bluuebunny/crossref_metadata_2025_split huggingface dataset:
Quantenchemische untersuchungen zum mechanismus der elektrophilen substitution. I. Zur potentialhyperfläche des systems benzol/h+
Authors: Heidrich D. , Grimmer M. 
Published: December 2004

AbstractResults of semiempirical calculations (CNDO/2‐FK and MINDO/2 methods) for the σ‐π complex problem on protonated benzene are given and compared with previous ones. The semiempirical methods were chosen according to the agreement of their results with new theoretical energy data (EHF + Ekorrel) concerning the classical–nonclassical problem of protonated ethylene. By these methods the corresponding part of the energy surface of the benzene/H+ system is simulated. The stationary points of this surface are found by a gradient method with complete optimization of the geometry. On the basis of this method we determined the energy profile of a reaction coordinate between the classical (σ‐complex) and nonclassical (π‐complex) cation. The so called strong π‐complex is a saddle point between two σ‐complex minima and can be interpreted as transition state of 1,2‐proton shifts. Hypotheses for possible minimum energy paths of electrophilic attacks in the given region of the surface are discussed.
Direct minimization of the energy by simultaneous variation of parameters in nonorthogonal basis functions. I. Method
Authors: Weltin E. E. 
Published: December 2004

AbstractA direct method to optimize parameters in nonorthogonal basis orbitals is discussed. The partial derivatives of the energy of the state of interest, not necessarily the ground state, with respect to the orbital parameters are calculated analytically. The required cofactors up to third order of the overlap matrices over spin‐space orbitals in pairs of Slater determinants are calculated by biorthogonalization. All parameters are varied simultaneiously in the direction of the negative gradient (steepest descent). A search logic with dynamically adjusted step‐size is shown, in the example of the minimum basis Ne ground state, to find the energy minimum in an efficient manner.
Ab initioLCAO‐MO‐SCF Calculation of chlorpromazine and promazine
Authors: Popkie Herbert E. , Kaufman Joyce J. 
Published: December 2004

AbstractAb initio LCAO‐MO‐SCF Gaussian basis function calculations have been performed for chlorpromazine and promazine. By prescreening for the size of the integrals before calculation, it was only necessary to calculate 12 million out of the possible 38.5 million integrals for chlorpromazine. By a novel procedure of processing the integral tapes for the SCF it was possible to cut down significantly on the amount of time for the SCF. The SCF calculations converged smoothly for both promazine and chlorpromazine. There is a sizeable energy gap between the energy of the highest occupied molecular orbitals in these molecules (which is of the order of −0.3 a.u.) and the lowest unoccupied molecular orbital (which is of the order of + 0.15 a.u.). The gross atomic populations of chlorpromazine and promazine resemble each other and differ only somewhat on the carbon atom to which the substituent is attached and the carbons and their hydrogens adjacent to it.
Ab initio molecular orbital calculations of the cobalt porphine complex. I. LCAO SCF MO calculation of low‐spin, high‐spin, and π‐ionized states of Co‐porphine
Authors: Kashiwagi Hiroshi , Takada Toshikazu , Obara Shigeru , Miyoshi Eisaku , Ohno Kimio 
Published: December 2004

AbstractAb initio LCAO SCF MO calculations are carried out on planar Co‐porphine with a basis set of roughly double zeta quality for Co and N and of single zeta quality for C and H. The net charge on Co and N and the overlap population between them are 1.78, −0.57, and 0.06, respectively, in the 2A1g, state, which is known to be the ground state by experiment. The bonding in this complex is thus largely ionic. The first and second calculated ionization potentials are 6.51 and 6.77 eV, respectively, and are in reasonable agreement with the observed ionization potentials of 6.44 and 6.62 eV for Ni‐tetraphenylporphine. CI calculations within the framework of the ligand field theory are also performed. The calculated order of the five lowest states is 4B2g ≈ 4Eg, 4A2g, 2A1g, 4Eg from below and is not in agreement with the semiempirical order of 2A1g ≈ 4B2G, 4A2g, 2Eg, 4Eg determined by Lin.
Conformational changes and excitation energy transfer in myosin–auramine O system
Authors: Bogach P. G. , Zyma V. L. , Shakhovsky A. M. , Danilova V. M. 
Published: December 2004

