C6 Isomers Research Articles

Density-functional calculations of the nuclear magnetic shielding and indirect nuclear spin–spin coupling constants of three isomers of C20

The parameters of the nuclear magnetic resonance (NMR) spectrum – shielding constants and indirect spin–spin coupling constants – of three isomers of C20 are studied using density-functional theory. The performance of different exchange–correlation functionals is analysed by optimising the geometry for the ring, bowl and cage isomers, followed by a computation of the NMR constants at the optimised structure. The results are analysed and rationalised by performing comparisons of the three isomers with one another and with related systems such as polyynes (for the ring), o-benzyne (for the bowl) and C60 (for the cage). The shielding and spin–spin parameters calculated using the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional are sufficiently reliable to assist in future experimental NMR studies of C20 and, in particular, the identification of its isomers.

Intrinsic Chirality in Bare Gold Nanoclusters: The Au34− Case

In this work, we provide theoretical evidence on the existence of energetically stable chiral structures for bare gold clusters. Density functional theory calculations within the generalized-gradient approximation were performed to systematically study structural, vibrational, electronic, and optical properties of the lowest-lying isomers of the Au34Z, (Z = +1, 0, −1), clusters. Our results show that for the neutral and charged clusters, the lowest-energy isomer has a C1 (chiral) structure. In addition, a C3 (chiral) isomer was found nearly degenerate in energy with the C1 isomer. These results are in agreement with previous theoretical-experimental studies done for the Au34− cluster; however, because our calculated molecular scattering functions for the C1 and C3 isomers of this cluster are almost indistinguishable, it is concluded that the actual resolution in trapped ion electron diffraction experiments is not enough to discriminate between them. On the other hand, the electronic density of states of t...

Surface Tensions of Linear and Branched Alkanes from Monte Carlo Simulations Using the Anisotropic United Atom Model

The anisotropic united atoms (AUA4) model has been used for linear and branched alkanes to predict the surface tension as a function of temperature by Monte Carlo simulations. Simulations are carried out for n-alkanes ( n-C5, n-C6, n-C7, and n-C10) and for two branched C7 isomers (2,3-dimethylpentane and 2,4-dimethylpentane). Different operational expressions of the surface tension using both the thermodynamic and the mechanical definitions have been applied. The simulated surface tensions with the AUA4 model are found to be consistent within both definitions and in good agreement with experiments.

Synthesis of Macrocycle Isomers via Metathesis Cyclization and Their Self-Assembly from Aqueous Solutions

Novel triangular macrocycle isomers were synthesized through metathesis cyclization with high yield (77%). HPLC and MALDI-TOF showed that the purity of the macrocycles was higher than 99%, while (1)H NMR clearly showed that these macrocycles contain C2 and C3 isomers in a ratio of 1:3. AFM and TEM showed that they spontaneously formed vesicular structures in a chloroform/water system with an average diameter of 460 nm, which was corroborated by DLS results.

Two Ih-symmetry-breaking C60 isomers stabilized by chlorination

One abiding surprise in fullerene science is that I(h)-symmetric buckminsterfullerene C(60) (ref. 1) (I(h)-C(60) or (#1,812)C(60), the nomenclature specified by symmetry or by Fowler's spiral algorithm) remains the sole C(60) species experimentally available. Setting it apart from the other 1,811 topological isomers (isobuckminsterfullerenes) is its exclusive conformity with the isolated-pentagon rule, which states that stable fullerenes have isolated pentagons. Although gas-phase existence of isobuckminsterfullerenes has long been suspected, synthetic efforts have yet to yield successful results. Here, we report the realization of two isobuckminsterfullerenes by means of chlorination of the respective C(2v)- and C(s)-symmetric C(60) cages. These chlorinated species, (#1,809)C(60)Cl(8)(1) and (#1,804)C(60)Cl(12)(2), were isolated in experimentally useful yields. Structural characterization by crystallography unambiguously established the unique pentagon-pentagon ring fusions. These distinct structural features are directly responsible for the regioselectivity observed in subsequent substitution of chlorines, and also render these unprecedented derivatives of C(60) isomers important for resolving the long-standing puzzle of fullerene formation by the Stone-Wales transformation scheme.

