Search

The Multi-Configurational Self-Consistent Field approach with the geometry optimization was applied to the calculation of electronic properties of active site of heme core of truncated hemoglobin N, with the inclusion of [ONOO] functional group and two water molecules. The localized molecular orbitals are employed as a starting set. Two subspaces of full interaction have been used by the construction of MCSCF wavefunction. The first one includes 3d orbitals of iron atom, and the second contains bonding and antibonding molecular orbitals of peroxynitrite with one unshared electronic pair of the O2 fragment. The reaction is characterized by two transition states; the products are nitrate anion and one unbound water molecule. There arise an evidence of NO2 and NO radicals as the reaction products.

Raman spectra of glycine in a crystalline form and in a water environment have been obtained. B3LYP/6-311++G(3df,2p) quantum-chemical calculations have shown that an adequate description of the Raman spectra of glycine is achieved using the discrete–continuum model of its water solvation shell. A model is proposed for the stabilization of the zwitterionic state of glycine involving the inclusion of a single water molecule that is hydrogen-bonded with the –СOO– and –NH $$_{3}^{ + }$$ groups. Calculations of the Raman spectra of glycine complexes Gly + nH2O (where n = 1–3), as well as of a glycine dimer, yield good agreement with experiment.

The L and D isomers of the tryptophan (Trp) molecule and the (Trp)+ cation in the gas phase and water are calculated at the DFT level to reveal the effect of water considered in the dielectric continuum approximation on the electronic characteristics of the molecule. The distribution of effective atomic charges and bond lengths enables the prediction of the most probable parts of the chemical bond cleavage during the fragmentation of the molecule under the ionizing particle flux. These data are supplemented with a calculation of fragmentation energies. Zwitterionic structures characterized by the appearance of considerable dipole moments and a change in their orientation with respect to the ground state are distinguished among the possible isomeric forms in water.

Electronic characteristics of trHbN hemoglobin whose composition contains the (ONOO) group with the structure close to the structure of 1) peroxynitrite and 2) a nitrate anion in the gas phase are calculated. Electron correlation is considered by the multiconfigurational self-consistent field (MCSCF) method during the optimization of the geometry of the whole structure. Localized molecular orbitals (MOs) are used as starting ones. In the wave function of the MCSCF method two complete active subspaces (CASs) are set. These are the subspace of iron atom 3d orbitals and the subspace describing chemical bonds in peroxynitrite (bonding and antibonding MOs plus the orbital of one lone pair on the O2 moiety. The composition of the system involves two water molecules. The peroxynitrite structure is considered in two different spin states that correspond to the singlet and triplet states of this anion in the gas phase where the vibrational spectrum is characterized by frequencies of about (70-30) cm−1. The protective reaction of the active center of the tubercule bacillus is discussed.

We use the polaron state function described in terms of coupled classical and quantum fields to calculate the cross section of phonon scattering on a polaron. The value of the resonance momentum is determined by asymptotic values of several integrals. Calculating them with crystal parameters taken into account leads to bounds on the maximum value of the coupling constant. We confirm that the applicability domain of the strong-coupling approximation is near zero.

The structure and spin moments of iron-silicon clusters FeSin are calculated by a multiconfigurational self-consistent field (MCSCF) method in order to refine the spin moments in the ground state and their changes during the transition from the ground electronic state to excited ones. Transition energies to the first singlet state are obtained for the first time; the degree of stability of magnetic properties with temperature is evaluated in a wide range. The limiting number of silicon atoms, which corresponds to the disappearance of the cluster magnetic moment is found. The non-empirical calculation is compared with the data of the elecron density functional method in its different variants. The presence of iron-silicon clusters in the substance can be the reason for a local alteration of the magnetic field.

Multiconfiguration quantum chemical calculation of geometry and electron properties of Fe2Si18 cluster indicates on the predictable change of spin states as a function of the excitation energy beginning from ground state with the total spin S = 4. The charges on the two Fe atoms are quite different as well as the charge distribution on the surrounding Si atoms. Nevertheless the total dipole moment of the cluster is a monotonically decreasing function of the excitation energy and it reaches practically zero value in the first singlet state in which the cluster represents a new version of a quibit system.

