C6 Isomers Research Articles

Comprehensive theoretical study of the correlation between the energetic and thermal stabilities for the entire set of 1812 C60 isomers

The thermal stability of fullerenes plays a fundamental role in their synthesis and in their thermodynamic and kinetic properties. Here, we perform extensive molecular dynamics (MD) simulations using an accurate machine-learning-based Gaussian Approximation Potential (GAP-20) force field to investigate the energetic and thermal properties of the entire set of 1812 C60 isomers. Our MD simulations predict a comprehensive and quantitative correlation between the relative isomerization energy distribution of the C60 isomers and their thermal fragmentation temperatures. We find that the 1812 C60 isomers span over an energetic range of over 400 kcal mol−1, where the majority of isomers (∼85%) lie in the range between 90 and 210 kcal mol−1 above the most stable C60-Ih buckminsterfullerene. Notably, the MD simulations show a clear statistical correlation between the relative energies of the C60 isomers and their fragmentation temperature. The maximum fragmentation temperature is 4800 K for the C60-Ih isomer and 3700 K for the energetically least stable isomer, where nearly 80% of isomers lie in a temperature window of 4000–4500 K. In addition, an Arrhenius-based approach is used to map the timescale gap between simulation and experiment and establish a connection between the MD simulations and fragmentation temperatures.

Metal-organic framework supported single-site nickel catalysts for butene dimerization

Homotopic sites in a well-controlled environment are not only ideal systems for mechanistic studies, but also allow optimal control of catalytic transformations. Sites having only a single metal cation and sites consisting of metal oxo complexes with few nickel (Ni) cations supported on the nodes of UiO-66 metal–organic framework (Ni-UiO-66) are studied for 1-butene dimerization. Monomeric Ni sites, which bind to the Zr6 node via two Zr-OH(μ3) linkages, are active and selective for the dimerization of 1-butene to linear and mono-branched C8 isomers. Ni oxo complexes with few Ni cations show lower activity and promote the oligomerization of transiently formed C8 isomers. Kohn-Sham density function theory calculations combined with spectroscopic measurements and kinetic analyses indicate that dimerization follows a Cossee-Arlman reaction mechanism.

Synthesis, Structure, and Theoretical Study of Trifluoromethyl Derivatives of the IPR Isomer of C84 Fullerene, C84(11)(CF3)10,12,14

AbstractC84 is the most abundant higher fullerene which is present in the arc‐discharge fullerene soot. Among 24 topologically possible Isolated‐Pentagon Rule isomers of C84, two major isomers, C84(22) and C84(23), and several minor ones have been identified experimentally. We report the synthesis, HPLC isolation, and synchrotron X‐ray structure elucidation of trifluoromethyl derivatives of the most abundant minor isomer of C84, C84(11)(CF3)10,12,14. The experimentally determined addition patterns are discussed in terms of the attachments in ribbons of edge‐sharing para C6(CF3)2 hexagons and the uniform distribution of CF3 groups on the C2‐C84(11) carbon cage. A general scheme of C84(11) trifluoromethylation pathways was proposed on the basis of theoretical DFT calculations of formation energies. Thus, trifluoromethylation pathways are now discussed in details for five different isomers of C84.

Volatile organic compounds emission in the rubber products manufacturing processes

