Geometry prediction of hafnocenes by quantum chemical methods

The performance of quantum chemical methods for geometry prediction of hafnocenes was evaluated. HF, B3LYP and MP2 in combination with nonrelativistic (MHF) and relativistic (MWB and LANL2DZ) basis sets for hafnium together with standard basis sets 3-21G*, 6-31G* and 6-311G** for other elements were applied. Five basic structural parameters of the optimized structures of the hafnocenes were compared with experimental crystal structures obtained from the Cambridge structural database. Altogether 80 hafnocenes were included in the analysis. The results show that relativistic corrections are necessary for Hf atom. However, even the Hartree–Fock (HF) method, when combined with relativistic pseudopotentials, reproduces the experimental crystal structures with significant accuracy. The good performance of the HF method can be understood to originate from the absence of significant near-degeneracy correlations for hafnium. On average, the B3LYP and MP2 methods provide structural parameters somewhat closer to the experimental ones.     

IR, Raman and SERS spectra of 2-phenoxymethylbenzothiazole

The FT-IR and FT-Raman spectra of 2-phenoxymethylbenzothiazole were recorded and analyzed. The surface enhanced Raman scattering (SERS) spectrum was recorded in a silver colloid. The vibrational wavenumbers of the compound have been computed using the Hartree–Fock/6-31G* basis and compared with the experimental values. The appearance of the Ag–O stretching mode at 237 cm⁻¹ in the SERS spectrum along with theoretically calculated atomic charge density, leads us to suggest that the molecule is adsorbed through the oxygen atom with the molecular plane tilted on the colloidal silver surface. The direction of charge transfer contribution to SERS has been discussed from the frontier orbital theory.     

IR, Raman and SERS studies of methyl salicylate

The IR and Raman spectra of methyl salicylate (MS) were recorded and analysed. Surface enhanced Raman scattering (SERS) spectrum was recorded in silver colloid. The vibrational wave numbers of the compound have been computed using the Hartree-Fock/6-31G* basis and compared with the experimental values. SERS studies suggest a flat orientation of the molecule at the metal surface.     

6‐31G* basis set for third‐row atoms

Medium basis sets based upon contractions of Gaussian primitives are developed for the third‐row elements Ga through Kr. The basis functions generalize the 6‐31G and 6‐31G* sets commonly used for atoms up to Ar. A reexamination of the 6‐31G* basis set for K and Ca developed earlier leads to the inclusion of 3d orbitals into the valence space for these atoms. Now the 6‐31G basis for the whole third‐row K through Kr has six primitive Gaussians for 1s, 2s, 2p, 3s, and 3p orbitals, and a split‐valence pair of three and one primitives for valence orbitals, which are 4s, 4p, and 3d. The nature of the polarization functions for third‐row atoms is reexamined as well. The polarization functions for K, Ca, and Ga through Kr are single set of Cartesian d‐type primitives. The polarization functions for transition metals are defined to be a single 7f set of uncontracted primitives. Comparison with experimental data shows good agreement with bond lengths and angles for representative vapor‐phase metal complexes. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 976–984, 2001     

Accurate atomic Gaussian basis functions for second-row atoms: Small split-valence 3-21SP and 4-22SP basis sets

Small split-valence Gaussian 3-21SP and 4-22SP basis sets, previously reported for the first-row atoms [Chem. Phys. Lett., 229 , 151 (1996)], have been extended for the second-row elements of the Periodic Table. The total energies of the ground states of the second-row atoms calculated with the new basis sets are significantly lower than those obtained with the well-known 3-21G (J. Am. Chem. Soc., 104 , 2797 (1982)] and 4-31G [J. Chem. Phys., 56 , 5255 (1972)] basis sets. This is because, as first noted in our previous work for first-row atoms, that the 3-21G and 4-31G basis sets only correspond to a local minimum of the Hartree–Fock energy functional, which is relatively far from its global minimum. The proposed basis sets have been tested by performing geometry optimizations and calculations of normal frequencies in the harmonic approximation of some diatomic and polyatomic molecules at the Hartree–Fock level. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1200–1210     

Accurate prediction of proton chemical shifts. II. Peptide analogues

Proton chemical shifts of eight cyclic amide molecules were measured in DMSO and D2O solutions. The magnetic shieldings of the corresponding aliphatic, aromatic, and amide protons were calculated by Hartree-Fock and DFT, using the 6-311G**, 6-311++G**, and TZVP basis sets. For aliphatic protons, all of these methods reproduce the experimental values in DMSO solutions excellently after linear regression. The Hartree-Fock method tends to give slightly better agreement than DFT. The best performance is given by the HF/6-311G** method, with an rms deviation of 0.068 ppm. The deviations from experimental chemical shifts in D2O solutions are only slightly larger than those in DMSO solutions. This suggests that we can use the calculated gas phase proton chemical shifts directly to predict experimental data in various solvents, including water. For amide protons, which exchange with water and form hydrogen bonds with DMSO, only modest agreement is obtained, as expected. The present studies confirm that the GIAO approach can reach high accuracy for the relative chemical shifts of aliphatic and aromatic protons at a low cost. Such calculations may provide constraints for the conformational analysis of proteins and other macromolecules.     

