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ABSTRACT: The band energy differences of solids calculated with screened hybrid density functionals, such as the functional of Heyd-Scuseria-Ernzerhof (HSE), reproduce experimental band gaps with a high degree of accuracy. This unexpected result is here rationalized by observing that band energy differences obtained from generalized Kohn-Sham calculations with screened (short-range) Hartree-Fock-type exchange approach the excitation energies obtained via time-dependent density functional calculations with the corresponding unscreened functional. The latter are expected to be the accurate predictions of the experimental optical absorption spectra. While the optimum screening parameter (omega) is system dependent, the HSE standard value of omega=0.11 bohr(-1) represents a reasonable compromise across diverse systems.
The Journal of chemical physics 08/2008; 129(1):011102. · 3.09 Impact Factor
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ABSTRACT: A comprehensive first-principles theoretical study of the electronic properties and half-metallic nature of finite rectangular graphene nanoribbons is presented. We identify the bisanthrene isomer of the C28H14 molecule to be the smallest graphene derivative to present a spin-polarized ground state. Even at this quantum dot level, the spins are predicted to be aligned antiferromagnetically at the two zigzag edges of the system. As a rule of thumb, we find that zigzag graphene edges that are at least three consecutive units long will present spin polarization if the width of the system is 1 nm or wider. Room temperature detectability of the magnetic ordering is predicted for ribbons with zigzag edges 1 nm and longer. For the longer systems studied, spin wave structures appear in some high spin multiplicity states. Energy gap oscillations with the length of the zigzag edge are observed. The amplitude of these oscillations is found to be smaller than that predicted for infinite ribbons. The half-metallic nature of the ribbons under an external in-plane electric field is found to be preserved even for finite and extremely short systems.
Phys. Rev. B. 01/2008; 77(3).
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ABSTRACT: We present a comprehensive theoretical study of the electronic properties and relative stabilities of edge-oxidized zigzag graphene nanoribbons. The oxidation schemes considered include hydroxyl, lactone, ketone, and ether groups. Using screened exchange density functional theory, we show that these oxidized ribbons are more stable than hydrogen-terminated nanoribbons except for the case of the etheric groups. The stable oxidized configurations maintain a spin-polarized ground state with antiferromagnetic ordering localized at the edges, similar to the fully hydrogenated counterparts. More important, edge oxidation is found to lower the onset electric field required to induce half-metallic behavior and extend the overall field range at which the systems remain half-metallic. Once the half-metallic state is reached, further increase of the external electric field intensity produces a rapid decrease in the spin magnetization up to a point where the magnetization is quenched completely. Finally, we find that oxygen-containing edge groups have a minor effect on the energy difference between the antiferromagnetic ground state and the above-lying ferromagnetic state.
Nano Letters 09/2007; 7(8):2295-9. · 13.20 Impact Factor
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ABSTRACT: We present a systematic density functional theory study of the electronic properties, optical spectra, and relative thermodynamic stability of semiconducting graphene nanoribbons. We consider ribbons with different edge nature including bare and hydrogen-terminated ribbons, several crystallographic orientations, and widths up to 3 nm. Our results can be extrapolated to wider ribbons providing a qualitative way of determining the electronic properties of ribbons with widths of practical significance. We predict that in order to produce materials with band gaps similar to Ge or InN, the width of the ribbons must be between 2 and 3 nm. If larger bang gap ribbons are needed (like Si, InP, or GaAs), their width must be reduced to 1-2 nm. According to the extrapolated inverse power law obtained in this work, armchair carbon nanoribbons of widths larger than 8 nm will present a maximum band gap of 0.3 eV, while for ribbons with a width of 80 nm the maximum possible band gap is 0.05 eV. For chiral nanoribbons the band gap oscillations rapidly vanish as a function of the chiral angle indicating that a careful design of their crystallographic nature is an essential ingredient for controlling their electronic properties. Optical excitations show important differences between ribbons with and without hydrogen termination and are found to be sensitive to the carbon nanoribbon width. This should provide a practical way of revealing information on their size and the nature of their edges.
Nano Letters 01/2007; 6(12):2748-54. · 13.20 Impact Factor
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ABSTRACT: We have calculated the geometrical structure, relative stability, and nitrogen chemical shifts of five boron nitride hollow octahedral cages using density functional theory. Our results show three typical ranges for nitrogen chemical shifts corresponding to each of the nonequivalent magnetic sites of the N atoms. The principal component of the electric field gradient tensor at each 14N site in boron nitride cages is predicted to be much smaller than the corresponding value in borazine, which should reflect in sharper spectral lines and much better resolution.
