bluuebunny/crossref_metadata_2025_split huggingface dataset:
Rotation about the CN bond in 2‐aza‐1,3‐butadiene and the NN bond in 2,3‐diaza‐1,3 butadiene: A molecular orbital study
Authors: Bock Charles W. , George Philip , Trachtman Mendel 
Published: December 2004

AbstractThe geometry and energy of 2‐aza‐1,3‐butadiene and 2,3‐diaza‐1,3‐butadiene have been calculated using the 6‐31G* basis set as a function of the CNCC and CNNC dihedral angles, respectively. With the 2‐aza derivative potential minima are located at 0° (trans) and at about 130° for a gauche structure approximately 9.5 kJ mol−1 less stable than the trans. Potential maxima are at about 75° giving a gauche barrier height of approximately 19 kJ mol−1 relative to the trans structure, and at 180° (cis) giving a barrier height of approximately 14.5 kJ mol−1 relative to the 130° gauche structure. With the 2,3‐diaza derivative the gauche barrier has disappeared and there are a series of gauche structures in the region 70°–100° of almost equal energy 12.5‐15 kJ mol−1 less stable than the trans. In addition the cis barrier is much greater, nearly 70 kJ mol−1 relative to the trans structure. Inclusion of electron correlation, accounting for about 50% of the correlation energy, produces no significant changes in the shape of the potential energy curves. There are systematic and progressive changes in almost all the geometrical parameters as the CH groups in butadiene are replaced by N. The outward tilt and compression within the methylene groups show adverse steric interactions to be operative in the cis structures. The values of Vnn indicate that gauche structures of both the 2‐aza and the 2,3‐diaza derivatives near the cis structure are more compact (as with butadiene), and gauche structures of the 2‐aza derivative near the trans structure are less compact (as with butadiene). Originating in the changes in bond lengths and bond angles, rotation‐independent nuclear–nuclear interactions again play an important role.
A rotation–torsion vibronic band contour program
Authors: Leugers M. A. , Seliskar C. J. 
Published: December 2004

AbstractA Fortran‐77 computer program has been written which calculates the rotation–torsion profile of a vibronic transition using an asymmetric top/free internal rotor Hamiltonian. The program is applicable to any molecule that is composed of a frame portion of C2v symmetry a free rotor portion of C3v symmetry. Rotation–torsion bands of A‐, B‐, and C‐type contours may be calculated within an assumed Boltzmann distribution of rotation–torsion level populations or within a specified non‐Boltzmann distribution of torsional level populations.
Examination of formamide, formamidic acid, amidine dimers, and the double proton transfer transition states involving these dimers
Authors: Zielinski Theresa Julia , Poirier Raymond Alcide 
Published: December 2004

AbstractStructures and relative energies were obtained for the hydrogen bonded dimers of formamide and formamidic acid using the 3‐21G basis set. A double proton transfer transition state is claimed to link these two dimers. While the structure of the transition state was intermediate between those of the two dimers, the energy was only 7.6 kJ/mol greater than the less stable formamidic acid dimer. The activation energy from the formamide dimer side of the reaction was found to be 125 kJ/mol of dimer. A similar transition state was found for the amidine dimer system. The activation energy for this model reaction was found to be 66.9 kJ/mol of dimer.
Vectorization of quantum chemical algorithms: MC‐SCF procedures
Authors: Rappé A. K. 
Published: December 2004

AbstractExamples of the types of algorithmic modification needed to achieve substantial enhancement for quantum chemical codes on vector processors such as the Cyber 205 are presented. Specific examples include matrix transformations, 4‐index integral transformations, and general multiconfiguration–self‐consistent‐field (MC‐SCF) codes.
Critical comparison of the ab initio and spectroscopic methyl‐CH bond length difference in acetyl compounds, CH3C(O) X
Authors: Klimkowski V. J. , Pulay P. , Ewbank J. D. , McKean D. C. , Schäfer Lothar 
Published: December 2004

