Hemispherical polyarenes represent attractive templates from which carbon nanotubes of the same diameter and rim structure (chirality) might be grown by repetitive annulation reactions. The resulting single-index (n,m) nanotubes would have one end open and the other end capped by the original template. Efforts in the author’s laboratory to synthesize (5,5) and (6,6) nanotube end-caps are described. Nitroethylene is shown to serve well as a “masked acetylene” for the conversion of polyarene bay regions into new unsubstituted benzene rings by a Diels-Alder cycloaddition/aromatization process. Benzyne reacts similarly, both in solution and in the gas phase. These annulation reactions are proposed as methods for elongating large-diameter templates that have bay regions on their rims into structurally uniform, single-walled carbon nanotubes. Unfortunately, the bay regions on the strongly curved rim of the small-diameter (5,5) nanotube end-cap 3 resist Diels-Alder cycloadditions with both nitroethylene and benzyne. Pentabenzocorannulene (14) is proposed as a promising candidate for surface-catalyzed cyclodehydrogenation to a surface-bound hemispherical polyarene that could serve as a template for synthesis of pure (5,5) carbon nanotubes.
Not long after the discovery of carbon nanotubes (CNTs) by Iijima in 1991,  scientists recognized the relationship between the (n,m) indices of these remarkable new all-carbon materials and their electronic properties , . Whereas most chiral (n≠m) and zig-zag (n>0, m=0) CNTs are semiconducting, all armchair CNTs (n=m) are conducting and exhibit “metallic” behavior. Dreams of using conducting CNTs as ultra-thin, super-strong, nanowires in molecular scale electronics quickly spread , but the realization of that vision was thwarted by the lack of sources for pure metallic CNTs, uncontaminated by their semiconducting counterparts. Iijima’s method for generating CNTs from graphite invariably produces mixtures of chiral, zig-zag, and armchair tubes of varying diameters and lengths, the separation of which has proven to be virtually impossible on a practical scale.
In the 20 years following Iijima’s landmark discovery, chemists, physicists, and engineers all over the world raced to find alternative methods for preparing CNTs. Several new protocols were developed, but they all likewise produce mixtures of conducting and semiconducting CNTs . It was not until the early 21st century that ideas about synthesizing structurally uniform, single-index CNTs “from the ground up” by controlled chemical methods in the laboratory began to surface , , . These ideas were based on the premise that small hydrocarbon templates could be “grown” into long, single-walled CNTs, the diameters and chiralities of which would be predetermined by the diameters and rim structures of the templates. By this strategy, cycloparaphenylenes  and aromatic belts  would give CNTs that are open at both ends, whereas hemispherical polyarene templates would give CNTs that are open only at one end, the template serving as an end-cap at the other end. Whether or not one or both ends of a CNT is capped should not affect its electrical conductivity; the electronic properties should be determined solely by the (n,m) index. The following account summarizes efforts in the author’s laboratory to synthesize armchair CNT end-caps and to develop chemical methods for the elongation of these and other small hydrocarbon templates into long, single-walled, single-index (n,m) CNTs.
Strategies for synthesizing hemispherical polyarene templates
Synthesis of a (5,5) carbon nanotube end-cap
Figure 1 outlines our original strategy for synthesizing a C50H10 geodesic polyarene (3) from corannulene (1) and for using it as a small hydrocarbon template from which to grow (5,5) CNTs . After considerable experimentation, the 5-fold indenoannulation of corannulene (1) was achieved in three steps, using microwave-induced, Pd-catalyzed, intramolecular arylation reactions to close all the new 5-membered rings (Fig. 2) , . Each cyclization reaction introduces additional strain in the molecule and further accentuates the curvature of the hydrocarbon bowl . The X-ray crystal structure of pentaindenocorannulene (2) reveals that the curvature of 2 (POAV angle of the pyramidalized hub carbon atoms=12.1°) exceeds not only that of corannulene (1, hub POAV angle=8.3°) but also even that of the smallest stable fullerene, C60 (POAV angle=11.6°) , .
