CNDO calculation of the π-electronic structure and barrier to internal rotation in benzenesulphonic acid

The CNDO calculation of the charge densities and bond orders show a distinction between two extreme geometrical configurations of the benzenesulphonic acid, but the barrier to internal rotation of the sulphonic group about the C-S bond, calculated by the same method, has revealed that they are energetically equivalent.     

A CNDO/INDO molecular orbital formalism for the elements H to Br. applications

The CNDO/INDO molecular orbital formalism introduced in the preceding paper has been applied to a large number of atom combinations up to bromine under the inclusion of the first transition metal series. The results are compared with experimental data (geometries, ionization potentials, dipole moments) or with the results of sophisticatedab initio calculations (one electron energies, net charges, atomic populations). The semiempirical model reproduces for a wide range of molecules the experimental andab initio data with remarkable success.     

The barrier to internal rotation in Ge2H6

High-quality SCF-MO calculations yield a barrier to internal rotation of 1.70 kJ mole^?1 for Ge₂H₆. A new contraction scheme is reported for Dunning's larger Ge basis set. Geometry optimization, although carried out, is unimportant in this particular calculation. Wavefunctions and properties are reported for GeH₄ and the staggered and eclipsed conformers of Ge₂H₆. The magnitude of the calculated barrier is more physically reasonable than those deduced from experimental data. Comparisons are made across the two series X₂H₆ and CH₃XH₃.     

The barrier to internal rotation in metallocenes

It has been shown by calculation that the eclipsed forms of ferrocene and ruthenocene are more stable than their staggered forms. The main contribution to the energy difference is the induction energy of the metal in the potential field of the rings. Direct ring-ring electrostatic energy also favours the eclipsed forms by a small amount. The calculated barrier in ferrocene is in good agreement with an experimental estimate from electron diffraction.     

A CNDO/INDO molecular orbital formalism for the elements H to Br. theory

A CNDO and INDO formalism is presented that can be used for any atom combination up to bromine under inclusion of the first transition metal series. The semiempirical parameters were chosen to reproduce results ofab initio calculations on metalorganic compounds. The calculational results are invariant to rotations of the coordinate system but not to a general transformation into other basis functions. The one-center Coulomb-expressions were selected in order to include intraatomic correlation contributions. Within the CNDO model this could be achieved by the scaled monopole termF ₀, while in the INDO framework the one-center Coulomb integrals are given as a sum of the monopole-contributionF ₀ and higher multipole contributions expressed as a linear combination of Slater-Condon parameters. The invariance problem in the case of local rotations within the INDO approximation was solved by considering the combination of one-center Coulomb and exchange integrals as a function ofl but independent ofm. The two-center electron-electron interaction terms were calculated via the Dewar-Sabelli, Ohno-Klopman relation. Penetration effects were treated according to Fischer and Kollmar. For the resonance integralH _v ^AB parameters are used which carry information related to the directed nature of the chemical bond by using optimized Klondyke functions. The core-core repulsion is constructed as a superposition of a soft potential function, describing polarization effects of the atomic cores, and a hard repulsion function, avoiding the collapse of the atomic cores with decreasing distance.     

Influence of molecular top deformations on the internal rotation barrier

The influence of molecular top deformations on the potential barrier is investigated. For some ethane-like molecules, it is shown that the deformation makes a pronounced contribution to the height of the torsion barrier. Orenburg State Pedagogical Institute. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 2, pp. 356–358, March–April, 1995. Translated by L. Smolina     

Gaussian molecular orbital calculations of the barrier to internal rotation in the ethyl cation

Extended Gaussian orbital basis set calculations have been carried out on an assumed staggered and eclipsed classical geometrical configuration of C₂H. The best total energies obtained for these geometries were ?78.170692 a.u. and ?78.170674 a.u. respectively, corresponding to a barrier to internal rotation of 1.8 × 10^?5 a.u. or 11 kcal/mole. An analysis of the charge density matrix indicates that charge is distributed in these molecules in a manner consistent with the concept of hyperconjugation.     

An electron diffraction and CNDO/2 investigation of the molecular structure and internal rotation of hexafluorodisilane,Si2F6

The molecular structure and internal rotation of Si₂F₆ were investigated by electron diffraction of gases. The following r⁰_α -values for the geometric parameters were obtained: r(Si-Si) = 2.317 ± 0.006 Å, r(Si-F) = 1.564 ± 0.002 Å and ∠FSiF = 108.6° ± 0.3°. The barrier to internal rotation was found to be between 0.51 ± 0.10 and 0.73 ± 0.14 kcal mol^?1, depending on different assumptions of temperature drop due to gas expansion in the nozzle. Attempts were made to calculate the potential barriers for Si₂X₆ molecules with X as H, F and Cl, using the CNDO/2 approximation. When the 3d orbitais of silicon are taken into account, these results differ widely from the experimental values in the case of Si₂Fg₆ and Si₂Cl₆. Neglecting the 3d orbitais of silicon the theoretical and experi- mental potential barriers agree very well.     

On the barrier to internal rotation in phosphineborane

An ab initio STO-3G calculation is carried out on PH₃BH₃. The electronic structure of the molecule is considered, and special attention is paid to the barrier to internal rotation.     