AbstractIn order to provide additional data for the relative lengths of methyl‐CH bond distances in acetyl derivatives, which are difficult to determine accurately by the conventional tools of structural chemistry, the geometries of CH3COH, CH3COF, CH3COCH3, CH3COOH, and CH3CONH2 were determined by ab initio SCF gradient optimization at the 5‐31G** level and compared with previous 4‐21G results. For acetaldehyde 6‐311G4* calculations were also performed and the correlated methyl‐CH stretching potential energy functions were determined. It is found that the calculated differences between the in‐plane and out‐of‐plane methyl‐CH bonds are practically independent of the computational scheme. The calculated results are in contrast to relative bond lengths obtained by some vibrational overtone spectroscopic studies, but are in perfect agreement with CH bond length differences determined from isolated CH stretching frequencies of partially deuterated compounds. The reliability of the latter, and other spectroscopic data concerning the assignment of the methyl‐CH vibrations are critically analyzed. On the basis of the available evidence we conclude: (1) the methyl groups of the CH3C(O)X systems here discussed contain one strong (in‐plane) and two weak (out‐of‐plane) CH bonds; (2) intensities of CH local mode spectra do not provide a reliable basis for assignment to individual bonds.
A MINDO/3 study of the hetero‐Diels–Alder reaction
Authors: Lee Ikchoon , Han Eun Sook , Choi Jae Young 
Published: December 2004

AbstractThe transition states involved in the hetero‐Diels–Alder reactions of acrylaldehyde–ethylene and butadiene–formaldehyde systems were characterized using an efficient method of locating a saddle point on the energy surface of two coordinates calculated with MINDO/3 method. It has been shown that parallel transition and rotation of the conjugate coordinate frame lead to the reaction and orthogonal coordinate system on the hyperbolic paraboloid energy surface. It was concluded that the method we adopted is similar to the “X‐method” but is much simpler to apply to real reaction systems.
Ab initio calculation of amine out‐of‐plane angles
Authors: Boggs James E. , Niu Zefu 
Published: December 2004

AbstractFully optimized geometries are computed at the Hartree–Fock level for formamide and methylamine with basis sets ranging up to ones that are triple‐zeta plus two sets of polarization functions in the valence shell. The effect of very diffuse s and p functions is also examined. The relation of basis set size and composition to the computed amino group out‐of‐plane angle and to the related localized molecular orbitals is systematically investigated and compared with scattered results previously reported for other systems. It is concluded that the amino group angle is the result of a delicate balance between lone pair character of the nitrogen electrons and delocalization of these electrons into other portions of the molecule. Polarization functions in the nitrogen basis set are necessary to provide for proper description of the lone pair character, but even in methylamine the delocalization is not properly described without an extensive basis set, either polarization functions or very diffuse s and p functions, on the other heavy atoms. The effect is extreme for molecules such as formamide, aniline, and PF2NH2, where very large basis sets are required to obtain the correct out‐of‐plane angle.
Ab initio studies of structural features not easily amenable to experiment. 42. Molecular geometries and conformational analysis of methylbutanoate
Authors: Schäfer Lothar , Klimkowski V. J. , Van Alsenoy C. , Ewbank J. D. , Scarsdale J. N. 
Published: December 2004

AbstractConformational energy profiles were calculated for τ1, the CCCO torsion, and τ2, the CCCC torsion, of methyl butanoate, using Pulay's ab initio gradient procedure at the 4‐21G level with geometry optimization at each point. In addition, the structures of seven conformations were fully relaxed, including the energy minima (τ1, τ2) = (0, −60), (0, 180), (120, 180), (120, −60), and the maxima (0, 0), (180, 180), and (60, −60). The calculated geometries confirm the previously formulated rule that, in saturated hydrocarbons, a CH bond trans to a CC bond (CHs) is consistently shorter than a CH bond (CHa) trans to another CH bond. Specifically, for XC(α) ( O)C(β)C(γ)C(δ) systems, the following rules can be formulated, incorporating results from previous studies of butanal, butanoic acid, and 2‐pentanone: (1) C(δ)Hs < C(δ)Ha in all the conformers in which the δ‐methyl group is remote from the ester group; whereas, in all the conformers in which nonbonded interactions are possible between the C(δ)‐methyl and the ester groups, the bonding pattern is affected by a CH ⃛OC interaction. (2) In the most stable conformers, (0, 60), C(β)Ha < C(β)Hs, and C(γ)Ha < C(γ)Hs, regardless of X. (3) The average CC bonds in the τ2 = 180° conformers are consistently shorter than those with τ2 = 60° (compared at τ1 constant). In the most stable conformations (τ1 = 0°, τ2 = 60° or 180°), the bonding sequence is consistently C(α)C(β) < C(β)C(γ) < C(γ)C(δ); whereas, when τ1 = 120°, C(α)C(β) < C(β)C(γ) > C(γ)C(δ).
Ab initioSCF MO calculations on the reactions of hydroxyl radical with imidazole and monoprotonated imidazole
Authors: Eatock Beverley G. , Waltz William L. , Mezey Paul G. 
Published: December 2004

