR motif in line with the aromatic annulene-within-an-annulene model, proposed by Scott et al.32 The rim carbons exhibit a decrease in the axial symmetry of the CSA tensor, which amounts to η = 0.32, owing mostly to the variation of the σ22 component oriented along the Chub− Crim bond, denoting a lesser electronic delocalization between such carbons, thus supporting the aromatic behavior of C20H104− as two independent concentric aromatic systems.

■

Article

AUTHOR INFORMATION

Corresponding Author

*A. Muñoz-Castro. E-mail: [email protected]. ORCID

Alvaro Muñoz-Castro: 0000-0001-5949-9449 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by FONDECYT 1140359 and MILLENNIUM PROJECT RC120001 grants.

■

REFERENCES

(1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene. Nature 1985, 318, 162−163. (2) Taylor, R.; Hare, J. P.; Abdul-Sada, A. K.; Kroto, H. W. Isolation, Separation and Characterisation of the Fullerenes C60 and C70: The Third Form of Carbon. J. Chem. Soc., Chem. Commun. 1990, 20, 1423. (3) Wang, W.; Dang, J.; Zhao, X. Role of Four-Membered Rings in C32 Fullerene Stability and Mechanisms of Generalized Stone-Wales Transformation: A Density Functional Theory Investigation. Phys. Chem. Chem. Phys. 2011, 13, 14629. (4) Fowler, P. W.; Heine, T. Stabilisation of Pentagon Adjacencies in the Lower Fullerenes by Functionalisation. J. Chem. Soc. Perkin Trans. 2 2001, 4, 487−490. (5) Hirsch, A.; Chen, Z.; Jiao, H. Spherical Aromaticity inIh Symmetrical Fullerenes: The 2(N+1)2 Rule. Angew. Chem., Int. Ed. 2000, 39, 3915−3917. (6) Kadish, K. M.; Ruoff, R. S. Fullerenes: Chemistry, Physics, and Technology; Wiley-Interscience: New York, 2000. (7) Bühl, M.; Hirsch, A. Spherical Aromaticity of Fullerenes. Chem. Rev. 2001, 101, 1153−1184. (8) Hirsch, A.; Brettreich, M. Fullerenes: Chemistry and Reactions; Wiley-VCH: Weinheim, 2005. (9) Kroto, H. Symmetry, Space, Stars, and C60(Nobel Lecture). Angew. Chem., Int. Ed. Engl. 1997, 36, 1578−1593. (10) Haddon, R. C.; Brus, L. E.; Raghavachari, K. Electronic Structure and Bonding in Icosahedral C60. Chem. Phys. Lett. 1986, 125, 459−464. (11) Elser, V.; Haddon, R. C. Icosahedral C60: An Aromatic Molecule with a Vanishingly Small Ring Current Magnetic Susceptibility. Nature 1987, 325, 792−794. (12) Kaminský, J.; Buděsí̌ nský, M.; Taubert, S.; Bouř, P.; Straka, M. Fullerene C70 Characterization by 13C NMR and the Importance of the Solvent and Dynamics in Spectral Simulations. Phys. Chem. Chem. Phys. 2013, 15, 9223. (13) Piskoti, C.; Yarger, J.; Zettl, A. C36, a New Carbon Solid. Nature 1998, 393, 771−774. (14) Heine, T.; Bühl, M.; Fowler, P. W.; Seifert, G. Modelling the 13C NMR Chemical Shifts of C84 Fullerenes. Chem. Phys. Lett. 2000, 316, 373−380. (15) Yannoni, C. S.; Johnson, R. D.; Meijer, G.; Bethune, D. S.; Salem, J. R. Carbon-13 NMR Study of the C60 Cluster in the Solid State: Molecular Motion and Carbon Chemical Shift Anisotropy. J. Phys. Chem. 1991, 95, 9−10. (16) Tycko, R.; Haddon, R. C.; Dabbagh, G.; Glarum, S. H.; Douglass, D. C.; Mujsce, A. M. Solid-State Magnetic Resonance Spectroscopy of Fullerenes. J. Phys. Chem. 1991, 95, 518−520. (17) Avent, A. G.; Dubois, D.; Pénicaud, A.; Taylor, R. The Minor Isomers and IR Spectrum of [84]fullerene. J. Chem. Soc., Perkin Trans. 2 1997, 10, 1907−1910. (18) Orendt, A. M.; Facelli, J. C.; Bai, S.; Rai, A.; Gossett, M.; Scott, L. T.; Boerio-Goates, J.; Pugmire, R. J.; Grant, D. M. Carbon-13 Shift Tensors in Polycyclic Aromatic Compounds. 8. 1 A Low-Temperature NMR Study of Coronene and Corannulene. J. Phys. Chem. A 2000, 104, 149−155.