In principle, it should be possible to stitch together the arms of pentaindenocorannulene (2) by flash vacuum pyrolysis (FVP); however, our years of experience with this technique had taught us the value of incorporating aryl radical precursors (Br or Cl substituents) at the sites of the intended C–C bond formation . Brute force thermal cyclodehydrogenations of unfunctionallized polycyclic aromatic hydrocarbons have been reported, but they rarely give good yields of cyclized products , . Consequently, rather than focus on trying to convert 2 to 3, we modified the plan and incorporated five additional chlorine atoms in our synthetic intermediates. In this way, we succeeded in synthesizing the (5,5) CNT end-cap 3 in three steps from corannulene, as shown in Fig. 3 , . Attempts to convert intermediate 4 to 3 by Pd-catalyzed, intramolecular arylation reactions (cfFig. 2) gave hydrocarbon products in which up to 6 of the 10 desired bonds had formed, as judged by mass spectral analysis , but only FVP closed all of the new rings to give 3.
The low yield of the FVP step makes it difficult to prepare more than a few milligrams of 3 at a time, but we were able to obtain enough of this C50H10 geodesic polyarene to grow crystals for X-ray analysis . Fortunately, the strategy of growing CNTs from small hydrocarbon templates does not demand large quantities of the templates. Elongating end-cap 3 to a length of even 1 μ would provide gram quantities of pure (5,5) CNTs, and 1 μ does not seem unreasonable, considering that individual CNTs have been grown by other methods to lengths of more than 20 cm!  Our efforts to grow long CNTs from template 3 are described in a later section below.
Attempted syntheses of a C3v (6,6) carbon nanotube end-cap
This plan relies on 1,3-peri-shifts of hydrogen atoms (Fig. 5) in the aryl radicals generated by homolysis of the C–Cl bonds closest to the hub in order to initiate closures to the bowl . Spontaneous thermal cyclodehydrogenations analogous to those that enabled our C60 synthesis , ,  and radical cyclizations initiated by homolysis of the other three C–Cl bonds would be required to close the remaining new rings. Unfortunately, FVP of the C60H24Cl6 synthetic precursor (5) at a variety of temperatures and pressures gave only small amounts of the desired C3v C60H12 geodesic polyarene (6), or none at all, depending on the FVP conditions. From the most encouraging experiments, the 1H NMR, UV-vis, and mass spectra recorded on the unpurified pyrolysates all showed peaks in good agreement with those calculated for 6, but the low solubilities of the components in the mixture rendered isolation and purification of the CNT end-cap hopeless .
An alternative, solution-phase route to (6,6) CNTs from a slightly larger C3v C66H24 geodesic polyarene (8) is outlined in Fig. 6 , . In this approach, all six 5-membered rings of the final target compound (8) are already present in the penultimate intermediate (7). It was anticipated that the challenge of stitching the arms together in this end-cap precursor might be less demanding than in 5 and not require FVP, because 7 already contains much of the pyramidalization strain and molecular curvature of the desired hemispherical polyarene (8). Unfortunately, all attempts to convert 7 to 8 by oxidative cyclodehydrogenation reactions (FeCl3 or AlCl3/Cu(OTf)2 or MoCl5), even at elevated temperatures, gave back only unchanged starting material and, in some cases, small amounts of partially chlorinated derivatives of 7 , . Anionic cyclodehydrogenation attempts (alkali metals or potassium naphthalenide or tetrapotassium corannulenide), even with microwave irradiation, likewise gave mostly unchanged starting material (7), after the reactions were quenched with O2 , . We speculate that the oxidative cyclodehydrogenation reactions may have failed because corannulenes and other curved polyarenes are more difficult to oxidize than planar polyarenes. On the other hand, curved polyarenes are more easily reduced than their planar counterparts. The failure of our anionic cyclodehydrogenation attempts may have resulted from over-reduction of 7 to a stable polyanion. If so, then milder reduction under more controlled conditions (e.g. electrochemical) might have a better chance. Notwithstanding the reasons for our failures to stitch up synthetic intermediates 5 and 7, we never succeeded in preparing a (6,6) CNT end-cap in isolable amounts. Fortunately, we were able to prepare enough of the (5,5) CNT end-cap (3) to move on to the final hurdle.
