Relativistic quantum chemistry and rigorous variational analysis

A brief review of relativistic quantum chemistry is given here. Relativistic effects and their importance in chemistry are discussed. An outline of different theoretical aspects is presented. Aspects of variation techniques relevant to relativistic calculations are discussed in detail. These involve the derivation of min-max theorems for Dirac, Dirac-Hartree-Fock and Dirac-Coulomb calculations. The consequence of relativistic Hamiltonians being unbounded are also discussed for other lines of investigation. The upper bounds derived are physically interpreted. Sample Dirac-Hartree-Fock results for the Be atom, calculated using both STO and GTO bases for the nonrelativistic orbitals and the upper components of the relativistic orbitals, are given. The inadequacy of the so-called kinetically balanced basis set is discussed and illustrated with these results. The importance of the variational or dynamical balance and hence the merit of the LCAS-MS scheme is pointed out. The possibility of calculating quantum electrodynamical pair energy from relativistic configuration interaction calculations on a two-electron atom is discussed and exemplified. The present status of relativistic molecular calculations is briefly reviewed. Conclusions on the aspects of variational analysis and molecular calculations are enclosed.     

Association of Haloforms in Condensed and Gas Phases. Ir Spectroscopy and Dft Calculations

Fourier-transform infrared (FTIR) spectra are recorded for the haloforms of fluorine, chlorine, and bromine atoms in gas and liquid phases, as well as for their low-temperature films at 20–200 K. The molecules of these compounds are shown to be connected by hydrogen bonds associated with the transfer of a hydrogen atom. According to ab initio DFT calculations, chloroform demonstrates two types of associated structures, one with the chlorine atom and the other with the central carbon atom binding as the proton acceptor in the intermolecular interaction. At the same time, calculations predict only one form of bonding for two other haloforms, namely, between the hydrogen atom and the fluorine atom for fluoroform, and between the hydrogen atom and the carbon atom for bromoform.     

Structure of 3,3′-bis-(tribromogermyl)propionic acid according to the results of ab initio calculations

The nonempirical quantum-chemical calculations of the molecule of 3,3-bis-(tribromogermyl)-propionic acid by RHF/6-31G(d) and MP2/6-31G(d) methods were performed with full optimization of its geometry. The results of calculations by the two methods do not differ fundamentally. The calculations showed that two forms of this molecule are stable: one containing two tetrahedrally coordinated Ge atom and another with one tetrahedrally coordinated and one pentacoordinated Ge atom. The first form is typical of the crystalline state of the matter, the second is the most energy-effective for the gas. At the intramolecular interaction between the carbonyl oxygen atom and the Ge atom in the second form leading to its pentacoordination the corresponding Ge-Br bonds are polarized in such a way that the negative charges on the Br atoms and a positive charge on the Ge increase. No transfer of electron density from the oxygen to the germanium was revealed.     

Surface vibrations of atoms physisorbed on a crystal surface: monolayer and bilayer xenon on (0001) graphite

Surface atom vibrations of monolayer and bilayer xenon physisorbed on (0001) graphite are studied with H atom scattering. The temperature dependence of the mean square displacement of surface atoms is obtain through measurements of the Debye-Waller factor. The monolayer data are consistent with LEED measurements and calculations using the Einstein model. The bilayer data lie between the monolayer and calculations for the (111) surface of solid xenon.     

The interaction between atoms of Au and Cu with clean Si(1 1 1) surface: A study combining synchrotron radiation grazing incidence X-ray fluorescence analysis and theoretical calculations

In order to evaluate the interactions between Au/Cu atoms and clean Si(1 1 1) surface, we used synchrotron radiation grazing incidence X-ray fluorescence analysis and theoretical calculations. Optimized geometries and energies on different adsorption sites indicate that the binding energies at different adsorption sites are high, suggesting a strong interaction between metal atom and silicon surface. The Au atom showed higher interaction than Cu atom. The theoretical and experimental data showed good agreement.     

Impact of Surface Dipole on Resonant Electron Injection in Scanning Tunneling Spectroscopy

Resonant electron injection and first-principles calculations are utilized to study single-adsorbed selenium (Se) atom on a Si(111)-7×7 surface. Theoretical calculations indicate that a negative dipole of 0.61 eV forms toward the adsorbed Se atom due to electron trans-fer from the associated Si atoms. The formation of surface dipole modifies the effective tunneling barrier height and causes a shift in the energy of the resonant state formed in the vacuum gap between the tip and the sample surface. The experimental data imply that an outward negative surface dipole of 0.61 eV causes a resonant electron injection bias shift to high voltage of about 0.45 V.     