AbstractIn the myosin–auramine O system 10 or 11 binding sites of auramine O on the rod‐like part of myosin were discovered. The myosin fluorescence decreases with an increase in auramine O concentration. This is evidence of the excitation energy transfer from tryptophanyls situated near the binding sites to auramine O. The effective distance of energy transfer is 35 Å. It is suggested that the light meromyosin has binding sites with periodicities of 77‐86 Å. Every binding site has a negative charge and hydrophobic locus.
Generalization of the second quantization operators: Electron attachment and detachment operators
Authors: Saute M. , Laforgue A. 
Published: December 2004

AbstractThis paper proposes a method for the generalization of the second quantization operators. The method is based on two main hypotheses: (i) the same form of the correlation operator, which is good for closed‐shell systems is chosen and (ii) a system of equations is given to calculate the differences (xk0−xk1), which characterize both attachment and detachment operators.
Effect of low‐molecular impurity on the melting of DNA‐type macromolecules
Authors: Kosevich A. M. , Galkin V. L. , Polyakov M. L. 
Published: December 2004

AbstractThermodynamics of the melting of a DNA‐type macromolecule is studied theoretically. Elements of the macromolecule are able to annex a low‐molecular impurity from the solvent (e.g., metal ions). Two models of the annexing of the impurity to the macromolecule are analyzed. It is shown that the concentration dependence of the helix‐coil phase transition temperature is describable by a non‐monotonic function in the case of certain relations between parameters.
Interrelations between electronic structure and spatial geometry of specific ligands in the functioning active site of some pyridoxal‐p‐dependent enzymes
Authors: Almazov V. P. , Morozov Yu. V. , Savin F. A. , Sukhareva B. S. 
Published: December 2004

AbstractUsing a spectrokinetic approach the absorption spectra of the short‐lived transient products in the enzymatic reaction of the glutamate decarboxylase with natural substrate are determined for the first time. The quantum‐chemical calculations of the electronic structure and spectra of various ionic species of numerous vitamin B6 derivatives allowed a hypothesis on the nature of the intermediate products detected to be suggested. A model of the enzyme functioning taking into account the charge equilibria and some electronic–conformational relations is proposed.
Spectroscopic and photochemical studies of model visual pigment chromophores
Authors: Kliger David S. 
Published: December 2004

AbstractThe primary step in the bleaching sequence of visual pigments has been shown to occur in picoseconds, while the isomerization of model visual pigment chromophores, protonated 11‐cis retinylidene Schiff bases, takes place on a time scale several orders of magnitude slower. Thus, the well‐accepted notion that the primary step in visual transduction involves a simple isomerization deserves closer examination. Studies of visual pigments and compounds which mimic visual pigment chromophores are discussed in terms of several alternatives for the nature of the primary step. In addition to discussion of photochemical studies, spectroscopic experiments are discussed. To fundamentally understand the nature of the primary processes in visual transduction it is important to understand the photochemistry, and therefore the electronic structure, of pigment chromophores. Spectroscopic studies aimed at elucidating the electronic structure of polyenic systems are thus discussed.
Electronic structure of the pyrimidine and purine components of nucleic acids in their ground and lower excited singlet and triplet states
Authors: Savin F. A. , Morozov Yu. V. , Borodavkin A. V. , Chekhov V. O. , Budowsky E. I. , Simukova N. A. 
Published: December 2004

AbstractQuantum‐chemical calculation of the energies of the electronic transitions and the electronic structures of the neutral and ionic species of the nucleic acids components in their ground and lower excited singlet and triplet ππ* and nπ* states has been carried out in the all‐valence‐electron approximation CNDO/S. The results of the calculation allow one to identify the most photoreactive sites of the molecules and to consider the dependence of the location of these sites on the ionic state of the molecules. The calculated data are compared with our previous results obtained in a π‐electron approximation. The individual absorption spectra of various ionic and tautomeric species of the nucleic acids components obtained by us earlier have been decomposed into bands corresponding to separate electronic transitions. As a rule, there is a good agreement between the calculated data in the two approximations and the experimental results.
Torsional isomerization of biologically active bicyclic molecules
Authors: Bolotin Adolph B. , Bolotin Victor A. , Gineityte Victoria L. 
Published: December 2004

