Electronic Structure of Tubular Aromatic Molecules Derived from the Metallic (5,5) Armchair Single Wall Carbon Nanotube

Electronic Structure of Pentagonal Carbon Nanocones: An ab Initio Study

In this work, we investigate the electronic structure of a particular class of carbon nanocones having a pentagonal tip and C5v symmetry. The ground-state nature of the wave function for these structures can be predicted by the recently proposed generalized Hückel rule that extends the original Hückel rule for annulenes to this class of carbon nanocones. In particular, the structures here considered can be classified as closed-shell or anionic/cationic closed-shells, depending on the geometric characteristics of the cone. The goal of this work is to assess the relationship between the electronic configuration of these carbon nanocones and their ability to gain or lose an electron as well as their adsorption capability. For this, the geometry of these structures in the neutral or ionic forms, as well as systems containing either one lithium or fluorine atom, was optimized at the DFT/B3LYP level. It was found that the electron affinity, ionization potential, and the Li or F adsorption energy present an intimate connection to the ground-state wave function character predicted by the generalized Hückel rule. In fact, a peculiar oscillatory energy behavior was discovered, in which the electron affinity, ionization energy, and adsorption energies oscillate with an increase in the nanocone size. The reasoning behind this is that if the anion is closed-shell, then the neutral nanocone will turn out to be a good electron acceptor, increasing the electron affinity and lithium adsorption energy. On the other hand, in the case of a closed-shell cation, this means that the neutral nanocone will easily lose an electron, leading to a smaller ionization potential and higher fluorine adsorption energy.

Computational Modeling of Gold Nanoparticle Interacting with Molecules of Pharmaceutical Interest in Water

We derived a theory of biomolecule binding to the surface of Aun clusters and of the Au plane based on the hard soft acid base (HSAB) principle and the free electron metallic surface model. With the use of quantum mechanical calculations, the chemical potential (μ) and the chemical hardness (η) of the biomolecules are estimated. The effect of the gold is introduced via the empirical value of the gold chemical potential (-5.77 eV) as well as by using the expression (modified here) for the chemical hardness (η). The effect of an aqueous environment is introduced by means of the ligand molecular geometry influenced by the PCM field. This theory allows for a fast and low-cost estimation of binding biomolecules to the AuNPs surface. The predicted binding of thiolated genistein and abiraterone to the gold surface is about 20 kcal/mol. The model of the exchange reaction between these biomolecules and citrates on the Au surface corresponds well with the experimental observations for thiolated abiraterone. Moreover, using a model of the place exchange of linear mercaptohydrocarbons on 12-mercaptododecane acid methyl ester bound to the Au surface, the present results reflect the known relation between exchange energy and the size of the reagents.

Structural, electronic and nonlinear optical properties, reactivity and solubility of the drug dihydroartemisinin functionalized on the carbon nanotube

Density functional theory (DFT) calculations of the antimalarial drug dihydroartemisinin (DHA) functionalized on the carbon nanotube (CNT) were carried out in gas phase and in solution to investigate the role of fCNTs as a nanovector for the targeted delivery of the DHA drug and to predict their chemical descriptors and electronic and nonlinear optical (NLO) properties. The results of the geometric optimization indicate that the functionalization does not change the molecular structure of DHA. Based on our findings of binding and solvation energies, two energetically stable configurations were identified in 1st (fCNT1-2) and 2nd (2fCNT1-2) functionalization. For these stable configurations, the energy gap value goes from 1.52 eV for the (5,5) single wall pristine CNT to 1.27 eV for the 1st functionalization and to 1.06 eV for the 2nd functionalization regardless of the considered media; which gives these nanostructures excellent semiconductor properties. Findings from global reactivity descriptors show that the reactivity of the functionalized CNT is strongly improved in solvent media and that the stability of DHA decreases while its reactivity increases during the functionalization. Thus, the fundamental gap (E_f) in gas phase decreases from 3.65 eV for the virgin CNT to 3.30 eV for fCNT2 and to 3.02 eV for 2fCNT2. On the contrary, in water E_f goes from 1.20 eV for the virgin CNT to 0.95 eV for fCNT2 and to 0.74 eV for 2fCNT2; demonstrating an improvement in the reactivity of our fCNTs as nanovectors for targeted delivery of DHA drug. Finally, our findings show that these nanostructures may also have good NLO properties and can be promising materials for NLO applications.