The phenoxy/phenol/copper cation: a minimalistic model of bonding relations in active centers of mononuclear copper enzymes

The "bare" complex [Cu(PhOH)(PhO)](+) with a phenol (PhOH) and a phenoxy (PhO) ligand bound to copper is studied both experimentally and computationally. The binding energies and structure of this complex are probed by mass spectrometry, infrared multi-photon dissociation, and DFT calculations. Further, the monoligated complexes [Cu(PhO)](+) and [Cu(PhOH)](+) are investigated for comparison. DFT calculations on the [Cu(PhOH)(PhO)](+) complex predict that a phenolate anion interacts with copper(II) preferentially through the oxygen atom, and the bonding is associated with electron transfer to the metal center resulting in location of the unpaired electron at the aromatic moiety. Neutral phenol, on the other hand, interacts with copper preferentially through the aromatic ring. The same arrangements are also found in the monoligated complexes [Cu(PhO)](+) and [Cu(PhOH)](+). The calculations further indicate that the bond strength between the copper atom and the oxygen atom of the phenoxy radical is weakened by the presence of neutral phenol from 2.6 eV in bare [Cu(PhO)](+) to 2.1 eV in [Cu(PhOH)(PhO)](+).     

Structure électronique des isologues oxygénés du trithia-1,6,6aSiv pentalène: I. Nature De L'interaction SβO

Valence electron CNDO/2-SCF-MO calculations have been performed on some oxygenated derivatives of 1,6,6aS^IV-trithiapentalene. The calculations demonstrate the relative importance of the d orbitals for the central sulphur atom and a dΣ-pΣ interaction between this atom and the neighbouring oxygen atom.     

Trajectory calculation results on the orientation of a diatomic molecule in collision with an atom

Orientational effects in collisions between a diatomic molecule and a noble gas atom are studied by classical trajectory calculations. A preferable ”perpendicular orientation“ of the molecule is found at the moment of closest approach of the atom. This orientational effect is more pronounced in collisions with heavy atoms.     

Ab initio study of (13)C(alpha) chemical shift anisotropy tensors in peptides

This study reports magnitudes and the orientation of the (13)C(alpha) chemical shift anisotropy (CSA) tensors of peptides obtained using quantum chemical calculations. The dependency of the CSA tensor parameters on the energy optimization of hydrogen atom positions and hydrogen bonding effects and the use of zwitterionic peptides in the calculations are examined. Our results indicate that the energy optimization of the hydrogen atom positions in crystal structures is necessary to obtain accurate CSA tensors. The inclusion of intermolecular effects such as hydrogen bonding in the calculations provided better agreement between the calculated and experimental values; however, the use of zwitterionic peptides in calculations, with or without the inclusion of hydrogen bonding, did not improve the results. In addition, our calculated values are in good agreement with tensor values obtained from solid-state NMR experiments on glycine-containing tripeptides. In the case of peptides containing an aromatic residue, calculations on an isolated peptide yielded more accurate isotropic shift values than the calculations on extended structures of the peptide. The calculations also suggested that the presence of an aromatic ring in the extended crystal peptide structure influences the magnitude of the delta(22) which the present level of ab initio calculations are unable to reproduce.     

Zeeman transitions in collisions of na with xe

Transitions between the Zeeman states of a sodium atom induced bv collision with a ground-state xenon atom have been investigated by quantum-mechanical methods. The calculations were carried out using the pseudo-potentials of Czuchaj and Sienkiewicz, and results pertinent to crossed atomic beam experiments are reported. Comparison with previous theoretical work indicates that a thermally averaged cross section (relevant to cell-type experiment) would be in excellent agreement with recent experimental results.     

吡啶-BH~3相互作用复合物的理论研究

对吡啶-BH~3复合物分别用MP2/6-31+G^*和B3LYP/6-31+G^*进行理论计算以预测该复合物的构型及解离能，得到四种构型，在MP2优化构型基础上作CCSD/6-31+G^*单点能量计算以验证MP2与B3LYP结果的可靠性，然后用B3LYP作振动频率分析，计算了各构型的垂直电离势，最后用更大基组作单点能量计算和自然键轨道(NBO)分析。结果表明，N-B直接相连的构型最稳定，其解离能为141.50kJ/mol,MP2和B3LYP对N-H接近的构型结果相关较大，另外两种构型稳定性介于二者之间，解离能分别为15.18kJ/mol,14.06kJ/mol(MP2/6-31+G^*)。     

1H, 13C and 15N NMR assignments of phenazopyridine derivatives

Phenazopyridine hydrochloride (1), a drug in clinical use for many decades, and some derivatives were studied by one- and two-dimensional (1)H, (13)C and (15)N NMR methodology. The assignments, combined with DFT calculations, reveal that the preferred protonation site of the drug is the pyridine ring nitrogen atom. The chemoselective acetylation of phenazopyridine (2) and its influence on the polarization of the azo nitrogen atoms were evidenced by the (15)N NMR spectra. Molecular calculations of the phenazopyridines 2-4 show that the pyridine and phenyl groups are oriented in an antiperiplanar conformation with intramolecular hydrogen bonding between the N-b atom and the C-2 amino group preserving the E-azo stereochemistry.     

