bluuebunny/crossref_metadata_2025_split huggingface dataset:
Polarizable continuum model of solvation for biopolymers
Authors: Frecer Vladimír , Miertuš Stanislav 
Published: December 2004

AbstractThe original polarizable continuum model (PCM) of solvation based on the boundary element method for the numerical solution of the Poisson equation for a realistic molecular shape is modified through a classical description of solute charge distribution and solute polarization. In this approach, the solute charge distribution is represented by point charges and polarizabilities and the solute–solvent polarization is evaluated. The solvent–solute back polarization is also considered. The effect of electrolyte ionic strength is included via a dielectric function based on the Debye–Hückel model. A more realistic description of molecular cavity boundaries is introduced with the help of variable van der Waals radii. Approaches of evaluation of dispersion–repulsion and cavitation contributions to solvation Gibbs free energy suitable for large biopolymers are also presented. Test calculations illustrating the performance of the model are given together with examples of its application for larger biopolymer structures.
Ab initio CPHF calculations of the static polarizability and second hyperpolarizability of small molecules: Comparisons between standard and moderately large basis sets augmented with diffuse functions
Authors: Dory M. , Beudels L. , Fripiat J. G. , Delhalle J. , André J. M. , Dupuis M. 
Published: December 2004

AbstractStatic polarizability and second hyperpolarizability have been calculated for a number of small moleculesCO2, OCS, CS2, C2H2, C2H6, C3H8, cyclo‐C3H6, C3H4, C3H6, SiH4, Si2H6in the framework of the coupled‐perturbed Hartree‐Fock (CPHF) theory. The linear and nonlinear coefficients have been calculated with standard Gaussian basis sets and 3‐21G bases moderately enlarged with diffuse functions. It is shown that the parallel component of the polarizability saturates rapidly, which suggests that a 3‐21G basis containing s and p diffuse functions is sufficient to reproduce αzz. For the αxx and αyy components, a 3‐21G basis with s, p, and d diffuse functions is required. In general, the concordance between α computed with this basis set and the experimental static polarizability is at least of the order of 80%. On the contrary, the computation of the second hyperpolarizability with the same basis set for CO2, CS2, and C2H2 gives values that are 30% too low, compared to the experimental value. Better results are observed for ethane, propane, and cyclopropane for which the error is lower than 50%. The better agreement observed for the saturated compounds can probably be explained by their saturated character.
Electronic structure and nonlinear optical properties of push–pull conjugated molecules
Authors: Meyers F. , Brédas J. L. 
Published: December 2004

AbstractWe present a review of ab initio calculations we recently performed on organic molecules presenting promising quadratic nonlinear optical properties. These molecules constitute so‐called push–pull conjugated compounds in which a conjugated segment is capped at one end by an acceptor group and at the other end by a donor group. We foucs on three series of systems: (i) p‐amino‐p′‐nitrodiphenylacetylenes for which “unusual” distorsion patterns have been recently reported; (ii) benzodithiapolyenals, which present among the largest second‐order polarizabilities ever measured; and (iii) 2‐methylene‐2H‐pyrrole derivatives. Our results are found to provide a detailed understanding of the avilable experimental data.
A strategic plan for molecular electronics
Authors: Aviram Ari 
Published: December 2004

AbstractThis paper describes the background of microelectronics science and technology that influenced the development of molecular electronics (ME) and includes a description of some thoughts on the development of molecular circuit elements. Three basic principles are proposed and exemplified by a particular example of a suitable molecule. It is pointed out that as the size of electronic devices approaches molecular dimensions, ME promises to have distinct advantages over other technologies because the component function can be an outcome of the electronic structure of the molecule. A plan that can lead to the development of the ME concept with the scientific and technological knowledge of the present day is proposed.
A calculation of exciton energies in periodic systems with helical symmetry: Application to a hydrogen fluoride chain
Authors: Vracko M. G. , Zaider M. 
Published: December 2004

AbstractAb initio Hartree–Fock crystal orbital results were used as input for the calculation of exciton energies in the Tamm–Dancoff and random‐phase approximation for polymers with helical symmetry. The calculations were applied to a hydrogen fluoride chain. We show that the Tamm–Dancoff method is a good approximation to the random‐phase theory. Furthermore, the influence of the band–band interaction in the exciton calculation has been investigated.
Orbitally resolved charge sensitivity analysis: Basic concepts and relations
Authors: Nalewajski Roman F. , Mrozek Janusz 
Published: December 2004