The Journal of Physical Chemistry A 10/2006; 110(37):10844-7. · 2.95 Impact Factor
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ABSTRACT: We present a detailed study of the work function of pristine and doped single-walled carbon nanotubes (SWCNTs) using a novel screened exchange hybrid density functional. We find that SWCNTs with diameters larger than 0.9 nm tend asymptotically and smoothly to the graphene limit of 4.6 eV. On the other hand, the work function of narrow tubes exhibits a strong dependence on their diameter and chiral angle. Boron or nitrogen doping, with concentrations from 1% to 2%, not only changes the electronic behavior by introducing new states around the Fermi level, but also produces a significant change of the work function that can vary between 3.9 (N doping) and 5.2 eV (B doping).
The Journal of Chemical Physics 02/2006; 124(2):024709. · 3.33 Impact Factor
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ABSTRACT: We report vertical electronic transitions of 20 metallic single-walled carbon nanotubes calculated as band energy differences from Kohn-Sham density functional theory. Our first-order transitions (E11) calculated with hybrid functionals (containing a portion of exact exchange) are in very good agreement with available experimental data. Recently, we have reported similar agreement between experiment and theory for semiconducting tubes. We find that the trigonal warping splitting in the band structure of metallic tubes is about 1.5 to 2 times larger than that reported previously.
Nano Letters 10/2005; 5(9):1830-3. · 13.20 Impact Factor
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ABSTRACT: We present a density functional theory study of optical transitions in semiconducting single-walled carbon nanotubes. We utilize recently developed exchange-correlation functionals in a set of 21 tubes that includes large and chiral nanotubes. The novel TPSSh meta-generalized gradient approximation hybrid functional accurately reproduces optical excitations with mean absolute errors of 0.024 and 0.065 eV for first and second transitions, respectively. We also report predictions for higher order optical transitions.
Nano Letters 09/2005; 5(8):1621-4. · 13.20 Impact Factor
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ABSTRACT: We benchmark the performance of 20 approximate density functionals for the calculation of one-bond carbon−hydrogen NMR spin−spin coupling constants (SSCCs). These functionals range from the simplest local-spin density approximation to novel meta-generalized gradient approximation and hybrid density functionals. Our testing set consists of 72 diverse molecules that represent multiple types of hybridization of the carbon atom corresponding to 96 experimentally measured one-bond carbon−hydrogen SSCCs. Our results indicate that generalized gradient approximations perform best for this type of coupling.
06/2005;
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ABSTRACT: In this work we present a systematic density functional theory study of the electronic properties of single-walled carbon nanotubes (SWNT) with diameters ranging from 3 to 5 A. In this work meta-generalized-gradient approximation, hybrid, and screened exchange hybrid functionals are utilized to compute energy band gaps in these narrow SWNT. Our calculations using hybrid functionals show that the only true exceptions to the zone folding predictions are the (4,0) and (5,0) SWNT. The remaining chiral SWNT are semiconducting with band gaps that can be as large as 1.7 eV. However, the calculated energy band gaps are significantly smaller than those predicted by the zone folding scheme. This difference is primarily attributed to the sigma-pi hybridization present in such narrow SWNT.
The Journal of Chemical Physics 01/2005; 121(21):10376-9. · 3.33 Impact Factor
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ABSTRACT: One- two- and three 13C, 13C (n = 1, 2, 3) scalar couplings, (n)J(C,C) in a set of pyrimidine derivatives were studied both experimentally at natural abundance and theoretically by their DFT calculation of all four contributions. Trends of non-contact terms are discussed and substituent effects are rationalized, comparing some of them with the corresponding values in benzene and pyridine. Although substituent effects on non-contact terms are relatively important, the whole trend is dominated by the Fermi contact term. According to the current literature, substituent effects on 1J(C,C) couplings in benzene derivatives are dominated by the inductive effect, which, apparently, is also the case in nitrogen heteroaromatic compounds. However, some differences observed in this work for substituent effects on 1J(C,C) couplings in pyrimidine derivatives suggest that in the latter type of compounds substituent effects can be affected by the orientation of the ring nitrogen lone pairs.
Magnetic Resonance in Chemistry 12/2004; 42(11):938-43. · 1.44 Impact Factor
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ABSTRACT: We calculate NMR spin-spin coupling constants in the C70 fullerene by means of density functional theory. We show that using a hybrid density functional (B3LYP) and an adequate basis set (cc-pCVDZ-sd), excellent agreement with experimental values can be achieved for one-bond couplings. These benchmark calculations suggest that theoretical predictions of NMR spin-spin couplings can be extremely valuable for discerning structural information of fullerenes.
Journal of the American Chemical Society 07/2004; 126(24):7428-9. · 9.91 Impact Factor
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05/2004; , ISBN: 9780470845011
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ABSTRACT: We have studied the interaction of atomic hydrogen with (5,5) and (10,0) single-walled carbon nanotubes (SWNT) using density functional theory. These calculations use Gaussian orbitals and periodic boundary conditions. We compare results from the local spin density approximation, generalized gradient approximation (GGA), and hybrid density functionals. We have first kept the SWNT geometric structure fixed while a single H atom approaches the tube on top of a carbon atom. In that case, a weakly bound state with binding energies from -0.8 to -0.4 eV was found. Full geometry relaxation leads to a strong SWNT deformation, weakening the nearest C-C bonds and increasing the binding energy by about 1 eV. Full hydrogen coverage of the (5,5) SWNT converts this metallic nanotube into an insulator with a band gap of 3.4 eV for the GGA functional and 4.8 eV for the hybrid functional. Hybrid functionals perform similar to pure density functional theory functionals for the calculation of binding energies while band gaps critically depend on the functional choice.