Possible causes for rippling in a multivacancy graphene system

The occurrence of ripples in an eight‐atom vacancy graphene system was analyzed through first principles calculations based on density functional theory. The study focuses on the conditions that should be considered in order that ripples could occur in the single layer defected graphene. The vacancies concentration, especially the pentagonal figures formed after relaxation of the vacant system, and the distances between neighbor pentagons, are found to be key aspects for rippling occurrence, with no charge distribution consequence. Two different configurations of rippling have been found where the position of the pentagonal figures is discussed as the driving force for any of them to occur.     

Carbon‐Atom Extrusion from Halobenzenes and Its Coupling with a Methylene Ligand to Form Acetylene

Mechanistic aspects of an unusual gas‐phase reaction of [LaCH₂]⁺ with halobenzenes have been investigated using Fourier‐transform ion cyclotron resonance (FTICR) mass spectrometry combined with density functional theory (DFT) calculations. In this thermal process a carbon‐atom from the benzene ring, most likely the ipso‐position, and the carbene ligand are coupled to form C₂H₂.     

Spin uncoupling in ethylene activation by palladium and platinum atoms

Spin‐dependent effects in complex formation reactions of the ethylene molecule with palladium and platinum atoms were studied by electron correlation calculations with account of spin–orbit coupling. Simple correlation diagrams illustrating spin‐uncoupling mechanisms were obtained, showing that the low spin state of the transition‐metal atom or the transition‐metal atom complex is always more reactive than are the high spin states because of the involvement of the triplet excited molecule in the chemical activation. Spin–orbit coupling calculations of the reaction between a platinum atom and ethylene explain the high‐spin Pt(³D) reactivity as due to an effective spin flip at the stage of the weak triplet complex formation. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 581–596, 1999     

Relativistic effective potential SCF calculations of AgH and AuH

Relativistic effective potentials (REP) are now widely used in molecular electronic structure calculations. Tests of these REP are needed to assess their accuracy. This can now be done for AgH and AuH since Lee and McLean have published Dirac-Fock calculations for these molecules. Comparative SCF calculations have been performed for two types of effective potential. Satisfactory agreement between the effective potential results and Dirac-Fock values is found for spectroscopic constants and dipole moments, which supports the use of these potentials for heavy atom containing molecules.     

Combined theoretical and experimental deep-UV resonance raman studies of substituted pyrenes

The results of time-dependent density functional theory (TDDFT) calculations of resonance Raman intensities are combined with experimental deep-ultraviolet resonance Raman measurements at a single wavelength, i.e., 244 nm, in order to test the possibility to distinguish several very similar compounds. Pyrene and three of its substituted derivatives, in which a single hydrogen atom has been replaced by a halogen atom, are compared. The fixed 244 nm excitation wavelength overlapped with the same electronic transition of the four pyrenes. Ground-state calculations using the BP86 exchange-correlation functional were used to predict the Raman frequencies, whereas excited-state calculations have been carried out employing the "statistical averaging of (model) orbital potentials" (SAOP) potential within a linear-response TDDFT framework in combination with the short-time approximation of resonance Raman intensities. In view of the simplistic theoretical approach, we find a surprisingly good agreement between the simulated and measured resonance Raman spectra of pyrene and its substituted analogues in terms of frequencies and intensities, which shows that the calculations can be used reliably to interpret the experimental spectra. With this combined information, it is possible to find criteria to distinguish the compounds under investigation, although many features of their vibrational spectra are similar.     

Methanol adsorption on the beta-Ga2O3 surface with oxygen vacancies: theoretical and experimental approach

Methanol adsorption on beta-Ga2O3 surface has been studied by Fourier transform infrared spectroscopy (FTIR) and by means of density functional theory (DFT) cluster model calculations. Adsorption sites of tetrahedral and octahedral gallium ions with different numbers of oxygen vacancies have been compared. The electronic properties of the adsorbed molecules have been monitored by computing adsorption energies, optimized geometry parameters, overlap populations, atomic charges, and vibrational frequencies. The gallia-methanol interaction has different behaviors according to the local surface chemical composition. The calculations show that methanol can react in three different ways with the gallia surface giving rise to a nondissociative adsorption, a dissociative adsorption, and an oxidative decomposition. The surface without oxygen vacancies is very reactive and produces the methanol molecule decomposition. The molecule is nondissociatively adsorbed by means of a hydrogen bond between the alcoholic hydrogen atom and a surface oxygen atom and a bond between the alcoholic oxygen atom and a surface gallium atom. Two neighbor oxygen vacancies on tetrahedral gallium sites produce the dissociation of the methanol molecule and the formation of a bridge bond between two surface gallium atoms and the methoxy group.