Post-translational Modifications of Chicken Myelin Basic Protein Charge Components

Purified myelin basic protein (MBP) from various species contains several post-translationally modified forms termed charge components or charge isomers. Chicken MBP contains four charge components denoted as C1, C2, C3 and C8. (The C8 isomer is a complex mixture and was not investigated in this study.) These findings are in contrast to those found for human, bovine and other mammalian MBP's. Mammalian MBP's, each of which contain seven or eight charge components depending on the analysis of the CM-52 chromatographic curves and the PAGE gels obtained under basic pH conditions. Chicken MBP components C1, C2 and C3 were treated with trypsin and endoproteinase Glu-C. The resulting digests were analyzed by capillary liquid chromatography combined with either an ion trap tandem mass spectrometer or with a Fourier transform ion cyclotron resonance mass spectrometer. This instrumentation permitted establishing the amino acid composition and the determination of the post-translational modifications for each of the three charge components C1-C3. With the exception of N-terminal acetylation, the post-translational modifications were partial. The C1 component lacks any phosphorylated sites, a finding in agreement with the analysis of other MBP species. It also had a single methylation at R105 as did the components C2 and C3. The C2 component contains ten phosphorylated sites (S7, S18, S33, S64, S73, T96, S113, S141, S164, and S168), and modified arginine to citrulline residues at R24, and R165. Component C3 contains eight phosphorylated sites (S7, S33, S64, T96, S113, S141, S164, and S168), and citrulline residues at Arginine 41, R24 and R165. Partial deamidation of glutamine residues Q71, Q101 and Q146 were present in addition to asparagine N90 that was found in all three charge components. The glutamine at residue 3 is partially deamidated in isomers C1 and C2, whereas glutamine 74 and asparagine 83 were found not to be deamidated. Comparison of the PTM's of MBP's isolated from several vertebrate species reveals marked differences in their phosphate content. Chicken MBP does not share any phosphorylated sites with dogfish MBP; However, it does contain phosphorylated serine and threonine residues in common with mammalian MBP.

Synthesis and Structural Characterization of Four Isomers of C70(n‐C3F7)8

AbstractThe first four isomers of C70(n‐C3F7)8 synthesized by the ampoule reaction of C70 with n‐C3F7I, isolated by HPLC, and characterized by X‐ray crystallography possess unprecedented addition patterns among C70X8 compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Cu(II)-Catalyzed Direct and Site-Selective Arylation of Indoles Under Mild Conditions

We have developed a new site-selective Cu(II)-catalyzed C-H bond functionalization process that can selectively arylate indoles at either the C3 or C2 position under mild conditions. The scope of the arylation process is broad and tolerates broad functionality on both the indole and aryl unit, which makes it amenable to further elaboration. The mechanism of the arylation reaction is proposed to proceed via a Cu(III)-aryl species that undergoes initial electrophilic addition at the C3 position of the indole motif. We speculate that site of indole arylation arises through a migration of the Cu(III)-aryl group from C3 to C2, and this can be controlled by the nature of the group on the nitrogen atom; free (NH)- and N-alkylindoles deliver the C3-arylated product, whereas N-acetylindoles afford the C2 isomer, both with excellent yield and selectivity.

Determination of carbonyl compounds in the atmosphere of charcoal plants by HPLC and UV detection

