bluuebunny/crossref_metadata_2025_split huggingface dataset:
Cavity boundary effects within the onsager theory for dielectrics
Authors: Ehrenson S. 
Published: December 2004

AbstractMeans by which the Onsager theory of dielectrics may be modified in a general way to include important structurally nonspecific electrical effects exercised at or near the cavity boundary are developed and discussed. In essence they rely upon extraction of limiting numerical solutions of the Laplace equation potentials for multiple shells approximating regions of smoothly varying permittivity surrounding the dipole‐containing cavity. The results of several such modifications, which usefully retain the original reaction and cavity field forms, are applied to evaluate the dielectric constants of polar solvents acting as support media for mean‐space‐charge attenuation of intracavity electrical interactions, and in correlation of isolated molecule and condensed phase properties of formally nonassociated solvents. For the most part such predictions and correlations are found to be improved by a wide variety of physically reasonable continuity functions. Where they are not, more rational patterns of deviations consistent with possible modes of intramolecular charge transfer and weak intermolecular association in formally nonassociated liquids are detected. Some limited comparisons with particle‐ and structure‐dependent theories, e.g., the mean spherical model and discrete lattice formulations, also suggest that the present permittivity modifications can effectively simulate important short‐range nonspecific liquid ordering effects. Various factors relating to possible adaption of the limit procedures developed to other inhomogeneous dielectric medium calculations are also briefly outlined.
The electrical nature of CH bonds and its relationship to infrared intensities
Authors: Wiberg Kenneth B. , Wendoloski John J. 
Published: December 2004

AbstractThe charge distributions along the CH bonds of methane, ethylene, and acetylene have been obtained by numerical integration of 6‐31G** wavefunctions. Several criteria for assigning electron populations to carbon and hydrogen lead to negative charge on H for methane and ethylene and a positive charge on H for acetylene. These results are contrasted with the charges derived from the μ obtained from infrared intensity measurements. The difference between the results from infrared intensities and from the electron density calculations has been shown to result from incomplete orbital following in the molecular deformation and the creation of a “bent‐bond” moment. Its magnitude has been estimated.
A systematic preparation of new contracted Gaussian‐type orbital sets. V. From Na through Ca
Authors: Sakai Yoshiko , Tatewaki Hiroshi , Huzinaga Sigeru 
Published: December 2004

AbstractMinimal compact contracted Gaussian basis sets are constructed for the atoms from Na to Ca. They give satisfactory valence shell orbital energies, although they are minimal‐type basis sets. Split‐type basis sets are also derived from the minimal Gaussian basis sets in order to enhance the flexibility of the basis sets for molecular calculations.
A systematic preparation of new contracted Gaussian‐type orbital sets. VI. Ab initio calculation on molecules containing Na through Cl
Authors: Sakai Yoshiko , Tatewaki Hiroshi , Huzinaga Sigeru 
Published: December 2004

AbstractCompact contracted Gaussian basis sets introduced in the preceding article are tested for ab initio molecular calculations on molecules containing third‐row atoms (Na through Cl). It is found that the effect of splitting valence orbitals is essential for these molecules and addition of polarization functions to split basis sets can yield computed geometries, spectroscopic constants, and atomization energies in close agreement with the result of near Hartree–Fock calculations.
A theoretical approach to substituent effects. A comparison of the isoelectronic BH, CH3, and NH groups and their interaction with substituents in disubstituted benzenes
Authors: Kemister Gary , Pross Addy , Radom Leo , Taft Robert W. 
Published: December 2004

AbstractAb initio molecular orbital theory with the STO‐3G basis set is used to examine both charge and energy interactions in a series of meta‐ and para‐substituted phenylborate anions and toluenes. Comparison of the results is made with data for substituted anilinium cations. It is concluded that whereas NH is a powerful σ acceptor, with essentially no π interaction, BH is primarily a π donor, and, to a slight extent only, a π donor. CH3 is indicated to be both a weak σ and π donor. Energies of interaction of BH and NH with a series of substituents are an order of magnitude larger than corresponding values for CH3. Interaction energies for BH are of opposite sign to those for NH. The results may be understood qualitatively using perturbation molecular orbital (PMO) theory.
A systematic preparation of new contracted Gaussian‐type orbital sets. VIII. MINI‐1 and MIDI‐1 sets for Ga through Cd
Authors: Sakai Yoshiko , Tatewaki Hiroshi , Huzinaga Sigeru 
Published: December 2004

