Studies on the encapsulation of F- in single walled nanotubes of different chiralities using density functional theory calculations and Car-Parrinello molecular dynamics simulations

In this study, the encapsulation of F(-) in different nanotubes (NTs) has been investigated using electronic structure calculations and Car-Parrinello molecular dynamics simulations. The carbon atoms in the single walled carbon nanotube (CNT) are systematically doped with B and N atoms. The effect of the encapsulation of F(-) in the boron nitride nanotube (BNNT) has also been investigated. Electronic structure calculations show that the (7,0) chirality nanotube forms a more stable endohedral complex (with F(-)) than the other nanotubes. Evidence obtained from the band structure of CNT calculations reveals that the band gap of the CNT is marginally affected by the encapsulation. However, the same encapsulation significantly changes the band gap of the BNNT. The density of states (DOS) derived from the calculations shows significant changes near the Fermi level. The snapshots obtained from the CPMD simulation highlight the fluctuation of the anion inside the tube and there is more fluctuation in BNNT than in CNT.     

Protonation of carbon single-walled nanotubes studied using 13C and 1H-13C cross polarization nuclear magnetic resonance and Raman spectroscopies

The reversible protonation of carbon single-walled nanotubes (SWNTs) in sulfuric acid and Nafion was investigated using solid-state nuclear magnetic resonance (NMR) and Raman spectroscopies. Magic-angle spinning (MAS) was used to obtain high-resolution 13C and 1H-13C cross polarization (CP) NMR spectra. The 13C NMR chemical shifts are reported for bulk SWNTs, H2SO4-treated SWNTs, SWNT-Nafion polymer composites, SWNT-AQ55 polymer composites, and SWNTs in contact with water. Protonation occurs without irreversible oxidation of the nanotube substrate via a charge-transfer process. This is the first report of a chemically induced change in a SWNT 13C resonance brought about by a reversible interaction with an acidic proton, providing additional evidence that carbon nanotubes behave as weak bases. Cross polarization was found to be a powerful technique for providing an additional contrast mechanism for studying nanotubes in contact with other chemical species. The CP studies confirmed polarization transfer from nearby protons to nanotube carbon atoms. The CP technique was also applied to investigate water adsorbed on carbon nanotube surfaces. Finally, the degree of bundling of the SWNTs in Nafion films was probed with the 1H-13C CP-MAS technique.     

The "invisible" 13C NMR chemical shift of the central carbon atom in [(Ph3PAu)6C]2+: a theoretical investigation

The experimental 13C NMR chemical shift of the central carbon atom in the octahedral [(Ph3PAu)6C]2+ cluster was investigated on the basis of relativistic density functional calculations. In order to arrive at independent model conclusions regarding the value of the chemical shift, a systematic study of the dependence of the cluster structure on the phosphine ligands, the chosen density functionals, and the basis set size was conducted. The best structures obtained were then used in the NMR calculations. Because of the cage-like cluster structure a pronounced deshielding of the central carbon nucleus could have been expected. However, upon comparison with the 13C NMR properties of the related complex [C{Au[P(C6H5)2(p-C6H4NMe2)]}6]2+, Schmidbaur et al. have assigned a signal at delta=135.2 ppm to the interstitial carbon atom. Our calculations confirm this value in the region of the aromatic carbon atoms of the triphenylphosphine ligands. The close-lying signals of the 108 phenyl carbon atoms can explain the difficulties of assigning them experimentally.     

Computational studies of effects of tubular lengths on the NMR properties of pristine and carbon decorated boron phosphide nanotubes

Density functional theory (DFT) calculations were performed to investigate the effects of tubular lengths on the nuclear magnetic resonance (NMR) properties of boron phosphide (BP) nanotubes. To this aim, the properties of pristine and carbon decorated (C-decorated) models of representative zigzag and armchair BP nanotubes were investigated. The results indicated that the atoms at the edges of nanotubes do not detect any significant changes. The NMR properties of boron atoms only detect slight changes but those of phosphorous atoms are more notable.     

Estimation and prediction of 13C NMR chemical shifts of carbon atoms in both alcohols and thiols

Quantitative structure-spectrum relationship calculations of ¹³C NMR chemical shifts of both 302 carbon atoms in 56 alcohols and 62 carbon atoms in 15 thiols are described using several parameters: the atomic ionicity index (INI), the polarizability effect index (PEI), and stereoscopic effect parameters (?) of the compounds. The ¹³C NMR chemical shifts for these compounds of both alcohols and thiols can be estimated through the multiple linear regression (MLR). A MLR model was built with variable screening by the stepwise multiple regression and examined by validation on its stability. The correlation coefficient of the established model as well as the leave-one-out cross-validation was 0.9724 and 0.9716 respectively. The results obviously indicate that INI and ? are linearly related with ¹³C NMR chemical shifts, which provides a new method for calculating ¹³C NMR chemical shifts in the compounds of both alcohols and thiols.     