AbstractThe addition reactions of hydroxyl radical with imidazole and its protonated form to yield radical adducts have been investigated by ab initio SCF MO methods using STO‐3G and 4‐31G basis sets. Analogous radical species are of importance in radiation damage to biological systems. Of the possible radical products, the calculations indicate that the allylic species are generally favored energetically over the nonallylic forms. On an energetic basis, the results show that the allylic adducts formed by addition at the C2 and C5 positions are about equally favorable. Although the C5 species is generally identified as the experimentally observed product in aqueous media for both protonated and unprotonated imidazole, some experimental evidence exists indicating the presence of other forms. Our results suggest that this other form is the C2 adduct. The calculations also point to the protonated form of imidazole being less reactive than imidazole, which is in accord with experimental observations.
On the use of perturbation theory corrections to numerically selected MRDCI calculations
Authors: Ruttink P. J. A. , Van Schaik M. M. M. 
Published: December 2004

AbstractIf numerical configuration selection procedures are used in MRDCI calculations, the full MRDCI energy may be estimated by adding energy corrections obtained by perturbation theory. Accurate results may then be obtained by including all selected CFs in the zero‐order function instead of only the reference CFs.
Effects of alkylation upon the proton affinities of nitrogen and oxygen bases
Authors: Redfern Paul , Scheiner Steve 
Published: December 2004

AbstractThe protonation energies of alkylated derivatives of NH3 and OH2 are calculated at the Hartree–Fock level with the split‐valence 4‐31G basis set. The methyl, dimethyl, and ethyl amines are studied; oxygen bases include methanol, dimethylether, and ethanol. The geometries of each molecule and its protonated analog are fully optimized. It is found that protonation leads to significant changes in the molecular structures. In particular, the bonds to the N and O atoms are substantially elongated, especially when the other atom involved is C rather than H. The calculated absolute proton affinities are somewhat larger than the experimental values. However, the differences in protonation energies of the various molecules relative to one another agree quantitatively with experiment. Replacement of one H atom of the base by a methyl group induces an increase in proton affinity of some 10 kcal/mol. If a second methyl group is added to the N or O atom, a further increment of about 70% this amount is noted. On the other hand, placement of the second C atom on the first methyl group (to form an ethyl substituent) leads to a smaller increase (∼30%). The magnitudes of these alkyl substituent effects are somewhat larger for the oxygen bases than for the amines.
A systematic preparation of new contracted Gaussian‐type orbitals. IX [54/5], [64/5], [64/6], [74/6], [74/7] and MAXI‐1–MAXI‐5 from Li to Ne
Authors: Tatewaki Hiroshi 
Published: December 2004

AbstractThe new contracted Gaussian‐type orbitals (CGTOs) for molecular calculations have been developed from Li to Ne. The CGTOs are minimal type, i.e. composed of two s‐type CGTOs, s1, s2, and one p‐type CGTO, p1. They are new family of CGTOs given by Tatewaki and Huzinaga, and others. In the previous works three primitive GTOs are used for s2, which is the main part of the 2s orbital, whereas four primitive GTOs are employed in the present work. The sets generated are [54/5], [64/5], [64/6], [74/6], and [74/7]. In almost all the cases the errors in the 2s and 2p orbital energies are smaller than those of DZ. The resulting 2s orbitals are close to the orbitals of the uncontracted GTO sets, (13/n) and (14/n) of Duijneveldt. It is found that the 2s and 2p orbitals given by [64/6], [74/6], and [74/7] are satisfactorily near to those of Hartree–Fock. The basis sets [54/5], [64/6], and [74/7] are applied to the N2 molecule in the split valence forms of [5211/311], [6211/3111], and [7211/4111]. Adding the d‐type polarization functions from one through three, the quality of the basis sets has been examined. All of the three sets show good behavior and the sets augmented with three d‐type polarization functions give almost entirely the same results as the very extended basis set.
A theoretical study of α‐substituted cyclopropyl and isopropyl radicals
Authors: Lien M. H. , Hopkinson A. C. 
Published: December 2004