AbstractMinimal contracted Gaussian basis sets are presented for Ga through Cd. Characteristically these Gaussian‐based minimal sets give far better d orbital energies than those by minimal STO basis sets. These new basis sets were tested on Br2 for which a new benchmark calculation was also performed. The test result is satisfactory in that these basis sets produce good general agreement with the near Hartree–Fock calculation with respect to the molecular spectroscopic constants.
Pitfalls in the use of the torsion angle driving method for the calculation of conformational interconversions
Authors: Burkert Ulrich , Allinger Norman L. 
Published: December 2004

AbstractSeveral deficiencies and potential sources of error in the torsion angle driving method, the approach employed most frequently for the simulation of conformational interconversions, have been studied. A general explanation of the observed effects is given in terms of the energy surface and of the effects brought about by “side valleys.” Several examples of molecular mechanics calculations of conformational interconversions, among them the cyclohexane ring inversion, illustrate the problems.
Calculation of macromolecular force constants
Authors: Putnam B. F. , Van Zandt L. L. 
Published: December 2004

AbstractForce constant refinement may be carried under a system of constraints imposed on well characterized modes as the refinement proceeds. Resulting stability permits the accurate calculation of many independent macromolecular force constants. A useful formalism for this technique is developed. Prediction of initially poorly characterized modes can emerge from this stability. A brief discussion of currently used refinement schemes has been included. We show that the standard least‐squares procedure is a special case of our present more general method, the former being useful when the molecule is not too large.
Structures of some fluorinated benzenes determined by ab initio computation
Authors: Boggs James E. , Pang Frank , Pulay Péter 
Published: December 2004

AbstractThe structures of fluorobenzene, 1,3‐difluorobenzene, 1,3,5‐trifluorobenzene, 1,2,3‐trifluorobenzene, pentafluorobenzene, and hexafluorobenzene have been determined by the ab initio gradient method with a 4‐21 basis set. Approximate corrections have been developed to yield re and r0 structures so that comparison can be made with the available experimental information. The CF bond distance decreases in a uniform manner with increasing fluorination of the ring, either on adjacent or on meta sites. The ring structure also shows systematic deformation, with a decrease of about 0.01 Å in the CC bond length adjacent to the point of substitution and an increase of about 2.5˚ in the ring angle at the substitution site in those cases where the requirement of ring closure permits.
Mechanisms of conformational chirality inversion in bicyclo[4.2.1]nonan‐9‐one and bicyclo[4.2.2]decane as studied in two‐parametric torsional energy surfaces
Authors: Ōsawa Eiji 
Published: December 2004

AbstractWith the purpose of deciphering conformational inversion processes of typical mobile bicyclic molecules, torsional energy surfaces near the enantiomers of bicyclo[4.2.1]nonan‐9‐one (1) and bicyclo‐[4.2.2]decane (2) were prepared using molecular mechanics with an improved two‐bond drive technique. Inversion of 1 takes place most favorably via a Cs transition state with the tetramethylene chain over the ethano bridge [1B, ΔH± 6.1 (calculated) vs. 6.8 (observed) kcal/mol]. An alternative pathway involving a Cs local energy minimum (1C), in which the tetramethylene chain is bent over the carbonyl, has a barrier 2.4 kcal higher than 1B. The global energy minimum conformation of 2 has boat–chair cyclooctane and twist–boat cyclohexane rings (BCTB), and enantiomerizes into its mirror image (BCTB') via three intermediates: TCTB, CB, and TCTB'. The highest point in the proposed pathway, a saddlepoint CB, is calculated to lie 8.0 kcal/mol above BCTB (observed ΔH± 7.8 kcal/mol). The advantage of the two‐parametric over the one‐parametric torsional energy surface is discussed.
MNDO Calculations of silicon‐containing molecules
Authors: Verwoerd W. S. 
Published: December 2004