The Journal of Chemical Physics 05/2004; 120(15):7169-73. · 3.33 Impact Factor
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ABSTRACT: Substituent effects for 2J(F,F) couplings in aliphatic and olefinic CF2 moieties and 3J(F,F) couplings in fluorinated derivatives of ethylene were studied using both high level ab initio and DFT/B3LYP calculations. Where possible, J variations have been compared with experimental values. In general, the SOPPA (second-order polarization propagator approximation) methodology matches absolute experimental values reasonably well, whereas the DFT/B3LYP approach performs poorly in describing 2J(F,F) couplings. Fortunately, substituent effects for DFT J couplings are notably better reproduced. For a vinyl CF2 moiety, the accurate prediction of 2J(F,F) couplings is a challenging task even for high level ab initio methods such as SOPPA and SOPPA(CCSD) (second-order polarization propagator approximation with coupled cluster singles and doubles amplitudes). Aliphatic 2J(F,F) couplings are very sensitive to the electronegativity of substituents placed α to the CF2 group. The latter J perturbations are dominated largely by the noncontact PSO and SD Ramsey contributions, whereas the influence of the FC term is rather small. Substituent effects on 2J(F,F) and 3J(F,F) couplings in fluorinated derivatives of ethylene are also dominated by non-Fermi contributions. Because DFT/B3LYP strongly underestimates the FC contribution, but generally assesses the non-Fermi terms similar to SOPPA, the latter accounts for DFT's ability to predict substituent effects reasonably well.
05/2003;
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ABSTRACT: Numerous well-behaved Karplus curves correlating dihedral angle (phi) and NMR coupling constants (3JXY) exist for X-C-Z-Y fragments with X or Y = H, C, F, and P. Absent is the enigmatic F-C-C-F moiety. By calculating the four Ramsey contributions to 3JFF for F-CH2-CH2-F, a pleasing phi/3JFF curve with both positive and negative regions arises. Unlike the H-C-C-H curve, F-C-C-F is PSO vs FC dominated. The latter and the F lone electron pairs cause both the negative J values and the substituent-induced J-leveling effect.
Journal of the American Chemical Society 09/2002; 124(33):9702-3. · 9.91 Impact Factor
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ABSTRACT: All four isotropic contributions to the NMR fluorine−fluorine coupling constants (Fermi contact, FC, spin-dipolar, SD, paramagnetic spin−orbit, PSO, and diamagnetic spin−orbit, DSO) have been calculated for 2,6-difluoropyridine, 2,4,6-trifluoropyridine, perfluoropyridine, and 2-Br-3,4,5,6,7,8-hexafluoroquinoline by means of density functional theory in combination with the rather modest 6-311G** basis set. Experimental values ranging from −20.3 to +45.8 Hz are semiquantitatively reproduced for three- to seven-bond couplings, suggesting that the different electronic effects responsible for the spin−spin interactions are adequately taken into account. In all cases, the relative importance of noncontact terms was examined. With few exceptions, the sum of the SD and PSO noncontact terms is larger than the FC contact contribution, even though in most cases the two noncontact values have opposite signs. The widespread assumption that the Fermi contact term dominates scalar spin−spin couplings in the case of light atoms would appear to be an oversimplification for JFF in polyfluorinated organic molecules. In addition, the CPU performance of the Fermi contact contribution calculated separately by the coupled-perturbed and the finite-perturbation methods was investigated showing the latter to be slightly more efficient.
05/2002;
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ABSTRACT: The known lone pair orientation effect of an α-nitrogen atom on 1J(C,C) couplings was studied using the natural J coupling (NJC) dissection method for the Fermi contact (FC) term. This dissection was performed at the DFT-B3LYP level using three different basis sets. It was verified that non-contact contributions, which are much smaller than the FC term, do not depend on the orientation of the N lone pair. Acetone oxime (1), protonated acetone oxime (2) and ethylamine (3), were taken as model compounds to perform this study. In 1 the difference between 1J(Z) and 1J(E) is mainly determined by three NJC terms, i.e the nitrogen lone pair, the carbon–carbon bond containing the coupled carbon atoms and the carbon inner core orbitals contributions. In 3 the angular dependence of 1J(C,C) vs the amine group conformation is dominated by the same three contributions that define the 1J(Z) and 1J(E) difference in 1. Copyright © 2001 John Wiley & Sons, Ltd.
Magnetic Resonance in Chemistry 08/2001; 39(10):600 - 606. · 1.44 Impact Factor