AbstractRotational isomerization of bipyridines C5H4NC5H4N was studied by CNDO/2, PPP/CI, and CNDO/CI methods. It is shown that CNDO/2 overestimates the angle of rotation ϕ between the pyridine rings ca. two times. The angle ϕ was determined for 2,2′‐bipyridine by means of correlation of the theoretical (CNDO/2) and experimental dipole moment. It was also found from the correlation between the theoretical and experimental UV spectra. It is shown that there is an explicit dependence of the results upon the distance between heteroatoms (PPP/CI). It has been found that the CNDO/CI method correctly predicts the value of the rotational angles and their sequence in bipyridines.
Simulation of intermolecular and intramolecular interactions of nucleic acid subunits by means of atom–atom potential functions
Authors: Poltev V. I. 
Published: December 2004

AbstractAtom–atom potential functions for calculation of nonbonded interaction energies of nucleic acids are proposed. Quantitative reliability of the obtained results is checked by using the calculations of intermolecular interaction energy in crystals. Based on the calculations of the interaction energy of the nucleic acid bases the molecular mechanisms of spontaneous mutations are suggested.
Mechanisms of charge separation in bacterial photosynthesis
Authors: Kharkyanen V. N. , Khristoforov L. N. , Kukhtin V. V. , Petrov E. G. 
Published: December 2004

AbstractThe physical aspects of the primary charge separation processes in bacterial photosynthesis are discussed. The donor‐acceptor model of electron transfer due to participation of protein current states is used. The kinetics of photosynthetic reaction center (PRC) processes is investigated and the PRC energetic scheme is constructed using the nonequilibrium density matrix method. It is shown that with allowance for the effect of vibrational sublevels of states participating in transitions the theory describes well experimental data.
Orbit–orbit interaction in the NO–Fe(II)Hemoproteins studied by the low‐temperature magnetic circular dichroism: Dynamic Jahn–Teller effect detected by MCD
Authors: Mineyev A. P. , Sharonov Yu. A. , Sharonova N. A. , Figlovsky V. A. 
Published: December 2004

AbstractThis paper analyzes the low‐temperature magnetic circular dichroism of NO–Fe(II)hemoproteins in detail. We include the Jahn–Teller effect in our consideration of the low‐temperature MCD results.
Coordinate measurements and operators
Authors: Broyles A. A. 
Published: December 2004

AbstractRelativistic quantum‐field theory provides the machinery for calculating wave functions or probability amplitudes depending upon space‐time coordinates. The currently accepted theory, however, fails to provide position operators and a means of measuring particle coordinates that are consistent with Dirac's properties of physical observables. This is because it calls for a space position probability distribution at a specified time. This paper shows, however, that space‐time event coordinate operators, together with a corresponding measurement procedure, can be found that are consistent with Dirac's requirements. This is done through a reinterpretation of the amplitudes computed by field theory and does not involve any change in that mathematical formalism. The measurement of the space‐time coordinates of an event is accomplished by detecting the absorption of a photon by a particle from each of two light pulses designed to overlap at a given point at a given time. If a final emitted photon has an energy whose sum with the final particle energy approximately equals the sum of the mean energies of the pulses, then the absorption of the two pulse photons must certainly have taken place within a distance the order of a Compton wavelength of the small space‐time region of overlapping pulses. This is clear from the fact that the high energy required to confine the pulses to very small volumes must throw a particle absorbing them far off the mass shell. Thus the absorption of the two photons throws the particle into a narrowly confined spatial wave function that must decay extremely rapidly—to within a Compton wavelength, a delta function in space‐time. This delta function is the eigenfunction of space‐time coordinate operators Xμ and is the scalar product of vectors in a Hilbert space spanned by spin–space‐time kets large enough to contain the operators of the Poincaré group. These event operators transform properly under the action of Poincaré operators but do not commute with the mass. If the Compton wavelength is not negligible compared to the accuracy desired in the coordinate measurements, individual coordinate measurements are no longer possible. Nevertheless, a large number of repeated coordinate measurements can be carried out to produce a coordinate probability distribution. This distribution can be unfolded to find a true coordinate probability distribution if the charge form factor is known from basic theory. An analysis of laboratory particle detection techniques shows that they actually determine space coordinates and energy rather than spatial coordinates at a given time. When this fact is included, the Klein–Nishina formula can be derived using the electromagnetic four‐vector potential as the photon probability amplitude wave. To clarify the meaning of the observables, a mass‐momentum measurement is described.
Multiple scattering mass operator method for molecular orbital calculations
Authors: Pavãto A. C. , Braga M. , Fazzio A. , Leite J. R. 
Published: December 2004

