Teijiro Yonezawa

A Molecular Orbital Theory of Reactivity in Aromatic Hydrocarbons

In the search for a quantitative correlation between reactivity and electronic configuration of aromatic hydrocarbons, the electron density, at each carbon atom, of the highest occupied π‐orbital in the ground state of the molecule is calculated by means of the LCAO method. Comparing the result of such a calculation on fifteen condensed aromatic hydrocarbons with their chemical reactivities, we find that the position at which the electron density is largest is most readily attacked by electrophilic or oxidizing reagents.It is, therefore, concluded that distinct from other π‐electrons the pair of π‐electrons occupying the highest orbital, which is referred to as frontier electrons, plays a decisive role in chemical activation of these hydrocarbon molecules. The theoretical significance of this discrimination of the frontier electrons in relation to the chemical activation is discussed.

Molecular Orbital Theory of Orientation in Aromatic, Heteroaromatic, and Other Conjugated Molecules

As to the LCAO MO treatment of the orientation problem in chemical reactions of π‐electron systems, the frontier electron concept which has been previously introduced by the authors for explaining the reactivities in aromatic hydrocarbons is subjected to an extension in the sense that the frontier orbitals are specified according to the type of reaction. Thus, fundamental postulates relating to the reactivities of π‐electron systems are set up, which are believed to include general principles involved in the mechanism of both substitution and addition of electrophilic, nucleophilic, or radical type. On the basis of these postulates it is possible to predict the position of attack in conjugated molecules in all the three types of substitution as well as addition in a consistent manner. There is a nearly perfect agreement between the theoretical conclusions and experimental results hitherto reported.

MO‐Theoretical Approach to the Mechanism of Charge Transfer in the Process of Aromatic Substitutions

An MO‐theoretical investigation on the mechanism of aromatic substitution is made. Supposing that the approaching reagent and the atom to be replaced can be treated as an atom (pseudoatom) which has a π‐type orbital in the course of reaction, an important role of hyperconjugation taking place between the pseudoatom and the aromatic compound is remarked. Since the electron density at the pseudoatom relates to the amount of charge transfer through this hyperconjugation and varies as the reaction proceeds, the proceeding of the reaction can be measured by this electron density. Then, the electron density is expressed, in the form of a contour diagram, in terms of the Coulomb integral of the pseudoatom and the resonance integral between the pseudoatom and carbon atom to be attacked. Thus, a reaction path can be represented by a locus plotted on this diagram. The discriminating property of frontier orbitals can be observed in this diagram. It has been confirmed that an electrophilic, or a nucleophilic, substitution takes place only when a certain condition is satisfied with respect to the energy of the reagent. By determining the transition state which corresponds to the end point of the locus, several reactivity indices, i.e., frontier electron density, superdelocalizability, its one‐term approximation and a generalized reactivity index are derived from the hyperconjugation energy at the transition state, which is also available as a generalized reactivity index. Correlation between these and the existing indices is examined.

Interrelations of Quantum‐Mechanical Quantities Concerning Chemical Reactivity of Conjugated Molecules

It is attempted here to disclose the interrelations underlying the parallelism found in several quantum‐mechanical methods previously proposed to explain chemical reactivity in conjugated molecules. This is carried out mathematically in alternant hydrocarbons and hetero‐alternant molecules mainly by the use of Coulsons integral formulas. In an alternant hydrocarbon, provided a condition is satisfied between minors of the secular determinant, the orders of reactivity of each position predicted by the self‐polarizability, free valence, superdelocalizability, Whelands localization energy, and Dewars approximate localization energy by NBMO are shown to be in complete agreement with one another. The relation of the frontier electron density and superdelocalizability is discussed. It is also demonstrated that a nonperturbation treatment for the so‐called static method leads us to the same conclusion as the perturbation treatment. The parallelism of various quantities is discussed in their relation to the eff...

Cluster expansion of the wave function. Valence and Rydberg excitations and ionizations of pyrrole, furan, and cyclopentadiene

The symmetry‐adapted cluster (SAC) expansion and the SAC‐CI theory have been applied to the calculations of the valence and Rydberg excitations and ionizations of the five‐membered ring compounds, pyrrole, furan, and cyclopentadiene. For almost all cases, the experimental values were reproduced to within 0.3 eV for the excitation energy, and to within 0.5–0.7 eV for the ionization potential. We have given several new assignments for the excitation spectra of pyrrole and furan, and the first ab initio assignments for cyclopentadiene. However, there were remarkable disagreements of about 1.2 eV for the singlet π → π* excitations of 2 1A1 state for furan, and of 1 1B2 state for cyclopentadiene. This is the same tendency as those due to the MRSD‐CI method. For ionization potential, our assignments of the peaks are essentially the same as those due to the Green’s function method by von Niessen et al., but the present SAC‐CI method reproduces the experimental data better than this method.

