Effects on the aromatic character of DNA/RNA nucleobases due to its adsorption onto graphene

In the last years, experimental/theoretical studies have shown that graphene has a strong affinity toward nucleobases, serving as a promising nanomaterial for self‐assembly, sensing, and/or sequencing of DNA/RNA constituents. Then, a complete picture of the properties of the nucleobase–graphene systems is required for its use in technological applications. This work describes a detailed quantum chemical analysis of the aromaticity of adsorbed nucleobases on graphene, comparing between aromaticity indexes based on magnetic, geometry, electron density, and electron delocalization properties of graphene–nucleobase systems. Contrary to the stated by magnetic‐based aromaticity criteria (such as nucleus‐independent chemical shifts), it is proved that the aromatic character of nucleobases is not increased/decreased upon binding on graphene. Therefore, magnetic aromaticity criteria are not recommended to analyze aromaticity in related systems, unless a fragmented scheme be adopted. Finally, these results are expected to expand the knowledge about the understanding of biomolecules‐graphene interactions.     

A comparison of approaches to estimate the resonance energy

We discuss Ab Initio approaches to calculate the energy lowering (stabilisation) due to aromaticity. We compare the valence bond method and the block-localised wave function approaches to calculate the resonance energy. We conclude that the valence bond approach employs a Pauling–Wheland resonance energy and that the block-localised approach employs a delocalisation criterion. The latter is shown to be more basis set dependent in a series of illustrative calculations.     

Quenching of local magnetic moment in oxygen adsorbed graphene nanoribbons

The electronic and magnetic properties of oxidized zigzag and armchair graphene nanoribbons, with hydrogen passivated edges, have been investigated from ab initio pseudopotential calculations within the density functional scheme. The oxygen molecule in its triplet state is adsorbed most stably at the edge of a zigzag nanoribbon. The Stoner metallic behavior of the ferromagnetic nanoribbons and the Slater insulating (ground state) behavior of the antiferromagnetic nanoribbons remain intact upon oxygen adsorption. The formation of a spin-paired C-O bond drastically reduces the local atomic magnetic moment of carbon at the edge of the ferromagnetic zigzag ribbon.     

Rational design of three-dimensional nitrogen and phosphorus co-doped graphene nanoribbons/CNTs composite for the oxygen reduction

In the present work, we report nitrogen and phosphorus co-doped 3-D structured carbon nanotube intercalated graphene nanoribbon composite. The graphene nanoribbons are prepared via partial exfoliation of multi-walled carbon nanotubes. In the graphene nanoribbons/CNTs composite, carbon nanotubes play a role of skeleton and support the exfoliated graphene nanoribbons to form the stereo structure. After high temperature heat-treatment with ammonium dihydrogen phosphate, the unique structure reserves both the properties of carbon nanotube and graphene, exhibiting excellent catalytic performance for the ORR with excellent onset and half-wave potential, which is similar to commercial Pt/C electrocatalysts.     

Pyrido-annellated diazaphospholenes and phospholenium ions

Pyrido-annulated 1,3,2-diazaphospholenium ions and P-bis(trimethylsilyl)amino substituted 1,3,2-diazaphospholenes were synthesised and characterised by spectroscopic methods and single-crystal X-ray diffraction studies. The distribution of bond distances provided evidence for pi-electron delocalisation in the fused ring system. This hypothesis was confirmed by calculations of magnetic (NICS, nucleus independent chemical shift) and geometrical (Bird index, bond shortening index) aromaticity indexes which attest aromatic character of the five-membered rings in the cations that is lost upon introduction of a substituent at the phosphorus atom. Computation of isodesmic reaction energies suggest that the reclamation of aromatic stabilisation energies in the products facilitates reaction of P-amino-subsituted annulated diazaphospholenes under cleavage of the fused ring system.     