A chromatographic quantification method with two different mobile phases (elution conditions 1 and 2) was developed to determine carbonyl compounds (CCs) in air samples collected from charcoal production workplaces, using C18 cartridges coated with 2,4-dinitrophenylhydrazine (DNPHi). Several 2,4-dinitrophenylhydrazones (DNPHo) were separated and quantified using an HPLC system and UV detection. In 16 min, elution condition 1 successfully separated and quantified the DNPHo of 14 CC including acetaldehyde, acrolein, formaldehyde, and furfural, and estimated the sum of C4 isomers, butanal-isobutanal-butanone. This elution condition was able to resolve the pairs acrolein/furfural and propanone/propanal, which have been cited in the literature as difficult mixtures to be separated. The elution condition 2 allowed separation and quantification, in less than 30 min, of 13 out of the 17 CC listed above. This elution condition was also able to separate propanone from propanal and butanone from the other components of the C4 mixture. When the two mobile phases were used together, they allowed confirmation of the presence of the DNPHo in the real samples. Thus, both elution conditions have been shown to be appropriate to determine CC, in personal and stationary samples, collected in charcoal production plants.

Modulating chemical reactivity using a photoresponsive molecular switch

The kinetics of an alkylation reaction are used to probe the effect of an electron-withdrawing pyridinium group on the nucleophilicity of a free pyridine in a photoresponsive dithienylcyclopentene (DTCP) derivative in order to demonstrate effective and reversible control over chemical reactivity. The kinetic data support the hypothesis that the ring-open isomer of the DTCP ( 1o) is more reactive than its ring-closed counterpart ( 1c) due to electronic communication between the two pyridine groups existing only in the latter isomer. The rates of the alkylation reactions of bis(pyridine) versions of the photochromic compounds are also evaluated to provide a better understanding of the through-bond and the through-space effects of the groups located at the ends of the linearly conjugated π-electron backbone. The use of the two DTCP isomers ( 1o and 1c) as nucleophilic catalysts is also suggested in preliminary investigations.

Dynamics and magnetic resonance properties of Sc3C2@C80 and its monoanion

We report density functional theory (DFT) studies on the endohedral scandium carbide fullerene Sc3C2@C80 and its monoanion [Sc3C2@C80](-). The system consisting of a Sc3C2 moiety inside the Ih C80 fullerene has been studied by using first principles molecular dynamics simulations at the DFT level. On the picosecond time scale, the triangle defined by the scandium atoms is seen to jump between orientations along the equatorial six-membered ring belt of the cage. The confined carbide unit, in turn, is engaged in a flipping motion through the Sc3 plane. In contrast to the equilibrium geometry optimisations using large basis sets that predict a trigonal bipyramidal structure, a planar Sc3C2-moiety is preferred during the finite-temperature simulation. In the molecular dynamics picture, Sc3C2@C80 is best described as an equilibrium between the two static minimum structures. Calculations of the vibrational frequencies show that the earlier predicted C2 and C2v symmetric isomers are in fact saddle points, with one imaginary normal mode frequency that is related to the flipping motion of the confined carbon dimer. Reoptimisation revealed two new minimum energy structures where the C2 unit is tilted with respect to its orientation in the earlier suggested higher-symmetry structures. The nature of the bonding in the static structures of the two isomers of Sc3C2@C80 has been investigated using the electron localisation function and natural population analysis. Some increased electron pair localisation is detected on the six-membered rings closest to the Sc atoms. 13C nuclear magnetic resonance (NMR) chemical shifts have been calculated for the closed-shell monoanion of Sc3C2@C80. The 13C shifts were also calculated for Sc2C2@C84, for further comparison to experimentally measured spectra. The confined carbon atoms are strongly deshielded in these metallofullerenes, implying an incorrect earlier interpretation of the experimental 13C NMR spectrum of Sc2C2@C84. The neutral Sc3C2@C80 system with one unpaired electron is further characterised by calculating the hyperfine coupling constants, the g tensor, as well as paramagnetic NMR (pNMR) 13C shifts for both static isomers. The chemical shifts of the confined carbon atoms and the hyperfine coupling constants of all the confined atoms are strongly dependent on the conformation of the Sc3C2 moiety. Consequently, dynamical effects are expected to be important in the modelling of the magnetic properties of endohedral scandium carbide fullerenes. The two low-lying isomers have rather different pNMR 13C shifts, implying the potential of this method in structural assignment.

