Recent work on approximating ground states of Heisenberg spin clusters by projected Hartree–Fock theory (PHF) is extended to a cluster-based ansatz (cPHF). Whereas PHF variationally optimizes a ...site–spin product state for the restoration of spin- and point-group symmetry, cPHF groups sites into discrete clusters and uses a cluster-product state as the broken-symmetry reference. Intracluster correlation is thus already included at the mean-field level, and intercluster correlation is introduced through symmetry projection. Variants of cPHF differing in the broken and restored symmetries are evaluated for ground states and singlet-triplet gaps of antiferromagnetic spin rings for various cluster sizes, where cPHF in general affords a significant improvement over ordinary PHF, although the division into clusters lowers the cyclical symmetry. In contrast, certain two- or three-dimensional spin arrangements permit cluster groupings compatible with the full spatial symmetry. We accordingly demonstrate that cPHF yields approximate ground states with correct spin- and point-group quantum numbers for honeycomb lattice fragments and symmetric polyhedra.
To assess the accuracy of different quantum mechanical methods for biochemical modeling, the reaction energies of 20 small model reactions (chosen to represent chemical steps catalyzed by commonly ...studied enzymes) were calculated. The methods tested included several popular Density Functional Theory (DFT) functionals, second-order Møller Plesset perturbation theory (MP2) and its spin-component scaled variant (SCS-MP2), and coupled cluster singles and doubles and perturbative triples (CCSD(T)). Different basis sets were tested. CCSD(T)/aug-cc-pVTZ results for all 20 reactions were used to benchmark the other methods. It was found that MP2 and SCS-MP2 reaction energy calculation results are similar in quality to CCSD(T) (mean absolute error (MAE) of 1.2 and 1.3 kcal mol
−1
, respectively). MP2 calculations gave a large error in one case, and are more subject to basis set effects, so in general SCS-MP2 calculations are a good choice when CCSD(T) calculations are not feasible. Results with different DFT functionals were of reasonably good quality (MAEs of 2.5–5.1 kcal mol
−1
), whereas popular semi-empirical methods (AM1, PM3, SCC-DFTB) gave much larger errors (MAEs of 11.6–14.6 kcal mol
−1
). These results should be useful in guiding methodological choices and assessing the accuracy of QM/MM calculations on enzyme-catalyzed reactions.
The electronic structure of heterometallic cubane-type clusters with core {Mo
3
S
4
M′} (M′ = Cu
+
, Ni
0
, Pd
0
) was investigated by the Atoms in Molecules and Electron Localization Function ...topological methods as well as Energy Decomposition Analysis. It was found that {Mo
3
S
4
Cu} and {Mo
3
S
4
M′} (M′= Ni, Pd) clusters have a different interaction between molybdenum and heterometallic atoms. In case of the complexes with Ni/Pd atom, the disynaptic basins on the Mo–M′ bonds were observed in contrast to the Cu containing complex. The complex formation between heterometallic fragments and trinuclear molybdenum complex {Mo
3
S
4
} were found to be hindered due to steric interactions. Nevertheless, the formation of the heterometallic cubane-type clusters occurs due to the multicenter and pair covalent interactions between Mo and M′ atoms.
Abstract
Theoretically speaking
: The mechanistic details associated with the generation and reaction of CuO
+
species from Cu
I
–α‐ketocarboxylate complexes, especially with respect to modifications ...of the ligand supporting the copper center, were investigated (see scheme). Theoretical models were used to characterize the electronic structures of different CuO
+
species and their reactivity in CH activation and O‐atom transfer reactions.
magnified image
A mechanism for the oxygenation of Cu
I
complexes with α‐ketocarboxylate ligands that is based on a combination of density functional theory and multireference second‐order perturbation theory (CASSCF/CASPT2) calculations is elaborated. The reaction proceeds in a manner largely analogous to those of similar Fe
II
–α‐ketocarboxylate systems, that is, by initial attack of a coordinated oxygen molecule on a ketocarboxylate ligand with concomitant decarboxylation. Subsequently, two reactive intermediates may be generated, a Cu–peracid structure and a CuO
+
species, both of which are capable of oxidizing a phenyl ring component of the supporting ligand. Hydroxylation by the CuO
+
species is predicted to proceed with a smaller activation free energy. The effects of electronic and steric variations on the oxygenation mechanisms were studied by introducing substituents at several positions of the ligand backbone and by investigating various N‐donor ligands. In general, more electron donation by the N‐donor ligand leads to increased stabilization of the more Cu
II
/Cu
III
‐like intermediates (oxygen adducts and CuO
+
species) relative to the more Cu
I
‐like peracid intermediate. For all ligands investigated, the CuO
+
intermediates are best described as Cu
II
O⋅
−
species with triplet ground states. The reactivity of these compounds in CH abstraction reactions decreases with more electron‐donating N‐donor ligands, which also increase the CuO bond strength, although the CuO bond is generally predicted to be rather weak (with a bond order of about 0.5). A comparison of several methods to obtain singlet energies for the reaction intermediates indicates that multireference second‐order perturbation theory is likely more accurate for the initial oxygen adducts, but not necessarily for subsequent reaction intermediates.
