The chemical bond is always considered from different points of view, depending or on the chemical and physical aspects to be examined. In both cases, description of the chemical bond are chosen that are appropriate for the particular research or application. Therefore, there are significant differences in the understanding as what constitutes chemical bonding. This is acceptable in practice but proves to be a hindrance for true interdisciplinarity. The concept of the chemical bond offer a firm basis upon which to forge links not only within chemistry but also to all related sciences. The general desire and the growing necessity for interdisciplinary collaboration requires a careful treatment of these concepts and, if possible, a tightening and standardization to a level that is widely acceptable and beneficial. In the age of tremendously fast development of computers and computer science, we believe that the electron localization function (ELF) provides a new description of the chemical bond for almost all classes of compounds. Its graphical language earns it the ultimate qualification for enhanced interdisciplinarity.

ELF: The Electron Localization Function

Modern quantum chemistry can make quantitative predictions on an immense array of chemical systems. However, the interpretation of those predictions is often complicated by the complex wave function expansions used. Here we show that an exceptionally simple algebraic construction allows for defining atomic core and valence orbitals, polarized by the molecular environment, which can exactly represent self-consistent field wave functions. This construction provides an unbiased and direct connection between quantum chemistry and empirical chemical concepts, and can be used, for example, to calculate the nature of bonding in molecules, in chemical terms, from first principles. In particular, we find consistency with electronegativities (χ), C 1s core-level shifts, resonance substituent parameters (σR), Lewis structures, and oxidation states of transition-metal complexes.

/pdf/intrinsic-atomic-orbitals-an-unbiased-bridge-between-quantum-3xsf9tmc08.pdf

Intrinsic Atomic Orbitals: An Unbiased Bridge between Quantum Theory and Chemical Concepts.

Many concepts used for a qualitative description of chemical bonding that originated in the early days of theoretical chemistry have been vindicated recently by quantum chemical calculations, at least as far as first row elements are concerned. However, many concepts that have been justified for first row elements (Li to Ne) cannot—contrary to widespread belief—be generalized to the higher main group elements. This applies particularly to the concept of hybridization, which should be viewed with considerable caution. The essential difference between the atoms of the first and higher rows is that the cores of the former contain only s-AOs, whereas the cores of the latter include at least s- and p-AOs. As a consequence, the s and p valence AOs of first row atoms are localized in roughly the same region of space, while the p valence AOs of higher row atoms are much more extended in space. This has the consequence that for the light main group elements both lone-pair repulsion and isovalent hybridization play a greater role than for the heavy main group elements. Furthermore, this implies that single bonds between first row elements are weak and multiple bonds are strong, whereas for the second or higher row elements single bonds are strong and multiple bonds weak. The “extended valence” (violation of the octet rule) observed in compounds of higher main group elements has very little to do with the availability of d-AOs but is due rather to the size of these atoms and thus to the reduced steric hindrance between ligands and, to a lesser extent, also to the lower electronegativity of the heavy atoms. A model based on the concept of electron-rich multicenter bonds is certainly closer to reality than one involving hybrids with the participation of d-AOs. The XO bonds in phosphane oxides, sulfoxides, oxo acids and related compounds are better formulated as semipolar rather than as true double bonds, even if they behave in some respects like double bonds.—The growing interest of theorists in compounds of higher main group elements parallels new and, in some instances, spectacular results of experimental research on the chemistry of these elements.

Chemical Bonding in Higher Main Group Elements

First, different approaches to detect hydrogen bonds and to evaluate their energies are introduced newly or are extended. Supermolecular interaction energies of 256 dimers, each containing one single hydrogen bond, were correlated to various descriptors by a fit function depending both on the donor and acceptor atoms of the hydrogen bond. On the one hand, descriptors were orbital-based parameters as the two-center or three-center shared electron number, products of ionization potentials and shared electron numbers, and the natural bond orbital interaction energy. On the other hand, integral descriptors examined were the acceptor-proton distance, the hydrogen bond angle, and the IR frequency shift of the donor-proton stretching vibration. Whereas an interaction energy dependence on 1/r3.8 was established, no correlation was found for the angle. Second, the fit functions are applied to hydrogen bonds in polypeptides, amino acid dimers, and water cluster, thus their reliability is demonstrated. Employing the...

Estimating the Hydrogen Bond Energy

By the end of the last century there were already the first indications of the possible existence of Al1 halides. However, it was only through the pioneering works of W. Klemm, who would have celebrated his 100th birthday on January 6, 1996, that detailed spectroscopic investigations became possible. Since the end of the 1970s the reactivity of AlX and GaX species in solid noble gases has been confirmed by numerous examples. In recent years formally monovalent Al and Ga species have been successfully synthesized on a preparative scale. In addition to the first halides, organometallic compounds with metal–metal bonds have been isolated and investigated with regard to their chemical properties. The fundamental importance of such species has been documented in this journal among others in the form of two highlight articles in which experimental and theoretical aspects have been examined with examples, and parallels and differences with respect to boron chemistry have been illustrated. This review is intended to give an account of the chronological development of this research area over the last few years, but an attempt is also made to categorize the experimental results achieved not only with respect to structure, thermodynamics, and reactivity, but also with the aid of quantum chemical calculations and by comparative considerations.

Aluminum(I) and Gallium(I) Compounds: Syntheses, Structures, and Reactions

A new interpretational scheme is proposed for the analysis of molecular wavefunctions. Starting from the molecular density operator we first construct a minimal set of MAOs from the requirement that the MOs can be represented as closely as possible by the MAOs. We then use the MAOs to compute atomic occupation numbers N and shared electron numbers σ. The molecular density is then discussed in terms of N and σ. This approach has the following advantages: 1) it is generally applicable, 2) the quantities N and σ are virtually basis set independent, 3) the quantities N and σ fulfil the intuitively expected boundary conditions, 4) the simultaneous consideration of N and σ allows for a more reliable description of chemical bonding than consideration of atomic charges only.

Population analysis based on occupation numbers of modified atomic orbitals (MAOs)

Stability and reactivity of the silicon-carbon double bond

Extended basis set ab initio computations are performed on HF, PNO-CI and CEPA level to determine the structure of P2H4 and the potential curve E(ϕ) for rotation around the P-P axis. The structure parameters are optimized for dihedral angles ϕ of 0 ° (cis), 50 °, 80 ° (gauche or semi-eclipsed), 130 °, and 180 ° (trans). It turns out that P2H4 has a gauche equilibrium structure, a local minimum for trans which is ≈2.5 kJ/mol above gauche, a rather large cis barrier of ≈20 kJ/mol and a gauche → trans barrier of ≈ 3.5 kJ/mol. The potential E(ϕ) is extremely flat in the region 50 ° < ϕ < 310 °, where E(ϕ) varies by less than 5 kJ/mol. Electron correlation tends to reduce the barriers but has no drastic effect on E(ϕ).

Conformation analysis of diphosphine

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Population analysis based on occupation numbers of modified atomic orbitals (MAOs)

Stability and reactivity of the silicon-carbon double bond

Conformation analysis of diphosphine