Strategies for elongating hemispherical polyarene templates
“Masked Acetylene” Diels-Alder cycloaddition/aromatization strategy
The rims of armchair CNTs and their end-caps (e.g. 3) are characterized by continuous strings of polyarene bay regions. One strategy for growing an armchair CNT from an end-cap such as 3 requires a reagent that will add two carbon atoms across the mouths of all the bay regions and convert them into new, unsubstituted benzene rings. In principle, Diels-Alder cycloaddition of acetylene, followed by aromatization of the new 6-membered ring (symmetry-allowed thermal 1,4-loss of H2, Fig. 7) would produce an elongated CNT with a continuous string of bay regions on the new rim created by the new benzene rings . As long as all the steps are purely thermal and require no intervention by the chemist, the process should repeat itself indefinitely, until the supply of reagent is exhausted. In this scenario, the growing hydrogen-terminated CNT is a living polymer, and the iterative Diels-Alder cycloaddition/aromatization process is a polymerization that requires only a small hydrocarbon template, such as end-cap 3, to get started.
Despite the attractiveness of this strategy on a formal level, two serious problems make it look rather unrealistic on a practical level. One problem is that the “diene” partners of these Diels-Alder cycloadditions are buried in benzene rings, the aromaticity of which will be energetically costly to disrupt by the conversion of sp2 carbon atoms to sp3 carbon atoms, even though just temporarily. The second problem is that acetylene (HC≡CH) is a notoriously poor dienophile for Diels-Alder cycloaddition reactions . Nevertheless, we decided to pursue this strategy.
Diels-Alder cycloadditions on biphenyl and phenanthrene are unknown, even with the most potent dienophiles, because the accompanying loss of aromaticity is simply too much to overcome. On the other hand, Diels-Alder cycloadditions at the bay regions of larger polyarenes, though rare, are feasible , because the loss of aromaticity in just two rings out of many becomes energetically less costly as the size of the polyarene increases (Fig. 8) .
To validate the predictions of these calculations, we prepared a soluble, air-stable derivative of bisanthene (9) and heated it with a standard dienophile, diethyl acetylenedicarboxylate. We were rewarded by production of the anticipated ovalene derivative (10) formed by two-fold Diels-Alder cycloaddition/aromatization (Fig. 9) . We did not verify that the hydrogen atoms originating in the bay regions of 9 were actually lost as molecular H2, but we searched for reduced dienophile (diethyl maleate and diethyl fumarate) and confirmed that none was formed. In a competition experiment between 9 and perylene (the next smaller polyarene in Fig. 8), only the bisanthene (9) reacted with the diethyl acetylenedicarboxylate; the perylene was recovered unchanged .
Encouraged that Diels-Alder cycloadditions should be possible on the rims of hydrogen-terminated CNTs, we turned our attention to the second problem, the poor dienophilicity of acetylene (HC≡CH). One solution to this problem would be to use a “masked acetylene” as the dienophile, rather than acetylene itself, i.e. a dienophile that is more potent than HC≡CH but would still leave new HC=CH bridges across the original bay regions after completion of all the post-cycloaddition thermal reactions. Paquette introduced phenyl vinyl sulfoxide as a masked acetylene for Diels-Alder cycloadditions several decades ago , and it is commercially available, so we studied its Diels-Alder reactivity with the bay regions of bisanthene 9. Unfortunately, phenyl vinyl sulfoxide proved to be a much weaker dienophile than diethyl acetylenedicarboxylate, although it did give low yields of the two-fold Diels-Alder cycloaddition/aromatization product from 9 under forcing conditions (180-fold molar excess of phenyl vinyl sulfoxide, 155°C, 6 days) , .
