Calculation of electrostatic and polarization energies from electron densities

We investigate procedures for calculating the electrostatic and polarization energies, Ees and Epol, associated with noncovalent interactions. The starting points are the electron densities of the isolated components and the complex; these could be obtained either computationally or experimentally. A slightly modified version of a scheme proposed by Gavezzotti is used to carry out numerical integrations over these electron densities. Our approach to estimating Epol is based upon partitioning the charge distributions of the components into overlapping and nonoverlapping regions. The effects of varying the integration parameters, computational techniques and basis sets are examined in detail for several noncovalently bound molecular dimers. Our results are in good agreement with the values of Ees and Epol produced by other methods, which require analytical integrations over interaction Hamiltonian matrix elements.     

Electrostatic potentials and covalent radii

We begin with a brief overview of the electrostatic potential V(r) as a fundamental determinant of the properties of systems of electrons and nuclei. The minimum of V(r) along the internuclear axis between two bonded atoms is a natural and physically meaningful boundary point, at which the electrostatic forces of the two nuclei upon an element of charge exactly cancel. We propose that the distances from nuclei to V(r) bond minima provide the basis for a well-defined set of covalent radii. Density functional calculations at the B3PW91/6-311+G** level were carried out for 59 molecules to locate the V(r) minima in 95 bonds and use these as the basis for determining single- and multiple-bond covalent radii for eight first- and second-row atoms plus hydrogen. It was found to be unrealistic to assign a single covalent radius to each atom; different values are needed for bonds to first- and second-row atoms, as well as to hydrogen. Using these results, we are able to predict the bond lengths of 33 single and multiple bonds with average errors of less than 0.04 A relative to experimental data.     

Thin-layer chromatography of laser dyes and dye analogs with cyclodextrins in the mobile phase

The addition of beta-cyclodextrin and substituted hydroxyethyl and hydroxypropyl-beta-cyclodextrins to aqueous urea-containing mobile phases, can be used to enhance the migration of various laser dyes on thin layer chromatography. Selected laser dyes from the coumarin, rhodamine and bimane families were examined, as well as a number of dye analogs. Silica gel, polyamide and C18 reverse phase plates were utilized. Overall, the substituted hydroxypropyl-beta-cyclodextrins appeared to be the most effective for increasing dye migration.     

Computational Determination of Nitroaromatic Solid Phase Heats of Formation

Homodesmic heats of reaction are computed at the B3PW91/6-31G* level as a means of determining gas phase heats of formation. These are then converted to solid state values by combining with H _sub, the latter being either experimental or calculated, with an empirical relationship based on molecular surface electrostatic potentials. Our focus, after evaluating the approach in four test cases, is upon six nitroaromatic compounds for which the literature H _f(solid) are ambiguous, i.e. two or more values have been reported that differ by at least 6 kcal/mol. The calculated results provide clear support for some of the experimental data at the expense of the remainder. This appears to be an effective approach to estimating H _f , provided that appropriately designed homodesmic reactions can be used.     

X–NO2 rotational energy barriers: Local density functional calculations

The X–NO₂ rotational energy barriers of nitromethane, nitroethylene, nitrobenzene, and a group of nitramines have been computed using a local density functional (LDF ) procedure, using ab initio Hartree–Fock (HF )-optimized structures of the ground and rotational transition states. The results have been discussed in relation to HF and some correlated ab initio values and the available experimental data. Our LDF barriers are overall quite reasonable, in generally satisfactory agreement with the experimental and correlated ab initio results. © 1993 John Wiley & Sons, Inc.     

Internal kink instability during off-axis electron cyclotron current drive in the DIII-D tokamak

Experimental evidence is reported of an internal kink instability driven by a new mechanism: barely trapped suprathermal electrons produced by off-axis electron cyclotron heating on the DIII-D tokamak. It occurs in plasmas with an evolving safety factor profile q(r) when q(min) approaches 1. This instability is most active when ECCD is applied on the high field side of the flux surface. It has a bursting behavior with poloidal/toroidal mode number = m/n = 1/1. In positive magnetic shear plasmas, this mode becomes the fishbone instability. This observation can be qualitatively explained by the drift reversal of the barely trapped suprathermal electrons.     

Variation of parameters in Becke‐3 hybrid exchange‐correlation functional

We have investigated the consequences of varying the three parameters in Becke's hybrid exchange‐correlation functional, which includes five contributions: Hartree–Fock exchange, local exchange, Becke's gradient exchange correction, local correlation, and some form of gradient correlation correction. Our primary focus was upon obtaining orbital energies with magnitudes that are reasonable approximations to the electronic ionization potentials; however, we also looked at the effects on molecular geometries and atomization enthalpies. A total of 12 parameter combinations was considered for each of three different gradient correlation corrections: the Lee–Yang–Parr, the Perdew‐86, and the Perdew–Wang 91. Five molecules were included in the study: HCN, N₂, N₂O, F₂O, and H₂O. For comparison, a Hartree–Fock calculation was also carried out for each of these. The 6‐31+G** basis set was used throughout this work. We found that the ionization potential estimates can be greatly improved (to much better than Hartree–Fock levels) by increasing the Hartree–Fock exchange contribution at the expense of local exchange. In itself, this also introduces major errors in the atomization enthalpies. However, this can be largely or even completely counteracted by reducing or eliminating the role of the gradient exchange correction. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 227–238, 2000     

Density functional analysis of a decomposition of 4-nitro-1,2,3-triazole through the evolution of N2

A density functional computational study shows that 4-nitro-1,2,3-triazole, which is highly impact sensitive, can decompose through ring opening and subsequent N₂ evolution, with the net release of 12 kcal/mol. An input of 52 kcal/mol is required to initiate the process. © 1997 John Wiley & Sons, Inc.     

Reaction energetics of tetrahedrane and other hydrocarbons: Ab initio and density functional treatments

Reaction energetics of the highly strained tetrahedrane molecule, as well as some smaller hydrocarbons, were computed at the ab initio MP4, QCISD (T), G1, and G2 levels and also by several density functional (DF) approaches: VWN(LDA), PW86, BLYP, and B3LYP. A variety of basis sets were used in the DF calculations. For atomization processes, nonlocal DF procedures are superior to MP4 and QCI. For the other reactions, B3LYP is the most reliable DF approach and is overall competitive with MP4 and QCISD (T), although the MP4/6-311G(2df, p) results are generally the closest to the experimental and/or the G2 values. There is no consistent correlation between the accuracy of the DF results and the size of the basis set. © 1996 John Wiley & Sons, Inc.