Volatile organic compounds (VOCs) emission from rubber products manufacture processes, mixing, shaping and vulcanization were investigated in four rubber products factories in China. The source emission air was passively sampled by pre-vacuumized stainless steel canister and analyzed by gas chromatography-mass spectrometry-flame ionization detection (GC/MS-FID). The species profile of 107 VOCs in the emission processes were obtained. We calculated the photochemical ozone formation potential (OFP) and carcinogenic risk (CR) of the VOCs for each manufacture process. The results showed that mixing process mainly released dichloromethane (14.53%), carbon disulfide (CS2) (6.88%), styrene (5.72%), 4-methyl-2-pentanone (5.22%) and naphthalene (3.69%) for solvents used and raw rubber degradation in the process. The C6–C8 alkanes, especially heptane and isomers of heptane (44.71%), were dominated in shaping process. The major species released from vulcanization process were carbon disulfide (29.72%), naphthalene (8.17%), acetone (7.73%) and dichloromethane (4.26%). VOCs emitted from vulcanization process had the highest OFP, which contributed by naphthalene, m/p-xylene, o-xylene and carbon disulfide. VOCs emission from mixing process had the highest CR, and 1,2-dibromoethane, 1,2-dichlorethane and 1,3-butadiene were the main contributors to CR. We also estimated the total VOCs emissions into the atmosphere from tires manufacturing in China, which were 7.58 × 105 t in 2018 and contributed about 9% of total industry processes VOCs emissions.

Clar and Fries structures for fullerenes

Fries and Clar numbers are qualitative indicators of stability in conjugated π systems. For a given Kekulé structure, call any hexagon that contains three double bonds benzenoid. The Fries number is the maximum number of benzenoid hexagons, whereas the Clar number is the maximum number of independent benzenoid hexagons, in each case taken over all Kekulé structures. A Kekulé structure that realises the Fries (Clar) number is a Fries (Clar) structure. For benzenoids, it is not known whether every Fries structure is also a Clar structure. For fullerenes Cn, it is known that some Clar structures in large examples correspond to no Fries structure. We show that Fries structures that are not Clar occur early: examples where some Fries structure is not Clar start at C34, and examples where no Fries structure is Clar start at C48. Hence, it is unsafe to use fullerene Fries structures as routes to Clar number. However, Fries structures often describe the neutral fullerene better than a Clar structure, e.g. in rationalising bond lengths in the experimental isomer of C60. Conversely, an extension of Clar sextet theory suggests the notion of anionic Clar number for fullerene anions, where both pentagons and hexagons may support sextets.

Structural exploration of selected C6 and C7-substituted coumarin isomers as selective MAO-B inhibitors

Monoamine Oxidase B is considered a successful target for developing antiparkinsonian drugs. Due to the side effects of current MAO-B inhibitors, there’s an urgent need for novel potent and highly selective MAO-B inhibitors. A recent study has shown that coumarins tend to be more selective towards MAO-B than MAO-A when connected to a hex-5-ynyloxy chain at position 6 in contrast to their C7-isomers. The present study describes the mode of interaction of the C6 and C7-substituted coumarin isomers characterized by their difference in selectivity towards MAO-B through molecular docking and molecular dynamics simulations in an effort to elucidate the structural components and molecular interactions that may be responsible for MAO-B selectivity. Three isomeric coumarin pairs connected to ether chain at position 6 or 7 were taken from the literature and modelled according to their IUPAC nomenclature. Molecular docking study revealed one π- π stacking interaction with Tyr-326 in common between the selective coumarin C6-isomers. Resulting complexes of one isomeric coumarin pair that displayed the highest selectivity shift towards MAO-B were subject to 100 ns molecular dynamics simulations study to analyze the stability of the docked complexes. Molecular dynamics revealed that the C7-isomer is relatively stable in both MAO isoforms through the simulation duration, whereas the C6-isomer deemed unstable for MAO-A which may be due to the bulky Phe-208 residue in MAO-A. Our results might be applied for further development and optimization of coumarin derivatives into a successful drug against Parkinson’s disease. Communicated by Ramaswamy H. Sarma

Cage‐Opened C60 Isomers with Different Reactivities

AbstractWe synthesized two cage‐opened C60 isomers with different arrangement of an imine moiety on their orifices and investigated reactivities of them toward nucleophiles. Both isomers underwent the enlargement of the opening by the reaction with N‐methylmorpholine N‐oxide in comparable yields while the reaction with MeOH caused an orifice contraction via a transannular cyclization resulting in obviously different yields. The products in the latter reaction have a new hexagonal or heptagonal ring on their skeletons. The observed difference in reactivity could be explained by the greater thermodynamic stabilization in the hexagon‐containing isomer compared with the other.

