Galina Orlova

Density functional theory with fractionally occupied frontier orbitals and the instabilities of the Kohn–Sham solutions for defining diradical transition states: Ring-opening reactions

Density functional theory with fractionally occupied frontier orbitals was combined with an analysis of the instability of the Kohn–Sham solutions and applied to the study of the ring-opening reactions of 1,2-dioxetene, 1,2-dithiete, 1,2-diselenete, and 1,2-ditellurete. These molecules possess transition structures and reactive intermediates for the ring-opening reactions which exhibit strong nondynamical electron correlation. All restricted density functional theory (DFT) solutions for the closed shell transition state structures for all these species are unstable. The solutions with the hybrid Hartree–Fock DFT functionals, B3LYP and B3PW91, are triplet unstable, while for the pure DFT functional BLYP the instability is due to a violation of the Aufbau principle. The same types of instabilities were found for the 1,2-diselenete and 1,2-ditellurete intermediates. Lower energy stable solutions for the diradical transition structures were found with unrestricted DFT methods allowing fractionally occupied or...

Catalytic oxidation of H2 by N2O in the gas phase: O-atom transport with atomic metal cations.

Twenty-five atomic cations, M (+), that lie within the thermodynamic window for O-atom transport catalysis of the oxidation of hydrogen by nitrous oxide, have been checked for catalytic activity at room temperature with kinetic measurements using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Only 4 of these 25 atomic cations were seen to be catalytic: Fe (+), Os (+), Ir (+), and Pt (+). Two of these, Ir (+) and Pt (+), are efficient catalysts, while Fe (+) and Os (+) are not. Eighteen atomic cations (Cr (+), Mn (+), Co (+), Ni (+), Cu (+), Ge (+), Se (+), Mo (+), Ru (+), Rh (+), Sn (+), Te (+), Re (+), Pb (+), Bi (+), Eu (+), Tm (+), and Yb (+)) react too slowly at room temperature either in their oxidation with N 2O to form MO (+) or in the reduction of MO (+) by H 2. Many of these reactions are known to be spin forbidden and a few actually may lie outside the thermodynamic window. Three alkaline-earth metal monoxide cations, CaO (+), SrO (+), and BaO (+), were observed to favor MOH (+) formation in their reactions with H 2. A potential-energy landscape is computed for the oxidation of H 2 with N 2O catalyzed by Fe (+)( (6)D) that vividly illustrates the operation of an ionic catalyst and qualitatively accounts for the relative inefficiency of this catalyst.

Pyramidane and isoelectronic pyramidal cations

Abstract Ab initio STO-3G, 4-31G and semi-empirical MINDO/3 calculations were performed on a series of C 4 H 4 X compounds to analyze the possibility of existence of their nonclassical C 4V pyramidal isomeric forms. It was shown that with the four-electron group X (C, N + , P + , O 2+ , S 2+ ) in an apex, C 4V pyramidal structures are indeed stable, i.e., conform to local minima on the corresponding potential energy surfaces. Relative energies, molecular geometries and charge density distribution for various isomeric forms of the C 4 H 4 X compounds are presented. The theoretically derived strategies for an experimental trapping of the pyramidane 1 , X = C and the C 4V pyramidal thiophene dication 1 , X = S 2+ are discussed.

Is density functional theory free of spatial symmetry breaking? The case of the linear carbon radical cations: C3+, C5+, C7+, and C9+

Abstract Spatial symmetry breaking is examined numerically for the linear Cn+ (n=3, 5, 7, 9) radical cations with hybrid B3LYP and local BP86 functionals. B3LYP clearly breaks spatial symmetry for 2 Σ u + D ∞ h structures for C3+ and C5+. BP86 does not break spatial symmetry but generates lower-symmetry lower-energy 2 Σ C ∞ v solutions which are not connected with the 2 Σ u + D ∞ h structures starting from C5+. The 2 Π g and 2 Π u electronic states do not suffer from spatial symmetry breaking. The bent C3+, kinked C5+, and quasi-linear C2v and C2h structures of C7+ and C9+ are the lowest energy chains.

Singularities in the behavior of density functionals in predictions of singlet biradicals: The 1,2-dichalcogenins

The performance of conventional exchange-correlation functionals for the description of singlet biradicals which possess a small admixture of an auxiliary electronic state was examined using a broken symmetry spin unrestricted density functional approach. A pure density functional, BLYP, and the hybrid B3LYP and BHandHLYP exchange-correlation functionals were employed to study the thermal ring opening of 1,2-dichalcogenins and the subsequent formation of the bicyclic products. A stepwise mechanism was predicted for the reaction of the 1,2-dithiin and 2-selenathiin rings and no biradical structures were located. For 1,2-diselenin, the biradical stepwise mechanism competes energetically with a concerted reaction which is slightly favored. For 1,2-ditellurin, a high-energy open intermediate with considerable biradical character was located. The hybrid functionals show an increase in biradical character with an increase in the percentage of Hartree–Fock exchange incorporated into the exchange functional. The ...

Practical failures from the inclusion of exact exchange: how much exact exchange is appropriate?