Clearly, a more potent masked acetylene is needed for Diels-Alder cycloadditions to polyarene bay regions. Guided by computational experiments,  our attention was drawn to nitroethylene as an attractive candidate. Nitroethylene polymerizes readily and is not commercially available, but methods for generating it in situ and trapping it in Diels-Alder cycloadditions with aliphatic dienes have been known for many years . To our delight, bisanthene 9 reacts readily with nitroethylene to give excellent yields of the two-fold Diels-Alder cycloaddition/aromatization product (11), in which the original bay regions have been spanned by HC=CH bridges and transformed thereby into new, unsubstituted benzene rings . Figure 10 shows the steps that presumably account for this new annulation reaction. The final thermal loss of HONO resembles a Cope elimination but is not a common reaction for ordinary nitroalkanes. In this case, however, several special structural features facilitate the thermal elimination. Specifically, the C–N and C–H bonds that break during this elimination are both benzylic and, therefore, much weaker than those in ordinary nitroalkanes. Furthermore, the new C=C bond being formed completes the π-system of a new benzene ring, which is also energetically favorable.
It should be noted that the Diels-Alder cycloaddition/aromatization method ought to work not only for the growth of armchair CNTs but also for the growth of chiral CNTs (Fig. 11) , . As long as the conversion of a bay region into a new unsubstituted benzene ring creates a new bay region, the growth process should continue indefinitely. Additionally, the small hydrocarbon templates suitable for elongation by the Diels-Alder cycloaddition/aromatization strategy need not be limited to hemispherical polyarenes; aromatic belts that are sufficiently wide to enable bay region Diels-Alder cycloadditions should serve well, too.
During the course of the computational experiments that led us to nitroethylene, we were intrigued to discover that the reactivity of unsubstituted acetylene (HC≡CH) in bay region Diels-Alder cycloadditions is predicted to be only marginally less than that of phenyl vinyl sulfoxide . Would it be possible to use acetylene itself, perhaps under high pressure and/or elevated temperatures, rather than nitroethylene or some other masked acetylene, to grow CNTs from small hydrocarbon templates? We felt compelled to look into this question experimentally and were amazed to find that heating a solution of bisanthene 9 in dimethylformamide under 1.8 atm of acetylene gas in a pressure-regulated reactor gives the same primary Diels-Alder cycloaddition/aromatization product as the one formed in the nitroethylene reaction (Fig. 12) ! Subsequent Diels-Alder cycloaddition to the second bay region is actually more difficult and was not observed under the conditions employed. Nevertheless, with sufficiently reactive hemispherical polyarenes or aromatic belts, acetylene might prove to be the most attractive 2carbon annulation reagent of all for the growth of CNTs from small hydrocarbon templates.
In our efforts to elongate the C50H10 hemispherical polyarene (3) into (5,5) CNTs, we chose to use the most reactive dienophile of those studied, nitroethylene. When template 3 was exposed to nitroethylene under the same conditions that work well for the double annulation of bisanthene 9 (Fig. 10), however, the starting material (3) was recovered unchanged. All of the nitroethylene simply polymerized. We tried increasing the amount of nitroethylene, raising the temperature, and running the reaction longer, but polyarene 3 never showed any signs of engaging in even one Diels-Alder cycloaddition (Fig. 13). For some reason, the bay regions of 3 are far less reactive toward nitroethylene than the bay regions of the bisanthene practice compound (9).