Exploring the Properties of H2O@C60 with the Local Second‐Order Møller‐Plesset Perturbation Theory: Blue or Red Shift in C60 and H2O Fundamentals to Expect?

AbstractThe main structural, energetic and vibrational properties of H2O@C60 were computationally investigated using the density‐fitting local second‐order Møller–Plesset (DF‐LMP2) theory. Two low‐energy isomers, of C2 and C1 symmetry, were initially considered and only the latter one was identified as a true minimum with the lowest energy. The energy difference between these isomers was rather small (only 0.002 kcal/mol) and the respective interaction energy of any of the H2O@C60 isomers was −14.26 kcal/mol. These isomers were structurally similar having larger values of O−H bond lengths and angles by 0.0082 Å and 0.9 degrees, respectively, compared to a free water molecule. The calculated harmonic IR spectrum of the C1 isomer was characterized by: a) the presence of water frustrated translation frequencies at 90.5‐94.3 cm−1; b) the red shift in frequency of the first radial C60 mode (by −(1.8–2.7) cm−1) and the blue shift for the other three IR‐active C60 modes (by up to 2.2 cm−1); c) the red shift in H2O fundamentals by −19.2, −110.4 and −123.8 cm−1 for the bending, symmetric and antisymmetric mode, respectively, in excellent agreement with recently reported experimental results. The zero‐point energy contributed only 0.15 kcal/mol to the ZPE‐corrected value of H2O@C60 interaction energy which became now equal to −14.11 kcal/mol.

Breathing-Assisted Selective Adsorption of C8 Alkyl Aromatics in Zn-Based Metal-Organic Frameworks.

Invited for the cover of this issue are Purna Chandra Rao, Minyoung Yoon and co-workers at Kyungpook National University, Gachon University, POSTECH, Korea Atomic Energy Research Institute and the University of Sydney. The image depicts how single C8 isomers are selectively isolated from a mixture. Read the full text of the article at 10.1002/chem.202102640.

Front Cover: Breathing‐Assisted Selective Adsorption of C8 Alkyl Aromatics in Zn‐Based Metal‐Organic Frameworks (Chem. Eur. J. 60/2021)

A key point of our work is selective adsorption of a C8 isomer with the assistance of metal–organic framework (MOF) breathing, which can be further applied to C8 isomer separation. When a C8 isomer mixture (colored balls) is applied to the MOF from an inlet bottle, the MOF starts to preferentially adsorb a single isomer (green balls) in the pores of MOF by breathing, as depicted by the expanding of the empty cages (MOFs) by incorporating the balls. As a result, the single C8 isomer is selectively isolated from the mixture, which is collected in the other bottle. More information can be found in the Full Paper by P. C. Rao, M. Yoon et al. (DOI: 10.1002/chem.202102640).

Electrochemically Induced Dimerization of p9mp-C70(CF3)12 Trifluoromethylated Fullerene.

We report a comprehensive study of a novel isomer of C70(CF3)12, p9mp-C70(CF3)12, whose electrochemical behavior differs from most of the other trifluoromethylated fullerenes. The addition pattern of p9mp-C70(CF3)12 is established by means of 19F-19F COSY NMR spectroscopy and DFT calculations. Like p7mp-C70(CF3)10, the new isomer p9mp-C70(CF3)12 undergoes dimerization to the [C70(CF3)12]22- upon single-electron reduction. The electrochemical observations are supported by the DFT calculations of dimerization energy and the temperature dependence of the CW X-band EPR spectroscopy data. Experimentally determined dimerization energies of p9mp-C70(CF3)12-• and p7mp-C70(CF3)10-• in solution are ca -8 and -26 kJ mol-1, respectively, in good correspondence with DFT data.

