Molecular Orbital Concepts Research Articles

Effect of kinetic and electronic energy on the reactions of Cr+ with H2, HD, and D2

The reactions of atomic chromium ions with H2, HD, and D2 are examined using guided ion beam tandem mass spectrometry. The ground electronic state (6S) is found to react inefficiently. The thresholds of these data are analyzed to yield a 0 K bond dissociation energy for CrH+ of 1.37±0.09 eV (31.6±2.0 kcal/mol). In the reaction of Cr+(6S) with HD, formation of the metal deuteride ion is strongly favored over that of the metal hydride ion in the threshold region but not at higher energies. These results indicate that Cr+(6S) reacts largely via an impulsive mechanism. No evidence for reaction of Cr+(6D), the first excited state, is found but higher lying excited states, 4D and 4G (produced by electron impact), are found to react at their thermodynamic threshold. In reaction with HD, these states produce CrH+ preferentially by a factor of 2–4 over CrD+. The differences in reactivity among these states can be explained by using simple molecular orbital concepts which have been developed for other atomic transition metal ions.

Effect of kinetic and electronic energy on the reactions of cobalt, nickel, and copper ions with hydrogen, hydrogen deuteride, and deuterium

Reactions of atomic cobalt, nickel, and copper ions with H/sub 2/, HD, and D/sub 2/ have been examined by using guided ion beam tandem mass spectrometry. The ground electronic states of all three ions are found to react efficiently above their thermodynamic thresholds. In the reactions of these states with HD, formation of the metal hydride ion is strongly favored over that of the metal deuteride ion. These data are analyzed to yield 0 K bond dissociation energies for CoH/sup +/ of 1.98 +/- 0.06 eV (45.7 +/- 1.4 kcal/mol), NiH/sup +/ of 1.68 +/- 0.08 eV (38.6 +/- 1.8 kcal/mol), and CuH/sup +/ of 0.92 +/- 0.13 eV (21.2 +/- 3.0 kcal/mol). Excited states of the metal ions produced by electron impact are found to react very inefficiently. These results fit in nicely with those of other atomic transition-metal ions and can be explained by using simple molecular orbital concepts.

Effect of kinetic and electronic energy on the reactions of iron(1+) with hydrogen, deuterium hydride, and deuterium: state-specific cross sections for Fe+(6D) and Fe+(4F)

Reactions of atomic iron ions with H/sub 2/, HD, and D/sub 2/ have been examined by using guided ion beam mass spectrometry. Individual reaction cross sections for the ground and first excited electronic states of Fe/sup +/ are determined by utilizing three different ion sources. The ground state, Fe/sup +/(/sup 6/D), is found to be relatively unreactive and, in reaction with HD, greatly favors production of FeD/sup +/. In contrast, the first excited state, Fe/sup +/(/sup 4/F), reacts efficiently and forms approx. =4 times as much FeH/sup +/ as FeD/sup +/ in reaction with HD. This disparate behavior is compared with that of their atomic transition-metal ions and is explained by using simple molecular orbital concepts. The results are analyzed to yield a bond dissociation energy at 0 K for FeH/sup +/ of 2.12 +/- 0.06 eV (48.9 +/- 1.4 kcal/mol).

Effect of kinetic and electronic energy on the reactions of Mn+ with H2, HD, and D2

Reactions of several electronic states of Mn+ with H2, HD, and D2 have been examined using guided ion beam mass spectroscopy. The excitation function for the ground state of Mn+(7S) has two regions: one of very low reactivity at threshold and another more efficient pathway at higher energies. In contrast, the 5S and 5D states react efficiently at their thermodynamic thresholds. In reaction with HD, the 5S and 5D states produce ≊3 times as much MnH+ as MnD+ in the threshold region. This isotope effect is similar to that seen in previous studies of transition metal ion reactions. Reaction of Mn+(7S) with HD, on the other hand, exhibits an extreme isotope effect such that MnD+ is formed almost exclusively. The state dependence of the reactivity and reaction mechanisms is explained using simple molecular orbital concepts. The results are analyzed to yield a bond dissociation energy at 0 K for MnH+ of 2.06±0.15 eV (47.5±3.4 kcal/mol).