AbstractThe structures of α‐X‐cyclopropyl and α‐X‐isopropyl radicals (X = H, CH3, NH2, OH, F, CN, and NC) are reported at the RHF 3‐21G level of theory. The isopropyl radicals are pyramidal with out‐of‐plane angles varying from 12° (X = CN) to 39° (X = NH2), and barriers to inversion ranging from 0.4 kcal/mol (X = H) to 4.0 kcal/mol (X = NH2). The cyclopropyl radicals have larger out‐of‐plane angles, from 39.9° (X = CN) to 49.4° (X = NH2), and their barriers to inversion, which increase with the inclusion of polarization functions, vary from 5.5 kcal/mol (X = H) to 16.7 kcal/mol (X = F). In both types of radicals the amino group is the most stabilizing substituent, while the α‐fluoro has little effect. The β‐fluoro group is weakly destabilizing in the cyclopropyl radical. The strain energies of the cyclopropyl radicals (36–43 kcal/mol) are compared with those of similarly substituted anions, cations, and cyclopropanes.
Orbital relaxation in the Rydberg series of the He atom
Authors: Chang T. C. , Pao Chien Hua , Hsue Chen‐Shiung 
Published: December 2004

AbstractThe orbital behavior of both the singlet and triplet Rydberg series for 1sns configurations of the He atom has been investigated using one‐ and two‐configuration Hartree–Fock wave functions. The core orbital within the triplet series changes negligibly; however, a contract trend is noticeable as the Rydberg orbital penetrates more into the core region, which is in contrast to that found in the 1s2ns 2S Rydberg series of Li atom. Although some properties of the whole atom can be calculated, the core relaxation of the singlet series can not be drawn without ambiguity because a two‐configuration correlated wave function has to be used, and thus the orbital picture is lost.
Basis set and correlation effects on computed proton affinities of some oxygen and nitrogen bases
Authors: Del Bene Janet E. 
Published: December 2004

AbstractBasis set expansion and correlation effects on the computed proton affinities of the oxygen and nitrogen bases CH3OH, H2CO, CO, CH3NH2, CH2NH, and HCN have been evaluated. Basis set enhancements lead to systematic changes in computed proton affinities. These effects appear to be additive, and are greater for correlated proton affinities than for Hartree‐Fock energies. Inclusion of correlation decreases proton affinities, with fourth‐order Møller‐Plesset energies bracketed by second and third order energies.
General expressions for monocenter repulsion integrals in a basis of real atomic orbitals
Authors: François J. P. , Voets R. , Van Poucke L. C. 
Published: December 2004

AbstractGeneral expressions for monocenter electron repulsion integrals in a basis of real atomic orbitals are derived in terms of the radial integrals R. The final expressions for these integrals can be classified into five main classes which are characterized by the angular part of the real atomic orbitals. For a basis of real s, p, d, and f AO's the total number of monocenter repulsion integrals is 65536, from which 6652 are different from zero. The nonzero integrals can be classified into 430 groups which contain integrals of equal value.
Ab initioSTO‐3G optimization of planar, perpendicular, and twisted molecular structures of biphenyl
Authors: Häfelinger Günter , Regelmann Claus 
Published: December 2004

AbstractThe complete molecular structure of biphenyl, characterized by 12 independent parameters, has been derived by ab initio gradient techniques using a STO‐3G basis set for coplanar, perpendicular, and minimum energy conformations with the constraint of planar phenyl ring units and a C2 symmetry axis along the CC interring bond. The minimum torsional angle obtained was ϕ = 38.63° with torsional energy barriers of 8.59 and 10.04 kJ/mol for ϕ = 0° and ϕ = 90°, respectively.
Theoretical studies on pteridines. 2. Geometries, tautomer, ionization and reduction energies of substrates and inhibitors of dihydrofolate reductase
Authors: Gready Jill E. 
Published: December 2004