The shape selectivity of zeolites in isobutane alkylation: An investigation using CBMC and MD simulations

The adsorption and diffusion behaviors of C8 isomers and C9 component as main products of the isobutane alkylation catalyzed by zeolites with different topologies were investigated using configurational bias Monte Carlo (CBMC) and MD simulations in this work. The adsorption capacity of zeolites at 353 K and 2.0 MPa relies on their pore volume with the order of FAU > BEA > LTL > MOR. The diffusion coefficient of C8 and C9 molecules increases with the decrease of loading and the increase of temperature, and the more compact molecules diffuse slower than the slimmer ones at low loadings while quicker at higher loadings. It was found that for zeolites, small adsorption heat of C8 isomers, or large molecular diffusion coefficient would correspond to a high selectivity of target alkylates. Particularly, BEA zeolite with large RPLD/LCD shows high reaction selectivity, rendering it a good candidate catalyst for C4 alkylation.

Flexible Zn-MOF with Rare Underlying scu Topology for Effective Separation of C6 Alkane Isomers.

Adsorptive separation by porous solids provides an energy-efficient alternative for the purification of important chemical species compared to energy-intensive distillations. Particularly, the separation of linear hexane isomers from its branched counterparts is crucial to produce premium grade gasoline with high research octane number (RON). Herein, we report the synthesis of a new, flexible zinc-based metal-organic framework, [Zn5(μ3-OH)2(adtb)2(H2O)5·5 DMA] (Zn-adtb), constructed from a butterfly shaped carboxylate linker with underlying (4,8)-connected scu topology capable of separating the C6 isomers nHEX, 3MP, and 23DMB. The sorbate-sorbent interactions and separation mechanisms were investigated and analyzed through in situ FTIR, solid state NMR measurements and computational modeling. These studies reveal that Zn-adtb discriminates the nHEX/3MP isomer pair through a kinetic separation mechanism and the nHEX/23DMB isomer pair through a molecular sieving mechanism. Column breakthrough measurements further demonstrate the efficient separation of linear nHEX from the mono- and dibranched isomers.

Virtual vibrational spectrometer for sp 2 carbon clusters. 2. Fullerene C60 and its isomers

The article presents the first consideration of virtual vibrational spectrometry of big molecules as a branch of the overall digitalization. The concept ability is illustrated by the example of fullerene C60. Connected in a single chain, digital twins, virtual devices and IT products related to the molecule and its isomers significantly enrich standard pool of molecular spectroscopic knowledge. Currently, the approach removes the problem of the C60 ‘silent’ vibrational modes providing a convincing interpretation of the fine structure of vibrational spectra and makes it possible to resolve the issue of the isolation of the only structure from numerous C60 isomers in practice. The practical implementation of the concept is provided by a virtual vibrational spectrometer HF Spectrodin exploiting semi-empirical Hartree–Fock approximation.

Regioisomers of magnesium(II) phthalocyanine bearing menthol substituents - Synthesis, spectral, electrochemical and computational studies

Three regioisomers of L-menthol-substituted magnesium(II) phthalocyanine were obtained and characterized. Nuclear magnetic resonance studies were used to unambiguously assign the regioisomers to be with C1, C2 and C4 symmetry, while D2 regioisomer was not isolated. Additionally, the crystal structure of the C1 isomer was determined. Maxima of the Q-band were observed in the range of 703–710 nm, quantum yields of fluorescence varied from 0.11 to 0.25, while quantum yields of singlet oxygen generation varied from 0.20 to 0.37. The phthalocyanine derivatives displayed, in the voltammetric study, two one-electron oxidations and two one-electron reductions, which were assigned to ring based one electron processes. DFT methods (B3LYP/6-31G) were used to calculate IR and UV–Vis spectra and optical band gap.