Geometry-dependent electrochemistry of graphene oxide family

The influence of graphene oxide geometry on electrochemical performance is of great interest, but there are few reports on this subject. Three different members of the graphene oxide family, graphene oxide nanosheets, graphene oxide nanoribbons, and graphene oxide quantum dots were comparatively investigated as electrode materials to systematically study the effect of geometric structure. The results showed that, as the geometric structure varies, the three graphene oxide materials possess different electrical conductivities, various defect densities and oxygen contents, as well as diverse electrode surface chemistry and microstructures, which combine together to result in the distinct electrochemical responses for the modified electrodes, depending on the redox system involved. This work broadens the method of studying the electrochemical performance of many other materials from the perspective of geometry.     

How π-electron delocalisation reflects replacement of H+ with Li+ in variously substituted malonaldehydes

Correlations were studied between the properties of Bader's ring and bond critical points calculated for variously substituted malonaldehyde (containing intramolecular H-bond) and its π-electron delocalisation expressed by aromaticity indices HOMA and NICSs. It was observed that π-electron delocalisation of the system strongly depends on the substituent and its position. Replacement of the H⁺ with Li⁺ in malonaldehyde allowed us to study the role of unoccupied 2p orbital in π-electron delocalisation. In the case of lithium system the aromaticity is increased as compared with the malonaldehyde itself and moreover with the malonaldehyde anion. This proves that the unoccupied 2p orbital of Li⁺ may play a significant role in delocalisation of π-electrons due to its low orbital energy. In the case of lithium system the substituent effect is negligible, which resembles the situation in benzene derivatives. Dedicated to Prof. Karl Jug on the occasion of his 65th birthday anniversary.     

Cover picture: transition-metal-catalyzed unzipping of single-walled carbon nanotubes into narrow graphene nanoribbons at low temperature (angew. Chem. Int. Ed. 35/2011)

Narrow, smooth-edged graphene nanoribbons are needed for graphene electronics to replace the current silicon technology. In their Communication on page?8041?ff., J. Wang, F. Ding, et?al. report a smart strategy for cutting single-walled carbon nanotubes (gray) into narrow graphene nanoribbons in H(2) gas (green) with a single transition-metal atom (Cu, red) as the chemical scissors.     

Marked adsorption irreversibility of graphitic nanoribbons for CO2 and H2O

Graphene and graphitic nanoribbons possess different types of carbon hybridizations exhibiting different chemical activity. In particular, the basal plane of the honeycomb lattice of nanoribbons consisting of sp(2)-hybridized carbon atoms is chemically inert. Interestingly, their bare edges could be more reactive as a result of the presence of extra unpaired electrons, and for multilayer graphene nanoribbons, the presence of terraces and ripples could introduce additional chemical activity. In this study, a remarkable irreversibility in adsorption of CO(2) and H(2)O on graphitic nanoribbons was observed at ambient temperature, which is distinctly different from the behavior of nanoporous carbon and carbon blacks. We also noted that N(2) molecules strongly interact with the basal planes at 77 K in comparison with edges. The irreversible adsorptions of both CO(2) and H(2)O are due to the large number of sp(3)-hybridized carbon atoms located at the edges. The observed irreversible adsorptivity of the edge surfaces of graphitic nanoribbons for CO(2) and H(2)O indicates a high potential in the fabrication of novel types of catalysts and highly selective gas sensors.     

Retaining Hückel Aromaticity in the Triplet Excited State of Azobenzene

The implication of the potential concept of aromaticity in the relaxed lowest triplet state of azobenzene, an efficient molecular switch, using elementary aromaticity indices based on magnetic, electronic, and geometric criteria has been discussed. Azobenzene exhibits a major Hückel aromatic character retained in the diradical lowest relaxed triplet state (T₁) by virtue of a twisted geometry with partial delocalization of unpaired electrons in the perpendicular p-orbitals of two nitrogen atoms to the corresponding phenyl rings. The computational analysis has been expanded further to stilbene and N-diphenylmethanimine for an extensive understanding of the effect of closed-shell Hückel aromaticity in double-bond-linked phenyl rings. Our analysis concluded that stilbene has Hückel aromatic character in the relaxed T₁ state and N-diphenylmethanimine has a considerable Hückel aromaticity in the phenyl ring near the carbon atom while a paramount Baird aromaticity in the phenyl ring near the nitrogen atom of the C=N double bond. The results reveal the application of excited-state aromaticity as a general tool for the design of molecular switches.     

