bluuebunny/crossref_metadata_2025 huggingface dataset:
Absorption UV spectroscopy and electronic structure of ionic and tautomeric forms of hydroxy and methoxy derivatives of cytosine and adenine, and of some 5‐substituted analogs of pyrimidines
Authors: Borodavkin A. V. , Chekhov V. O. , Dolin Yu. S. , Morozov Yu. V. , Savin F. A. , Budowsky E. I. , Yakovlev D. Yu.
Published: December 2004

The individual absorption spectra of various ionic and tautomeric species of some derivatives of nucleic‐acid components have been determined by computer handling of the summary spectra of these compounds measured at different pH values. These individual spectra have been decomposed into separate bands represented by log‐normal curves and corresponding to different electronic transitions. Using the parameter system calculated earlier for the molecules of the vitamin B6 family, the calculation of the electronic spectra and electronic structure of various ionic and tautomeric species of the molecules studied have been carried out in the π‐electron approximation. These properties have also been calculated by the all‐valence electronic method CNDOS. The calculated results correlate well with both approximations and with our experimental spectral data. The localization of the molecular electronic excitation studied on the transition to the S1 and T1 states is considered in connection with the reactivity in these states.
New derivation and a k‐particle generalization of SCF‐type theories
Authors: Kutzelnigg Werner
Published: December 2004

A hierarchy of necessary conditions that an exact density matrix of a pure state or an ensemble has to satisfy is derived, namely the hermiticity of certain operators F(k). For k = 1 this reduces to the well‐known Hartree‐Fock condition. It is then shown that the kth set of conditions is equivalent to stationarity of the energy with respect to unitary k‐particle transformations. k‐Particle generalizations of Hartree‐Fock theory are then discussed both in the spirit of k‐particle pseudoeigenvalue equations and in the framework of a Newton–Raphson‐type constructive scheme.
Modified virtual orbitals of Hartree–Fock method in calculation of second‐order perturbation energy
Authors: Hiroike Eiko
Published: December 2004

An advantage of modified virtual orbitals of Hartree–Fock method is discussed in the calculation of the second‐order perturbation energy. All the modified virtual orbitals can be fitted for the intermediate virtual states in the perturbation expansion, only if the molecular orbitals are expanded in terms of infinite basis functions and the set of molecular orbitals is infinite and complete. If the molecular orbitals are expanded in terms of finite basis functions, only the modified virtual orbitals with lower energies are appropriate to describe the intermediate virtual states, but the modified virtual orbitals with higher energies become inadequate. To explain the usefulness of the modified virtual orbitals, the calculation by the modified Hartree–Fock method without CI are compared with the calculation by the traditional Hartree–Fock method with complete CI.
Conservation of intrinsic dual and single symmetries of molecular orbitals
Authors: Au‐Chin Tang , Chia‐Chung Sun
Published: December 2004

In this paper, a fresh attempt has been made to discuss dual‐symmetry groups by which the supra–antarafacial cycloaddition and the chelatropic reactions can be correctly described, as a direct consequence of which the corresponding energy correlation diagrams are successfully constructed. Furthermore, the energy correlation diagrams of sigmatropic reactions are also established by the use of another kind of single‐symmetry group.
Comparison between the symmetry‐correlation examination and the perturbation method
Authors: Lee Tieh‐Sheng , Chu San‐Yan
Published: December 2004

A comparison between the construction of symmetry‐correlation diagrams and the perturbation method for studying chemical reactions is carried out. The perturbation method consists of decomposing the system Hamiltonian H into a sum, H = H0 + H′. Various symmetry correlation schemes appearing in the literature may be explained by the nonuniqueness of the decomposition scheme. All symmetry selection rules may be viewed as the varieties. By examining the symmetry‐correlation diagrams, processes under investigation may be called “forbidden” or “allowed,” depending on the topological feature. Of particular importance is the topology associated with the “avoided crossing.” By making the comparison, we can establish the correspondence of the two methods and conclude that the perturbation order furnishes the origin of the “forbiddenness” of a process.
CNDO calculation of the diamagnetic susceptibilities of cyclic conjugated molecules
Authors: Boucekkine A.
Published: December 2004

We present the results obtained within the CNDO/2 method for the diamagnetic susceptibilities of a series of conjugated molecules including benzene, fulvene, pyridine, pyrrole, and furan. The calculations have been carried out, according to Bley's method, in the framework of coupled Hartree–Fock perturbation theory, using gauge invariant atomic orbitals and the London approximation. The two usual CNDO parametrizations, due respectively to Pople, Santry, and Segal and to Del Bene and Jaffe, did not enable one to obtain the principal susceptibilities of the molecules under consideration. We investigated the effect of the CNDO parametrizations on the different contributions to the susceptibility, and gave a first improvement to Bley's method, which permits the calculation of the mean susceptibility of conjugated molecules with less than 9% error.
Theoretical study of photochemical reactions: Electron assignment and the state correlation diagram
Authors: Yamabe Shinichi , Minato Tsutomu , Osamura Yoshihiro
Published: December 2004