1,3,7,10,14,17,21,28,31,42,52,55-Dodecakis(trifluoromethyl)- 1,3,7,10,14,17,21,28,31,42,52,55-dodecahydro(C60–Ih)[5,6]fullerene

The title compound, C72F36, is one of four isomers of C60(CF3)12 for which crystal structures have been obtained. The fullerene mol­ecule has an idealized Ih C60 core with the 12 CF3 groups arranged in an asymmetric fashion on two ribbons of edge-sharing C6(CF3)2 hexa­gons, a para–meta–para–para–para–meta–para ribbon and a para–meta–para ribbon, giving an overall pmp 3 mp,pmp structure. There are no cage Csp 3—Csp 3 bonds. The F atoms of two CF3 groups are disordered over two positions; the site occupancy factors are 0.85/0.15 and 0.73/0.27. There are intra­molecular F⋯F contacts between pairs of CF3 groups on the same hexa­gon that range from 2.521 (3) to 2.738 (4) Å.

1,4,10,19,25,41,55,60,67,69-Decakis(trifluoromethyl)-1,4,10,19,25,41,55,60,67,69-decahydro(C70-D5h)[5,6]fullerene

The title compound, C80F30, is one of seven isomers of C70(CF3)10 that have been isolated. The C2-symmetry fullerene mol­ecule has an idealized D5h C70 core with the ten CF3 groups arranged on a `pole-to-pole' para-meta-para-para-para-para-para-meta-para (pmp5mp) ribbon of edge-sharing C6(CF3)2 hexa­gons. There are no cage Csp3—Csp3 bonds. There are intra­molecular F⋯F contacts between pairs of CF3 groups on the same hexa­gon that range from 2.560 (3) to 2.876 (3) Å. Two CF3 groups are disordered with almost equal occupancies.

Thermal Operation of Four-Zone Simulated Moving Beds

Simulation results with Aspen Chromatography 2004 show that thermal four-zone simulated moving beds (SMBs) operated in the traveling wave mode (fluid is heated or cooled) can significantly improve the separation without radial heat transfer limitations. For toluene/p-xylene separation (linear isotherms) in a four-zone SMB, the average purity increased from 84.8% to 99.2% and from 92.0% to 99.99% with four heat exchangers for one and two columns per zone, respectively. The average purities increased from 84.8% to 93.2% and from 92.0% to 99.0% when cold feed and hot desorbent were used with no heat exchangers for one and two columns per zone, respectively. The additional dead volumes introduced by heat exchangers had an insignificant effect. For separation of p-xylene from other C8 isomers (nonlinear isotherms), a p-xylene purity of 99.9% and recovery of 98.6% were obtained with a thermal SMB with significantly fewer columns/sections than an isothermal SMB.

1,4,7,11,18,21,24,31,35,39,51,58,61,64-Tetradecakis(trifluoromethyl)-1,4,7,11,18,21,24,31,35,39,51,58,61,64-tetradecahydro(C70–D5h)[5,6]fullerenep-xylene trisolvate

The title compound, C84F42·3C8H10, features one of five isomers of C70(CF3)14 that have been isolated. The fullerene mol­ecule, which has crystallographic C2 symmetry, consists of an idealized D5h–C70 core with the 14 CF3 groups arranged in an asymmetric fashion on a para–para–para–para–para–para–para–meta–para (p7mp) ribbon and a para–meta–para (pmp) ribbon of edge-sharing C6(CF3)2 hexa­gons such that the two ribbons connect to one another, forming two 1,3-C5(CF3)2 penta­gons. There are no cage Csp3—Csp3 bonds. There are intra­molecular F⋯F contacts between pairs of CF3 groups on the same hexa­gon that range from 2.560 (3) to 2.876 (3) A.