Formation of Octameric Methylaluminoxanes by Hydrolysis of Trimethylaluminum and the Mechanisms of Catalyst Activation in Single‐Site α‐Olefin Polymerization Catalysis

Hydrolysis of trimethylaluminum (TMA) leads to the formation of methylaluminoxanes (MAO) of general formula (MeAlO)_n(AlMe₃)_m. The thermodynamically favored pathway of MAO formation is followed up to n=8, showing the major impact of associated TMA on the structural characteristics of the MAOs. The MAOs bind up to five TMA molecules, thereby inducing transition from cages into rings and sheets. Zirconocene catalyst activation studies using model MAO co‐catalysts show the decisive role of the associated TMA in forming the catalytically active sites. Catalyst activation can take place either by Lewis‐acidic abstraction of an alkyl or halide ligand from the precatalyst or by reaction of the precatalyst with an MAO‐derived AlMe₂⁺ cation. Thermodynamics suggest that activation through AlMe₂⁺ transfer is the dominant mechanism because sites that are able to release AlMe₂⁺ are more abundant than Lewis‐acidic sites. The model catalyst system is demonstrated to polymerize ethene.     

Additivity and transferability of exchange energy

Exchange energy of molecules at equilibrium geometries is shown to be highly independent of the basis set and nearly equal to the sum of the exchange energies of isolated atoms. Furthermore, molecular exchange can be predicted along a chemical series with high accuracy in terms of empirical exchange contributions associated to the constituent atoms, whose values, obtained by fitting, are similar to those of the isolated atoms. It is concluded that molecular exchange is essentially a sum of atomic contributions whose differences from those of the isolated atoms are small and characteristic of the neighboring atoms. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Magnetization Slow Dynamics in Ferrocenium Complexes

The single-molecule magnet (SMM) properties of a series of ferrocenium complexes, [Fe(η⁵-C₅R₅)₂]⁺ (R=Me, Bn), are reported. In the presence of an applied dc field, the slow dynamics of the magnetization in [Fe(η⁵-C₅Me₅)₂]BAr_F are revealed. Multireference quantum mechanical calculations show a large energy difference between the ground and first excited states, excluding the commonly invoked, thermally activated (Orbach-like) mechanism of relaxation. In contrast, a detailed analysis of the relaxation time highlights that both direct and Raman processes are responsible for the SMM properties. Similarly, the bulky ferrocenium complexes, [Fe(η⁵-C₅Bn₅)₂]BF₄ and [Fe(η⁵-C₅Bn₅)₂]PF₆, also exhibit magnetization slow dynamics, however an additional relaxation process is clearly detected for these analogous systems.     

Alkenyl selenols and selenocyanates: synthesis, spectroscopic characterization by photoelectron spectroscopy, and quantum chemical study

Vinyl, allyl, and homoallyl selenols were easily prepared by a chemoselective reduction of the corresponding selenocyanates with aluminum hydrides. Two stable vinyl and five stable allyl conformers of both series were predicted on the potential-energy surface. The interaction of SeH or SeCN groups with the vinyl group has been investigated with UV photoelectron spectroscopy and quantum chemical calculations, using the MP2/cc-pVTZ and B3LYP/cc-pVTZ levels. In the vinyl derivatives, a surprisingly strong direct conjugation of the selenium lone electron pair and the C=C double bond was observed. On the other hand, in allyl position the selenium lone pair is independent on the C=C double bond, and the hyperconjugation between the Se-C bond and the double bond is the ruling effect. Thus is clarified the type and extent of the interaction between the SeH or SeCN group and the unsaturated moiety.     