Theoretical Study on the Electro-Reduction of Carbon Dioxide to Methanol Catalyzed by Cobalt Phthalocyanine

Density functional theory (DFT) calculations have been conducted to investigate the mechanism of cobalt(II) tetraamino phthalocyanine (CoPc-NH₂) catalyzed electro-reduction of CO₂. Computational results show that the catalytically active species 1 (⁴[Co^II(H₄L)]⁰) is formed by a four-electron-four-proton reduction of the initial catalyst CoPc-NH₂. Complex 1 can attack CO₂ after a one-electron reduction to give a [Co^III-CO₂^2-]^- intermediate, followed by a protonation and a one-electron reduction to give intermediate [Co^II-COOH]^- (4). Complex 4 is then protonated on its hydroxyl group by a carbonic acid to generate the critical species 6 (Co^III-L^•--CO), which can release the carbon monoxide as an intermediate (and also as a product). In parallel, complex 6 can go through a successive four-electron-four-proton reduction to produce the targeted product methanol without forming formaldehyde as an intermediate product. The high-lying π orbital and the low-lying π* orbital of the phthalocyanine endow the redox noninnocent nature of the ligand, which could be a dianion, a radical monoanion, or a radical trianion during the catalysis. The calculated results for the hydrogen evolution reaction indicate a higher energy barrier than the carbon dioxide reduction. This is consistent with the product distribution in the experiments. Additionally, the amino group on the phthalocyanine ligand was found to have a minor effect on the barriers of critical steps, and this accounts for the experimentally observed similar activity for these two catalysts, namely, CoPc-NH₂ and CoPc.

Sorting and decoration of semiconducting single-walled carbon nanotubes via the quaternization reaction

A study for the selective separation and functionalization of alcohol-soluble semiconducting single-walled carbon nanotubes (sc-SWCNTs) is carried out by polymer main-chain engineering. Introducing tertiary amine groups endows the functionalized sc-SWCNTs with alcohol-soluble properties and introducing the pyrimidine rings allows to increase the selective purity of sc-SWCNTs. In this study, a series of poly[(9,9-dioctylfluorene)-2,7-(9,9-bis(3'-(N,N-dimethylamino)propyl)-fluorene)] m -alt-[2-methylpyrimidine-2,7-(9,9-dioctylfluorene)] n (PFPy) are used for the selective dispersion of semiconducting single-walled carbon nanotubes, where n and m are the composition ratio of the copolymer. When m = n, the effective isolation of sc-SWCNTs with purity greater than 99% is achieved. The alcohol-soluble sc-SWCNTs with a diameter in the range of 1.1-1.4 nm are obtained through designing reasonable molecular structure. Moreover, the particular preference of PFPy (m = n) for sc-SWCNTs was studied via density functional theory (DFT) calculations and it was proved to be a promising method for the separation and functionalization of sc-SWCNTs.

Enhancing hydrogen evolution activity by doping and tuning the curvature of manganese-embedded carbon nanotubes

Doping heteroatoms (Mn and N) and tuning the curvature of carbon nanotubes could efficiently elevate the C p-band center, lower the absolute electrode potential, and thus enhance the HER performance.

Regioselective multistep reconstructions of half-saturated zigzag carbon nanotubes

The open edge reconstruction of half-saturated (6,0) zigzag carbon nanotube (CNT) was introduced by density functional calculations. The multistep rearrangement was demonstrated as a regioselective process to generate a defective edge with alternating pentagons and heptagons. Not only the thermal stability was found to be enhanced significantly after reconstruction but also the total spin of CNT was proved to be reduced gradually from high-spin septet to close-shell singlet, revealing the critical role of deformed edge on the geometrical and magnetic properties of open-ended CNTs. Kinetically, the initial transformation was confirmed as the rate-determining step with relatively the largest reaction barrier and the following steps can take place spontaneously. © 2016 Wiley Periodicals, Inc.

Calculation of Raman parameters of real-size zigzag (n, 0) single-walled carbon nanotubes using finite-size models

Structural and selected Raman features of real-size single-walled carbon nanotubes (SWCNTs) were studied using finite-size pristine SWCNT models at the DFT level.