AbstractFollowing the recent developments of the charge sensitivity analysis (CSA) in the atoms‐in‐molecules (AIM) resolution, the corresponding CSA quantities in the orbital (or shell) resolution (OR) are defined. The OR electron population variables, in the ordinary closed‐shell SCF problem, are the elements of the bond‐order matrix P, and their conjugates, “chemical potentials,” FT = ∂E/∂P, are the respective Fock matrix elements, appropriate for the representation in question; here E is the SCF energy. The second derivatives ∂2E/∂P∂P define the OR hardness tensor from which all related OR CSs, e.g., the hardness, softnesses, Fukui function (FF) indices, etc., can be determined. The rigid potentials and hardness tensor, corresponding to the “frozen” orbital approximation, are examined in more detail, and the decoupled representation of the normal orbitals (NoO) is introduced, in which the rigid hardness tensor becomes diagonal. Illustrative valence‐shell NoO contours for the water molecule are given and discussed. The new approximation for the OR FF indices, as the orbital occupation probabilities, is proposed on the basis of the density matrix functional development of Donnely and Parr for natural orbitals, and the relevant expressions for the molecular fragment (collection of orbitals) quantities are summarized.
An AM1 study of the coupling reaction of radicals with the acetone enolate ion and some related nucleophiles
Authors: Pierini A. B. , Borosky G. L. , Baumgartner M. T. 
Published: December 2004

AbstractWe here report an AM1 study for the reaction of different types of radicals with the acetone enolate ion 1 and other ambident nucleophiles. The theoretical reactivity order determined is phenyl > methyl > tert‐butyl ≅ bicyclo‐[2.2.1]hept‐1‐yl. The activation energy for the coupling of aliphatic radicals is ascribed mainly to nonbonded interactions. Considering possible solvent effects, we propose the same order of reactivity in solution. The coupling reaction of phenyl radical with 1 is an exothermic process with nonactivation energy in the gas phase. Any energy barrier in solution is here ascribed to desolvation of the anion. The regiochemistry of the coupling of radicals with ambident nucleophiles such as 1, phenoxide, or phenylamide anions is also explained on a theoretical basis. © 1992 John Wiley & Sons, Inc.
Semiempirical calculations of molecular polarizabilities and hyperpolarizabilities of polycyclic aromatic compounds
Authors: Lu Yuh‐Jy , Lee Shyi‐Long 
Published: December 2004

AbstractThe correlation of 2D conjugation and the nonlinear properties of a series of polycyclic aromatic compounds were studied by performing semiempirical calculations using the MOPAC package. For linear polyacenes, both linear and nonlinear properties are found to be increased as the number of six‐membered rings increases. In general, the polarizabilities, 〈α〉, and second hyperpolarizabilities, 〈γ〉, are found to correlate very well with total resonance energies. In the case of first hyperpolarizabilities, 〈β〉, symmetry is the dominating factor. © 1992 John Wiley & Sons, Inc.
Extension of MNDO to d orbitals: Parameters and results for the halogens
Authors: Thiel Walter , Voityuk Alexander A. 
Published: December 2004

AbstractA recently proposed extension of the MNDO formalism to d orbitals has been parameterized for the halogens CI, Br, and I. Extensive test calculations indicate slight consistent improvements for normalvalent molecules and dramatic improvements for hypervalent molecules, in comparison with established MNDO‐type methods without d orbitals. The mean absolute errors in calculated heats of formation are 3.9 kcal/mol for 155 normalvalent compounds and 2.8 kcal/mol for 23 hypervalent compounds. The predicted structures of the hypervalent molecules are qualitatively correct, with a mean absolute error of 2° in 19 bond angles.
Calculation of the long‐range coupling between electrons in lone pairs and double bonds using semiempirical and ab initio methods
Authors: Braga Manuel , Larsson Sven 
Published: December 2004

AbstractThe electronic coupling between symmetrically placed lone pairs or π electrons are calculated using semiempirical and ab initio methods and the results compared to experimental numbers. The agreement is satisfactory in cases when there is a small tendency for cancellation between contributions of different signs. We have found nonexponential behavior when the number of intervening cyclohexane units is increased in the case of phenyl endgroups. In this case as well as in the cases with exponential decrease, the CNDO/S method shows a similar behavior as that of the ab initio Hartree–Fock method, which suggests that the former method may be used to calculate the electronic factor in electron transfer systems. © 1992 John Wiley & Sons, Inc.
On the agostic interaction in lanthanide phenylene complexes: An INDO study of [(CpM)2C6H4] (M = Sc, Lu) employing an energy‐partitioning analysis
Authors: Jahns V. , Köstlmeier S. , Kotzian M. , Röusch N. , Watson P. L. 
Published: December 2004