“Complete” nuclear magnetic resonance spectrum of 4,6-diamino-3,5-dicyano-2-cyanomethylpyridine

The structure of the molecule of 4, 6-diamino-3, S-dicvano-2-cyanomethylpyridine is confirmed by the 13C NMR spectrum which, together with the t H and¹⁵N NMR spectra (the "complete" NMR spectrum), allows almost unambiguous assignment (with the exception of the virtually coinciding paired signals of the atoms of carbon and nitrogen of the 3- and 5-CN groups). The applicability of the method of increments in the 13C NMR was shown in the assignment of the signals of the carbon atoms in pyridine derivatives. The ratio of the chemical shifts of the nitrogen and hydrogen atoms of the amino groups, known from the literature for aminobenzenes, was confirmed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 508–511, April, 1996.     

Fast determination of 13C NMR chemical shifts using artificial neural networks

Nine different artificial neural networks were trained with the spherically encoded chemical environments of more than 500000 carbon atoms to predict their 13C NMR chemical shifts. Based on these results the PC-program "C_shift" was developed which allows the calculation of the 13C NMR spectra of any proposed molecular structure consisting of the covalently bonded elements C, H, N, O, P, S and the halogens. Results were obtained with a mean deviation as low as 1.8 ppm; this accuracy is equivalent to a determination on the basis of a large database but, in a time as short as known from increment calculations, was demonstrated exemplary using the natural agent epothilone A. The artificial neural networks allow simultaneously a precise and fast prediction of a large number of 13C NMR spectra, as needed for high throughput NMR and screening of a substance or spectra libraries.     

13C NMR Quantitative Study-Part 1: Relationships between the Conformation of Amino Acids, Peptide, Carboxylic Acids and Integration Intensity of 13C NMR

In proton broad band decoupling 13C NMR, carbon atoms have different integration intensity because of NOE effects and their different relaxation time(T1), thus it makes a 13C NMR quantitative analyses very difficult. To acquire a 3C NMR quantitative analyses, a gated decoupling with suppressed NOE technology, i.e., an inversed gated decoupling pulse (IGDP), must be used. In IGDP relay time (tR) between two acquisition cycles must be more than 5T1, the time needed for a acquisition cycles is so long that makes the total 13C NMR quantitative analyses time much longer. For this reason, the 13C NMR quantitative analyses is paid less attention.     

Structural and electronic characteristics of perhydrogenated carbon nanotubes

The structural and electronic characteristics of fully hydrogenated armchair and zigzag carbon nanotubes have been determined by quantum chemical methods. With use of line group symmetries, the structures of nanotubes up to 10 nm in diameter could be optimized by periodic B3LYP calculations. “In–out” isomerism is shown to significantly stabilize perhydrogenated carbon nanotubes, the energetically most favorable structures being those with 1/3–1/2 of the carbon atoms endo-hydrogenated. In favored nanotubes the ratio of endo- to exo-hydrogens is 1:1, the stabilities increasing as a function of the diameter of the nanotube. The calculated band gaps indicate that the perhydrogenated carbon nanotubes are insulators.     

Quantum chemical calculation of spectroscopic and photoelectronic characteristics of [<Emphasis Type="Italic">n</Emphasis>]staffanes

Structural parameters, IR spectra, ¹H and ¹³C NMR spectra, quadrupole moments, and dipole polarizabilities of seven [n]staffanes with a distance between the terminal carbon atoms of up to 22.0 Å have been determined by DFT quantum chemical calculations at the PBE0/cc-pVTZ level of theory.     

Linear augmented cylindrical wave Green’s function method for perfect nanotubes and nanotubes with regular and point impurities

Nanotubes are giant cage molecules looking like closed hollow cylindrical shells. This review deals with basic principles of the linear augmented cylindrical Green’s function method and its applications to calculation of the electronic structure of perfect nanotubes and those containing substitutional impurities. A major argument for using cylindrical waves to describe nanotubes is that such a choice of the basis set makes it possible to explicitly consider the actual cylindrical geometry of nanotubes, which, in particular, ensures rapid convergence of iterative procedures. A computation technique has been described and the results of calculations of the band structure and densities of states of carbon and boron nitride nanotubes have been reported. Special attention has been paid to the changes in the electronic properties of nanotubes induced by the substitution of nitrogen, boron, or oxygen for C atoms in the carbon nanotubes, as well as to the isoelectronic substitution of P, Sb, or As for the nitrogen and of Al, In, or Ga for the boron in boron nitride nanotubes.     