CONCLUSION

In summary, the variation of the chemical environment for carbon atoms as given by the 13C NMR patterns along the bowl-to-bowl interconversion in C20H10 behaves differently for hub, rim, and protonated carbons. For hub atoms, σ33 and σ11 components of the CSA tensor vary to a large extent, and for the rim carbons, σ33 and σ22 are more affected. Moreover, for the protonated carbons the σ11 component mainly varies. Hence, the observed variation of δiso comes from different variations of the main components of the shielding tensor, owing to both geometrical and electronic aspects. Structurally, the bowl-inversion barrier is given by the strain introduced by the planar conformation, which, despite the more favorable orbital and electrostatic character than the bowl conformation, the increase in the Pauli repulsion term (ΔEPauli) of 93.94 kcal·mol−1 is enough to overcome the aforementioned stabilizing terms, resulting in a structure 9.52 kcal·mol−1 less stable than the bowl-shaped conformation. Thus, the increase in strain destabilizes the planar conformation, despite the more favorable stabilizing terms. Interestingly, such a barrier is decreased in the tetraanionic counterpart, owing to a lesser strain on the planar conformation driven by their similar bonding and strain situations in both geometrical ground and transition states. This points out that the formation of the concentric aromatic ring or annulene-within-an-annulene structure in C20H104− induces a pronounced variation of the CSA tensor orientation in the carbons in the IPR motif, i.e., both hub and rim carbons. This leads to a more axially symmetric CSA tensor for the hub carbons, which seems to be an inherent characteristic for aromatic fused rings, accounting for the aromatic behavior in such a structure in a local manner. Such an observation can be extended to the evaluation of the aromatic properties in other IPR motifs embedded in fullerene cages. The comparison between the bowl-shaped and planar conformations is informative to unravel characteristic patterns in the planarization of isolated-pentagon structural motifs in fullerene cages, which can be evaluated from the high field shift rim and hub carbons. E


Article


Screening Energy and Its Gradient. J. Chem. Soc., Perkin Trans. 2 1993, 105, 799−805. (41) Klamt, A.; Jonas, V. Treatment of the Outlying Charge in Continuum Solvation Models. J. Chem. Phys. 1996, 105, 9972. (42) Wolinski, K.; Hinton, J. F.; Pulay, P. Efficient Implementation of the Gauge-Independent Atomic Orbital Method for NMR Chemical Shift Calculations. J. Am. Chem. Soc. 1990, 112, 8251−8260. (43) Schreckenbach, G.; Ziegler, T. Calculation of NMR Shielding Tensors Using Gauge-Including Atomic Orbitals and Modern Density Functional Theory. J. Phys. Chem. 1995, 99, 606−611. (44) Wolff, S. K.; Ziegler, T.; van Lenthe, E.; Baerends, E. J. Density Functional Calculations of Nuclear Magnetic Shieldings Using the Zeroth-Order Regular Approximation (ZORA) for Relativistic Effects: ZORA Nuclear Magnetic Resonance. J. Chem. Phys. 1999, 110, 7689. (45) Kaupp, M.; Bühl, M.; Malkin, V. G. Calculation of NMR and EPR Parameters: Theory and Applications; John Wiley & Sons, Inc.: New York, 2006. (46) Handy, N. C.; Cohen, A. J. Left-Right Correlation Energy. Mol. Phys. 2001, 99, 403−412. (47) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (48) Camacho Gonzalez, J.; Muñoz-Castro, A. Alternation of Aromatic−nonaromatic Rings in Belt-like Structures. The Behavior of [6.8] 3 Cyclacene in Magnetic Fields. Phys. Chem. Chem. Phys. 2015, 17, 17023−17026. (49) Ziegler, T.; Rauk, A. On the Calculation of Bonding Energies by the Hartree Fock Slater Method. Theor. Chim. Acta 1977, 46, 1−10. (50) Filatov, A. S.; Spisak, S. N.; Zabula, A. V.; McNeely, J.; Rogachev, A. Y.; Petrukhina, M. A. Self-Assembly of Tetrareduced Corannulene with Mixed Li−Rb Clusters: Dynamic Transformations, Unique Structures and Record 7 Li NMR Shifts. Chem. Sci. 2015, 6, 1959−1966. (51) Zabula, A. V.; Spisak, S. N.; Filatov, A. S.; Petrukhina, M. A. SelfAssembly of Charged Supramolecular Sandwiches Formed by Corannulene Tetraanions and Lithium Cations. Organometallics 2012, 31, 5541−5545. (52) Kaupp, M. Interpretation of NMR Chemical Shifts. In Calculation of NMR and EPR Parameters; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, FRG, 2004; pp 293−306.