The Structure of carbodiimide has been studied by using quantum chemical methods. Carbodiimide (HNCNH) has been detected towards Sagittarius B2 (N) in interstellar medium (ISM). Two reaction ...mechanisms have been proposed to study the formation of interstellar Carbodiimide. The first reaction mechanism is based on molecule-radical and the second one is a radical-radical mechanism, through previously detected interstellar molecules or radicals. Quantum chemical calculations have been performed by using density functional theory (DFT) and Moller-Plesset second order perturbation (MP2) theory, in gas phase as well as in polarizable continuum model (PCM). The proposed reaction paths are exothermic and barrierless which indicates the possibility of carbodiimide formation in ISM. Several basis sets have been used to verify the validity and accuracy of the results. The isotropic and anisotropic polarizabilities of carbodiimide have been calculated from relevant tensor components for both reaction mechanisms with the help of data obtained by DFT/B3LYP and MP2 methods using aug-cc-pVTZ basis sets in gaseous phase as well as in PCM.
The singlet and triplet potential energy surfaces for the reaction of HO
2
radical with Cl atom are explored by using high-level combination W1BD and W1U methods. Statistical rate theories are ...employed to compute the rate coefficients of the main product channels including HCl + O
2
(
1
Σ
u
-
), HCl + O
2
(
3
Σ
g
+
), OH + ClO and HOOCl as a function of pressure and temperature. It is found that the rate coefficients for the product channels OH + ClO and HOOCl are strongly depended on temperature and pressure. At lower pressures and higher temperatures, the product OH + ClO is important, while at higher pressures and lower temperatures, HOOCl is the dominant product. At some moderate temperatures and pressures, the rate coefficients for the formation of OH + ClO and HOOCl could be equal. The product channel HCl + O
2
(
3
Σ
g
+
) proceeds via formation of a van der Waals complex Cl..HOO and, next, passes through a saddle point structure with a negative energy of − 5.0 kJ mol
−1
relative to the reactants. It is shown that a two transition state model suitably describes the kinetics of this product channel.
The electronic structures and bonding patterns for a new class of radical cations, HnE‐H‐H‐EHn+ (EHn=element hydride, E=element of Groups 15–18), have been investigated by applying quantum‐chemical ...methods. All structures investigated give rise to symmetric potential energy minimum structures. We envisage clear periodic trends. The HH bond length is shorter for elements toward the bottom of the periodic table of elements, and a short HH bond corresponds to accumulation of electron density in the central HH region. All HnE‐H‐H‐EHn+ of Groups 15–17 are thermodynamically unstable towards loss of either H2 or H. The barriers for these dissociations are rather low. The Group 18 congeners, except E=Xe, appear to be global minima of the respective potential energy surfaces. The findings are discussed in terms of H2 bond activation, and a general mechanistic scheme for the standard reduction process 2H+ + 2e− → H2 is given. Finally, it is proposed that some of the symmetric radical cations are likely to be observed in mass spectrometric or matrix isolation experiments.
Activated dihydrogen bridges main‐group elements: A new class of radical cations, HnE‐H‐H‐EHn+ (see graphic), has been investigated by applying quantum‐chemical methods. These HnE‐H‐H‐EHn+ ions are unstable towards loss of H2 or H, so there is potential for these types of structures as building blocks for solid‐state devices for hydrogen storage.
The surface energy of metals Vitos, L.; Ruban, A.V.; Skriver, H.L. ...
Surface science,
08/1998, Letnik:
411, Številka:
1-2
Journal Article
Recenzirano
We have used density functional theory to establish a database of surface energies for low index surfaces of 60 metals in the periodic table. The data may be used as a consistent starting point for ...models of surface science phenomena. The accuracy of the database is established in a comparison with other density functional theory results and the calculated surface energy anisotropies are applied in a determination of the equilibrium shape of nano-crystals of Fe, Cu, Mo, Ta, Pt and Pb.
The spectral-luminescent properties of the charged (anionic and cationic) forms of three biologically active substituted benzaldehydes, namely,
o
-anisaldehyde (2-methoxybenzaldehyde), syringaldehyde ...(3,5-dimethoxy-4-hydroxybenzaldehyde), and vanillin (3-methoxy-4-hydroxybenzaldehyde), are studied by quantum-chemical methods. Calculations show that the
S
1
state of the charged forms of the studied molecules, in contrast to the neutral forms, is of the ππ* type, and its localization is similar to the localization of the
S
2
(ππ*) state of the neutral molecules (anisaldehyde and vanillin) or of the
S
3
(ππ*) state of syringaldehyde. According to the calculation results, the spectral range of 240–420 nm contains no new absorption bands corresponding to electronic transitions differing in nature and localization from the electronic transition of the neutral molecules. The calculated fluorescence characteristics of the charged forms of the studied molecules show that the radiative decay rate of the charged forms is considerably higher than that of the neutral forms, which is related to the different orbital nature of the
S
1
state in the neutral and charged forms. Analysis of the calculated and experimental data on the fluorescence of the studied substituted benzaldehydes in alcohol solutions reveals that the fluorescence at 410 nm belongs to the cationic forms. Vanillin and syringaldehyde may also exhibit weak fluorescence of their anionic forms.
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