AbstractAn exchange‐correlation potential model to be used in connection with the multiple scattering method is presented. Retaining the main advantages of the multiple scattering method with the Xα potential, particularly its low computational requirements, this new formalism does not require any adjustable parameter. Test calculations on the NiF64− system are reported and compared with experimental and ab initio results.
Rydberg states in chemistry
Authors: Sandorfy C. 
Published: December 2004

AbstractThe excited states of molecules can be divided into three categories: valence‐shell, Rydberg, and intermediate. Reasons are given why this classification is important for photochemistry. The photolytic primary steps should be different for excited states falling into these different categories. Rydberg states should be taken into account when the photochemical reaction path is interpreted in terms of orbital symmetry conservation or diradical intermediates. They might play a role in certain photobiological events.
Study of molecular polarizabilities of some amino and polyamino acids
Authors: Gupta V. P. , Gupta V. D. , Mehrotra C. 
Published: December 2004

AbstractMolecular polarizabilities of glycine,L‐alanine,L‐proline, and polygylcine I and II have been calculated by the Lippincott delta function potential model, the atomic polarization method, and the atom‐dipole interaction model. In the atom‐dipole interaction model the molecular polarizabilities are dependent on atomic (or group) polarizabilities and positional coordinates of the atoms (or groups) constituting the molecule and are conformation dependent. Those obtained from the other two methods are conformation independent and dependent on bond properties only. The results are compared with those obtained from the Kerr effect and molar refraction data.
Electrostatic molecular potential contour maps generated from ab initioMODPOT/VRDDO/MERGE wave functions of carcinogenic 3‐methylcholanthrene and its metabolites
Authors: Hariharan P. C. , Kaufman Joyce J. , Petrongolo Carlo 
Published: December 2004

AbstractThe electrostatic molecular potential contour maps were calculated for carcinogenic 3‐methylcholanthrene (3‐MCA) and a number of its metabolites {3‐MCA 7,8‐oxide and 3‐MCA 9,10‐oxide; 3‐MCA 7,8‐dihydrodiols[several stereoisomers: A trans(equatorial, equatorial) and A cis(equatorial, axial)]; 3‐MCA 9,10‐dihydrodiol–7,8‐epoxide A βββ and 3‐MCA 9,10‐dihydrodiol–7,8‐epoxide A αβα}. The maps were generated from our ab initio MODPOT/VRDDO/MERGE wave functions calculated for these species. The results of these maps for 3‐MCA [similarly to our results for the maps we generated for benzo(a)pyrene (BP)] show that these electrostatic molecular potential contour maps can be used to indicate favored positions of attack for electrophilic species, such as “electrophilic” oxygen to form an epoxide as well as for positive ion attack. The 3‐MCA maps indicate the favored site for attack and the pathways. The maps around 3‐MCA 9,10‐oxide and around 3‐MCA 9,10‐dihydrodiol‐7,8‐epoxide indicate the directional preferences for proton assisted epoxide ring opening. The maps around the 3‐MCA dihydrodiols indicate that while for certain stereoisomers the “electrophilic” oxygen will prefer to attack from below, for other isomers it will prefer to attack from above. This gives great insight into the stereochemical preference for formation of different 3‐MCA 9,10‐dihydrodiol–7,8‐epoxides.
Diagram approach to group algebraic methods
Authors: Black R. J. , Stedman G. E. 
Published: December 2004