The effect of solvent on the internal rotation of formamide: A CNDO/2-solvaton method study

The effect of solvent on the barrier to internal rotation of formamide had been studied using a solvaton method within the CNDO/2 parameterization. The experimental trend of increasing rotational barrier with increasing solvent dielectric constant has been reproduced. A critical examination of the manner in which interactions between solute and solvent were allowed to modify solute energies and the polarization of the solute wavefunction was performed.Work performed while A.J.D. was a visting scientist at the Polymer Institute of the Slovak Academy of Sciences sponsored under the exchange program between the U.S. National Academy of Sciences and the Czechoslovak Academy of Sciences during the summer of 1980.     

Electron diffraction,x-ray diffraction,and CNDO/2 analysis of the conformations,molecular geometry,and internal rotation potentials of aryldichloro- and aryldibromoarsines

A structural study was carried out on o-ClC₆H₄AsCl₂ (I), o-BrC₆H₄AsBr₂ (II), C₆H₅AsBr₂ (III), and p-BrC₆H₄AsBr₂ (IV). The normal mode frequencies and shapes were calculated and gas-phase electron diffraction was used to establish the geometric parameters of III and IV. The structure of IV was determined by x-ray diffraction structural analysis. The experimental results for I–IV (I and II were studied in previous work) were interpreted on the basis of CNDO/2 calculations for I–III.A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 5, pp. 61–69, September–October, 1989.     

Hindered internal rotation in molecular systems: quantum statistics of equilibrium and rate constants in the Wigner function formalism

Summary The quantum statistics of a symmetric hindered internal rotator in a molecule or molecular complex is developed within the Wigner function formalism. Different shapes of the rotational barrier are considered. The partition function and the thermodynamic functions are given as Wigner-Kirkwood series expansions in terms of powers of Planck's constant squared. One gets simple closed expressions containing the modified Bessel functionsJ ₀ andJ ₁ of the argumentiV ₀/2kT whereV ₀ is the barrier height. Some problems concerning the evaluation of equilibrium and rate constants of chemical reactions are discussed.Supported by the Alexander von Humboldt-Stiftung     

Torsional transitions and barrier to internal rotation of 1,2-butadiene

The far i.r. spectrum of 1,2-butadiene (methyl allene) has been recorded in the gas phase from 370 to 40 cm⁻¹ with a resolution of 0.1 cm⁻¹. The methyl torsional fundamental has been observed for the first time at 154.3 cm⁻¹, along with some accompanying torsional hot bands. From these data the barrier to internal rotation has been calculated to be 556 cm⁻¹ (1.59 kcal/mol). Detailed K-structure has also been observed for both A—A and E—E torsional transitions and considered in the analysis. SCF calculations have been made for the structure and energies of conformers, so that both kinetic and potential constants for internal rotation have been obtained. The a′ skeletal fundamental is observed at 201.8 cm⁻¹ as a much stronger band than the torsional mode, and the a″ skeletal fundamental gives rise to an even stronger band at 319.8 cm⁻¹.     

An extension of the CNDO/2 formalism for the study of transition metal complexes

The electronic structure and the conformational analysis of some Ni(PX₃)₄ (X = F, OCH₃, Cl, CH₃, H) complexes are investigated within the framework of the previously described extended CNDO/2 method in order to interpret their magneto-optical behaviour and to propose a suitable scheme for the electronic content of a metal-ligand ( + ) dative bond. Chatt and Wilkinson's pattern, used up to now, is criticized and a new scheme is proposed which — taking accurately the role of electronegativity into account and abiding by Pauling's principle of electroneutrality — fits well the sets of available physico-chemical data and allows us to understand whyab initio bonds overlap populations as well as IR force constants are low for any metal-ligand ( + ) dative bond.     

An extension of the CNDO/2 formalism for the study of transition metal complexes

A previously described extended CNDO/2 method is used for investigating the energy level distribution and electronic structure of trifluorophosphine metal complexes: Cr(PF₃)₆, Fe(PF₃)₅, Ni(PF₃)₄. The results are compared with the few experimental data which are known for these complexes. The metal-phosphorus bonds show large (P M) and (M P) charge transfers but small total charge transfers (M P) which induce on the metal in any case a small positive charge.Such ( + ) coordination bonds seem to be generally characterized by small bond overlap populations (or small Wiberg indices).     

A study of the ethane internal rotation barrier

In an effort to deduce the source of the ethane internal rotation barrier, we have investigated the contributions of exchange energy and orthogonality: two effects that are required by the Pauli principle. Fully antisymmetrized, partially antisymmetrized and non-antisymmetrized optimized orbital product wavefunctions were determined. Results show that the exchange energy contribution to the barrier is negligible only when it is evaluated from energy-localized orbitals; even in this case the small total results from cancellation of large contributions. The barrier is apparently caused by the orthogonality that is required between CH orbitals on opposite ends of the molecule. The CC bond has insignificant participation in this effect.     

CNDO/2几何优化算法的改进

几何优化在量子化学中已被广泛采用.Pulay和Török首先应用梯度法解决CNDO/2几何优化的问题^[1].作者曾采用Davidon变尺度法^[2]编出了CNDO/2几何优化程序^[3].现将该程序改编为BD-200算法语言以用于DJS-18型计算机并对算法作了进一步的研究,取得了明显的改进.