AbstractA comparison is made of MNDO and MINDO/3 calculations for saturated silicon‐containing molecules, and with experimental values, for heats of formation, molecular geometries, charge distributions, and ionization potentials. Except for bond angles, it is found that with the published parameter values the MINDO/3 program gives more reliable results than MNDO. For unsaturated molecules, a comparison of bond lengths and stabilities of Si multiple bonds as given by the two programs and ab initio methods is made, and large discrepancies between predicted structures are pointed out. Some reasons for the dicrepancies are discussed.
Quasi‐one‐dimensional systems and localized defects: Some results of the Green's matrix formalism
Authors: Seel M. , Del Re G. , Ladik J. 
Published: December 2004

AbstractThe application of the self‐consistent field (SCF) local‐impurity formalism to quasi‐one‐dimensional systems is discussed. We describe a general procedure for an accurate numerical determination of the Green's function matrix elements of the unperturbed system. An application to a local impurity in a model chain with two orbitals per unit cell is reported. The changes in the charge‐bond‐order matrix and in local and total density of states due to the impurity are discussed with special emphasis on the changes at the critical points (van Hove singularities) at the band edges. The Green's matrix approach is used to reexamine long‐range Friedel oscillations caused by an impurity in a strictly one‐dimensional metal. The extent of the long‐range tail of the perturbed charge density is in an inverse relation to the localization length of the impurity state: the stronger the perturbation the more localized is the bound state and the more extended are the oscillations in the charge distribution. The results for the model chain with two orbitals per unit cell indicate that the impurity‐induced change in charge distribution may be locally screened by redistribution of the population of the on‐site orbitals, therefore damping possible oscillations and leading to a faster decay than in strictly one‐dimensional systems.
The structure and acid–base properties of methyl and silyl amines and phosphines: An ab‐initio SCF study
Authors: Glidewell Chistopher , Thomson Colin 
Published: December 2004

AbstractThe structures of the methyl and silyl amines and phosphines and their ions have been calculated using ab‐initio SCF theory and the 3‐21G basis set. The computed structures give excellent agreement with the available experiment data without the inclusion of d functions, with the exception of (SiH3)2N– and the isoelectronic molecules (SiH3)2O and (SiH3)2C2− where d functions are essential. The observed trends in computed basicities and acidities are reproduced by the calculations.
The 6‐31G++ basis set: An economical basis set for correlated wavefunctions
Authors: Nobes Ross H. , Rodwell William R. , Radom Leo 
Published: December 2004

AbstractThe 6‐31G++ basis set is described. This basis set is very similar to the existing 6‐31G** set but is somewhat smaller through the use of five (rather than six) second‐order Gaussians (d functions) and has polarization function exponents optimized for correlated rather than Hartree–Fock wavefunctions. The performance of 6‐31G++ is compared with that of the 6‐31G** and 6‐31G** basis sets through calculation of the geometries and atomization energies for the set of molecules LiH, FH, H2O, NH3, CH4, N2, CO, HCN, and HCCH.
Computational study on the interaction of water with 2‐ and 3‐hydroxypyridine and the corresponding pyridones
Authors: La Manna Gianfranco , Venuti Eduardo 
Published: December 2004

AbstractSCF–MO–LCAO ab‐initio computations were carried out for the four tautomers 2‐pyridone, 2‐hydroxypyridine, 3‐pyridone, and 3‐hydroxypyridine interacting with a water molecule placed at different positions and orientations around each of them. The computed interaction energies were fitted by an analytical potential consisting of a 6–12 Lennard‐Jones term plus a Coulombic one. The root‐mean‐square deviation of the fit is 2.5 kJ/mol. We report isoenergy contour maps for some important cross sections and obtain a qualitative trend of the solvation entropy, in agreement with available experimental data. The obtained potentials were used for Monte Carlo simulation of four clusters consisting of a tautomer molecule as solute and 50 water molecules, at 300 K. The problem of the transferability of fitting constants is also examined.
Comparison of RHF, NDDO, and MOM molecular one‐electron expectation values calculated using weighted and unweighted STO–NG (ω) basis functions
Authors: Zeiss G. D. , Whitehead M. A. 
Published: December 2004

AbstractThe effect of basis functions on molecular one‐electron property expectation values calculated by approximate methods is examined using weighted and unweighted least‐squares Gaussian‐type orbital function expansions of Slater‐type orbital functions.
Ab initio studies of a proposed mechanism for N‐Nitrosamine formation
Authors: Herzig L. , Sapse A. M. , Snyder G. 
Published: December 2004