Impurity interactions and pseudo-molecule formation in metals

It is shown that the concept of molecular orbitals is a useful one also in describing interactions between impurities in metals. The conclusion is based on first principles, self-consistent calculations for H2 in jellium. The dissociation energy is primarily determined by the position of the Fermi energy relative to an antibonding molecular resonance. In a dense metal like Aℓ this is completely filled and the dissociation energy is an order of magnitude smaller than for free H2.

Model organometallic systems. The interaction of 2S beryllium(1+) ion, 1S beryllium, and 3P beryllium atoms with acetylene and ethylene

To model the interaction between metal atoms and unsaturated hydrocarbons, several electronic states of the Be-C/sub 2/H/sub 2/ and Be--C/sub 2/H/sub 4/ systems have been investigated using ab initio electronic structure theory. Double basis sets have been used (after some experimentation with polarization functions) at the self-consistent-field level of theory. For ten different electronic arrangements, potential energy curves have been predicted for the perpendicular approach of the Be nucleus to the midpoint of the carbon--carbon bond. The interaction of Be/sup +/ ion (ground /sup 2/S state) with both acetylene and ethylene is attractive, yielding equilibrium Be midpoint separations of 2.03 and 2.09 A and binding energies of 30 and 33 kcal/mole. In contrast the /sup 1/S ground state of the neutral Be atom has an essentially repulsive interaction. The threefold degenerate excited /sup 3/P state splits into /sup 3/A/sub 1/, /sup 3/B/sub 1/, and /sup 3/B/sub 2/ components as it approaches the ethylene molecule. The /sup 3/B/sub 2/ state is rather strongly bound, 19 kcal for Be--C/sub 2/H/sub 2/ and 25 kcal for Be-C/sub 2/H/sub 4/. the /sup 3/A/sub 1/ state is strongly repulsive, while the /sup 3/B/sub 1/ state assumes an intermediate role. These trends are readily explained bymore » qualitative molecular orbital concepts.« less

The path to molecular orbital theory

Abstract

Molecular X-ray Spectra of Sulfur and Chlorine Bearing Substances

AbstractThe interpretation and use of x-ray photon spectra of substances containing second row elements has utilized a number of theoretical models. These models may be divided into three basic categories, namely, the isolated atom model, various molecular models, and a number of solid state models, it is the purpose of this paper to examine critically the validity and limitations of molecular models for interpreting published x-ray photon spectra and spectra obtained by this group on chlorine and sulfur bearing substances.Chlorine and sulfur bearing substances were chosen for at least three important reasons. First, a great deal of published experimental data already exists on the Kα, Kβ, and L2, 3 transitions of these substances. Second, motivated in part by the long standing controversy concerning possible 3d orbital participation in the bonding of second row elements, there are extensive quantum mechanical calculations for ions containing sulfur and chlorine via simple molecular orbital concepts. Thirdj the availability of accurate photoelectron spectroscopic data on these substances now permits a detailed quantitative comparison of x-ray photon transitions with quantum mechanical calculations.Detailed evaluation along these lines indicates that for many substances the theoretically calculated energy values are frequently within a few electron volts (or less) of the experimentally observed energies. This study, therefore, tends to substantiate a viewpoint suggested by some recently; namely, that for many substances the starting point in interpreting most of the basic features of soft x-ray spectra should be based upon molecular bonding approaches.

Use of Molecular Orbital Theory to Interpret X-Ray K-Absorption Spectral Data

This paper demonstrates how molecular orbital theory can be useful in obtaining chemical information from x-ray absorption spectra. Basic molecular orbital theory concepts such as symmetry and overlap of wavefunctions are used to explain the K-absorption spectra of first-row transition metal complexes. In x-ray K-absorption spectroscopy, 1s electrons are excited to high-energy unoccupied orbitals; thus, the main absorption maxima (for first row transition metals) are due to excitations to the triply degenerate 4p antibonding orbitals. The shifts in these peaks are due to changes in the orbital energy levels, which, of course, are a function of chemical bonding. A splitting or a broadening of a peak is an indication of a splitting of the degenerate energy levels, which in turn is an indication of unsymmetrical metal to ligand bonding. It is also shown how x-ray absorption spectra can be useful in determining the lattice sites of atoms of nearly equal x-ray scattering power.

