Bond functions for AB initio calculations. MCSCF results for CH,NH, OH and FH

Separate optimized s and p bond functions (BFs) were added to the corresponding Dunning basis sets (11s,6p15s) → (5s,4p13s) for the four hydrides. Properties calculated with these basis sets are quasi-identical to those obtained with conventional polarization functions (d_lp). The computer time ratios found are: t(BF)/t(dlp) = $\text{1}\text{2}$ for UHF calculations, and $\text{2}\text{3}$ for MC SCF calculations.     

Conformational analysis of primary ethylene ozonide by gradient and multiconfigurational scf calculations

Four conformers of ethylene primary ozonide have been studied by ab initio gradient and MC SCF calculations, using gaussian-type basis functions. The MC SCF results indicate that the conformers are not as close in energy as suggested from single-determinant SCF calculations. The oxygen-oxygen and carbon-oxygen half-chair structures are much lower in energy than the carbon-carbon half-chair.     

Transition structures for the reaction of difluorocarbene with propene

Two $\text{ab}$$\text{initio}$ transition structures for the reaction of difluorocarbene with propene have been located with gradient techniques and the 3-21G basis set. The activation energy is 1.3 kcal/mol lower than for the reaction with ethylene. Two transition structures with the fluorines approaching $\text{syn}$ or $\text{anti}$ to the methyl group are identical in energy     

Application of the MC SCF method to the π → π* excitation energies of ethylene

The MC SCF method is employed to calculate the N → T and N → V π → π^* vertical excitation energies of ethylene. To obtain accurate excitation energies it is found to be necessary to utilize an expanded valence space containing two π and two π^* orbitals. Relatively small MC SCF calculations, allowing at most one-electron excitations from the sigma space, are found to yield excitation energies and spatial extents of the excited states in excellent agreement with the predictions of large multi-reference or iterative-natural-orbital CI calculations. These results show that within an MC SCF framework σ-σ correlation is unimportant for describing the π → π^* processes. We also conclude that the neglect of the effects of unlinked cluster terms in some of the CI calculations may have introduced small, but important, errors in the excitation energies and predictions of the spatial extent of the V state.     

An MNDO SCF MO investigation of the electronic structures and conformational preferences of model polycarbonate systems

Theoretical calculations within the MNDO SCF MO formalism have been carried out to investigate the relative heats of formation, conformation preferences and excitation energies of model polycarbonate systems. It is shown that the trans-trans and cis-trans configurations of diphenyl carbonate (DPC) are of comparable energy and have a strong energetic preference for orthogonal forms. The energy barrier for the interconversion of trans-trans and cis-trans orthogonal forms is small. Comparison of the electronic structure of DPC with simple carbonyl, carboxy ester and carbonate systems indicates that the nature of the first excited singlet of polycarbonate is of $\text{π → π}{}^1$ character in contrast to the $\text{n → π}{}^1$ assignment which has been described in the literature.     

On the interchain interaction in the (SN)x polymer

The interchain interactions in the (SN)_x polymer are discussed on the basis of one-dimensional tight-binding SCF MO calculations of two chains of (SN)_x. We conclude that the interchain interaction in the (100) crystallographic plane is more significant than that in the ($\text{1}$02) plane in accordance with the previous result by Messmer and Salahub and that, from more quantitative analysis, these interactions are mainly caused by the shortest interchain SS coupling in both planes.     

Ab initio study of the rotational energy barrier in carbonylylide

The rotational energy barrier in carbonylylide CH₂OCH₂ is studied using RHF CI calculations. Depending on the size of the CI and the basis set (STO-3G and 4–31G), values in the range 13–17 $\text{kcal}\text{mol}$ are found. At this level of calculation, the mid-point of the isomerization process can be mainly described by the diradical rather than the zwitterion.     

Matrix representation of the relativistic kinetic energy operator: Two-component variational procedure for the treatment of many-electron atoms and molecules

A matrix representation for the relativistic free-particle energy (p²c² + m²c⁴)^{$\text{1}\text{2}$} is proposed. Unlike the usual expansion term - $\text{p}{}^4\text{8}\text{m}{}^3\text{c}{}^2$, which is shown to lead to very compact states far below the ground state, this operator yields satisfactory charge distributions when included in the LCAO SCF and MRD CI procedure. Variational calculations employing the square-root matrix representation and a Coulomb potential (plus one-electron Darwin term) are carried out for a series of one-electron atoms (Z = 1, 5 and 8) and very good agreement is obtained with experiment for at least the 2s and 2p IPs; the corresponding 1s values are in somewhat worse agreement, however. When the same method is applied to the many-electron systems Br and I, results for the ²P_u valence-shell spin-orbit splittings are obtained in first-order perturbation theory which agree within 10% of their experimental values, indicating that such an ab initio variational treatment retains a high degree of reliability for the calculation of valence-shell properties even for relatively heavy elements.     

Katalytische reaktionen von aminen mit olefinen

Tetramethylethylenediamine (TMED) reacts with ethylene (propene) under the influence of catalytic amounts of alkyllithium to give dimethylvinylamine and dimethylethylamine (dimethylisopropylamine). In competitive reactions alkyl-lithium and the amides formed by this with primary or secondary amines in presence of tetramethylethylenediamine enhance the addition of primary or secondary amines to ethylene more strongly than in the absence of the diamine. The pressures are $\text{1}\text{5}$ to $\text{1}\text{10}$ of those needed when sodium catalysts are used and the temperatures required are 50–100° lower. Dimethylamine but not diethylamine, which however reacts with cycloolefins containing strained double bonds, adds to propene.     