Design and First-principles Study of a Fullerene Molecular Device

By using open-ended armchair (6, 6) single-wall carbon nanotubesas electrodes, we investigate the electron transportproperties of an all-carbon molecular junction based on the C82molecule. We find the most stable system among differentisomers by performing structural optimization calculations of theC82 isomers and the C82 extended molecules. Thecalculated results show that the C82–C2(3) isomer andthe C82 extended molecule with C82–C2v isomer aremost stable. For the all-carbon hybrid system consisting ofC82–C2v extended molecules, it is shown that theLandauer conductance can be tuned over several orders of magnitude bothby changing the distance between two electrodes and by changing theorientation of the C82 molecule or rotating one of the tubesaround the symmetry axis of the system at a fixed distance. Also, wefind the most stable distance between two electrodes from the totalenergy curve. This fact could make this all-carbon molecular system apossible candidate for a nanoelectronic switch. Moreover, we interpretthe conductance mechanism for such a molecular device.

1,6,11,16,18,24,27,36-Octakis(trifluoromethyl)-1,6,11,16,18,24,27,36-octahydro(C60–Ih)[5,6]fullerene deuterochloroform solvate

The title compound, C68F24·CDCl3, is a solvate of one of five isomers of C60(CF3)8 that have now been isolated. The fullerene mol­ecule has an idealized Ih C60 core with the eight CF3 groups arranged in an asymmetric fashion on a para–para–para–meta–para–meta–para ribbon of edge-sharing C6(CF3)2 hexa­gons. There are no cage Csp3—Csp3 bonds. There are intra­molecular F⋯F contacts between pairs of CF3 groups on the same hexa­gon, in the range 2.544 (2)–2.641 (2) A.

Photophysical Properties of C84 Major Isomers

Photophysical properties of C84 isomers have been measured separately and compared. Intrinsic triplet state lifetimes at room temperature are found to differ dramatically: 4.5 μs for the D2d(II) isomer, 125 μs for Cs(a), and 640 μs for D2(IV). The triplet lifetime of D2(IV) C84 represents the second longest found to date among fullerenes. Measurements between 77 and 320 K reveal that the triplet decay kinetics of D2d(II) is largely temperature-independent, whereas thermally activated channels dominate triplet decay in the D2(IV) isomer. Oxygen-quenching studies suggest that the T1 energy of D2d(II) lies near 7600 cm-1 but that of D2(IV) exceeds 8000 cm-1. These results highlight the importance of isomeric separation in exploring the rich photophysics of higher fullerenes.

Multicomponent sorption of hexane isomers in zeolite BETA

AbstractBreakthrough curves of single, binary, ternary, and quaternary mixtures of hexane (C6) isomers n‐hexane (nHEX), 3‐methylpentane (3MP), 2,3‐dimethylbutane (23DMB), and 2,2‐dimethylbutane (22DMB) were performed in commercial pellets of zeolite BETA (BEA structure), covering the temperature range between 423 and 523 K and partial pressures up to 30 kPa. From these data, single and multicomponent adsorption equilibrium isotherms were collected. A tri‐site Langmuir model (TSL) was developed to interpret the equilibrium data based on considerations about zeolite structure, and a dynamic adsorption model was tested predicting with a good accuracy the behavior of multicomponent fixed‐bed experiments. At the partial pressures studied, the sorption hierarchy in the zeolite BETA is nHEX >>> 3MP > 23DMB >> 22DMB. BEA structure demonstrates a significant selectivity between C6 isomers, especially at low coverage, giving a good perspective regarding their separation by adsorption processes. © 2007 American Institute of Chemical Engineers AIChE J, 2007

1,6,11,18,24,27,33,51,54,60-Decakis(trifluoromethyl)-1,6,11,18,24,27,33,51,54,60-decahydro(C60-Ih)[5,6]fullerene

The title compound, C70F30, is one of six isomers of C60(CF3)10 that have now been isolated. The fullerene mol­ecule has an idealized Ih C60 core with the ten CF3 groups arranged in an asymmetric fashion on two ribbons of edge-sharing C6(CF3)2 hexa­gons, a para–para–para–meta–para ribbon and a para–meta–para ribbon, giving an overall p3mp,pmp structure. There are no cage Csp3—Csp3 bonds. There are intra­molecular F⋯F contacts between pairs of CF3 groups on the same hexa­gon that range from 2.568 (3) to 2.682 (3) A.