Effect of the heavy atom on the nuclear shielding constant. I. The proton chemical shifts in hydrogen halides

The abnormal upfield bias of the proton chemical shift in hydrogen halides, HX, on going from X = F to I is examined theoretically in terms of the third‐order perturbation theory including the spin‐orbit interaction (λL · S). The expression for the new type of shielding constant σLS is presented in its complete form and in simplified form with the average excitation energy approximation. The numerical calculations using semiempirical SCF wavefunctions predict substantial contribution (51%) of this shielding effect (σLS) to the total shielding of the proton in hydrogen iodide.

Abinitio theoretical study on the reactions of a hydrogen molecule with small platinum clusters: A model for chemisorption on a Pt surface

Reactions of a hydrogen molecule with small platinum clusters Ptn (n=1,2,3) are studied theoretically by ab initio methods. This provides a cluster model study for hydrogen chemisorption on a Pt surface. The results suggest that the Pt atom and the linear Pt3 cluster will react with H2 and dissociatively adsorb it, making two Pt–H bonds, whereas the Pt2 cluster will not react with H2 because of a high energy barrier. The dissociative adsorption of a hydrogen molecule occurs at a side‐on, on‐top site of a surface Pt atom and molecular adsorption does not seem to occur. Essentially only one Pt atom is involved in the initial adsorption process. These findings are in contrast to the palladium case previously reported. Almost no energy barrier exists for the hydrogen migration from one Pt atom to an adjacent one, with a preference being shown for one H atom on each Pt atom rather than two H atoms on one Pt atom. The heat of adsorption, the stability of the catalytic surface, etc., are best represented by the ...

Catalysis by amorphous metal alloys. Part 1.—Hydrogenation of olefins over amorphous Ni–P and Ni–B alloys

Amorphous Ni–P (Ni81P19) and Ni–B (Ni62B38) alloys have been prepared by the rapid quenching method. The untreated alloys are inactive for the hydrogenation of olefins, but successive pretreatments of the alloys with the dilute HNO3, oxygen and hydrogen bring about greater catalytic activity than that possessed by the crystalline alloys. Measurements of the ESCA spectra of the surfaces of the alloys have indicated that the alloys become active by the partial oxidation of the nickel and phosphorus or boron on the surface. Buta-1,3-diene is hydrogenated to give isomeric butenes of constant composition, and butenes isomerise rapidly to give a similar composition to that obtained on the hydrogenation of buta-1,3-diene. The hydrogenation of buta-1,3-diene is first and zeroth order with respect to the partial pressures of hydrogen and buta-1,3-diene, respectively. These results are explained by assuming that the hydrogen and the olefins are not competing for adsorption on the same sites and that hydrogen is activated on electron-deficient nickel atoms formed by electron transfer from nickel to the oxidised species.

On the Unrestricted Hartree–Fock Wavefunction

The unrestricted Hartree–Fock (UHF) wavefunction is analyzed and interpreted in configuration‐interaction (CI) language. The results of the present study are as follows. (i) The UHF wavefunction includes only one type of the singly excited configurations [Eq. (20)], and thus the correlation effects included are very limited ones, compared with the usual CI treatment. (ii) The weight of the lowest contaminating spin function, included in the UHF wavefunction, decreases with increasing spin multiplicity. (iii) The annihilation of the lowest contaminating spin function little affects the electron density distributions and other physical quantities, the operators of which commute with the annihilation operator. (iv) In the UHF method, the “spin‐appearing” (spin‐polarization and spin‐delocalization) mechanisms are clearly divided, and an approximate method to separate these contributions is generalized, and some discussions about spin annihilation are made.

Coupling parameters from the 13CH satellite nuclear magnetic resonance spectra of 1-substituted aziridines

Abstract An analysis of the 13 CH satellite resonances in the NMR spectra of 1-substituted aziridines is reported. The interproton coupling constants as well as the 13 CH couplings associated with the cis and trans ring methylene protons are separately determined. Considerably different values of vicinal cis HH coupling for the cis and trans protons as well as the corresponding directly bonded 13 CH coupling are obtained. The effect of the orientation of the nitrogen lone pair on the adjacent 13 CH coupling constant is also discussed.