Si-Doped single-walled carbon nanotubes interacting with isoniazid-a density functional and molecular docking study

The interaction of antitubercular drug isoniazid (INH), with pristine and Si-doped (5,5) and (9,0) single-walled carbon nanotubes (SWNTs) have been reported.

Self-Trapping of Charge Carriers in Semiconducting Carbon Nanotubes: Structural Analysis

The spatial extent of charged electronic states in semiconducting carbon nanotubes with indices (6,5) and (7,6) was evaluated using density functional theory. It was observed that electrons and holes self-trap along the nanotube axis on length scales of about 4 and 8 nm, respectively, which localize cations and anions on comparable length scales. Self-trapping is accompanied by local structural distortions showing periodic bond-length alternation. The average lengthening (shortening) of the bonds for anions (cations) is expected to shift the G-mode frequency to lower (higher) values. The smaller-diameter nanotube has reduced structural relaxation due to higher carbon-carbon bond strain. The reorganization energy due to charge-induced deformations in both nanotubes is found to be in the 30-60 meV range. Our results represent the first theoretical simulation of self-trapping of charge carriers in semiconducting nanotubes, and agree with available experimental data.

Size, dimensionality, and strong electron correlation in nanoscience

In electronic structure theory, electron-electron repulsion is normally considered only in an average (or mean field) sense, for example, in a single Hartree-Fock determinant. This is the simple molecular orbital model, which is often a good approximation for molecules. In infinite systems, this averaging treatment leads to delocalized electronic bands, an excellent description of bulk 3D sp(3) semiconductors. However, in reality electrons try to instantaneously avoid each other; their relative motion is correlated. Strong electron-electron repulsion and correlation create new collective states and cause new femtosecond kinetic processes. This is especially true in 1D and 2D systems. The quantum size effect, a single electron property, is widely known: the band gap increases with decreasing size. This Account focuses on the experimental consequences of strong correlation. We first describe π-π* excited states in carbon nanotubes (CNTs). To obtain the spectra of individual CNTs, we developed a white-light, right-angle resonant Rayleigh scattering method. Discrete exciton transitions dominate the optical absorption spectra of both semiconducting and metallic tubes. Excitons are bound neutral excited states in which the electron and hole tightly orbit each other due to their mutual Coulomb attraction. We then describe more generally the independent roles of size and dimensionality in nanoelectronic structure, using additional examples from graphene, trans-polyacetylene chains, transition metal dichalcogenides, organic/inorganic Pb iodide perovskites, quantum dots, and pentacene van der Waals crystals. In 1D and 2D chemical systems, the electronic band structure diagram can be a poor predictor of properties if explicit correlation is not considered. One- and two-dimensional systems show quantum confinement and especially strong correlation as compared with their 3D parent systems. The Coulomb interaction is enhanced because the electrons are on the surface. One- and two-dimensional systems can exhibit essentially molecular properties even though they are infinite in size. Zero-dimensional Qdots show quantum confinement and modest electron correlation. Correlation is weak in 3D bulk semiconductors. Strongly correlated electronic states can behave as if they have fractional charge and effectively separate the spin and charge of the electron. This is apparent in the "soliton" state of polyacetylene, the fractional charge quantum Hall state of graphene, and the Luttinger electrical conductivity of metallic CNTs.

Potentiometric, electronic, and transient absorptive spectroscopic properties of oxidized single-walled carbon nanotubes helically wrapped by ionic, semiconducting polymers in aqueous and organic media

We report the first direct cyclic voltammetric determination of the valence and conduction band energy levels for noncovalently modified (6,5) chirality enriched SWNTs [(6,5) SWNTs] in which an aryleneethynylene polymer monolayer helically wraps the nanotube surface at periodic and constant morphology. Potentiometric properties as well as the steady-state and transient absorption spectroscopic signatures of oxidized (6,5) SWNTs were probed as a function of the electronic structure of the aryleneethynylene polymer that helically wraps the nanotube surface, the solvent dielectric, and nanotube hole polaron concentration. These data: (i) highlight the utility of these polymer-SWNT superstructures in experiments that establish the potentiometric valence and conduction band energy levels of semiconducting carbon nanotubes; (ii) provide a direct measure of the (6,5) SWNT hole polaron delocalization length (2.75 nm); (iii) determine steady-state and transient electronic absorptive spectroscopic signatures that are uniquely associated with the (6,5) SWNT hole polaron state; and (iv) demonstrate that modulation of semiconducting polymer frontier orbital energy levels can drive spectral shifts of SWNT hole polaron transitions as well as regulate SWNT valence and conduction band energies.