AbstractINDO ground‐state calculations have been performed to elucidate the electronic structure of the two para‐dimetallated σ‐phenylene complexes [(CpM)2C6H4], M = Sc, Lu. The lutetium complex exhibits an extraordinarily small MCC angle of about 88°. This unusual geometry is attributed to an agostic interaction between the metal center and the (CH)β group of the σ‐bonded aryl ring. For the analogous scandium compound, the corresponding angle is 118°, rather close to the “normal” value of 120°. Both experimental findings are very well reproduced in the computational models. An energy‐partitioning analysis that exploits the features of the INDO formalism is applied in order to rationalize this geometry difference. The bending of the aryl ligand out of the plane defined by the metal coordination is favored by covalent ligand–metal interactions, but is counteracted by electrostatic repulsion effects. For the lutetium complex, the covalent interactions dominate due to the extended nature of the Lu 5d orbitals and an agostic distortion of the geometry results.
A critical examination of H‐bonding interactions calculated using the AM1 molecular orbital method
Authors: Dannenberg J. J. , Evleth E. M. 
Published: December 2004

AbstractAM1 calculations for various intermolecular and internally hydrogen‐bonding species are compared with experiment. The calculations generally provide a useful, but not perfect, means for modeling H‐bonds. The average stabilization energy for 33 gas‐phase hydrations of ions was 1.4, whereas that for amination of 10 ions was 4.2 kcal/mol, too weak when compared to experimental results. H‐bonding distances have a tendency to be long. The tendency of AM1 to predict multicentered H‐bonds is discussed in the context of recent surveys of crystallographic data banks. Caution is recommended in comparing AM1 results with inappropriate experiments and inadequate ab initio calculations. © 1992 John Wiley & Sons, Inc.
Valence‐bond calculations on ZNO and HGO using integrals computed through the semiempirical AM1 method
Authors: Sola Miquel , Balcells Merce , Duran Miquel , Lledos Agusti , Bertran Juan 
Published: December 2004

AbstractValence‐bond calculations have been carried out on ZnO and HgO using a basis set of Slatertype atomic orbitals and the one‐ and two‐electron integrals as computed in the semiempirical AM1 molecular orbital method. The zero differential overlap approximation has been used to calculate integrals between atomic orbital Slater determinants using the rules for matrix elements between determinants formed by orthogonal orbitals. Diabatic and adiabatic curves have been analyzed for the two systems, and results compared with molecular orbital AM1 results. © 1992 John Wiley & Sons, Inc.
Ab initio study of low‐lying electronic states of the FNO2 molecule
Authors: Cai Z.‐L. , Wang Y.‐F. , Xiao H.‐M. 
Published: December 2004

AbstractAb initio electronic structure calculations are reported for low‐lying electronic states, 1A1, 1A2, 3A2, 1B1, 3B1, 1B2, and 3B2 of the FNO2 molecule. Geometric parameters for the ground state 1A1 are predicted by MRSDCI calculations with a double‐zeta plus polarization basis set. The vertical excitation energies for these electronic states are determined using MRSDCI/DZ+P calculations at the ground‐state equilibrium conformation. The oscillator strengths and radiative lifetimes for some electronic states are calculated based on the MRSDCI wave functions. © 1993 John Wiley & Sons, Inc.
Hylleraas–CI with linked correlation terms
Authors: Kleindienst H. , Lüchow A. 
Published: December 2004

AbstractHylleraas–CI calculations with linked correlation terms of the form rr are discussed. Formulas for the integration of the angular part are deduced and a method for the reduction of the radial part to auxiliary integrals is given. In the case of the Li atom, it is shown that for the calculation of the ground‐state energy an ansatz for the wave function with at most two factors rr is sufficient to achieve spectroscopic accuracy. © 1993 John Wiley & Sons, Inc.
Direct solution of the many‐body schröudinger equation in the hyperspherical formalism: Application of the HH–GLF method to the positronium ion e+e−e+
Authors: Bian Wensheng , Deng Conghao 
Published: December 2004