A general way for drawing the state correlation diagram and seeking the reaction path is presented. If a high‐symmetry reacting system is given, its least‐motion path that maintains the symmetry is primarily examined. For a given state, it is judged whether the least‐motion path is symmetry allowed or forbidden. If allowed, it is called the direct process. If forbidden, the symmetry imposed on the system should be relaxed, resulting in the mixing of MO's. Then, the energy barrier of the avoided crossing for some excited states is removed and the possible reaction path is found. After this procedure, the symmetry‐allowed paths may be sought by the geometry optimization with a suitable wave function. By the use of such a procedure, the dissociation of diazomethane and (3H‐)diazirine is found to proceed via the Cs and C2 symmetries.
Field correlation effects on the resonant light scattering
Authors: Kôno H. , Fujimura Y. , Nakajima T.
Published: December 2004

Effects of incident photon field coherence on resonant light scattering have been investigated. In order to obtain the scattering intensity and the photon counting rate, an expression for the reduced density matrix for the scattered field has been derived. The expression involves the first‐order correlation function of the incident field. The relation between the line shape of the scattered light and the bandwidth of the incident field has been clarified. Model calculations of the photon counting rate have been performed in the case of an incident field without first‐order coherence. In our treatment, the transverse and longitudinal relaxation constants have been taken into account by using the impact approximation.
SCF CI MO calculations on the base–base interactions in dinucleoside monophosphates
Authors: Gumbinger H. G. , Kaiser P. M.
Published: December 2004

In order to elucidate further details of RNA conformations we have studied base stacking in dinucleoside monophosphates (DNP's) using UV difference spectra and the hypochromic effect. SCF CI MO calculations according to PPP and MIM approximations were carried out for six pyrimidine‐containing DNP's in each of several stacking geometries. The calculated and plotted difference spectra were fitted to the experimental spectra. Different DNP's showed distinct geometries in the range of the helix angle of 35°. From our results we conclude that there is a microstructure in the helix. This would imply an additional content of information in this macromolecule, a higher precision in nucleic acid interactions, and make possible the prediction of the conformation of polynucleotides.
MO Study of flavin–protein interactions in flavodoxin catalysis
Authors: Teitell Murray F. , Fox J. Lawrence
Published: December 2004

MINDO/3 calculations have been performed on the Clostridium MP flavodoxin active site (a complex of the redox active coenzyme flavin mononucleotide sandwiched between the side chains of methionine and tryptophan) at various redox levels using coordinates derived from x‐ray diffraction studies of the holoenzyme. Frontier orbital indices were calculated and indicate that reduction of the flavin is accompanied by induced polar states in the amino acid side chains. This stabilization of charge by the amino acid side chains could account for the reaction rate enhancement of flavin reduction catalyzed by flavodoxin. Frontier orbitals for free flavin, for the flavodoxin bound flavin without the amino acid side chains, and for the oxidized Desulfovibrio vulgaris flavodoxin active site were computed for comparison.
MO Study of the photochemical behavior of the imine bond
Authors: Osamura Yoshihiro , Yamabe Shinichi , Nishimoto Kichisuke
Published: December 2004

The photoisomerization of imine compounds is studied in terms of an ab initio MO CI calculation. The potential curves of the syn‐anti isomerization via the rotation and the inversion are examined for benzaldimine. It is suggested that the photoisomerization is initiated through the rotation around the CN bond in both singlet and triplet states. The ease of the photoisomerization is found to be determined by both the conformation of phenyl ring in the ground state and the energy difference of vertically excited states between two isomers.
MO Study on the photochemical isomerization of isoxazole
Authors: Tanaka H. , Matsushita T. , Osamura Y. , Nishimoto K.
Published: December 2004

The reaction mechanism of photoisomerization of isoxazoles to oxazoles through azirine intermediates is investigated theoretically by means of ab initio MO CI calculations. Azirine intermediates in S1 state [(n →*) state of the carbonyl chromophore] cause the CN bond rupture of azirine ring and transform to isoxazoles via intersystem crossing to T1 state. On the other hand, azirine intermediates in S2 state [(n →*) state of the ketimine chromophore] lead to the CC bond rupture of azirine ring and convert to oxazoles via intersystem crossing to T1 state.
Ab initio molecular orbital studies of polyethylene deformation
Authors: Brower A. L. , Sabin J. R. , Crist B. , Ratner M. A.
Published: December 2004

A molecular LCAO Hartree‐Fock procedure was used to calculate total energies of axially stretched normal paraffins containing up to nine carbon atoms. The results are used to model the mechanical properties of polyethylene.
Gaussian matrix elements of the operator X: Reduction to confluent hypergeometric functions
Authors: Niukkanen A. W.
Published: December 2004