Theoretical calculations revealed the reason for the low reactivity of 3. At the B3LYP/6-31G* level of theory, the activation energy for Diels-Alder cycloaddition of acetylene in the bay region of bisanthene 9 is 24.2 kcal/mol, whereas that for polyarene 3 is 55.1 kcal/mol . We suspected that the curvature of the rim of 3 might be responsible for much of this difference, and that hypothesis was borne out by further calculations. Replacing the (5,5) CNT template (3) with a less severely curved (6,6) CNT template lowered the activation energy to 52.9 kcal/mol, and for a (10,10) CNT template the activation energy dropped down to 27.1 kcal/mol , . Reaching the transition state for Diels-Alder cycloaddition on the rim of a hemispherical polyarene apparently requires the introduction of considerably more strain than is required for the corresponding reaction at the bay region of a planar polyarene. It is worth noting that the activation energy for Diels-Alder cycloaddition on the rim of a (10,10) CNT template lies very close to that for Diels-Alder cycloaddition to bisanthene 9, which occurs readily. Thus, although the growth strategy using nitroethylene fails with our (5,5) CNT template (3), the method still holds promise for elongating large-diameter (≥2 nm) hydrocarbon templates, both hemispherical polyarenes and wide aromatic belts. A synthesis of a (10,10) CNT end-cap has been initiated but remains unfinished , .
Benzyne Diels-Alder cycloaddition/aromatization/cyclodehydrogenation strategy
As an alternative to the repetitive 2-carbon annulation strategy described in the previous section, we also considered a 6-carbon annulation strategy based on bay region Diels-Alder cycloadditions of benzyne (Fig. 14) .
For this “polymerization” to work, thermal cyclodehydrogenations in the newly formed fjord regions on the rim must join together the new 6-carbon units after they have become attached in order to restore an armchair rim on the growing CNT . Fortunately, such uncatalyzed thermal cyclodehydrogenations of  helicenes are well known to occur at high temperatures .
Reports of benzyne cycloadditions in the bay regions of polycyclic aromatic hydrocarbons are scarce , but we found that benzyne adds readily to our bisanthene (9) to give the dibenzovalene derivative (12, Fig. 15) , .
These Diels-Alder cycloadditions and all of those in the previous section have been performed in solution. There may be some concern, however, about the ability of the synthetic CNTs to continue growing once they reach a size that makes them insoluble. To these concerns we have two responses. First, even an insoluble CNT should be able to continue engaging in Diels-Alder cycloadditions at the solid-liquid interface, as long as the bay regions on its rim are still exposed to the solution that contains the dienophile. Furthermore, there may be no need for solvent at all. Perhaps the Diels-Alder cycloaddition/aromatization strategy could be carried out at high temperatures under solvent-free conditions, similar to the ways that CNTs are prepared by previous methods. To test this latter concept, we studied the feasibility of benzyne Diels-Alder cycloaddition/aromatization in the bay region of perylene under solvent-free conditions. FVP of phthalic anhydride has long been known as a clean thermal source of benzyne, , ,  so we sublimed a mixture of phthalic anhydride and perylene through a FVP tube. As anticipated, this co-pyrolysis gives the same annulated hydrocarbon (13) as the one formed when benzyne (from a different precursor) adds to perylene in solution (Fig. 16) , .
The high reactivity of benzyne as a dienophile gave us hope that we might be able to elongate our (5,5) CNT template (3) by this strategy. Unfortunately, all attempts to add benzyne to 3 failed and gave back the starting material unchanged (Fig. 17). Thus, even with powerful dienophiles, the bay regions on the rim of 3 resist Diels-Alder cycloadditions. Larger diameter templates will no doubt be required in order to make this strategy work, too.
Surface-catalyzed growth strategy
It was always our ambition to develop methods for the chemical synthesis of CNTs from hemispherical polyarenes that rely entirely on metal-free organic reactions, such as those described above. As a backup plan, however, we knew that CNTs could be grown on nanoparticles of catalytically active metals  and hypothesized that attachment of hemispherical polyarenes to such metal surfaces should lead to the preprogramed growth of single-index CNTs. The structures of CNTs grown in this manner would be dictated by the template chosen. Attaching a template such as 3 to a metal surface by replacement of the hydrogens with carbon-metal bonds, however, is not a trivial task.