(19) Muñoz-Castro, A. Axis-Dependent Magnetic Behavior of C60 and C6010+ . Consequences of Spherical Aromatic Character. Chem. Commun. 2015, 51, 10287−10290. (20) Barth, W. E.; Lawton, R. G. Dibenzofluoranthene. J. Am. Chem. Soc. 1966, 88, 380−381. (21) Hanson, J. C.; Nordman, C. E. The Crystal and Molecular Structure of Corannulene, C20H10. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1976, 32, 1147−1153. (22) Greene, A. K.; Scott, L. T. Rapid, Microwave-Assisted Perdeuteration of Polycyclic Aromatic Hydrocarbons. J. Org. Chem. 2013, 78, 2139−2143. (23) Nishida, S.; Morita, Y.; Ueda, A.; Kobayashi, T.; Fukui, K.; Ogasawara, K.; Sato, K.; Takui, T.; Nakasuji, K. Curve-Structured Phenalenyl Chemistry: Synthesis, Electronic Structure, and BowlInversion Barrier of a Phenalenyl-Fused Corannulene Anion. J. Am. Chem. Soc. 2008, 130, 14954−14955. (24) Dubceac, C.; Filatov, A. S.; Zabula, A. V.; Petrukhina, M. A. Addition of Dihalocarbenes to a π-Bowl: First Structural Study. Cryst. Growth Des. 2015, 15, 778−785. (25) Stępień, M.; Gońka, E.; Ż yła, M.; Sprutta, N. Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications. Chem. Rev. 2017, 117, 3479−3716. (26) Li, X.; Kang, F.; Inagaki, M. Buckybowls: Corannulene and Its Derivatives. Small 2016, 12, 3206−3223. (27) Butterfield, A. M.; Gilomen, B.; Siegel, J. S. Kilogram-Scale Production of Corannulene. Org. Process Res. Dev. 2012, 16, 664−676. (28) Seiders, T. J.; Baldridge, K. K.; Grube, G. H.; Siegel, J. S. Structure/Energy Correlation of Bowl Depth and Inversion Barrier in Corannulene Derivatives: Combined Experimental and Quantum Mechanical Analysis. J. Am. Chem. Soc. 2001, 123, 517−525. (29) Juríček, M.; Strutt, N. L.; Barnes, J. C.; Butterfield, A. M.; Dale, E. J.; Baldridge, K. K.; Stoddart, J. F.; Siegel, J. S. Induced-Fit Catalysis of Corannulene Bowl-to-Bowl Inversion. Nat. Chem. 2014, 6, 222− 228. (30) Sygula, A.; Abdourazak, A. H.; Rabideau, P. W. Cyclopentacorannulene: π-Facial Stereoselective Deuterogenation and Determination of the Bowl-to-Bowl Inversion Barrier for a Constrained Buckybowl. J. Am. Chem. Soc. 1996, 118, 339−343. (31) Seiders, T. J.; Baldridge, K. K.; Elliott, E. L.; Grube, G. H.; Siegel, J. S. Synthesis and Quantum Mechanical Structure of SymPentamethylcorannulene and Decamethylcorannulene. J. Am. Chem. Soc. 1999, 121, 7439−7440. (32) Ayalon, A.; Rabinovitz, M.; Cheng, P.-C.; Scott, L. T. Corannulene Tetraanion: A Novel Species with Concentric Anionic Rings. Angew. Chem., Int. Ed. Engl. 1992, 31, 1636−1637. (33) Baumgarten, M.; Gherghel, L.; Wagner, M.; Weitz, A.; Rabinovitz, M.; Cheng, P.-C.; Scott, L. T. Corannulene Reduction: Spectroscopic Detection of All Anionic Oxidation States. J. Am. Chem. Soc. 1995, 117, 6254−6257. (34) Amsterdam Density Functional (ADF) Code; Vrije Universiteit: Amsterdam, The Netherlands, http://www.scm.com. (35) Te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; van Gisbergen, S. J. a.; Snijders, J. G.; Ziegler, T. Chemistry with ADF. J. Comput. Chem. 2001, 22, 931−967. (36) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (37) Perdew, J. P. Density-Functional Approximation for the Correlation Energy of the Inhomogeneous Electron Gas. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33, 8822. (38) Vosko, S. H.; Wilk, L.; Nusair, M. Accurate Spin-Dependent Electron Liquid Correlation Energies for Local Spin Density Calculations: A Critical Analysis. Can. J. Phys. 1980, 58, 1200−1211. (39) Grimme, S. Density Functional Theory with London Dispersion Corrections. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2011, 1, 211−228. (40) Klamt, A.; Schüürmann, G. COSMO: A New Approach to Dielectric Screening in Solvents with Explicit Expressions for the F


Computational Study of 13C NMR Chemical Shift Anisotropy Patterns

Recommend Documents