A new method has been proposed for calculating the Gaussian matrix elements of the interaction operator represented by an arbitrary degree of interelectron distance X. The method is based on the expansion of two‐electron integrals as the sum of one‐electron integrals which in turn admit compact operator representation in terms of confluent hypergeometric functions. The generating differential operator has been shown to be related to the modified Hermitian polynomials. The standard structure of the special functions encountered in this approach is useful in studying the analytical behavior of the integrals and makes it possible to obtain for these integrals recurrence relations, direct algebraic expressions in the forms of finite sum of confluent hypergeometric functions, integral representations, and asymptotic properties. Unlike the usual methods based on integral transformation of the interaction operator, the proposed approach has a wider field of application, and in addition, leads to compact and convenient analytical expressions. The idea of using differential properties of integrals to simplify the integrand structure gives the proposed approach a certain resemblance to that suggested by Boys but not developed in detail in his pioneer work.
Evaluation of MINDO/3 calculations in simple amides, ureas, and heterocycles of biological interest
Authors: Defina J. A. , Andrews P. R.
Published: December 2004

The performance of the MINDO/3 method has been evaluated in simple heteroatomic molecules that are representative of several important biological systems. Molecular geometries are accurate to within 0.04 Å for bond lengths, and 3° for most bond angles. Dipole moments and charge distributions are estimated satisfactorily, and ionization potentials are predicted to an accuracy of 0.5 eV. The method is less accurate in estimating the ionization of some upper level σ orbitals. The overall evaluation shows that MINDO/3 provides a good description of the geometries and electronic properties of the molecules studied. The results of MINDO/3 calculations in several other chemical systems are also reviewed.
Simple technique to obtain correlated energy curves: LiH calculations
Authors: Moscardó Federico , Delgado‐Barrio Gerardo
Published: December 2004

The possibility of applying the half‐projected Hartree–Fock method together with the Coulomb hole model to obtain cofrelated potential‐energy curves is investigated. Here we report calculations performed on the LiH molecule. The results show that this combined technique can provide potential curves very near to the experimental ones with a similar effort to that involved in the Hartree–Fock calculations.
The fermi surface of beryllium
Authors: Oriade Joseph
Published: December 2004

The Fermi surface (FS) of beryllium has been calculated using the augmented plane wave energy bands for the material. The extremum area and the FS caliper distances are in excellent agreement with the de Haas–van Alphen measurements, and are better than existing ab initio calculations.
Coupled perturbation theory within the antisymmetrized product of separated geminals (APSG) framework
Authors: Dmitriev Yu. , Peinel G.
Published: December 2004

A coupled perturbation theory for the antisymmetrized product of separated geminals (APSG) approximation is derived. The variational principle for the APSG wave function in an external oscillating field is employed and a set of equations of the form analogous to the normal RPA is obtained. At this level the reduced resolvent in the form of a spectral expansion is written and it is used for the evaluation of the second‐order properties.
Structures, stabilities, and energies of isomerization of some alkyl cyanides and isocyanides and their corresponding α‐cyanocarbonium and α‐isocyanocarbonium ions
Authors: Moffat J. B.
Published: December 2004

Ab initio STO‐3G geometry‐optimized calculations have been performed on methyl, ethyl, and isopropyl cyanides and isocyanides, and the cations formed on abstraction of an α‐hydride ion. Stabilization energies of the cations and cyanide–isocyanide isomerization energies have been calculated and correlated with structural features and population analyses.
Ghost orbitals in semiempirical methods. Estimation of basis set superposition error
Authors: Lipiński Józef , Chojnacki Henryk
Published: December 2004

Based on the all‐valence ZDO SCF approximation a procedure for estimating the basis set superposition error (BSSE) in semiempirical CNDO/INDO methods has been proposed. The results of the calculation show that the BSSE effect may improve the results obtained from the standard CNDO/INDO supermolecule calculation. The estimated BSSE effect enables one to explain some recently reported artificial structures for water and ethylene dimers.
A possible basis for structure–function relationship of estrogens
Authors: Durani S. , Anand Nitya
Published: December 2004

It is well known that a wide variety of molecules compete for binding to the estrogen receptor and act as estrogens and/or antiestrogens. These molecules such as estradiol, diethylstilbestrol, doisynolic acid, the triarylethylenes, and cyclofenyl apparently share little resemblance which could account for their interaction with a common estrogen binding site. Knowledge of the receptor binding of triarylethylene and cyclofenyl prototypes, in particular, relative to that of the other estrogens is critical for understanding their structure‐function relationship. We have carried out a study on the receptor binding specificity of triarylethylene and cyclofenyl prototypes. This study has revealed that these molecular types share considerable resemblance in their receptor binding specificity and differ from estradiol and other similar molecules in some important respects. However, comparison of their substructural binding specificities reveals the possibility that the triarylethylene and estradiol prototypes may interact with at least some common regions of the estrogen binding site. Based on this reasoning the comparative receptor binding of estrogens has been rationalized on the basis of a subsite hypothesis for the estrogen binding site. According to this hypothesis, the composite estrogen binding site is composed of essentially five subsites, and that the structurally different estrogenic prototypes can interact with different set of subsites, and thus differ in their binding orientation. The essential difference in the activity profile of estradiol prototypes and the triarylethylene antiestrogens reveals the possibility of a causal relationship between the binding orientation of a ligand and its activity profile. This model for receptor site can thus serve as a working hypothesis to rationalize the structure‐function relationship of estrogens.
Structure–Activity relationships in auxins
Authors: Raghunathan S. , Pattabhi Vasantha
Published: December 2004