Density functional calculations of the 13C NMR chemical shifts in (9,0) single-walled carbon nanotubes

The electronic structure and (13)C NMR chemical shift of (9,0) single-walled carbon nanotubes (SWNTs) are investigated theoretically. Shielding tensor components are also reported. Density functional calculations were carried out for C(30)-capped and H-capped fragments which serve as model systems for the infinite (9,0) SWNT. Based on the vanishing HOMO-LUMO gap, H-capped nanotube fragments are predicted to exhibit "metallic" behavior. The (13)C chemical shift approaches a value of approximately 133 ppm for the longest fragment studied here. The C(30)-capped SWNT fragments of D(3d)/D(3h) symmetry, on the other hand, are predicted to be small-gap semiconductors just like the infinite (9,0) SWNT. The differences in successive HOMO-LUMO gaps and HOMO and LUMO energies, as well as the (13)C NMR chemical shifts, converge slightly faster with the fragment's length than for the H-capped tubes. The difference between the H-capped and C(30)-capped fragments is analyzed in some detail. The results indicate that (at least at lengths currently accessible to quantum chemical computations) the H-capped systems represent less suitable models for the (9,0) SWNT because of pronounced artifacts due to their finite length. From our calculations for the C(30)-capped fragments, the chemical shift of a carbon atom in the (9,0) SWNT is predicted to be about 130 ppm. This value is in reasonably good agreement with experimental estimates for the (13)C chemical shift in SWNTs.     

DFT calculations of structures, 13C NMR chemical shifts,and Raman RBM mode of simple models of small‐diameter zigzag (4,0) carboxylated single‐walled carbon nanotubes

Linearly conjugated benzene rings (acenes), belt‐shaped molecules (cyclic acenes), and models of single‐walled carbon nanotubes (SWCNTs) with one carboxylic group at the open end were fully optimized at the B3LYP/6‐31G* level of theory. These models were selected to obtain some insight into the nuclear isotropic changes resulting from systematically increasing the basic building units of open‐tip‐monocarboxylated SWCNTs. In addition, the position of radial breathing mode (RBM), empirically correlated with the SWCNT diameter, was directly related with the radius of model cyclic acene rings. A regular convergence of selected structural, NMR, and Raman parameters with the molecular system size increase was observed, and a simple two‐parameter mathematical formula enabled their estimation in infinity. The predicted ¹³C NMR chemical shifts of carbon atoms close to the substituted rim of carboxylated models of zigzag (4,0) SWCNTs differed significantly from the pristine nanotubes. Copyright © 2012 John Wiley & Sons, Ltd.     

A computational study of oxygen-termination of a (6,0) boron nitride nanotube

Abstract  

We performed density functional theory (DFT) calculations to investigate the properties of electronic structures of representative armchair and zigzag silicon carbide nanotubes (SiCNTs). The model structures were optimized and the NMR parameters were calculated at the sites of silicon-29 and carbon-13 atoms in these structures. Our results indicated that different electronic environments could be detected by using the atoms of nanotubes in which the atoms of tips, especially for zigzag SiCNT, exhibit distinctive properties among other atoms.     

Determination of the relative stereochemistry of flexible organic compounds by Ab initio methods: conformational analysis and Boltzmann-averaged GIAO 13C NMR chemical shifts

Ab initio calculations at the Hartree-Fock level with full-geometry optimization using the 6-31G(d) basis set, and GIAO (gauge including atomic orbitals) (13)C NMR chemical shifts, are presented here as a support in the study of the stereochemistry of low-polar organic compounds having an open-chain structure. Four linear stereoisomers, fragments of a natural product previously characterized by experimental (13)C NMR spectra, which possesses three stereogenic centers, 11 carbon atoms, and 38 atoms in total, were considered. Conformational searches, by empirical force-field molecular dynamics, pointed out the existence of 8-13 relevant conformers per stereoisomer. Thermochemical calculations at the ab initio level in the harmonic approximation of the vibrational modes, allowed the evaluation, at 298.15 K, of the standard Gibbs free energy of the conformers. The (13)C NMR chemical shift of a given carbon atom in each stereoisomer was considered as the average chemical shift value of the same atom in the different conformers. The averages were obtained by the Boltzmann distribution, using the relative standard free energies as weighting factors. Computed parameters related to linear correlation plots of experimental (13)C chemical shifts versus the corresponding computed average data allowed us to distinguish among the four stereoisomers.     