AbstractWe apply the HH–GLF method, a new simple hyperspherical harmonic method proposed recently by one of us, to directly solve the three‐body Schrödinger equation for e+e−e+. Uniformally convergent energy eigenvalues are obtained with only several GLF and the obtained ground‐state energy with 200 HH and 6 GLF is −0.26124 au, which is very near the exact value of −0.26200 au. Energy results for maximum global momentum Km ≤ 20 are compared with those from some other hyperspherical techniques carefully, and we find that, in the example of e+e−e+, the HH–GLF method can yield results as accurate as the best available other HH method, but is conceptually simpler and more convenient for practical calculations with a large number of hyperspherical harmonics. © 1994 John Wiley & Sons, Inc.
Vibrational Stark effect and vibrational static electric properties of N2O
Authors: Andrés José Luis , Bertrán Juan , Duran Miquel , Marti Josep 
Published: December 2004

AbstractThe vibrational Stark effect together with nuclear relaxation and vibrational contributions to the static molecular electric properties of the N2O molecule are computed using ab initio molecular orbital thoery. Contributions to the molecular properties are computed by finite‐difference techniques involving the energy vs. the uniform electric‐field strength. © 1994 John Wiley & Sons, Inc.
Ab initio calculations to the reactions of HF and HCl with Si(OH)4 and (HO)3SiOSi(OH)3: Modeling of SiO2 etching reactions
Authors: Jenichen A. 
Published: December 2004

AbstractThe local mechanisms of dissociative chemisorption and reactive etching are investigated by molecular models modeling SiOH groups and SiOSi bridges at SiO2 surfaces. The elementary reaction paths for the reactions of HF and HCl with SiO bonds of the models, Si(OH)4 and (HO)3SiOSi(OH)3, are calculated by the Hartree–Fock method. The local transition‐state structures are nearly plane quadrangles with neighboring pentacoordinated silicon and tricoordinated oxygen atoms. For the reaction and activation energies, the electron correlation is considered by the Møller–Plesset perturbation theory. The results agree with experimental findings. © 1994 John Wiley & Sons, Inc.
Iso‐energy cutoff for the calculation of interionic potential of mean force in water
Authors: Mezei Mihaly 
Published: December 2004

AbstractAn iso‐energy cutoff scheme is introduced for the calculation of the potential of mean force between two ions in water. The cutoff criterion is based on the optimal interaction of the water dipole with the ion pair, for which analytical expressions are derived. Formulas are also derived to characterize the solvent reorganization contribution to the potential of mean force. Treatment of the contributions from waters outside the cutoff is also discussed. © 1994 John Wiley & Sons, Inc.
Nuclear spin–spin coupling and nuclear motion
Authors: Raynes W. T. , Geertsen J. , Oddershede J. 
Published: December 2004

AbstractThe vibration and rotation of molecules affects nuclear spin–spin coupling constants. This manifests itself as a temperature dependence of the coupling and also as an isotope effect (after allowing, where necessary, for differing magnetogyric ratios of the two nuclei involved in the isotopic substitution). Within the Born–Oppenheimer approximation, a nuclear spin–spin coupling surface can be defined for each pair of coupled nuclei. This surface is sampled by the nuclei as they undergo the excursions about equilibrium geometry that are governed by the force field. An accurate ab initio carbon–proton spin–spin coupling surface for the methane molecule has been calculated. This was obtained by summing the surfaces for each of the four contributions—Fermi contact, spin–dipolar, orbital paramagnetic, and orbital diamagnetic—expressed as power series in terms of symmetry coordinates. Preliminary calculations for 13CH4 and 13CD4 give a difference of only 6% between the calculated and observed nuclear motion contributions. The observed temperature dependence is also accounted for by the calculations. For these isotopomers, bond stretching plays the dominant role. © 1994 John Wiley & Sons, Inc.
Theoretical study of the structure and vibrational spectra of the (H2O)2…HF and H2O…(HF)2 molecular complexes
Authors: Rovira Carme , Constans Pere , Whangbo M.‐H. , Novoa Juan J. 
Published: December 2004

AbstractUsing the Hartree–Fock and MP2 methods with bases of up to 6‐31++G(2d, 2p) quality, the optimum geometry of the 1:2 and 2:1 (H2O)n… (HF)m complexes of water and hydrogen fluoride is searched in a systematic way. Two minimum‐energy conformations are found for the 1:2 complex connected through a low‐energy transition state. For the 2:1 complex, only one minimum‐energy structure is obtained. The analysis of the geometries of the minima and their vibrational frequencies shows that none of them can be used to explain the existence of the H …FH reverse complex detected experimentally. © 1994 John Wiley & Sons, Inc.
Structure and stability of BN microclusters: Ab initio calculations for (BN)n (n = 2–4)
Authors: Sutjianto Amin , Pandey Ravindra , Manuel Recio J. 
Published: December 2004