Of the various theories proposed for auxin activity, the conformational change theory and the charge separation theory have received considerable attention. However, no conclusive theory collating molecular structure with hormonal activity has yet been established. We have studied the structural requirements for hormonal activity in plants through single crystal x‐ray studies, as well as through conformational and charge density calculations on a series of plant hormones. We find that the existing theories are inadequate in explaining the activity of several auxins. We propose that the size of the auxin molecule in conjunction with the charge separation serves as a possible mechanism of action in plant hormones.
Biological activity of the nucleoside analogs: A theoretical study
Authors: Sanyal Nitish K. , Roychoudhury Mihir , Ojha Rajendra P.
Published: December 2004

It has been observed through the conformational analysis of nucleoside analogues that in most of the cases these molecules possess similar conformations to their parent molecules. Therefore, it is, generally believed that these molecules are incorporated as such in the nucleic acids during various biological functions, viz., replication, transcription, etc. A theoretical study for such incorporation has been carried out. Charge distribution of the base parts of these molecules has been calculated by using CNDO/2 method wherever they were not available in literature. Electrostatic energies for various base pairs with these bases have been calculated. Also a possible incorporation during transcription has been studied in view of the model developed by Sanyal, Kumar, and Roychoudhury by taking formycin as an examples. The results of these studies are discussed.
Probing possible left‐ and right‐handed poly(dinucleotide) helical conformations from (n–h) plots. models for polysequential nucleotides
Authors: Jayaraman S. , Yathindra N.
Published: December 2004

Introducing the concept of the “dinucleotide” as the helical repeat, theoretical attempts have been made to determine possible single and double stranded helical structures by using helical parameter calculations and model building investigations. By virtue of its flexible framework, the dinucleotide repeat offers a much greater scope of finding new secondary structural forms for nucleic acids. Considering only those conformations which show tendency for at least partial base overlap as does the dinucleotide helical repeat, it has been possible to predict poly(dinucleotide) helical models in which successive phosphodiesters as well as nucleotide conformations alternate. More important, the recently found left‐handed Z‐type polynucleotide helix is characterized rather uniquely on the helical parameter plot. The results further suggest the possibility of other Z‐type helices obtainable by alternative conformations for the exocyclic C4'–C5' bond and sugar pucker. Near neighbor long range conformational correlations between the dinucleotide repeat and the phosphodiester linking them have been established similar to poly(mononucleotide) helices. Need for considering higher repeats such as trinucleotide has been suggested to obtain models for looped out helical conformations.
Virtual bond probe to study ordered and random coil conformations of nucleic acids
Authors: Malathi R. , Yathindra N.
Published: December 2004

Recognition of the stereochemical features inherent to a nucleotide, viz., the preferred trans character of the two CO bonds and the approximate chemical and conformational symmetry, has facilitated the representation of a nucleotide in terms of two nearly equal blocks or two virtual bonds spanning the atoms P to C4' on the 5' side and C4' to P on the 3' side. The scheme, by virtue of its unique ability to account for the main sources of flexibility and to incorporate their interdependence, could be effectively applied to probe ordered, as well as random coil conformations of polynucleotide chains. Using this scheme, conformations of nucleotide, dinucleotides, helices, and yeast tRNAPhe have been characterized by virtual bond parameters. The two blocks of nucleotide have also been described in terms of two approximate planes similar to peptides. Unperturbed end‐to‐end dimensions and persistence lengths of random coil polynucleotides have been computed by considering short‐range as well as near‐neighbor bond long‐range correlations and are in excellent agreement with the experimentally determined values. A striking finding of these studies is the relevation that random coils comprise of a large proportion of stacked A‐type helical segments in sharp contrast to earlier interpretations which invoked a high fraction of unstacked extended conformations.
Molecular electrostatic potential on the surface envelopes of macromolecules: B‐DNA
Authors: Lavery Richard , Pullman Bernard
Published: December 2004

A procedure for calculating the molecular electrostatic potentials on surface envelopes surrounding macromolecules is presented. This new representation of potential is employed in studying B‐DNA double helices and, from the results, deductions are drawn on the interaction specificities of B‐DNA with electrophiles.
Calculation of the density of states of tight‐binding d band for simple cubic lattice
Authors: Modrak P.
Published: December 2004

The electronic density of states for d bands was calculated accurately in the tight‐binding approximation for a simple cubic lattice. Numerical values of the density of states for five subbands as well as of the total density of states were tabulated for the set of values of two‐center integrals of the magnitudes corresponding to those estimated for real crystal.
Hypervirial analysis of enclosed quantum mechanical systems. III. Unsymmetrical boundary conditions
Authors: Fernández Francisco M. , Castro Eduardo A.
Published: December 2004

New theoretical developments based upon hypervirial analysis of enclosed systems are given. Systems which obey unsymmetrical boundary conditions are analyzed. Several results which were given by the authors in two previous communications are generalized.
Calculation of periodic molecular systems with perturbed periodicity within the PCILO framework. I. Method
Authors: Lochmann R. , Weller Th.
Published: December 2004