AbstractThe geometry of a proposed intermediate (α‐dialkylamino nitrite ester) of N‐nitrosamine synthesis from nitrites is studied by ab initio SCF methods. The energy of a zwitterion is calculated and found to be high.
Ground states of molecules. 53.MNDO calculations for molecules containing chlorine
Authors: Dewar Michael J. S. , Rzepa Henry S. 
Published: December 2004

AbstractMNDO calculations of heats of formation, dipole moments, ionization potentials, and structures are reported for a wide range of compounds containing chlorine in its characteristic valence state (ClI) and one or more of the elements H, B, Be, C, N, O, and F. The calculated errors in the heats of formation and the dipole moments are not significantly greater than those previously reported for compounds containing no chlorine. First vertical ionization potentials were on average 0.95 eV too high. The ordering of higher cationic states was found to be correct, even for species such as Cl2O, Cl2, and HOCl, where ab initio–Koopmans' theorem calculations predict the incorrect ordering. The calculated energies and geometries of compounds such as CIF3 are qualitatively incorrect, probably because of the lack of 3d atomic orbitals in the orbital basis set.
Gaussian basis sets for the fourth‐row main group elements, In–Xe
Authors: Strömberg Ann , Gropen Odd , Wahlgren Ulf 
Published: December 2004

AbstractGaussian basis sets, consisting of 15 s‐type, 11 p‐type, and 6 d‐type functions, for the fourth‐row main group elements, In‐Xe, are presented. In order to compare these basis sets with larger ones, calculations have been performed in I2 and TeO2.
Molecular orbital theory of the hydrogen bond. XXX. Water‐cytosine complexes
Authors: Del Bene Janet E. 
Published: December 2004

AbstractAb initio SCF and SCF‐CI calculations with the STO‐3G basis set have been performed to investigate the structures and energies of water–cytosine complexes and the intermolecular water–cytosine surface in the cytosine molecular plane. Although there are six nominal hydrogen‐bonding sites in this plane, only three dimers are distinguishable in the ground state. The most stable has an energy of −10.7 kcal/mol, and is found in the N1H and O2 region. An asymmetric cyclic structure in which the water molecule bridges adjacent N1H and O2 sites is the preferred form of this dimer. The dimer in the region between O2 and N4H′ of the amino group is slightly less stable at −10.4 kcal/mol, and also has an asymmetric cyclic structure as the preferred structure, with the water molecule bridging amino N4H′ and N3 hydrogen‐bonding sites. The third dimer has the amino group as the proton donor to water through the hydrogen cis to C5, and a stabilization energy of −7.0 kcal/mol. The water‐cytosine surface is characterized by deeper minima and higher barriers than the water‐thymine surface and by a decreased mobility of the water molecule between adjacent hydrogen‐bonding sites. Absorption of energy by the C2O group leads to the first n → π* excited state in which interactions of water with O2 are broken. The water‐cytosine dimers remain bound in this state, but may change structurally. In the second n → π* state interactions between water and N3 are no longer stabilizing. As a result, the dimer in the O2 and N4H′ region collapses to either a dimer with water the proton donor to O2, or one with N4H′ the proton donor to water. The other two dimers remain bound. All excited dimers are destabilized on vertical excitation relative to the ground state.
Computer‐assisted pseudorotation analysis of five‐membered rings by means of proton spin–spin coupling constants: Program PSEUROT
Authors: De Leeuw Frank A. A. M. , Altona Cornelis 
Published: December 2004

AbstractA computer method for the calculation of the pseudorotational parameters in five‐membered rings from vicinal proton spin–spin coupling constants is described. Some typical problems met in practice are discussed. Applications of the program in the conformational analysis of some substituted cyclopentanes are presented.
Determination and characterization of a transition structure for water–formaldehyde addition
Authors: Spangler Dale , Williams Ian Henderson , Maggiora Gerald M. 
Published: December 2004