Systematic quantum chemistry calculations of electronic and geometric characteristics of four channels for interaction of a Mgn (n = 4, 10) cluster with RHal organohalides, where R = CH3, C2H5, i-C3H7, t-C4H9, C2H3, C6H5 and Hal = Cl (and partially F, Br), were performed using the DFT method (B3PW91, 6–311+G(d)). The reaction channels are classified as radical or non-radical according to the nature of their reaction products. Regarding to Mg10⋯RHal, the calculated values of Gibbs free energies of activation are close to experimental values. For all the investigated cases, radical channels are preferred, but for vinyl and phenyl chlorides, this preference is only true due to the entropy factor.

The reaction channels of Znn and Mgn clusters (n = 4, 10) with RCl organochlorides are compared to the investigation of relative activity of metals in the reaction of Grignard reagent formation within a framework of quantum chemical modeling of a reaction path. R radicals are chosen from a list of alkyl (CH3, C2H5, i-C3H7, and t-C4H9) and residues with π-bond (C2H3 and C6H5). Quantum chemistry calculations are made using the density function theory (DFT) approach at the B3PW91/6-311+G(d) level. The reaction channels for both zinc and magnesium can be classified as radical and nonradical according to the nature of reaction products. The calculation of activation energies shows much similarity in their dependence on the nature of R radical but in the case of zinc, barriers are significantly higher. Both for zinc and magnesium clusters, the radical channels are more preferable in the case of alkyl residues, for vinyl and phenyl residues barriers of radical and nonradical channels are comparable. © 2012 Wiley Periodicals, Inc.

Dry etching of silicon nitride (SiN) is a challenging process in the semiconductor industry and requires high etch selectivity. Fluorine containing chemistry is widely used to etch silicon based materials, and one of the methods to increase the etch selectivity is to use reactants enhancing SiN etching in the presence of atomic fluorine. In this paper, quantum chemistry simulations have been carried out using the density functional theory method [B3LYP, 6-31 + G(d,p)] in order to determine such reactants. These simulations are based on the assumption that such reactants, for example, NO, can increase SiN etch rate via interacting with F–N bonds of the modified SiNFx surface. Silicon nitride cluster with F–N bonds is used here as a model of the modified SiNFx surface. Several new reactants (HF, Cl, H, Br, and FNO) enhancing SiN etch rate have been found by these simulations. The fact that such species as NO, HF, and H atoms can enhance SiN etching explains qualitatively some previously published experimental data.Dry etching of silicon nitride (SiN) is a challenging process in the semiconductor industry and requires high etch selectivity. Fluorine containing chemistry is widely used to etch silicon based materials, and one of the methods to increase the etch selectivity is to use reactants enhancing SiN etching in the presence of atomic fluorine. In this paper, quantum chemistry simulations have been carried out using the density functional theory method [B3LYP, 6-31 + G(d,p)] in order to determine such reactants. These simulations are based on the assumption that such reactants, for example, NO, can increase SiN etch rate via interacting with F–N bonds of the modified SiNFx surface. Silicon nitride cluster with F–N bonds is used here as a model of the modified SiNFx surface. Several new reactants (HF, Cl, H, Br, and FNO) enhancing SiN etch rate have been found by these simulations. The fact that such species as NO, HF, and H atoms can enhance SiN etching explains qualitatively some previously published experiment...

The subject of investigation is the fragmentation of variously charged molecular ions arising in col-lisions of several kiloelectronvolt H+, He2+, and Ar6+ ions with molecules of the simplest alkanes (from methane to butane). Using the method of time-of-flight mass spectrometry, the formation cross sections of dissociation-induced fragment ions are measured. The dissociation takes place when an incident ion captures an electron from a methane, ethane, or propane molecule. The role of additional ionization of the molecule, which accompanies the electron capture by the incident ion, is elucidated. The kinetic energy spectrum for protons resulting from the fragmentation of multiply charged alkane ions is determined. The most plausible kinetic energies of protons depending on the degree of ionization and molecule size fall into the range 1–25 eV. It is shown that, when the molecule loses several electrons, the kinetic energies of protons are governed by Coulomb interaction between all fragment ions and are determined by their flying apart from the relative spatial arrangement of corresponding atoms in a parent molecule.

