Search

Since the 1960s, dithiolene complexes have been intensely studied; however, the same cannot be said about diselenolene complexes. Thus, the chemistry associated with the reduction of several Mo- and W-tris(diselenolene) complexes was investigated. In particular, relative reduction potentials, changes in key geometrical properties, the contribution of the metal center to the redox active MO, and HOMO–LUMO energy gaps are investigated. It is noted that the results obtained for the tris(diselenolene) complexes are compared to analogous Mo- and W-tris(dithiolene) complexes to understand the effect of substituting the sulfur atoms with selenium atoms. The reduction potentials of the complexes are more dependent upon the choice of the ligand than the metal. Overall, it is found that the substitution of chalcogen atom for the tris complexes investigated herein has only a subtle effect on the calculated reduction potentials between analogous redox couples. Upon reduction of the neutral and mono-anionic complexes, it is found that changes in key bond lengths, fold angles (θ), and trigonal twist angles (ΦAvg) are very similar for the tris(diselenolene) complexes investigated. Such changes have been previously observed for several Mo- and W-tris(dithiolene) complexes. Comparing the HOMO–LUMO energy gaps of the tris(diselenolene) complexes to the tris(dithiolene) complexes, the former complexes have on average a 0.07-eV smaller energy gap and are thus expected to be slightly more reactive to reduction. Lastly, in the neutral complexes, the Mo and W atoms contribute at most 26% to the redox active MO; thus, in the case of the tris(diselenolene) complexes, it can be concluded that the redox active MO is predominantly ligand based. The contribution of the metal-based AO becomes less as the complexes are reduced. In summary, given the high interest of dithiolene complexes in the areas of alternative energy and material science, the results presented herein provide motivation to further investigate the chemistry of diselenolene complexes.

The local value of the single-particle momentum provides a direct three-dimensional representation of bonding interactions in molecules. It is given exclusively in terms of the electron density and its gradient, and therefore is an ideal localized electrons detector (LED). The results introduced here extend to molecular systems our study of the single-particle local momentum in atomic systems (Bohorquez and Boyd in J Chem Phys 129:024110, 2008; Chem Phys Lett 480:127, 2009). LED is able to clearly identify covalent and hydrogen bonding interactions by depicting distinctive regions around the bond critical points, emerging as a complementary tool in conventional atoms in molecules studies. The local variable we introduce here is an intuitively interpretable 3D electron-pairs locator in atoms and molecules that can be computed either from theoretical or experimentally derived electron densities.

Protein tyrosine phosphatase 1B (PTP1B) functions by removing the phosphoryl group from tyrosinephosphorylated proteins in insulin signaling and metabolism. The regeneration of the active site involves a sulphenylamide intermediate derived from the intrastrand cross-linking between the catalytic serine and the neighboring backbone nitrogen. Two mechanisms have been proposed for the formation of the sulphenylamide intermediate and the subsequent reactivation of the catalytic site. In the current work, the proposed mechanisms have been investigated by the use of density functional theory calculations. Our results suggest that these two mechanisms have similar overall energy barriers and that the preferred route will be determined by the availability of hydrogen peroxide or other oxidizing reagents.

This communication outlines the development of a novel method for the ab initio computation of molecular systems wherein the Born–Oppenheimer approximation is not invoked. In the current method, a common Hamiltonian is employed to operate on the electrons and nuclei simultaneously. In addition, an inseparable wavefunction is generated to describe the non-Born–Oppenheimer behaviour. The physical implications of the new method are discussed.

The purpose of this overview is to highlight the broad scope and utility of current applications of density functional theory (DFT) methods for the study of the properties and reactions of biomolecules. This is illustrated using examples selected from research carried out within our research group and in collaboration with others. The examples include the hyperfine coupling constants of amino acid radicals, the use of an amino acid as a chiral catalyst for the formation of carbon–carbon bonds in the aldol reaction, hydrogen-bond mediated catalysis of an aminolysis reaction, radiation-induced protein–DNA cross-links, and the mechanism by which an antitumor drug cleaves DNA. We demonstrate that DFT-based methods can be applied successfully to a broad range of problems that remain beyond the scope of conventional electron-correlation methods. Furthermore, we show that contemporary computational quantum chemistry complements experiment in the study of biological systems.

Mean values of r 1 and r 12 for the ground and several excited states of the helium isoelectronic sequence are used to demonstrate that a simple scaling which superimposes the distribution function f(r 12) as a function of the atomic number leads to a similar result for the electron density distribution D(r1). On the basis of a screening interpretation of the scaling parameter δ, it is concluded that screening is greater in the singlet than the triplet state of a particular configuration, that screening is greater in the P states than the corresponding S states, and that the screening approaches the limiting value of 1 for the highly excited states. The perturbation expansions of Scherr and Knight are used to determine the limiting value of δ when Z→∞ and the relationship between the scaling parameter and the scale factor, chosen so that a trial wave function satisfies the virial theorem, is discussed. A brief discussion of the scaling of the Coulomb hole is presented.

The unrestricted Hartree-Fock (UHF) method of Snyder and Amos is used to calculate, in the π-electron approximation, the spin density distributions in radical anions of heterocyclic amine N-oxides. The computed spin densities are observed to be in good agreement with the experimental values. The computed spin density distribution of the radical anion of pyridine N-oxide is consistent with the greater susceptibility of pyridine N-oxide relative to pyridine to electrophilic nitration. Also, the calculations are consistent with the lower basicity of the N-oxides relative to the parent bases.

The Fermi and Coulomb holes of the 21S state of the helium isoelectronic sequence are investigated. Several interesting differences between the results obtained and those which might be expected on the basis of the corresponding 23S state are pointed out and discussed.

STRUCTURAL studies with X rays1,2 of five and six-coordinated iron porphyrin complexes have shown that the distance from the iron atom to the nitrogen atoms of the porphyrin rings is less for low spin Fe(III) than for high spin Fe(III). As a result, it is often assumed that the Fe(III) atom is larger in its high spin state than in its low spin state. Similar size differences for the Fe(II) system form the basis of the Perutz model3 for the cooperativity of oxygen binding in haemoglobin. I discuss here the relative sizes of the high and low spin states of atoms in terms of the results of quantum mechanical calculations on isolated atoms. Contrary to earlier interpretations of structural studies, I have found that in the atoms I have studied the lowest lying high spin state of an atom in a given configuration is smaller than any state of lower spin.

Hund's multiplicity rule, according to which a high spin state has a lower energy than any other state of lower spin arising from the same configuration, was deduced from atomic spectra immediately before the advent of modern quantum mechanics. Since then, the lower energy of the state of higher multiplicity has generally been explained by assuming greater electron repulsion in the lower spin states. Numerous calculations1–13 have shown, however, that the traditional explanation of Hund's rule is invalid. In every neutral species for which the requisite calculations have been carried out, the electron–electron repulsion, Vee, is greater in the high spin state, and the lower energy of the high spin state is due to its greater electron–nucleus attraction, Ven. I show here that Hund's rule is a consequence of the Fermi correlation between electrons with parallel spin which leads to less screening of the nucleus in the high spin state.