Inspired by the work of Nuckolls et al. in 2007  and Otero et al. in 2008  on surface-catalyzed cyclodehydrogenation of polyarenes, we reasoned that the best way to attach a CNT end-cap to a catalytically active metal surface might be to create the hemispherical polyarene in situ by surface-catalyzed cyclodehydrogenation of an adsorbed planar or quasiplanar precursor. Still focusing on syntheses of (5,5) CNTs, we identified pentabenzocorannulene (14) as the ideal precursor (Fig. 18) .
Theoretical calculations predict that the central core of 14 should adopt a bowl-shaped conformation that is more shallow than that of corannulene (1) and that the barrier for bowl-to-bowl inversion of 14 should be much lower that that of 1 , . Thus, no matter which face of 14 (convex vs. concave) approaches the metal surface, the polyarene should flatten out on the surface to maximize its contact. Surface-catalyzed cyclodehydrogenation at elevated temperatures should then yield an attached hemispherical template, ready for elongation into a CNT.
We completed two different syntheses of pentabenzocorannulene (14) several years ago, but neither has yet been published. Some encouraging results have also been obtained from experiments to deposit 14 onto a Pt(111) surface under UHV conditions, transform it thermally into a metal-bound (5,5) CNT end-cap by surface-catalyzed cyclodehydrogenation, and grow those into long CNTs by successive insertions of 2-carbon units. This work is ongoing but is still too preliminary to report in detail here. We are, however, quite optimistic.
Our optimism about the viability of this approach received a major boost in August of 2014, when Fasel et al. reported its successful application to a template-directed synthesis of (6,6) CNTs from a 90-carbon polyarene on a Pt(111) surface under UHV conditions . To the best of our knowledge, that work represents the first and only controlled chemical synthesis to date of single-index CNTs from a small hydrocarbon template.
Conclusions and prospects for the future
We have successfully synthesized a hemispherical polyarene that has the diameter and armchair rim structure of a (5,5) CNT (3). A Diels-Alder cycloaddition/aromatization method for the one-step conversion of polyarene bay regions to new unsubstituted benzene rings has also been developed as a method for the growth of such small hydrocarbon templates into CNTs. Nitroethylene has been found to serve as a potent “masked acetylene” for this transformation, but in favorable cases, even acetylene itself can be used, under pressure at elevated temperatures.
Diels-Alder cycloaddition/aromatization of benzyne at the bay regions of polyarenes has been proposed as a possible alternative to the 2-carbon annulation approach and has been demonstrated to operate even under solvent-free conditions in the gas phase at high temperatures. Unfortunately, the (5,5) CNT template (3) resists Diels-Alder cycloadditions with both nitroethylene and benzyne and has not been elongated into (5,5) CNTs.
Calculations indicate that the high strain energy associated with Diels-Alder cycloadditions to 3 should be virtually absent in the corresponding reactions of larger-diameter (≥2 nm) hydrocarbon templates, and this bodes well for future chemical syntheses of (10,10) and wider CNTs by these methods.
Pentabenzocorannulene (14) has been identified as an attractive precursor for on-surface synthesis of CNT end-caps and their elongation to (5,5) CNTs. Preliminary experiments along these lines look promising; however, translating the sub-microgram scale chemistry of such UHV experiments to preparative scale syntheses of CNTs remains a major challenge in this area.
A collection of invited papers based on presentations at the 23rd IUPAC Conference on Physical Organic Chemistry (ICPOC-23), Sydney, Australia, 3–8 July 2016.
The research from my laboratory that is described herein was carried out by many dedicated students and postdoctoral associates whose names appear in the literature references cited. Financial support for this research from the National Science Foundation (CHE-1149096, CHE-0809494, CHE-0414066, CHE-0107051) and the Department of Energy (DE-FG02-93ER14359) is also gratefully acknowledged.
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