The Effect of Structural Distortions on the Electronic Structure of Carbon Nanotubes

We calculated the effects of structural distortions on the electronic structure of carbon nanotubes. The key modification of the electronic structure brought about by bending a nanotube involves an increased mixing of $\sigma$ and $\pi$-states. This mixing leads to an enhanced density-of-states in the valence band near the Fermi energy region. While in a straight tube the states accessible for electrical conduction are essentially pure C($2p_{\pi}$)-states, they acquire significant C($2sp_{\sigma}$) character upon bending. Bending also leads to a charge polarization of the C-C bonds in the deformed region reminiscent of interface dipole formation. Scattering of conduction electrons at the distorted regions may lead to electron localization at low temperatures.Comment: 11 pages and 4 figures, (figure 4 corrected

Nuclear magnetic resonance spin–spin coupling constants from density functional theory: Problems and results

Our recently developed method for the calculation of indirect nuclear spin-spin coupling constants is studied in more detail. For the couplings between nuclei other than N, O, and F ͑which have lone pairs͒ the method yields very reliable results. The results for 1 J͑Si-H͒ couplings are presented and their dependence on the basis set quality is analyzed. Also, The limitations of the method, which is based on density functional theory, are connected with the inability of the present LDA and GGA exchange-correlation functionals to describe properly the spin-perturbations ͑through the Fermi-contact mechanism͒ on atoms to the right of the periodic table ͑containing lone pairs͒. However, the deviations from experiment of the calculated couplings for such nuclei are systematic, at least for one-bond couplings, and therefore these calculated couplings should still be useful for NMR structure determinations

Conductance of Distorted Carbon Nanotubes

We have calculated the effects of structural distortions of armchair carbon nanotubes on their electrical transport properties. We found that the bending of the nanotubes decreases their transmission function in certain energy ranges and leads to an increased electrical resistance. Electronic structure calculations show that these energy ranges contain localized states with significant $\sigma$-$\pi$ hybridization resulting from the increased curvature produced by bending. Our calculations of the contact resistance show that the large contact resistances observed for SWNTs are likely due to the weak coupling of the NT to the metal in side bonded NT-metal configurations.Comment: 5 pages RevTeX including 4 figures, submitted to PR

DFT exchange: sharing perspectives on the workhorse of quantum chemistry and materials science

In this paper, the history, present status, and future of density-functional theory (DFT) is informally reviewed and discussed by 70 workers in the field, including molecular scientists, materials scientists, method developers and practitioners. The format of the paper is that of a roundtable discussion, in which the participants express and exchange views on DFT in the form of 302 individual contributions, formulated as responses to a preset list of 26 questions. Supported by a bibliography of 777 entries, the paper represents a broad snapshot of DFT, anno 2022

Origin of the high basicity of 2,7-dimethoxy-1,8-bis(dimethylamino)naphthalene: implications for enzyme catalysis

Density functional calculations on model systems are performed to understand the origin of the large increase of basicity from 1,8-bis(dimethylamino)naphthalene (1) to 2,7-dimethoxy-1,8-bis(dimethylamino)naphthalene (2). It is found that the increase of the gas-phase proton affinity (PA) on going from 1 to 2 mainly comes from the relief of steric repulsions of the methoxy groups with their neighboring amino groups as a result of protonation. It is suggested that this relief of the steric repulsions along with favorable electrostatic interactions involving the methoxy groups in 2H+ makes a major contribution to the enhanced basicity.NRC publication: Ye

SEMIEMPIRICAL MO CALCULATIONS OF THE ELECTRONIC SPECTRA OF NORMAL- AND BRANCHED CHAIN PARAFFINS

Author Institution: Department de chimie, Universit\'{e} de Montr\'{e}alCNDO, INDO and RCNDO (CNDO) including higher (Rydberg) atomic orbitals in the basis calculations completed by first order configuration interaction were performed on straight chain and branched chain paraffins. The results interpret reasonably the main characteristics of the observed electronic spectra. The importance of outer atomic orbitals is stressed and it is found that the first singlet-singlet transition of highly branched paraffins leads to an excited state with considerable Rydberg character