Vibrational Excitation in Electron Transport through Carbon Nanotube Quantum Dots

Electron transport in single-walled carbon nanotubes (SWCNTs) is extremely sensitive to environmental effects. SWCNTs experiencing an inhomogeneous environment are effectively subjected to a disorder potential, which can lead to localized electronic states. An important element of the physical picture of such states localized on the nanometer-scale is the existence of a local vibronic mainfold resulting from the localization-enhanced electron-vibrational coupling. In this Letter, scanning tunneling spectroscopy (STS) is used to study the quantum-confined electronic states in SWCNTs deposited on the Au(111) surface. STS spectra show the vibrational overtones identified as D-band Kekulé vibrational modes and K-point transverse out-of plane phonons. The presence of these vibrational modes in the STS spectra suggests rippling distortion and dimerization of carbon atoms on the SWCNT surface. The present study thus, for the first time, experimentally connects the properties of well-defined localized electronic states to the properties of their associated vibronic states.

Probing the chemical functionalization of single-walled carbon nanotubes with multiple carbon ad-dimer defects

Drying-tube-shaped single-walled carbon nanotubes (SWCNTs) with multiple carbon ad-dimer (CD) defects are obtained from armchair (n,n,m) SWCNTs (n=4, 5, 6, 7, 8; m=7, 13). According to the isolated-pentagon rule (IPR) the drying-tube-shaped SWCNTs are unstable non-IPR species, and their hydrogenated, fluorinated, and chlorinated derivatives are investigated. Interestingly, chemisorptions of hydrogen, fluorine, and chlorine atoms on the drying tube-shaped SWCNTs are exothermic processes. Compared to the reaction energies for binding of H, F, and Cl atoms to perfect and Stone-Wales-defective armchair (5,5) nanotubes, binding of F with the multiply CD defective SWCNTs is stronger than with perfect and Stone-Wales-defective nanotubes. The reaction energy for per F(2) addition is between 85 and 88 kcal mol(-1) more negative than that per H(2) addition. Electronic structure analysis of their energy gaps shows that the CD defects have a tendency to decrease the energy gap from 1.98-2.52 to 0.80-1.17 eV. After hydrogenation, fluorination, and chlorination, the energy gaps of the drying-tube-shaped SWCNTs with multiple CD defects are substantially increased to 1.65-3.85 eV. Furthermore, analyses of thermodynamic stability and nucleus-independent chemical shifts (NICS) are performed to analyze the stability of these molecules.

OH-functionalized open-ended armchair single-wall carbon nanotubes (SWCNT) studied by density functional theory

The structures of ideal armchair (5,5) single-wall carbon nanotubes (SWCNTs) of different lengths (3.7, 8.8, and 16.0 Å for C(40)H(20), C(80)H(20), and C(140)H(20)) and with 1-10 hydroxyl groups at the end of the nanotube were fully optimized at the B3LYP/3-21G level, and in some cases at the B3LYP/6-31G level, and the energy associated with the attachment of the OH substituent was determined. The OH-group attachment energy was compared with the OH functionalization of phenanthrene and picene models and with previous results for zigzag (9.0) SWCNT systems. In comparison to zigzag SWCNTs, the armchair form is more (by about 5 to 10 kcal mol(-1)) reactive toward hydroxylation.

Effect of Peierls transition in armchair carbon nanotube on dynamical behaviour of encapsulated fullerene

The changes of dynamical behaviour of a single fullerene molecule inside an armchair carbon nanotube caused by the structural Peierls transition in the nanotube are considered. The structures of the smallest C20 and Fe@C20 fullerenes are computed using the spin-polarized density functional theory. Significant changes of the barriers for motion along the nanotube axis and rotation of these fullerenes inside the (8,8) nanotube are found at the Peierls transition. It is shown that the coefficients of translational and rotational diffusions of these fullerenes inside the nanotube change by several orders of magnitude. The possibility of inverse orientational melting, i.e. with a decrease of temperature, for the systems under consideration is predicted.