A method within the PCILO framework for the study of periodic molecular systems involving impurities is described regarding a one‐dimensional chain. The calculation of the whole system is reduced to the calculation of subsystem pairs only. By use of the first‐neighbor approximation only one or two subsystem pairs must be considered for subsystems with identical or differing surroundings, respectively. The procedure described yields the ground state energies and charge distributions of the subsystems.
Analytic expression of the rotation harmonics in the vibration–rotation wave function of a diatomic molecule
Authors: Kobeissi Hafez , Korek Mahmoud
Published: December 2004

By using the vibration–rotation canonical functions, we show that the wave function for a vibration–rotation level (v, J) can be represented by ψvJ(r) = ψv (r)+Σ λlϕl(r) with λ = J(J + 1). The radial Schrödinger equation can be written (Hv + λHJ)ψvλ = (Ev + Σ λlεl)ψvλ, where ε1, ε2, ε3,… stand for the rotational constants Bv, Dv, Hv,…. The radial equation being satisfied for any value of λ, the rotation “harmonics” ϕ1, ϕ2,… are found to be the solutions of a set of inhomogenous differential equations of the form Hϕl = Evϕl + fl(r)ψv. An analytic expression of the harmonics ϕi is given for any potential. The numerical application shows that, for a given r, the harmonics decrease in absolute values like Bv, Dv, Hv,… and that the agreement between the values of ψvJ deduced from the computation of the harmonics on one hand, and the direct computation on the other hand, is very satisfactory.
Studies in Löwdin's partitioning technique in quantum theory. II. Operators T and Ω
Authors: Wilson R. G. , Sharma C. S.
Published: December 2004

Operator Ω and reaction operator T of Löwdin are studied with a slightly different approach, which leads to a clearer understanding of their nature and brings out the relevance of the conditions for the invertibility of K studied in the preceding work in this series.
Direct calculation of lattice sums. A method to account for the crystal field effects
Authors: Stolarczyk Leszek Z. , Piela Lucjan
Published: December 2004

A method of direct calculation of lattice sums in three‐dimensional crystals is reported. The method is based on annihilation of some lowest multipole moments of the unit cell by a redefinition of the unit cell content. As a result, properties of the infinite crystal can be calculated as usual by taking a finite cluster of unit cells, but surrounded by an additional surface layer of a charge density (e.g., a layer of point charges). This charge density distribution produces the electric field approximating that one of the rest of the infinite crystal. The method proposed is easily applicable in the SCFLCAO procedure as well as in any method using a cluster representation for an infinite crystal. The validity of the infinite crystal model for a finite crystal is also discussed.
Reorganization effects in the photoelectron spectra of transition metal compounds: A Green's function study in the framework of a variable INDO hamiltonian
Authors: Böhm Michael C.
Published: December 2004

The magnitude of reorganization energies in the photoelectron (PE) spectra of various transition metal compounds with Mn, Fe, and Ni as 3d center is studied by means of a variable INDO Hamiltonian. The Koopmans defects are analyzed as a function of the one‐electron resonance integral β and as function of the one‐ and two‐center electron–electron interaction integrals. β has the property of an inverse coupling constant; reorganization effects are enlarged with reduced β values. In the limit of very small resonance integrals a reduction of the calculated Koopmans defects due to modified localization properties of the orbital wave function is encountered. The two‐center electron‐electron interaction integrals γ have been calculated via an exponential formula with a variable range parameter. In the limit of long‐range potentials with flattened γ; gradients a significant reduction of relaxation and correlation is diagnozed; large defects are predicted in the short‐range limit with steep gradients in the repulsion potential. The one‐center Coulomb and exchange integrals (γ, K) have been modified by a multiplicative factor. With enlarged one‐center integrals enhanced Koopmans defects are encountered. The reorganization energies are determined by means of a Green's function approach with a renormalized approximation for the self‐energy part.
Study of an approximate relation between the energy of an atom and the electronic potential at the nucleus
Authors: Alonso J. A. , González D. J. , Balbás L. C.
Published: December 2004

This paper provides an analysis of the reasons for the approximate validity of the relation \documentclass{article}\pagestyle{empty}\begin{document}$ E = \frac{3}{7}NV(0) $\end{document}, between the total energy E of a neutral atom, the number N of electrons, and the electronic potential at the nucleus V(0). Using the density functional formalism we find that the right‐hand side of the above equation also appears (and is the leading term) in density functional approximations more sophisticated than the Thomas–Fermi (TF) approximation (the above equation is exact in the TF approximation). Systematic improvements to the equation appear to be difficult because the main corrections come from those terms which are more difficult to handle in the density functional formalism. After this analysis we propose a kinetic energy functional for neutral atoms in the Hartree–Fock approximation. The first term of this new functional is a rescaled Thomas–Fermi term

, where γ = −0.0063 for light atoms and γ = 0.0085 for the others. The second term is the first gradient correction due to Kirzhnits

.