AbstractUnrestricted Hartree–Fock calcuations coupled with second‐order Møller–Plesset correlation correction were performed to study the structures and energetics of microlusters. For (BN)2, linear and rhombus forms are almost isoenergetic, whereas cyclic forms are preferable for (BN)3 and (BN)4 clusters. As a general trend, linear isomers prefer the triplet spin state, whereas cyclic isomers prefer the singlet spin state. Total charge density plots show a strong dominace of the BN bond, indicating that the extent of its polar character becomes stronger with the increase in the cluster size. The loss of a BN monomer is shown to be the most likely fragmentation channel for both neutral and single‐ionized clusters. We find that neutral (BN)n clusters have the same structural configurations as those of their corresponding C2n counterparts. This similarity follows the isoelectronic principle and is of importance due to recent interest in the investigations of BN fullerene analogs. © 1994 John Wiley & Sons, Inc.
Internal dynamics of simple floppy molecules
Authors: Zülicke L. , Zuhrt Ch. , Chapuisat X. , Saint‐Espès C. 
Published: December 2004

AbstractThe dynamical behavior of simple nonrigid molecules still attracts much interest both from experimentalists and theoreticians. On the one hand, modern laser‐spectroscopic techniques like SEP allow for the detection of highly excited vibrational–rotational states of a molecule, and advanced theoretical methods, on the other hand, are more and more able to calculate accurate potential energy surfaces and to simulate the intramolecular dynamics. The aim of the present article was to contribute to the understanding of the dynamical properties of simple floppy molecules by means of a comparative study of the two triatomics HCN and HO2. Continuing our earlier work, we start from an analysis of the potential energy surface topography, then we investigate the classical dynamical behavior and the localization of the quantum states. Several conclusions of probably more general validity are drawn. © 1994 John Wiley & Sons, Inc.
The role of the bond midpoint electron density in homonuclear molecular binding
Authors: March N. H. 
Published: December 2004

AbstractElectron density theory is first employed to express the potential energy curve of the H molecular ion in terms of bond midpoint properties. For heavy homonuclear diatomics, low‐ordr density gradient theory is used, but now for the chemical potential, with a similar conclusion to that for H. Homonuclear clusters of alkali atoms are then treated in some detail. Finally the dissociation of doubly charged clusters is considered, by a study of supermolecular treated in some detail. Finally the dissociation of doubly charged clusters is considered, by a study of supermolecular ions (Na)2 and (K)2, again using a density gradient expansion in low order. The deviation between the barrier for fission and the Coulomb barrier is linear in the bond midpoint density over a substantial range of fragment separation. © 1994 John Wiley & Sons, Inc.
Lower and upper bounds of the energy spectrum for potentials with multiminima
Authors: Žitňan Peter 
Published: December 2004

AbstractAn extended Rayleigh–Ritz method for computing two‐sided eigenvalue bounds of the one‐dimensional Schrödinger equation without using a complementary method is presented. The method is based on the B‐spline approximation over the truncated domain, which results in a generalized banded matrix eigenvalue problem. Numerical results for two multiminima potentials of different natures are presented. © 1994 John Wiley & Sons, Inc.
Spectroscopic properties and potential energy curves of some low‐lying electronic states of AlO, AlO+, LaO, and LaO+: An ab initio CASSCF study
Authors: Márquez Antonio , Capitán Maria José , Odriozola José Antonio , Sanz Javier Fernández 
Published: December 2004

AbstractResults of CASSCF state‐averaged calculations on the lowest electronic states of LaO and LaO+ are reported in this work. For comparison, some low‐lying electronic states of AlO and AlO+ are also reported. The ground state of LaO was found to be the X2Σ+ (Re = 1.987 Å, ωe = 794 cm−1) with a low‐lying A2Δ excited state. Five more excited states below 26000 cm−1 were found. The first ionization potential (IP) is found at 5.16 eV, resulting in an X1Σ+ ground state for the LaO+ diatom, in opposition to AlO+ for which an X3 Π ground state has been found. Analysis of the wave functions, dipole moments, and Mulliken populations reveal that the bonding is quite ionic in both systems. © 1994 John Wiley & Sons, Inc.