Model Description of Some Molecular Properties by the Modified-Atom-in-Molecule (MAM) Approach

Conclusive evidence is presented whlch shows that the concept of modified atoms in molecule (MAM) is a viable model for a good description of numerous molecular properties. Atomic modification . can be decomposed to isotropic and anisotropic components. The isotropic change caused by molecular formation is given by the electric monopoles of atoms. It is a consequence of the charge drift accompanying chemical bonding. Atomic monopoles reproduce diamagneticshielding of the nuclei rJAd, diamagnetic susceptibility xd and ESCA shifts with an intriguing success. The atomic monopole model is easily extended to include higher local multipoles (i. e. anisotropic contribution), thus yielding satisfactory total molecular multipoles and extramolecular electrostatic potentials. Salient directional properties of covalent bonds are well described by the use of polarized atomic orbitals. It was shown that hybridization is the underlying concept which explains interrelations between steric features and local bond properties. Hybridization rationalizes in a natural and simple way the electron pair (Lewis) bond which is one of the corner stones of chemistry being particularly important for the first row atoms. It was concluded that the high information content of hybrid AOs can be ascribed to the fact that they conform to the local symmetry of the immediate molecular environment. Thus the HAOs are local wavefunctions of the zeroth order which describe atomic angular distortions. Although atoms can not be uniquely defined within molecules, the MAM model has high interpretative power yielding reasonable results. Special attention deserves a picture of charged atoms immersed in the »sea« of mixed electron density, because it is free of any arbitrariness in the slicing of molecular volume of partitioning of overlap charge. Finally, the definition of pseudo-observables is given. It was concluded that atomic monopoles and hybridization indices are pseudo-observables par exceHence. A.pparently there is colour, apparently sweetness, apparently bitterness; actuaUy there are only atoms and the void. Democritus, 420 B. C

Model Description of Some Molecular Properties by the Modified-Atom-in-Molecule (MAM) Approach

Conclusive evidence is presented whlch shows that the concept of modified atoms in molecule (MAM) is a viable model for a good description of numerous molecular properties. Atomic modification . can be decomposed to isotropic and anisotropic components. The isotropic change caused by molecular formation is given by the electric monopoles of atoms. It is a consequence of the charge drift accompanying chemical bonding. Atomic monopoles reproduce diamagneticshielding of the nuclei rJAd, diamagnetic susceptibility xd and ESCA shifts with an intriguing success. The atomic monopole model is easily extended to include higher local multipoles (i. e. anisotropic contribution), thus yielding satisfactory total molecular multipoles and extramolecular electrostatic potentials. Salient directional properties of covalent bonds are well described by the use of polarized atomic orbitals. It was shown that hybridization is the underlying concept which explains interrelations between steric features and local bond properties. Hybridization rationalizes in a natural and simple way the electron pair (Lewis) bond which is one of the corner stones of chemistry being particularly important for the first row atoms. It was concluded that the high information content of hybrid AOs can be ascribed to the fact that they conform to the local symmetry of the immediate molecular environment. Thus the HAOs are local wavefunctions of the zeroth order which describe atomic angular distortions. Although atoms can not be uniquely defined within molecules, the MAM model has high interpretative power yielding reasonable results. Special attention deserves a picture of charged atoms immersed in the »sea« of mixed electron density, because it is free of any arbitrariness in the slicing of molecular volume of partitioning of overlap charge. Finally, the definition of pseudo-observables is given. It was concluded that atomic monopoles and hybridization indices are pseudo-observables par exceHence. A.pparently there is colour, apparently sweetness, apparently bitterness; actuaUy there are only atoms and the void. Democritus, 420 B. C

A Greatly Under-Appreciated Fundamental Principle of Physical Organic Chemistry

If a species does not have a finite lifetime in the reaction medium, it cannot be a mechanistic intermediate. This principle was first enunciated by Jencks, as the concept of an enforced mechanism. For instance, neither primary nor secondary carbocations have long enough lifetimes to exist in an aqueous medium, so SN1 reactions involving these substrates are not possible, and an SN2 mechanism is enforced. Only tertiary carbocations and those stabilized by resonance (benzyl cations, acylium ions) are stable enough to be reaction intermediates. More importantly, it is now known that neither H3O+ nor HO− exist as such in dilute aqueous solution. Several recent high-level calculations on large proton clusters are unable to localize the positive charge; it is found to be simply “on the cluster” as a whole. The lifetime of any ionized water species is exceedingly short, a few molecular vibrations at most; the best experimental study, using modern IR instrumentation, has the most probable hydrated proton structure as H13O6+, but only an estimated quarter of the protons are present even in this form at any given instant. Thanks to the Grotthuss mechanism of chain transfer along hydrogen bonds, in reality a proton or a hydroxide ion is simply instantly available anywhere it is needed for reaction. Important mechanistic consequences result. Any charged oxygen species (e.g., a tetrahedral intermediate) is also not going to exist long enough to be a reaction intermediate, unless the charge is stabilized in some way, usually by resonance. General acid catalysis is the rule in reactions in concentrated aqueous acids. The Grotthuss mechanism also means that reactions involving neutral water are favored; the solvent is already highly structured, so the entropy involved in bringing several solvent molecules to the reaction center is unimportant. Examples are given