AbstractAb initio SCF calculations with STO‐3G and 3‐21G basis sets are reported for approximately 85 pteridines of interest to the study of the reaction and inhibition mechanisms of dihydrofolate reductase. These include tautomeric, protonated, deprotonated, reduced and 6‐substituted forms of the 2‐amino‐4‐oxo‐ and 2,4‐diamino‐pteridines, many of which are not easily amenable to experimental investigation. Full geometry optimizations at the SCF/STO‐3G level for 30 such pteridines have been performed. A step‐wise computational protocol designed to identify the minimum level of theory necessary for reliable prediction of relative tautomer, reduction and ionization energies has been developed in an effort to minimize the cost of calculations for this reasonably large N‐heterobicylic system. In general, SCF/STO‐3G results were found to be inadequate while SCF/3‐21G results obtained with STO‐3G optimized geometries agreed with all available experimental evidence with the exception of the relative acidity of 6‐methyl‐7,8‐dihydropterin. Correlation graphs relating experimental pKa's to the calculated ionization energies are presented: these are of potential predictive value. An analysis is given of the importance of resonance substructures, such as the guanidinium and extended‐guanidinium groups, in stabilizing some preferred tautomeric and ionized forms, and in explaining the observed geometry changes.
An efficient UMP2 program
Authors: Čársky P. , Fabian J. , Hess B. A. , Schaad L. J. 
Published: December 2004

AbstractTimings are reported for 6‐31G*/UMP2 calculations on planar allene, twisted ethylene, cyclopropenyl anion, allyl, ethyl, isopropyl and CF3 radicals performed by means of the programs GAUSSIAN 80 and HONDO UMP2. The comparison of the two programs is used for pointing out several factors affecting the speed of MP2 calculations.
Study of a medium‐size biological molecular association by means of a pair potential semiempirical approach: β‐carboline–lumiflavin
Authors: Sánchez‐Marín J. , Ortí E. , Tomás F. 
Published: December 2004

AbstractThe molecular association between some substituted β‐carbolines and a model flavin, lumiflavin, has been studied by means of a semiempirical approach proposed by Fraga based on a 1/R expansion of atom‐atom pair potentials. Only stacked minima have been considered because of their possible biological interest. The calculations characterize 15 different minimum stacked conformations, most of them occurring in the complexes considered. The structural and energetic effects of 1–Me, 6–OH and 7–OH substituents of β‐carboline are discussed. An equilibrium conformation involving geometric overlap between the pteridinic portion of flavin and the indole group of β‐carboline is predicted to be the most characteristic structure of the complexes. The recognition of this feature can serve to check the effect of substituents and substitution sites of β‐carboline in its association complexes.
Computational aspects of Laplace transform inversion of thermal unimolecular rate constant
Authors: Schoenenberger Christian , Forst Wendell 
Published: December 2004

AbstractThe thermal limiting high‐pressure unimolecular rate constant k∞ represents, operationally, the Laplace transform of the product of microcanonical rate constant for decomposition of molecules having specified energy E [k(E)] and the density of states [N(E)]. By inversion, it is possible to recover k(E)N(E), from which one can obtain the energy dependence of k(E) and the pressure dependence of kuni, the thermal general‐pressure unimolecular rate constant. This article examines numerical aspects of three methods of inversion, their reliability and dependence on sampling, i.e., on the number of available experimental data points, by comparing exact k(E) and kuni with those obtained by inversion. It turns out that the method of steepest descents is the best all‐round performer.
Comparative studies on the reactivity of molecules by atomic valences
Authors: Jug Karl , Buss Sabine 
Published: December 2004

AbstractSINDO1 calculations were performed to study the reactivity of molecules with a valence number derived from the density matrix. A comparison of valence numbers was made for selected examples of rotation barriers, substituted strained molecules, radical gas phase reactions, and acid‐base reactions. Relations between valence, geometry, and energy changes are discussed.
Compact basis sets for LCAO‐LSD calculations. Part II: Tests for Cr2 and Ni4
Authors: Radzio E. , Andzelm J. , Salahub D. R. 
Published: December 2004

AbstractThe compact orbital and auxiliary basis sets for LCAO‐LSD calculations introduced in Part I are tested in molecular calculations on Cr2 and Ni4. The present results for spectroscopic constants and valence orbital energies obtained using medium size orbital expansions with a double‐zeta representation for valence orbitals are in very good agreement with those previously calculated with very extended sets. Since the computational time of the present calculations is reduced severalfold compared with the extended basis set calculations, the present basis sets allow increased efficiency of the LCAO‐LSD calculations and allow the method to be extended to larger systems.
Unique description of chemical structures on hierarchically ordered extended connectivities (HOC procedures). II. Mathematical proofs for the HOC algorithm
Authors: Mekenyan Ovanes , Bonchev Danail , Balaban Alexandru T. 
Published: December 2004

AbstractThe basic notions and definitions, necessary for the better understanding of Part I of this series, are presented. The mathematical proof is given for sufficiency of the various HOC procedures for vertex canonical numbering and graph orbit finding.