Adsorption and co-ordination of unsaturated hydrocarbons with metal surfaces and metal atoms

A survey is first made of the co-ordination of olefins and chelating diolefins to metal atoms in organometallic complexes: with d8 and d10 complexes, the observed stability sequences for ethylene co-ordination (e.g., NiII<PdII<PtII; AgI<PdII<RhI) are interpreted in terms of the spatial direction and extension of the d-orbitals. Measurements of the power of diolefins to inhibit the isomerization of 1-pentene catalyzed by methanolic RhCl3 confirm their stronger co-ordination which is attributed to an entropy effect. The principal phenomena of homogeneous catalysis by organometallic complexes are consistent with the above stability sequences, optimum activity occurring with RhI and PdII complexes: comparison of these phenomena with those in heterogeneous hydrogenation catalysis leads to the working hypothesis that the reactive state of adsorbed unsaturated hydrocarbons is also a π-complex.A molecular orbital model appropriate to a face-centred cubic metal is outlined, and the direction of emergence of the eg and t2g orbitals at the (100), (110) and (111) planes obtained: the degree of occupation of the orbitals is discussed, and possible models for π-adsorbed states of olefins, diolefins and alkynes are sketched. The (111) plane is the least suited for their adsorption. A brief attempt is made to apply molecular orbital concepts to mechanisms in hydrogenation catalysis.

Would Mendeleev have predicted the existence of XeF4?

The author suggests that a contemporary Mendeleev might have used the concept of molecular orbitals to predict the existence of XeF4.

Molecular Orbitals for H3+

A somewhat more general concept of molecular orbitals than used heretofore is applied to H3+. It is shown to be superior to the Heitler-London approximation, but inferior to the HLSP-plus-ionic-terms treatment of Hirschfelder. Relations between the HLSP and molecular orbital approaches to wave functions are discussed.

Electronic Structures of Polyatomic Molecules and Valence. II. General Considerations

A description of electronic structures of polyatomic molecules in terms of atomic and molecular orbitals (one-electron orbital wave functions) is suggested, as follows. Unshared electrons are considered as occupying atomic orbitals associated with the various nuclei to which the electrons belong. Shared electrons are described from several points of view, whose simultaneous consideration should give a better understanding of their condition and functions in the molecule. In the first place, they are described, in terms of atomic orbitals, from the point of view of each nucleus in the molecule. In the second place, they are described from a unitary molecular point of view, in terms of molecular orbitals. It is pointed out that the chemical evidence which led Lewis to his concept of the electron-pair bond can now all be explained by the quantum theory without the necessity of using such a concept. It is noted that the Heitler-London, Pauling-Slater quantum-mechanical electron-pair bond concept differs markedly from Lewis's, also that it is of more restricted application to chemical data. It is pointed out, in agreement with Hund, that properties of the H. and L., S. and P. electron-pair bond which make it useful in dealing with chemical combination are also prossessed by the concept of molecular orbitals. For example, the Pauling-Slater criterion that bonds which correspond to a maximum overlapping of atomic orbitals are the strongest is just as characteristic of bonding molecular orbitals as of electron-pair bonds. An electron-pair bond is here interpreted as being little other than two electrons occupying a bonding molecular orbital. Or in general one must say that a set of $n$ electron-pair bonds is interpreted as a set of $2n$ electrons occupying $n$ molecular orbitals, because molecular orbitals are not necessarily localized between two nuclei like electron-pair bonds. It is concluded that the essential facts of molecular electronic structure can be qualitatively understood in terms of the mode of description stated in the first paragraph above and of a semi-empirical valence rule (not essentially new) which summarizes, in the light of quantum theory, the most important regularities in regard to the types of chemical compounds which are stable.