For lithium to krypton atoms, this new functional gives an average error of 0.22%.
Ab initio molecular orbital study of the tautomerism of 4‐hydroxy‐2‐pyridinone
Authors: Schlegel H. Bernhard
Published: December 2004

Ab initio calculations were performed on 2,4‐pyridinediol, 4‐hydroxy‐2‐pyridinone, and 2hydroxy‐4‐pyridinone at the HF/3‐21G level with full geometry optimization. Two conformations of the hydroxyl group were considered for each tautomer. Corrections for polarization functions, electron correlation, and zero point energy were made by comparison with previous calculations on 2‐ and 4‐pyridinone tautomerism which included these contributions. The most stable structure is 4‐hydroxy‐2‐pyridinone, in agreement with experiment. Relative to 4‐hydroxy‐2‐pyridinone, the energies of the other tautomers are estimated to be 1.9 kcal/mol for 2,4‐pyridinediol and 8.9 kcal/mol for 2‐hydroxy‐4‐pyridinone. These are in accord with the experimental values 0.3 ± 1.9 and 10.6 ± 1.9 kcal/mol, respectively, deduced from equilibration studies of the tautomeric methyl derivatives.
ESCA Peak intensities in the dipole and generalized sudden approximations using fourier transforms of GTO's
Authors: Morales J. , Palma A. , Poulain E.
Published: December 2004

Analytical expressions for the square of the spherical average of Fourier transforms of Gaussian type orbitals (GTO's) are given. A direct application of these expressions to the calculation of molecular photoionization cross sections is considered under the generalized sudden (GSA) and dipole (DA) Approximations. Numerical calculations were done on the CO molecule using bound orbitals obtained by ab initio LCAO–MO calculations with Gaussian basis sets. The results are in good agreement with experiments. Those obtained by the GSA method however, suggest a limitation in its use: the GSA method is only applicable when comparing photoionization intensities of neighboring ionization energy orbitals. Applications to other molecules are immediate.
Theoretical study of the effect of substituents on electrophilic addition to olefins: Ab initioMO approach with energy decomposition
Authors: Ruo‐Zhuang (R.Z.Liu) Liu , Jian‐Guo (J.G.Yu) Yu
Published: December 2004

The interaction energies of proton with ethylene and substituted ethylenes in a positive bridged ion have been calculated by ab initio MO method with STO‐3G basis set, and the energies were further decomposed according to decomposition scheme proposed by Kitaura and Morokuma [Int. J. Quantum Chem. 10, 325 (1976)].
Calculation of the coherent memory functions from the eigenvalues and eigenvectors of the corresponding stationary Schrödinger equation
Authors: Skála L.
Published: December 2004

Using some properties of the Bessel functions the algebraic representation of the Nakajima–Zwanzig generalized master equation was derived. This representation makes possible the calculation of the memory functions in terms of the probabilities. Using this approach, the formulas for the memory functions giving their dependence on the eigenvalues and eigenvectors of the corresponding stationary Schrödinger equation are given and discussed in detail.
The fragments‐in‐molecules method. II. Fim method for σ‐bonded systems
Authors: Klessinger Martin
Published: December 2004

A semiempirical SCF MO method has been developed in which the wave function of a composite molecule is written as a linear combination of localized fragment orbitals and which is formulated such that strictly transferable empirical data for the fragments may be introduced into the calculation. Results of FIM calculations in the CNDO/2 approximation for a number of RX molecules with R = alkyl and X = F, OH, NH2, and CH3 are presented and used to illustrate the possibilities and limitations of the method.
Photoelectron spectroscopy of chemisorbed atoms
Authors: Nicolaides C. A. , Andriotis A. N.
Published: December 2004

We outline a theory of UV and higher‐energy photoemission spectroscopy of chemisorbed atoms, that aims at the accurate calculation of inner electron binding energies and photoabsorption cross sections by including solid state and localized relativistic and correlation effects. It is based on an “atom on (in) solids” approach where one first extracts a surface potential and then uses it in a coupled Hartree–Fock theory to obtain self‐consistently the shifts and splittings of atomic levels. A first application of this theoretical program has been carried out on Na on the Al(100) system, by calculating from first principles the binding energies of the Na 1s and 2s electrons. For a coverage of 1.23 × 1014 adatoms/cm2 we find BE(1s) = 1075.2 eV and BE(2s) = 66.2 eV. Also, the Na 2p orbitals are found to split in the cylindrical symmetry by about 0.2 eV.
Reactivity comparison of one‐electron and two‐electron processes
Authors: Lee Tieh‐Sheng , Chu San‐Yan
Published: December 2004