AbstractClass sum theory, the duality with IRREP methods and tensor operators in the group algebra are discussed by generalizing the diagrammatic approach of conventional IRREP theory to include group label manipulation. Concepts such as invariant nodes and Jucys–Levinson–Vanagas reduction theorems generalize straightforwardly. The results are capable of unique simplification for certain nodes, when the group rearrangement theorem is useable or when a class sum is performed. A duality transformation (between IRREP–partner and class–element labels) emerges as an important concept.
Semiempirical molecular orbital calculations on octahedral aquo complexes of transition metal ions: σ‐donor ability of water
Authors: Bhattacharyya Sankar Prasad 
Published: December 2004

AbstractA comparative analysis is carried out on the strengths of binding of a series of transition metal ions to a water molecule using CNDO type MO theory developed earlier. Comparative features of the energetics of metal–ligand bonds in aquo and amine complexes are also analyzed.
Virial fragments and the Hohenberg–Kohn functional
Authors: Ludeña Eduardo V. , Mujica Vladimiro 
Published: December 2004

AbstractStarting from the Hohenberg‐Kohn functional we show that when the energy density is given as a function of ρ and ∇ρ, i.e., ξ = ξ(ρ, ∇ρ), the condition ∇ρ · n = 0 (which was found by Bader et al. to define virial fragments), appears as a natural boundary condition for the variation of this functional. We also show that when the energy density includes second order derivatives (∇2ρ) this condition is necessary but not sufficient to guarantee the vanishing of the variation. The implications of these results are discussed in the context of a density functional theory for virial fragments.
Method of recurrent construction of Löuwdin spin‐adapted wave functions. II. Local representation of creation–annihilation operators
Authors: Panin A. I. 
Published: December 2004

AbstractSome compositions of the addition and subtraction operators and recurrence relations for the Sanibel‐type coefficients cu, v (n, s, M) generated by these compositions are studied. A local representation of the fermion creation–annihilation operators via the addition and subtraction operators is obtained. Operators of single excitations, coupling, and decoupling operators, in terms of which the unitary group generators can be expressed are defined. The resulting representation of the nonelementary unitary group generators is much more simple than in the Gelfand–Tzetlin basis and in the most general case contains only six logically different terms, each of them possessing quite transparent physical significance.
Theoretical analysis of K‐region and bay‐region indicators of carcinogenicity
Authors: Lowe John P. 
Published: December 2004

AbstractThe correlation of the K‐region carcinogenicity indices of Pullman and of Mainster and Memory with bay‐region carbocation delocalization energy (BCDE) is displayed and shown to be maintained at the perturbational molecular orbital (PMO) level. PMO algebraic relations are examined to reveal the underlying relationships. Some correlation between BCDE and K‐region indices exists simply because carbocation formation occurs preferentially on carbons which are bonded to positions α (adjacent) to ring fusion centers. Sampling a K region is synonymous with sampling such α carbons. However PMO analysis shows that a very strong correlation exists between BCDE and indices for a certain type of K region (called “primitive”). It is further shown that high BCDE is favored by a straight‐line chain of benzene rings as one moves away from the bay region. Kinks in the chain will lower the BCDE, and the closer the kink is to the bay region, the greater is its effect. Effects of methylation upon BCDE are shown to correlate with K‐region indices if the methylation occurs at a carbon which is in the opposite class as the carbon to which the bay‐region carbocation is attached.
Calculated properties of some possible vinyl chloride metabolites
Authors: Politzer Peter , Proctor Timothy R. 
Published: December 2004

AbstractThere is considerable evidence indicating that the carcinogenic action of vinyl chloride involves metabolic conversion to the epoxide (chlorooxirane) as the initial step. In order to learn more about its subsequent behavior, we have computed structures, energies and other properties for two different protonated forms of the epoxide, and also for two possible rearrangement products, chloroacetaldehyde and acetyl chloride. An ab initio SCF‐MO procedure (GAUSSIAN 70) was used. Oxygen protonation is found to weaken both CO bonds, the effect being greater for the bond involving the carbon bearing the chlorine. Chlorine protonation leads to a marked weakening of the CCl bond; this suggests a possible loss of HCl, leaving behind a carbonium ion (and possible alkylating agent or rearrangement precursor). Thus, while CO bond breaking is doubtless an important reaction pathway for chlorooxirane, our results indicate that attention should also be focused upon the CCl bond; its rupture may conceivably be a key step in the biological action of vinyl chloride.