Quantum chemical investigation of lanthanum doping effects in tetragonal and rhombohedral PZT materials

Structural and electronic properties of lead–zirconate–titanate (PZT) materials doped with a lanthanum (La) impurity are studied using a quantum‐chemical approach based on the Hartree–Fock theory. Performed geometry optimization in the defective crystals shows that the atomic movements are predominantly outward with respect to the impurity position. It is found that the La impurity enhances a covalent character in the chemical bonding between the Ti and O atoms, as well as the Zr and O atoms situated in the neighborhood of the defect despite the fact that the La‐O interaction remains purely ionic. The occurrence of local energy levels within the band gap of the material is analyzed in light of the available experimental data on La concentration influence upon dielectric and piezoelectric properties in these crystals. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Asymmetric zirconocene precursors for catalysis of propylene polymerization

Racemic isopropylidene (1-η⁵-cyclopentadienyl)(1-η⁵-indenyl) dichlorozirconium and the 3-methylindenyl derivative have been synthesized and characterized. These precursors activated with methylaluminoxane produce poly(propylene) with hemiisotactic microstructures. © 1994 John Wiley & Sons, Inc.     

Modulation of Stacking Interactions by Transition‐Metal Coordination: Ab Initio Benchmark Studies

A series of ab initio calculations are used to determine the C? H???π and π???π‐stacking interactions of aromatic rings coordinated to transition‐metal centres. Two model complexes have been employed, namely, ferrocene and chromium benzene tricarbonyl. Benchmark data obtained from extrapolation of MP2 energies to the basis set limit, coupled with CCSD(T) correction, indicate that coordinated aromatic rings are slightly weaker hydrogen‐bond acceptors but are significantly stronger hydrogen‐bond donors than uncomplexed rings. It is found that π???π stacking to a second benzene is stronger than in the free benzene dimer, especially in the chromium case. This is assigned, by use of energy partitioning in the local correlation method, to dispersion interactions between metal d and benzene π orbitals. The benchmark data is also used to test the performance of more efficient theoretical methods, indicating that spin‐component scaling of MP2 energies performs well in all cases, whereas various density functionals describe some complexes well, but others with errors of more than 1 kcal mol^?1.     

Ab initio quantum chemical calculations of a cluster C8H12

Using unrestricted Hartree–Fock–Roothaan approximation and density functional theory on 6‐31G* basis, ab initio calculation cluster C₈H₁₂ has been carried out. It was shown as a result of calculations that a ground state for the conformation given is a septet state. This cluster can be used for investigation of the kinematic mechanism of magnetic exchange and magnetic ordering in polyradicals. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Theoretical study of one‐carbon unit transfer between methyl‐AICA and N1‐methyl‐N1‐acryloyl‐formamide

The mechanism of one‐carbon unit transfer between 1‐methyl‐5‐amino‐4‐carboxamide imidazole (M‐AICA) and N¹‐methyl‐N¹‐acryloyl‐formamide (the model molecule of 10‐f‐H4F) is investigated by the Hartree–Fock and DFT methods, respectively, at the 6‐31G* basis level. There are two different channels for the proton transfer, resulting in two reaction pathways with different properties. The results indicate that both channels can complete the reaction, but path a is slightly favored due to its lower active energy barrier. Furthermore, the influence of 4‐carboxamindde in M‐AICA is also discussed. This group can stabilize the reactant and intermediates, and reduce the active energy barrier through the intermolecular hydrogen bond. The intermolecular hydrogen bond results in an enlarged conjugation system and makes the transition states more stable. Our results are in agreement with experiments. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Restricted and unrestricted Hartree–Fock approaches applied to spherical quantum dots in a magnetic field

The Roothaan and Pople–Nesbet approaches for real atoms are adapted to quantum dots in the presence of a magnetic field. Single‐particle Gaussian basis sets are constructed, for each dot radius, under the condition of maximum overlap with the exact functions. The chemical potential, charging energy, and total spin expected values are calculated, and we have verified the validity of the quantum dot energy shell structure as well as Hund's rule for electronic occupation at zero magnetic field. At finite field, we have observed the violation of Hund's rule and studied the influence of magnetic field on the closed and open energy shell configurations. We have also compared the present results with those obtained within the LS‐coupling scheme for low electronic occupation numbers. We focus only on ground‐state properties and consider quantum dots populated up to 40 electrons, constructed by GaAs or InSb nanocrystals. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Quantum chemistry of quantum size‐effects in semiconductors: Small clusters electronic structure calculations

Ab initio RHF SCF calculations are used for some small clusters M_xX_y, where M=Cd, Ag; X=S, I; and x, y≤7. Variation of electronic structure with size for some clusters with the bulklike tetrahedral coordination and with the lower symmetry allows one to predict their possible geometries which are compared with experimental data on the existence of the clusters. The chemical‐bonding factor (the chemical nature of bounded atoms, coordination number for metal and nonmetal atoms, hybridization, etc.) is of more importance for properties of the clusters than is the familiar quantum confinement effect of semiconductor clusters. The essential difference in regularities of small cluster formation is analyzed for CdS‐ and AgI‐based structures. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 337–341, 1999