The (ONOO)− anion, known as peroxynitrite, is characterized by a singlet spin state. A new stable dimer form of peroxynitrite [NO-O2]− in the triplet state at distances close to r(O-N) ≈ 2.885 A between oxygen in the O2 structure and nitrogen has been established within the quantum-chemical CASSCF approximation. The vibrational motion of the dimer is significantly anharmonic; for the 16O and 14N isotopes, the differences in the energies of two neighboring levels correspond to frequencies of about 70–30 cm−1. The triplet dimer structure retains stability in the case of interaction with water molecules. The activation barriers caused by rearrangement of the peroxynitrite structure into the ground state of the NO 3 − anion with the symmetry D 3h are found.

The spectra of fragmentation of molecules under the action of pulsed laser radiation or ion beams are studied in the sudden-perturbation approximation. By the example of the fragmentation process of the propane molecular ion [C3H8]+, it is established by calculating the most significant potential-energy surface cross sections along the reaction pathway that there is a correlation between the probability of formation of ionic fragments and the breaking energetics of individual chemical bonds. A complex rearrangement of atoms in the methane detachment process is established. The fragmentation spectra are attributed to protons with an energy of approximately 10 keV.

In ab initio calculations, we determined the most probable routes of decomposition of the [CF3Cl]+, [CF2Cl2]+, [CFCl3]+, [CCl4]+ molecular ions of freons and [C3H8]+ ions of hydrocarbons formed by collision of neutral molecules with protons with energies of the order of 10 keV. The calculated potential surface sections are compared on a qualitative level with the probability of various ion fragments in experiments on fragmentation. The role of the charge transfer dynamics between the proton and the molecule is discussed.

Fragmentation accompanying the loss of electrons by butane and isobutane (C4H10) molecules in collisions with energy H+, He2+, and Ar6+ ions of kiloelectronvolt energies is studied. The electron density functional technique is applied to C n H2n+2 alkane molecules and their respective C n H 2+2 + ions to carry out quantum-chemical calculations of the atomic spacing, electron total energy for the initial configuration of the ionizing molecules and ions in the ground state, and atomic bond breaking energy necessary to produce different ion fragments. The fragmentation energy is correlated with the fragmentation probability. It is shown that the relative cross sections of ion fragmentation depend primarily on the related energy consumption. However, the process cross section is also strongly affected by the initial configuration of C4H10 isomer molecules, as well as by the amount of dangling and arising atomic bonds involved in the formation of each ion fragment.

Mass-to-charge spectra of ions formed in ionization of freon-12 molecules, CF2Cl2, by protons with energies of Ep = 10 keV and 1 MeV are studied for the first time. Relative probabilities of different dissociation channels of the ionized freon-12 molecule, are obtained and compared with similar data for freon-12 ionization by electrons, positrons and photons. Absolute total and partial experimental cross sections for the formation of different fragment ions are evaluated for the case Ep = 10 keV. Neutral chlorine atom detachment is found to be the main fragmentation channel at both energies studied. Calculations of geometry and binding energies of fragment ions as well as energies required for activating various dissociation channels are carried out. A qualitative explanation is given of the observed fragmentation spectra.

The virial theorem for the translation-invariant theory of a polaron [3] is discussed. It is shown that, in [3], Tulub made a nonoptimal choice of variational parameters in the strong-coupling limit, which led to the violation of the virial relations. The introduction of an additional variational parameter to the test function reduces the polaron energy and makes it possible to satisfy the relations of the virial theorem for a strong-coupling polaron (the Pekar 1 : 2 : 3 : 4 theorem).

Polarization characteristics of the paratellurite TeO2 crystal were studied in the model of the molecular cluster represented by the (TeO2)3 molecular chain. Nonempirical calculations of linear and nonlinear polarizability reveal a steep growth of the second hyperpolarizability when passing on from the single TeO2 molecule to the molecular chain (TeO2)3, which may be related to intermolecular contacts. Linear polarizability of isostructural molecules SO2, SeO2, and TeO2 have an equal order of magnitude, while the second hyperpolarizability of TeO2 is somewhat higher than that of SeO2 and exceeds the SO2 polarizability by an order of magnitude.

