Vedene H. Smith

Theoretical Determination and Analysis of Electronic Charge Distributions

The present status of the accurate calculation of molecular and crystalline charge densities is reviewed critically with emphasis on the role of basis sets and electron correlation. The analysis, interpretation and use of the various types of charge densities are considered in some detail.

Characterization of aromatic-thiol π-type hydrogen bonding and phenylalanine-cysteine side chain interactions through ab initio calculations and protein database analyses

In this study, the aromatic-thiol π hydrogen bonding and phenylalanine-cysteine side chain interactions are characterized through both molecular orbital calculations on a C6H6-HSCH3 model complex and database analyses of 609 X-ray protein structures. The aromatic-thiol π hydrogen bonding interaction can achieve a stabilization energy of 2.60 kcal mol−1, and is stronger than the already documented aromatic-hydroxyl and aromatic-amino hydrogen bonds. However, the occurrence of the aromatic-thiol hydrogen bond is rather rare in proteins. This is because most of the thiol groups participate in the formation of either disulphide bonds or stronger S—H…O (or N) ‘normal’ hydrogen bonds in a protein environment. Interactions between the side chains of phenylalanine and cysteine residues are characterized as the phenyl(Phe)(HSCH2-)(Cys) interaction. The bonding energy for such interactions is approximately 3.71 kcal mol-1 and is achieved in a geometric arrangement with an optimal phenyl(Phe)-(HS-)(Cys) π-type hydrogen bonding interaction. The interaction is very sensitive to the orientation of the two lone electron pairs on the sulphur atom relative to the π electron cloud of the phenyl ring. Accordingly, the interaction configurations that can accomplish a significant bonding energy exist only within a narrow configurational space. The database analysis of 609 experimental X-ray protein structures demonstrates that only 268 of the 1620 cysteine residues involve such phenylalanine-cysteine side chain interactions. Most of these interactions occur in the form of π (aromatic)-lone pair(sulphur) attractions, and correspond to a bonding energy less than 1.5 kcal mol−1. A few were identified as the aromatic-thiol hydrogen bond with a bonding energy of 2.0–3.6 kcal mol−1.

Internally folded densities

The 3q-dimensional Fourier transforms of the q-electron charge density and momentum density respectively are defined to be the q-electron form factor and internally folded density. For q = 1, these quantities are just the usual X-ray form factor and the B(r) function recently introduced for the analysis of Compton profiles. Numerous properties of the one-electron internally folded density B(r) are derived. B(r) functions for the atoms from He through Ne are calculated and examined for chemical content with the help of the concept of reduced Compton profiles.

On the relationship of the X-ray form factor to the 1-matrix in momentum space☆

Abstract The X-ray form factor F(μ) is shown to be proportional to the convolution of the momentum space charge density matrix.

Electronic, vibrational and magnetic properties of a novel C48N12 aza-fullerene

Abstract The structural, electronic, vibrational and magnetic properties of a novel C 48 N 12 aza-fullerene are studied by using density functional and Hartree–Fock methods. Optimized geometries and total energy of this aza-fullerene are calculated. The HOMO–LUMO gap of C 48 N 12 is found to be about 1 eV smaller than that of C 60 . Fifty-eight IR-active frequencies and 10 NMR spectral signals are predicted for C 48 N 12 . Diamagnetic shielding factor, polarizability and hyperpolarizability of C 48 N 12 are calculated. Our results suggest that C 48 N 12 may have potential applications as semiconductor components and possible building materials for nanometer electronics, photonic devices and diamagnetic superconductors.

A new analytic approximation to atomic incoherent X‐ray scattering intensities

A new analytic approximation to atomic incoherent X-ray scattering intensities is proposed. Unlike other approximations in the literature, the present function has the correct asymptotic behaviour at both large and small values of s. Fits to the incoherent intensities calculated by Cromer are presented for all atoms from He through Am.

The electron momentum distribution of molecular hydrogen

Abstract The (e, 2e) cross section for electron impact ionization of the hydrogen molecule has been measured with a precision of a few percent at 400 eV incident energy. The experimental values are compared to predictions based upon a broad range of theoretical wavefunctions. Discrepancies between theory and experiment are similar to those found when comparing experimental and theoretical Compton profiles.

Structural, electronic, and magnetic properties of heterofullerene C48B12

Bonding, electric (hyper)polarizability, vibrational, and magnetic properties of heterofullerene C48B12 are studied by first-principles calculations. Infrared- and Raman-active vibrational frequencies Of C48B12 are assigned. Eight C-13 and two B-11 nuclear magnetic resonance (NMR) spectral signals Of C48B12 are characterized. The average second hyper-polarizability of C48B12 is about 180% larger than that of C-60. Our results suggest that C48B12 is a candidate for photonic and optical limiting applications because of the enhanced third-order optical non-linearities

On a representation of the long-range interatomic interaction potential

A representation of the long-range dispersion series proposed by Smith and Thakkar (1972) and rediscovered by LeRoy (1974) requires a choice of n or equivalently chi =C10C6/(C8)2. chi is nearly constant for interactions between pairs of S-state atoms. This constancy is explained in terms of the single oscillator model, the hydrogenic model and the Starkschall-Gordon model.

Atomic information entropies beyond the Hartree-Fock limit

Atomic information entropies are computed from configuration interaction wavefunctions for members of the lithium isoelectronic series. For each member of the series, a sequence of wavefunctions built from increasingly larger basis sets is generated which satisfies a density convergence criterion, and exhibits a monotonic behaviour with respect to the density generated from the wavefunctions. Studies of the behaviour of the entropies along a sequence for a particular atom includes trends which have been shown to be present at the Hartree-Fock level. Results suggest that the position and momentum entropies are sensitive to small changes in density and thus can be useful indicators of the quality of the density. The sum of the entropies is also shown to be a useful indicator when density differences are larger. The nature of the information entropy-energy relationship is examined.