Torsion induced topological deformations in C60

Computational torsional deformation experiments were performed under the framework of density functional theory with C60 based on the anisotropy in the molecule inducing different topological defects. The effects of torsional deformation on spheroid cage and strain energy were investigated. Different isomers possessing different strain energies during the deformation experiments were noticed. Stable as well as reactive isomers of C60 which could withstand high pressures were identified including those induced with Stone-Wales defect.

Tuning the hexane isomer separation performances of Zeolitic Imidazole Framework-8 using mechanical pressure.

Hybrid osmotic Monte Carlo simulations were performed to anticipate the tunability of the separation performance of the flexible Zeolitic Imidazole Framework-8 (ZIF-8) via the application of an external mechanical pressure. This synergistic combination of mechanical control of the pore aperture/cage dimension and guest adsorption was applied to the challenging hexane isomers separation processes of vital importance in the field of petrochemical industry. The application of a mechanical pressure above 1 GPa was predicted to boost the linear hexane/2-methylpentane and 2-methylpentane/2,3-dimethylbutane selectivity by 40% and 17%, respectively, as compared to the pristine ZIF-8. We further unraveled the microscopic origin of this optimized performance with an in-depth analysis of the critical interplay between the structural changes of the ZIF-8 framework and the conformational rearrangements of C6 isomers under mechanical pressure.

Deactivation and regeneration of Pt‐modified zeolite Beta–Bindzil extrudates in n‐hexane hydroisomerization

AbstractBACKGROUNDThe metal–acid bifunctional catalysts are widely used in many industrially significant chemical processes, including hydroisomerization of n‐hexane. Deactivation and regeneration of metal–acid bifunctional extrudates was investigated in continuous n‐hexane hydroisomerization in a fixed‐bed reactor. Four Pt/H‐Beta‐25 catalysts containing 30% Bindzil binder were prepared with the same composition and controlled metal deposition.RESULTSDifferent preparation steps led to differences in the mechanical strength, Pt particle size, acidity and strength, metal‐to‐acid site (cPt/cAS) ratio and proximity between two types of active sites. A very slow deactivation and high selectivity to C6 isomers was obtained when Pt was deposited on H‐Beta‐25 zeolite.CONCLUSIONInitial selectivity to the desired products was correlated with the physicochemical properties of the catalysts. Location of the metal has a larger influence on catalyst deactivation of the bifunctional catalysts rather than the metal‐to‐acid site ratio or site proximity. Decline of n‐hexane conversion and selectivity to C6 isomers were correlated with changes in the textural properties and acidity of the deactivated catalyst, respectively. © 2021 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Crystal Flexibility Design through Local and Global Motility Cooperation.

Incorporating local mobility into a flexible framework promises to create cooperative properties unattainable in a conventional soft porous crystal. In this study, we propose a design strategy that integrates substituent moieties and a flexible porous crystal framework via intra-framework π-π interactions. This integration not only facilitates framework structural transitions but also gives the porous coordination polymers (PCPs) different guest-free structures that depend on the activation conditions. The incorporated flexibility gives the material the ability to discriminate C6 alkane isomers based on different gate-opening behaviors. Thus, the PCP has potential applications in C6 isomer separation, a critical step in the petroleum refining process to produce gasoline with high octane rating. This strategy, based on ligand designability, offers a new approach to flexible PCP structural and functional design.

Crystal Flexibility Design through Local and Global Motility Cooperation

AbstractIncorporating local mobility into a flexible framework promises to create cooperative properties unattainable in a conventional soft porous crystal. In this study, we propose a design strategy that integrates substituent moieties and a flexible porous crystal framework via intra‐framework π–π interactions. This integration not only facilitates framework structural transitions but also gives the porous coordination polymers (PCPs) different guest‐free structures that depend on the activation conditions. The incorporated flexibility gives the material the ability to discriminate C6 alkane isomers based on different gate‐opening behaviors. Thus, the PCP has potential applications in C6 isomer separation, a critical step in the petroleum refining process to produce gasoline with high octane rating. This strategy, based on ligand designability, offers a new approach to flexible PCP structural and functional design.