The reaction activity is analyzed in terms of an electronic factor and a Franck–Condon factor, in analogy to that in problems of spectroscopy. A scheme for codifying a number of physical and chemical processes based on magnitude of the two factors is proposed. The electronic factor not only determines the topological feature of the potential surfaces, but also is capable of revealing certain clues leading to the mechanistic understanding from the type of operator involved. Based on such an analysis, it is concluded that all processes involving a one‐electron operator, either concerted or stepwise, may compete favorably with processes involving two‐electron operators. A forbidden reaction with a two‐electron process may therefore become a more efficient one‐electron process by either choosing a nonleast motion pathway, by carrying out the overall transformation stepwise, or by changing the occupation number of certain orbitals in the ionic form. The mechanisms of energy transfer in the excited state is also briefly discussed in terms of one‐electron and two‐electron processes.
Calculation of NMR chemical shifts. III. Configuration interaction effects
Authors: Fukui H.
Published: December 2004

The theory for nuclear shielding including electron correlation effect is derived and applied to the hydrogen fluoride molecule. It is shown that about 20% of the paramagnetic contribution to nuclear shielding at nucleus F is due to electron correlation. Inclusion of electron correlation effect greatly improves the calculated values toward the experimental ones.
Some considerations on Ueff values entering the hubbard hamiltonian
Authors: Friedel J. , Noguera C.
Published: December 2004

One of the simplest ways to take into account correlations between electrons in solid state physics, beyond the usual one electron approximation, is to use the Hubbard Hamiltonian

Although it corresponds to very severe approximations, since intersite correlations are neglected, and since the whole problem is reduced to a single parameter U, it is already very difficult to solve, and exact solutions exist in only a few cases. It is thus not desired to introduce more complications in the Hamiltonian; but it could be interesting to understand, at least qualitatively, which processes influence the value taken by parameter U. The question arises in concrete cases, such as transition metals or organic molecules; comparing experiments and approximate solutions of the Hubbard Hamiltonian has shown that, in most cases, the value of U that yields the best fit is very different from what would be expected from atomic considerations. For instance, if one wants to study correlations on atoms of type M, a pure atomic point of view would affect to U the change in energy Uat in the redox‐type equation

. However it seems that solid state effects play an important role and we discuss, in this paper, three of them: (i) the finite time that one electron spends on a given atom due to the delocalization of the electrons in a band (effect of βij which increases U relatively to Uat); (ii) the mixing of bands of different orbital character which allow two electrons not to be on the same orbital of an atom (and thus decreases U); and (iii) the existence of Coulomb repulsion between electrons on neighboring sites which reduces their effective exclusion on the same site.
Cluster calculations of the H2O/Pt(111) system
Authors: Paul J. , Rosén A.
Published: December 2004

The electronic interaction between water and a Pt(111) surface as evaluated for different Ptx(H2O)y clusters is discussed. Hartree–Fock–Slater (HFS) one‐electron ground state energies, ionization potentials, partial densities of states, and Mulliken occupation numbers are related to bonding shifts, as well as initial and final state screening for different orientations of the molecule. The formation of PtH2O bonds are sensitive to the orientation since surface oriented H atoms bridge the spatial separation between O 2p and Pt 5d orbitals and thus increase the intermixing of metal and adsorbate orbitals. The dipole moment and the net charge of the H2O molecule is also discussed. Finally, approximations of the metal–H2O potential for use in statistical models of the liquid–metal interface are suggested.
Comparison of theoretical and thermodynamic values of interconstituent interactions in DNA
Authors: Shibata Masayuki , Kieber‐Emmons Thomas , Rein Robert
Published: December 2004

Experimental base stacking and hydrogen bonding enthalpies, as determined from double stranded DNA melting studies, are compared with theoretical calculations of interactions between the respective DNA interconstituent components. Comparisons are made in relation to the differences in magnitudes, sequence dependent spreads, and trends. It is shown that the use of an effective dielectric constant reconciles the differences between the experimental and theoretical values. Suggestions are made for calibration of potential functions suitable for nucleic acid conformational analysis.
Symmetry determined orbital and group overlap
Authors: Qianer Zhang
Published: December 2004

The MO theory of structural polyhedrons is one of the basic subjects closely related to the bonding theory of complex compounds and clusters. A series of valuable papers on this subject have been contributed by Hoffman, Wade, King, and Lauher et al. A method called “group overlap method” for constructing the symmetry orbitals and for calculating the group overlap integrals is proposed. It is hoped that the group overlap method may be used for the qualitative and quantitative MO study of structural polyhedrons.
Ab initioMODPOT/VRDDO/MERGE calculations on energetic compounds. III. Nitroexplosives: Polyaminopolynitrobenzenes (including DATB, TATB, and tetryl)
Authors: Hariharan P. C. , Koski Walter S. , Kaufman Joyce J. , Miller Richard S.
Published: December 2004