A comparison of empirical force field parameters for molecular mechanics calculations on saturated hydrocarbons

Nine empirical force fields used in molecular mechanics calculations are tested for their abilities to reproduce experimental geometric and energy data on $\text{cis-syn-cis}$-perhydroanthracene.     

On the validity of using the TKR sum rule to place zero angle electron impact spectra on an absolute scale

The use of the Thomas—Kuhn—Reiche (TKR) sum rule to place electron impact spectra on an absolute scale is only rigorously correct if the observed intensities can be extrapolated to their first Born zero momentum transfer limit. This note investigates the error involved in using the TKR sum rule at zero scattering angle instead of zero momentum transfer. By considering an expansion of the generalized oscillator strength at zero angle it is shown that the first order correction to the TKR sum rule can be written as ($\text{1}\text{3}\text{k}{}^2$)[|E_o|?($\text{4}\text{5}$)$\text{V}$_ee] for target systems randomly oriented in space where k² is the incident electron energy and |E_o| and $\text{V}$_ee are the magnitude of the total electronic energy and the electron—electron repulsive energy of the ground state of the target system respectively. Estimates based on Hartree—Fock calculations indicate a 0.27% error in the use of the TKR sum rule for F and 0.79% for Ar at an incident electron energy of 25 keV.     

Structure and conformation of heterocycles. 13. conformational analysis of 2,3-DI(R)OXY-1,4-dioxanes: anomeric and gauche effects

A series of $\text{trans}$- and $\text{cis}$-2,3-di R)oxy-1,4-dIoxanes (R = Me,Ph, Ac) and some 2,5-disubstituted analogis were prepared and analysed by various NMR techniques. The $\text{trans}$ isomers occur in more than 96% as diaxial conformers. Molecular mechanics calculations largely confirm these findings, which are interpreted -in terms of combined anomeric and gauche effects.     

Theoretical study of the 12A″ (X2II) State of N2 O+: implications for isotopic scrambling

Ab initio calculations suggest that the potential energy surface for the 1²$\text{A}$″ state of N₂O⁺ has a secondary minimum corresponding to a strongly bent structure. This structure is computed to lie below the $\text{A}$²Σ ⁺ state energetically and therefore may be responsible for the isotopic scrambling observed in this energy region.     

A quadratically convergent reference state optimization procedure

A quadratically convergent procedure for the optimization of multiconfigurational reference states is presented and analyzed. The optimal reference state is determined from the energy variation principle by means of a sequence of unitary transformations of the form exp(i$\text{Λ}$exp(i$\text{k}$, where the hermitean operators $\text{Λ}$ and $\text{k}$ generate transformations of orbitals and correlation coefficients, respectively. Sample calculations are carried out within the Pariser—Parr—Pople model using a correlated reference state suggested by Clementi and Veillard. For the systems considered so far, convergence is achieved after 2–4 transformations.     

An NMR and theoretical study on the conformational behaviour of benzylidenemalonaldehydes : A conformationally flexible unsaturated system

The conformational behaviour of the methylenemalonaldehyde fragment of the title compounds is characterized by solvent and temperture dependencies of ¹J($\text{CH}$O), ³J($\text{C}$HO,C$\text{H}$O), ³J($\text{C}$HO,C$\text{H}$), ⁴J(C$\text{H}$O,C$\text{H}$O) and ⁴J(C$\text{H}$O,C$\text{H}$) spin-spin couplings as well some $\text{ab}$$\text{initio}$ (STO-3G) and semiempirical level Molecular orbital calculations. The conformational behaviour of the couplings is also discussed on the basis of some INDO/FPT calculations. The conformations of the two formyls are strongly correlated, the $\text{trans}$-$\text{cis}$ arrangements being favored. The conformation of the Z-formyl is determined by the steric interaction with the aromatic nucleus. The conformations of the formyls are sensitive to solvent and substitution on the aryl. Approximate populations and couplings of the sites are derived. There is some evidence about the non-planarity of the methylenmalonaldehyde fragnent. Complex formation with Mg(C10₄)₂ is reported.     

Cyclic acetals as carbonyl blocking groups in the photo-fries rearrangement of acyl substituted aryl esters.

The ethylene acetals of $\text{o}$ and $\text{p}$-acetoxyacetophenone 3a,b, prepared from the corresponding hydroxyacetophenones by treatment with acetic anhydride in pyridine and subsequently with ethylene glycol and $\text{p}$-toluenesulphonic acid, give upon irradiation the expected photo-Fries products 4a,b (from 3a) or 6 (from 3b). These compounds are converted in part into diacetylphenols 5a,b as a consequence of a deacetalization, occurring to some extent during the irradiation and further in the chromatographic workup. The yields of the photoproducts and the $\text{ortho}$/$\text{para}$ ratio (in the case of 3a) are very similar to those given in the literature for phenyl acetate. By contrast, the acetoxyacetophenones 2a,b do not undergo any appreciable change upon irradiation,showing the deactivating effect of the acetyl side chain. Therefore, it is concluded that cyclic acetals are suitable carbonyl blocking groups in order to circumvent the deactivating effect of acyl substituents on the photo-Fries rearrangement of aryl esters.     

A study of the structure of fluoroethylenes ab-initio calculations on C2F4

Ab-initio calculations on C₂F₄, with full geometry optimization in a ($\text{7}\text{3}$) and a ($\text{9}\text{5}$) basis are reported. The predicted geometries, orbital energies and populations are discussed in view of the different basis sets and previous calculations of the other fluoroethylenes. The effects of substituting a hydrogen atom with a fluorine atom are rather local, the effects on the populations being additive in all fluoroethylenes except C₂F₄. A short comparison between the basis sets is presented.