AbstractQuantum mechanical calculations of the geometric, energetic, electronic, and vibrational features of a transition structure for gas‐phase water–formaldehyde addition (FW1‡) are described, and a new transition‐structure search algorithm is presented. Basis‐set‐dependent effects are assessed by comparisons of computed properties obtained from self‐consistent field (SCF) molecular orbital (MO) calculations with STO‐3G, 4‐31G, and 6‐31G** basis sets in the absence of electron correlation. The results obtained suggest that STO‐3G‐level calculations may be sufficiently reliable for the prediction of the transition structure of FW1‡ and for the transition structures of related carbonyl addition reactions. Moreover, the calculated activation energy for formation of FW1‡ from water and formaldehyde (−44 kcal mol−1) is very similar in all three basis sets. However, the energy of formaldehyde hydration predicted by STO‐3G (˜ −45 kcal mol−1) is about three times larger than that predicted by the other two basis sets, with the activation energy for dihydroxymethane dehydration also being too large in STO‐3G. Calculated force constants in all three basis sets are generally too large, leading to vibrational frequencies that are also too large. However, uniformly scaled force constants (in internal coordinates) give much better agreement with experimental frequencies, scaled 4‐31G force constants being slightly superior to scaled STO‐3G force constants.
Counting of resonance structures for large benzenoid polynuclear hydrocarbons
Authors: Brown R. L. 
Published: December 2004

AbstractA method is presented for counting the number of resonance structures for large benzenoid polynuclear hydrocarbons. Structure counts for even systems are made and compared with those of the odd systems resulting from the removal of one of the π centers from various points in the even structure. A computer program for performing the calculations is also given.
Ab initio evaluation of absorption and emission transitions for molecular solutes, including separate consideration of orientational and inductive solvent effects
Authors: Bonaccorsi R. , Cimiraglia R. , Tomasi J. 
Published: December 2004

AbstractAn ab initio computation procedure to obtain electronic transition wave numbers in solution is presented and discussed. The continuum description of the bulk solvent is adopted, and solute–solvent interaction is reduced to a closed‐form interaction operator added to the solute Hamiltonian. The resulting Schrödinger equations are solved variationally. No limitations are imposed on the shape of the solvent cavity or on the level of accuracy of the ab initio procedure.
Extended basis CNDO calculations of linear and nonlinear electric susceptibilities of some molecular dianions and carbonions using coupled Hartree–Fock perturbation theory
Authors: Waite J. , Papadopoulos M. G. 
Published: December 2004

AbstractThe results of calculating the average polarizabilities, first and second hyperpolarizabilities and molar Kerr constants of C5H, C6H, 2‐C10H, 2‐C14H, C8H and C8H are reported. The main elements of our computational scheme are McWeeny's coupled Hartree–Fock perturbation theory and an extended basis CNDO wave function. It is shown that the studied anions have nonlinearities within the same order of magnitude as their respective uncharged parent molecules. The Kerr constants of these anions are analyzed and the contribution of the various terms is appraised.
Operator technique for obtaining the recursion formulas of characteristic and matching polynomials as applied to polyhex graphs
Authors: Hosoya Haruo , Ohkami Noriko 
Published: December 2004

AbstractA simple and efficient method, called an operator technique, for obtaining the recurrence relation of a given counting polynomial, e.g., characteristic PG(x) or matching MG(x) polynomial, for periodic networks is proposed. By using this technique the recurrence relations of the PG(x) and MG(x) polynomials for the linear zigzag‐type and kinked polyacene graphs were obtained. For the lower members of these series of graphs, the coefficients of PG(x) and MG(x) polynomials are tabulated.
Ab initio studies on polymers. VI. Torsional potential in regular polyethylene chains
Authors: Karpfen Alfred , Beyer Anton 
Published: December 2004

AbstractAb initio crystal orbital calculations have been performed on regular polyethylene chains applying basis sets of minimal and double‐zeta quality. Relative stabilities of periodic all‐trans, all‐gauche, and alternating trans–gauche conformers have been evaluated, including extensive geometry optimization. Potential curves for a simultaneous rotation around CC single bonds from the all‐trans to the all‐gauche conformation have been computed applying the rigid‐rotor approximation, the flexible‐rotor approximation, and an additional reoptimization of CC distances. A rigid‐rotor potential curve from the all‐trans to the alternating trans‐gauche conformation has been computed as well. Results obtained are compared with ab initio calculations on butane and pentane and with semiempirical and empirical force‐field studies on polyethylene.