Optoelectronic Properties of Carbon Nanorings: Excitonic Effects from Time-Dependent Density Functional Theory

The electronic structure and size-scaling of optoelectronic properties in cycloparaphenylene carbon nanorings are investigated using time-dependent density functional theory (TDDFT). The TDDFT calculations on these molecular nanostructures indicate that the lowest excitation energy surprisingly becomes larger as the carbon nanoring size is increased, in contradiction with typical quantum confinement effects. In order to understand their unusual electronic properties, I performed an extensive investigation of excitonic effects by analyzing electron-hole transition density matrices and exciton binding energies as a function of size in these nanoring systems. The transition density matrices allow a global view of electronic coherence during an electronic excitation, and the exciton binding energies give a quantitative measure of electron-hole interaction energies in the nanorings. Based on overall trends in exciton binding energies and their spatial delocalization, I find that excitonic effects play a vital role in understanding the unique photoinduced dynamics in these carbon nanoring systems.

Structural determination of large molecules through the reassembly of optimized fragments

The accurate determination of the optimized structures of large molecules is, computationally quite expensive. As an alternate to the conventional approaches to structural optimization, we have explored the accuracy and speed-up obtained when variants of the fragmentation optimization and recombination method (FORM) are used. Specifically, the method was applied to eight prototypical molecules -n-decane, hexa-alanine, a long conjugate hydrocarbon molecule, a large polar conjugated molecule, large (5,5) armchair single-walled carbon nanotubes, a salen-aluminum complex and a multiply H-bonded system (two conformers of vancomycin aglycon with Di-N-acetyl-l-Lys-d-Ala-d-Ala - without optimizing the structure of the whole molecules. We find that FORM can predict the structure of these molecules with an acceptable accuracy, all at a computational cost that is 2-11 times less than conventional quantum mechanical methods at the Hartree-Fock (HF), density functional theory (DFT) and MP2 level of accuracy. FORM may therefore represent a viable approach for the fast structural predictions of large molecules.

Finite-length models of carbon nanotubes based on Clar sextet theory

Finite-length models of metallic and semiconducting carbon nanotubes (CNTs) based on Clar sextet theory of aromatic systems are proposed. For metallic CNTs, the electronic properties of finite-length models converge monotonically to the values expected for quasi-monodimensional metallic systems. For semiconducting CNTs, the use of finite-length models as proposed in this work leads to a fast convergence of the electronic properties to the values expected for the corresponding infinite-length nanotube.

Nanoscale fracture mechanics

Theoretical calculations on undefected nanoscale materials predict impressive mechanical properties. In this review we summarize the status of experimental efforts to directly measure the fracture strengths of inorganic and carbon nanotubes and discuss possible explanations for the deviations between the predicted and observed values. We also summarize the role of theory in understanding the molecular-level origin of these deviations. In particular, we consider the role of defects such as vacancies, Stone-Wales defects, adatoms and ad-dimers, chemical functionalization, and oxidative pitting.

Local Modifications of Single-Wall Carbon Nanotubes Induced by Bond Formation with Encapsulated Fullerenes

Defected fullerenes in nanopeapods form bonds with the encapsulating single-walled carbon nanotubes when irradiated by an electron beam leading to changes in the guest (fullerene) and the host (nanotube). Intrinsic reaction coordinate (IRC) analysis based on B3LYP hybrid density functional theory shows that a C1-C59 defect with a single protruding C atom is initially formed from the C60(Ih) cage. The high activation energy for this step (8.37 eV (193.0 kcal/mol)), being assumed to be accessible during irradiation, is lower than that of the Stone-Wales rearrangement on the sp2 network. The binding of the defected fullerene to the nanotube is preferential, orthogonal bonds relative to the tube axis being slightly preferred. Because of the covalent bonds formed between the guest and host, the carbon network on the nanotube is locally perturbed in the vicinity of the binding site. As a result of the new bonds, bisnorcaradiene-like as well as quinonoid-like patterns appear near the binding site. These results are interpreted using orbital interaction and Clar diagram arguments. The changes in the bonding pattern on the nanotube should be significant in further functionalization of carbon nanotubes.