The influence of exact exchange incorporated into exchange—correlation functionals on the predictions of relative energies, structures, electronic states, and vibrational spectra is examined numerically. Failures of widely used hybrid exchange—correlation functionals due to either the physical unacceptability of including exact exchange or an unbalanced mixing of exact exchange are considered. One set of examples involves tetraatomic chalcogen clusters and charge transfer complexes between diatomic chalcogens and diatomic oxygen. Poor energetic predictions from Hartree-Fock rule against the inclusion of exact exchange into the exchange—correlation functionals for these systems with significant left—right electron correlation effects. The energies of the conformers of [10]annulene are considered from an unusual viewpoint, namely, the empirical adjustment of the admixture of exact exchange to match the predictions of very high level theoretical methods. For this annulene with insignificant left-right electron correlation effects, a greater (50%) percentage of exact exchange should be included. The relationship of symmetry breaking to the inclusion of exact exchange is examined for seven linear radicals, OXO (X = B, Al, Ga, In, TI). AIOS, and OAIS. exchange—correlation functionals generate symmetry adapted solutions at the expense of an unusual ordering of the Kohn-Sham orbitals, which can cause uncharacteristic electronic states, incorrect vibrational spectra, and poorer predictions of energetics. These effects are greater when exact exchange is included. In all the examples considered, the appropriate focus for a detailed discussion of molecular properties involves consideration of the effects of exact exchange.

Experimental and computational studies of the macrocyclic effect of an auxiliary ligand on electron and proton transfers within ternary copper(II)-Histidine complexes

The dissociation of [CuII(L)His]•2+ complexes [L=diethylenetriamine (dien) or 1,4,7-triazacyclononane (9-aneN3)] bears a strong resemblance to the previously reported behavior of [CuII(L)GGH]•2+ complexes. We have used low-energy collision-induced dissociation experiments and density functional theory (DFT) calculations at the B3LYP/6-31+G(d) level to study the macrocyclic effect of the auxiliary ligands on the formation of His•+ from prototypical [CuII(L)His]•2+ systems. DFT revealed that the relative energy barriers of the same electron-transfer (ET) dissociation pathways of [CuII(9-aneN3)His]•2+ and [CuII(dien)His]•2+ are very similar, with the ET reactions of [CuII(9-aneN3)His]•2+ leading to the generation of two distinct His•+ species; in contrast, the proton transfer (PT) dissociation pathways of [CuII(9-aneN3)His]•2+ and [CuII(dien)His]•2+ differ considerably. The PT reactions of [CuII(9-aneN3)His]•2+ are associated with substantially higher barriers (>13 kcal/mol) than those of [CuII(dien)His]•2+. Thus, the sterically encumbered auxiliary 9-aneN3 ligand facilitates ET reactions while moderating PT reactions, allowing the formation of hitherto nonobservable histidine radical cations.

Density functional theory predictions for small radicals containing boron and aluminium: broken symmetry problems and solutions

Selected exchange-correlation functionals were employed to study certain radicals for which unrestricted (U) Hartree–Fock and post-Hartree–Fock methods showed spatial symmetry breakings. For AlO, BO2 and BS2 all functionals produced symmetry adapted solutions and predicted geometries and vibrational spectra in good accord with experiment. The USVWN and UB3LYP solutions for the D∞h O–Al–O structure break spin symmetry. Mixing of these spin-unrestricted Kohn–Sham orbitals yielded stable solutions with reasonable geometries and energetics but with large errors for the vibrational spectra. Only UBHandHLYP results in a broken spatial symmetry solution and yields an anomalous vibrational spectrum. The UBLYP solution does not show a tendency to instability and predicts a Dinfin;h O–Al–O species with a 2Πg electronic state.

Exploiting the photocatalytic activity of gold nanoparticle-functionalized niobium oxide perovskites in nitroarene reductions

Novel gold nanoparticle-doped niobium perovskites are synthesized using a chemical reduction technique, affording supported nanoparticles on the order of 8–10 nm. The UVA-initiated photocatalytic activity of the nanocomposites was studied using nitroarene reduction as a probe reaction which yielded the corresponding aminoarenes in as little as 3 hours with yields up to 92%. Analyses of kinetic data, using Hammett parameters and computational methods, show that electron-withdrawing groups accelerate the photocatalytic process and suggest that electron transfer from the gold nanoparticle surface to the nitro group is an integral part of the rate-limiting step of this reaction. The linearity of the Hammett plot shows that no change in the photocatalytic mechanism can be expected upon variation of the para-aryl substitution. Investigation of the reaction mechanism using CH3OH-d4 illustrates that CH3OH is the most likely source of protons, as both electrons and protons are required for successful photoreduction. Moderate recyclability of the heterogeneous nanomaterial over three catalytic cycles is observed.

Covalent nature of the hydrogen bond

ConclusionsThe main conclusion derived from this work is that the H bond in all three FHF−, FHFH, and HFHFH+ systems is a three-centered, two-electron, covalent chemical bond formed at the expense of 2a1g (2a1) MO bonding. The 1a1g MO bonding has little effect on H-bond stabilization. Thus the H bond is a one-orbital chemical bond with its formation corresponding, to that of a three-centered MO, as distinguished from molecules bonded by a two-centered MO (e.g., F2 or HOOH [41]; hence the H bond is much weaker. The uniqueness of the H bond lies in its being the weakest covalent bond. It is precisely the covalent nature of the H bond that gives it its characteristic properties, i.e., saturability and strict compliance to structural requirements. In addition, the low dissociation barrier makes it easy to control the H bond under mild conditions, which is very important in biological systems.