Mercury(II) cyanide complexes of thioureas and the crystal structure of [(N-methylthiourea)2 Hg(CN)2]

Mercury(II) cyanide complexes of thioureas (Tu), N-methylthiourea (MeTu), and N,N′-dimethylthiourea (DmTu)) have been prepared and characterized by IR and NMR (¹H and ¹³C) spectroscopy, and the crystal structure of one of them was determined by X-ray crystallography. An upfield shift in ¹³C NMR and downfield shifts in ¹H NMR are consistent with the sulfur coordination to mercury(II). The appearance of a band around 2200 cm^?1 in IR and a resonance around 145 ppm in ¹³C NMR indicates the binding of cyanide to mercury(II). The NMR data show that the [(Thione)₂Hg(CN)₂] complexes are stable in solution and undergo no redistribution reactions. In the crystal structure of the title complex, mercury atom is coordinated to two thione sulfur atoms of MeTu and to two cyanide carbon atoms in a distorted tetrahedral mode with the bond angles in the range of 90.2(2)°–169.3(3)°.     

Soft-x-ray photoemission spectroscopy and ab initio studies on the adsorption of NO2 molecules on defective multiwalled carbon nanotubes

The adsorption of NO(2) molecules on defective multiwalled carbon nanotubes has been studied by soft-x-ray photoemission. The valence band and carbon core-level spectra have been acquired before, during, and after NO(2) exposure. The spectra show a reversible decrease of the density of states at the top of the valence band when NO(2) molecules are adsorbed on the (carbon nanotubes) CNTs. No shift of the C 1s spectra has been observed. Theoretical calculations, using density-functional theory, have been performed on the CNT + NO(2) system, considering semiconducting nanotubes with different diameters and introducing a Stone-Wales [Chem. Phys. Lett. 128, 501 (1986)] defect. The calculation confirms the decrease of the density of states at the top of the valence band in the CNT + NO(2) system, while close to the adsorption site new states appear very close to the Fermi level.     

DFT calculation of structures and NMR chemical shifts of simple models of small diameter zigzag single wall carbon nanotubes (SWCNTs)

Linearly conjugated benzene rings (acenes), belt‐shape molecules (cyclic acenes) and model single wall carbon nanotubes (SWCNTs) were fully optimized at the unrestricted level of density functional theory (UB3LYP/6‐31G*). The models of SWCNTs were selected to get some insight into the potential changes of NMR chemical shift upon systematic increase of the molecular size. The theoretical NMR chemical shifts were calculated at the B3LYP/pcS‐2 level of theory using benzene as reference. In addition, the change of radial breathing mode (RBM), empirically correlated with SWCNT diameter, was directly related with the radius of cyclic acenes. Both geometrical and NMR parameters were extrapolated to infinity upon increase in the studied systems size using a simple two‐parameter mathematical formula. Very good agreement between calculated and available experimental CC bond lengths of acenes was observed (RMS of 0.0173 Å). The saturation of changes in CC bond lengths and ¹H and ¹³C NMR parameters for linear and cyclic acenes, starting from 7–8 conjugated benzene rings, was observed. The ¹³C NMR parameters of individual carbon atoms from the middle of ultra‐thin (4,0) SWCNT formed from 10 conjugated cyclic acenes differ by about 130 ppm from the corresponding open end carbon nuclei. Copyright © 2011 John Wiley & Sons, Ltd.     

Topology and electronic structure of nanotube junctions of tetrapod shape

We propose a series of carbon nanostructures in the shape of tetrapod as a kind of three-dimensional junction for carbon nanotubes. The tetrapod junctions are such open networks that are made of sp² carbon atoms only, have negative Gaussian curvature, and connect four nanotubes together. We define the structure of standard tetrapod junctions, the simplest one, that have 12 heptagons other than hexagons and have the T_d symmetry.Our tight-binding energy-band calculations for the standard tetrapod junctions of smaller sizes show that their electronic property mainly depends on one particular topological factor: the junctions having a carbon atom in the center of each triangular face of tetrahedron exhibit metallic band structure while the junctions having a benzene ring in the center of the faces are semiconductors. We also find that tetrapod junctions connecting (6,0) nanotubes exhibit a flat band near the Fermi energy in a particular momentum region. The origin of the flat band states can be figured out from the wavefunction distribution. We also show the possibility to extend the standard tetrapod junctions to some non-standard ones that can connect nanotubes of different kinds and/or radii.     

Manipulating the electronic structures of silicon carbide nanotubes by selected hydrogenation

We show that the electronic and atomic structures of silicon carbide nanotubes (SiCNTs) undergo dramatic changes with hydrogenation from first-principles calculations based on density-functional theory. The exo-hydrogenation of a single C atom results in acceptor states close to the highest occupied valence band of pristine SiCNT, whereas donor states close to the lowest unoccupied conduction band appear as a Si atom being hydrogenated. Upon fully hydrogenating Si atoms, (8,0) and (6,6) SiCNTs become metallic with very high density of states at the Fermi level. The full hydrogenation of C atoms, on the other hand, increases the band gap to 2.6 eV for (8,0) SiCNT and decreases the band gap to 1.47 eV for (6,6) SiCNT, respectively. The band gap of SiCNTs can also be greatly increased through the hydrogenation of all the atoms.