Calculations of the potential energy surfaces of X-OH 2 + cations (X = C, O) by the restricted and unrestricted Hartree-Fock methods (RHF, UHF) taking into account the correlation energy (MP-2) and by the CASSCF method are presented. All cations are found to be rather stable against X-O bond rupture and intermolecular rearrangement. The reaction of electron capture by these cations is studied.

A method for solving the vibrational problem by using an effective energy operator containing Morse potentials and new model atom approximations is developed. The approach is explained by reference to the trifluoromethylacetylene molecule where the model atom is a CCC or CC atomic group. Using certain assumptions for parameters in the spectrum, we managed to confirm the known experimental result of bond strengthening of the terminal hydrogen atom when the CC bond order increases and to predict the CH bond energy (462.5±4.6 kJ/mole). This procedure seems to be also effective for more complex adsorption problems where the model atom is a crystal. Given that the surface effect is predicted by the parameters of the method, the corresponding model energy operator is constructed. With appropriate spectral data, the approach guarantees very accurate estimations of adsorption bond energies.

For ions and polar molecules located near a surface, we derived and analyzed the general formulas to calculate their solvation energies.

Analytic formulas are derived for the solvation energy of molecules adsorbed in the surface layer of a liquid. These are analogs of the Born and Onsager formulas for the bulk phase. Deriving the solvation energy involves decomposing the molecular characteristics, which is done by quantum-chemical calculation.

A new cavity model is considered as a modification of spherically symmetric Born-Onsager model. For the analytical treatment the cavity surface is represented as the surface of two intersecting spheres. The general solution of electrostatic problem is obtained in toroidal coordinate system and after the application of Mehler-Fock integral transform it is reduced to a solution of the two coupled Fredholm equations of the second kind with a positive defined symmetric kernel. The solution of the external and of the internal Dirichlet problem takes very simple form for the rational values of angles which define the parameters of the both spheres. The total charge induced on each sphere is leisurely changing function of the position of a moving charge.

The end surface of a cylindrical nanotube is considered as an annular and is replaced by a torus by solving the electrostatic problems, they are 1) the interaction of a charged particle with a torus and 2) the interaction of a charged torus with a neutral particle. Solution of the Poisson equation is given in the toroidal coordinate system and expressed by a half-order Legendre function of the first and second kind. The convergence of series expansion is improved by a given transformation of corresponding hypergeometric functions. The calculations are represented for the axially symmetric case.

Es wird die Wahrscheinlichkeit fur die Anregung von zwei Atomen durch ein Photon untersucht. Weiterhin wird die Wahrscheinlichkeit fur die Energieubertragung von zwei Donatoren auf einen Akzeptor betrachtet, wobei sich die Anregungsenergien der Donatoren addieren. Durch Vergleich dieser Wahrscheinlichkeiten wird gezeigt, warum bisher die kooperative Emission experimentell nur in Anwesenheit eines Akzeptors beobachtet wurde. Der Einflus der Kristallsymmetrie auf diese Prozesse wird berucksichtigt. The probability of the excitation of two atoms by a single photon is studied theoretically. Further the probability of energy transfer from two donors whose exitation energies are added, to one acceptor has been considered. Comparison of these probabilities shows why the cooperative emission is observed only in the presence of one acceptor. The effect of crystal symmetry on these processes is also taken into account.

The analysis of the electronic structure of 1,3-diphenylallene C6H5HC=C=CHC6H5, which the authors performed on the basis of SCF-MO-LCAO-CNDO/S2 calculations in earlier work under the assumption of a planar structure for the CHC6H5 fragment, attests to the double degeneracy of some of the one-electron and many-electron energy levels. Since the symmetry point group of the molecule (C2) does not have two-dimensional irreducible representations, this degeneracy, which is maintained at small rotation angles of the phenyl groups (theta less than or equal to 15) may be called accidental. Its consequences include several features of the electrical properties, which are discussed in the present work.