Quantum chemical ab initio MODPOT/VRDDO calculations have been carried out on the following aminonitrobenzenes for which crystal structures had been determined experimentally: 4‐nitroaniline; N,N‐dimethyl‐p‐nitroaniline; 2,4,6‐trinitroaniline; 1,3‐diamino‐2,4,6‐trinitrobenzene (DATB—Form I); 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB); 2,3,4,6‐tetranitroaniline; N‐methyl‐N,2,4,6‐tetranitroaniline (Tetryl); and N‐(β,β,β‐trifluoroethyl)‐N,2,4,6‐tetranitroaniline. These quantum chemical calculations were performed on the molecules in their conformations as found in their crystal structures. The calculations were carried out with our own ab initio programs which also incorporate as options several desirable features for calculations on large molecules: ab initio effective core model potentials (MODPOT) which enable calculations of valence electrons only explicitly, yet accurately, and a charge conserving integral prescreening evaluation (which we named VRDDO‐variable retention of diatomic differential overlap) especially effective for spatially extended molecules. Aminonitrobenzenes are especially interesting since there are inherent intramolecular ring distortions and deviations from planarity and intramolecular hydrogen bonds as well as intermolecular hydrogen bonds causing further deviations from planarity. The theoretical indices resulting from the quantum chemical calculations are relevant to a number of properties and behavioral characteristics of these molecules, both intramolecular and intermolecular. The charges on the atoms [from the gross atomic populations (GAP's)] are needed for calculation of the atomic multipole–atomic multipole electrostatic contributions (a dominant factor) to the intermolecular interaction energies. These electrostatic interaction energies are part of the input necessary for calculations on the crystal packing and densities of these molecules. These GAP's are also of value in interpreting the experimental photoelectron and ESCA spectra of these molecules. The total overlap populations (TOP's) between atoms are related to the inherent bond strengths and can serve as a quantitative replacement for the old empirical bond length‐bond order‐bond energy relationship still used by explosives chemists to identify the “target bonds” (the weakest bonds). The TOP's are of considerable value in predicting and tracing initiation and subsequent steps of explosive phenomena. The molecular orbital energies of the lowest unoccupied orbitals are of interest since nitroexplosives have been implicated in testicular toxicity and the initial metabolic activation appears to proceed through a one‐electron reduction of the nitroexplosive.
Photoreactions on finely divided semiconductors
Authors: Peshev O.
Published: December 2004

Photoreactions on finely divided semiconductors are discussed. Experimental results on photo‐oxidation of CO, isobutane, and NH3 over UV‐irradiated n‐TiO2 (anatase) particles of different size are discussed. The possibility of using size‐quantized films for spectroscopy of adsorbates is also considered.
Theory of living State. VII. Bose–Einstein‐like ordering in temperature and time domain
Authors: Mishra R. K. , Bhowmik K.
Published: December 2004

The Bose–Einstein‐like ordering was earlier proposed as an important mechanism of energy handling in the living systems. In this regard two questions arise at once: (1) is such ordering feasible at body temperature and (2) what is the time that may be taken for establishing coherence? In this paper we hope to answer and satisfy these two questions.
Three‐body nonadditive effects for hydrogen bonded water molecules: SCF study of hydrated species including H2S and various metal ions
Authors: Novaro O.
Published: December 2004

The relative importance of purely nonadditive three‐body terms in solvated systems is assessed through ab initio LCAO–MO–SCF studies of hydrogen bonded structures containing two or more water molecules plus a solvate species. Among the latter the cases of Li+, Ca2+, and Mg2+ ions (of biological interest for aminoacid release processes in membrane transport) and of HDS molecules (of industrial importance in processes for the production of heavy water) are studied.
Quantum chemical study of the molecular patterns of MAO inhibitors and substrates
Authors: Martin M. , Sanz F. , Campillo M. , Pardo L. , Pérez J. , Turmo J.
Published: December 2004

Similarities and differences between mitochondrial monoamine oxidase (MAO) substrates and inhibitors (both A and B types) are considered, studying quantitatively two molecular properties: electron density and molecular electrostatic potential (MEP). The following molecules are considered: substrates: PHEA, BZA, tele‐N‐methyl‐histamine, phenylethanolamine, phenylpropylamine, tryptamine, dopamine, phenoxylethylamine, noradrenaline, serotonin, and p‐nitro‐phenylethanolamine; inhibitors: Deprenil, Clorgyline, and Lilly 51641 (only the moiety involved in the A–B differentiation is considered). The wave functions needed to calculate the analyzed properties are of ab initio quality, and have been calculated in analogous conformations, all near the energetic minima. Electron densities distributions are qualitatively compared by means of a correlation coefficient defined over the whole space. Otherwise, patterns of the possible zones of electrostatic interactions are described by means of the distances and angles between minima, in order to differentiate MAO‐A and MAO‐B substrates. The results reproduce efficiently the experimental classification and enable us to predict the type of enzymatic action of molecules not yet experimentally classified.
Rayleigh–Schrödinger perturbation theory for many‐electron systems with partitioning of configurations by orders of excitation
Authors: Taylor William J.
Published: December 2004

Rayleigh–Schrödinger perturbation theory for many‐electron systems has been rederived by an extension of Löwin's partitioning method in which excited configurations are further partitioned by their orders (two equivalent definitions of the order of a configuration are given). The resulting equations show explicitly how first‐, second‐, and third‐order configurations contribute to the first three orders of perturbative corrections to the ground‐state wave function, and the first six orders of corrections to the energy. Because the partitioning is by orders of configurations (rather than level, i.e., single, double, etc., excitations) the formalism is unaffected by the applicability, or nonapplicability, of Brillouin's theorem.