Bond energy, aromatic stabilization energy and strain in IPR fullerenes

Various models applied to DFT structures and energies of 2-D and 3-D aromatic molecules shed new light on the effects of strain and aromaticity in these systems. The cyclic pi electron delocalisation does not stabilize the fullerene C60 formation; and 5-6 and 6-6 CC bonds have near-identical bond stretch potentials.     

Analytical carbon-oxygen reactive potential

We present a reactive empirical potential with environment-dependent bond strengths for the carbon-oxygen (CO) system. The distinct feature of the potential is the use of three adjustable parameters characterizing the bond: the strength, length, and force constant, rather than a single bond order parameter, as often employed in these types of potentials. The values of the parameters are calculated by fitting results obtained from density functional theory. The potential is tested in a simulation of oxidative unzipping of graphene sheets and carbon nanotubes. Previous higher-level theoretical predictions of graphene unzipping by adsorbed oxygen atoms are confirmed. Moreover, nanotubes with externally placed oxygen atoms are found to unzip much faster than flat graphene sheets.     

Electronic and magnetic properties of armchair and zigzag graphene nanoribbons

The electronic properties, band gap, and ionization potential of zigzag and armchair graphene nanoribbons are calculated as a function of the number of carbon atoms in the ribbon employing density functional theory at the B3LYP6-31G* level. In armchair ribbons, the ionization potential and band gap show a gradual decrease with length. For zigzag ribbons, the dependence of the band gap and ionization potential on ribbon length is different depending on whether the ribbon has an unpaired electron or not. It is also found that boron and nitrogen zigzag and armchair doped graphene nanoribbons have a triplet ground state and could be ferromagnetic.     

Communication: A dramatic transition from nonferromagnet to ferromagnet in finite fused-azulene chain

One-dimensional fused-azulene oligomers (n = 2-6) are studied with the effective valence bond as well as density functional theory methods. A nonferromagnetic (closed-shell singlet) to ferromagnetic (triplet) ground state transformation is witnessed with increasing length of oligomers. The computational results are interpreted in terms of spin coupling between the unpaired electrons of two nonbonding molecular orbitals localized, respectively, on the top and bottom chains of the oligomers. The present study provides a theoretical suggestion for understanding the ferromagnetic spin polarizations that has been observed very recently in graphene nanoribbons.     

Aromaticity‐promoted C−F Bond Activation in Rhodium Complex: A Facile Tautomerization

Fluorine is the most electronegative element in the periodic table. Thus, activation of the carbon–fluorine (C?F) bond, the strongest single bond to carbon, has attracted considerable interest from both experimentalists and theoreticians. In comparison with numerous approaches to activate C?F bonds, the aromaticity‐promoted method is less developed. Herein, we demonstrate that the C?F bond activation could be achieved by a facile tautomerization, benefitting from aromaticity, which can stabilize both the transition states and products. Our findings highlight an important application of aromaticity in the C?F bond activation, providing experimentalists with an alternative approach to activate C?F bonds.     

On heteroaromaticity of nucleobases. Bond lengths as multidimensional phenomena

Three hundred and nine carbon-carbon, carbon-nitrogen, and carbon-oxygen pi-bond lengths in high precision crystal structures of 31 purine and pyrimidine nucleobases were related to the Pauling pi-bond order, its analogues corrected to crystal packing effects, the numbers of non-hydrogen atoms around the bond, and the sum of atomic numbers of the bond atoms. Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) demonstrated that the bond lengths in the nucleobases are three-dimensional phenomenon, characterized by nine distinct classes of bonds. Bond lengths predicted by Linear Regression models, Pauling Harmonic Potential Curves, Multiple Linear Regression, Principal Component, and Partial Least Squares Regression were compared to those calculated by molecular mechanics, semiempirical, and ab initio methods using PCA-HCA procedure on the calculated bond lengths, statistical parameters, and structural aromaticity indices. Incorporation of crystal packing effects into bond orders makes multivariate models to be competitive to semiempirical results, while further improvement of quantum chemical calculations can be achieved by geometry optimization of molecular clusters.