Polyatomic structures

It seems clear that Armstrong s motivation for taking the polyatomic structure hypothesis seriously was indeed to defend the periodic table. Even on this score, however, there is nothing of the feeling of a Kuhnian commitment to a paradigm. For example, Rucker, the President of the Royal Society, while ready to consider non-rotating polyatomic molecules, was, like Rayleigh and Ramsay themselves, also ready to see the periodic table rejected if necessary ... 

The principles of electronic spectroscopy have been discussed by Herzberg (1950) for diatomic molecules, and in a classic review by Sponer and Teller (1941) for the more general case of polyatomic structures. Recent developments are described in articles appearing regularly in the Annual Reviews of Physical Chemistry. Triatomic molecules and radicals have been intensively studied, the latter by the powerful method of flash photolysis (Herzberg, 1959). As triatomic structures have been comprehensively reviewed recently (Ramsay, 1962) we include in this article only those triatomic systems that are of particular interest in organic chemistry. Otherwise attention will be directed to molecules of four or more atoms, including all known representatives of the important chromophores. 

The Laser Raman microprobe constitutes a physical method of microanalysis based on the vibration spectra characteristic of polyatomic structures. A focused laser beam excites the sample. The light diffused by the Raman effect is used for identification and localisation of the molecular constituents present in the sample. An optical microscope allows a survey of the interesting structures and the placing of the laser beam. The spectra obtained from fossil organic particles generally match well the corresponding IR-spectra, but the features in particular yield additional information, which will be discussed below with the given examples (Fig. 23, p. 36). 

The interatomic and intennolecular interactions of adsorbed species and their state on the catalyst surface are the basis of all elementary steps of the catalytic process. The importance and reliability of the modeled results depend on the correct choice of the potential of the interatomic interaction. The question about the type and nature of interatomic forces between adsorbed species is the focus. Interatomic forces are diverse and usually anisotropic. Adsorbed species do not form structures at low coverage. When the number of adsorbed species and the rate of their surface diffusion increase, the probabilities of their interaction and formation of surface polyatomic structures increases. These structures can be rather stable and they form islands of adsorbed species. 

This is a formal definition since the bonds in polyatomic structures are generally partially covalent. The electrons are therefore only partially shifted towards the most electronegative atom. Moreover, this notion becomes more difficult to grasp in the case of bonds between atoms with similar electronegativities. 

If we regard each electronic formula as dynamic rather than static, there is little to be gained by emphasizing the concept of resonance hybrids. Furthermore, the detailed theory of resonance has come under heavy fire. It is claimed by A. Burawoy that .. . the interpretation of polyatomic structures does not require the hypothesis of resonance among several idealized valence-bond structures, a speculative application of the quantum-mechanical conception of resonance, which is in disagreement with numerous facts. In view of this controversy, we shall avoid detailed application of the theory of resonance. The theory of molecular orbitals, an alternative treatment, is too complex for the present discussion. 

Herzberg G 1966 Molecular Spectra and Molecular Structure III Electronic Spectra and Electronic Structure of Polyatomic Molecules (New York Van Nostrand-Reinhold)... 

Pack R T 1976 Simple theory of diffuse vibrational structure in continuous UV spectra of polyatomic molecules. I. Collinear photodissociation of symmetric triatomics J. Chem. Phys. 65 4765... 

Herzberg G 1945 Molecular Spectra and Molecular Structure II. Infrared and Raman Spectra of Polyatomic Molecules (Princeton, NJ Van Nostrand)... 

Yarkony D R 1995 Electronic structure aspects of nonadiabatic processes in polyatomic systems Modern Electronic Structure Theory vo 2, ed D R Yarkony (Singapore World Scientific) pp 642-721... 

Schinke R, Weide K, Heumann B and Engel V 1991 Diffuse structures and periodic orbits in the photodissociation of small polyatomic molecules Faraday Discuss. Chem. Soc. 91 31... 

G. Herzberg, Moleculer Spectra and Molecular Structure III. Electronic Spectra of Polyatomic Molecules, Van Nostrand, New York, 1967. 

Another example of reduced symmetry is provided by the changes that occur as H2O fragments into OH and H. The a bonding orbitals (ai and b2) and in-plane lone pair (ai) and the a antibonding (ai and b2) of H2O become a orbitals (see the Figure below) the out-of-plane bi lone pair orbital becomes a" (in Appendix IV of Electronic Spectra and Electronic Structure of Polyatomic Molecules, G. Herzberg, Van Nostrand Reinhold Co., New York, N.Y. (1966) tables are given which allow one to determine how particular... 

Free Radicals. In the formula of a polyatomic radical an unpaired electron(s) is(are) indicated by a dot placed as a right superscript to the parentheses (or square bracket for coordination compounds). In radical ions the dot precedes the charge. In structural formulas, the dot may be placed to indicate the location of the unpaired electron(s). 

These arguments can be extended to linear and non-linear polyatomic molecules for which zero-point structure, in terms of bond lengths and angles, is isotope-dependent but for which equilibrium structure is not. 

The molecular orbital (MO) approach to the electronic structure of diatomic, and also polyatomic, molecules is not the only one which is used but it lends itself to a fairly qualitative description, which we require here. 

As is the case for diatomic molecules, rotational fine structure of electronic spectra of polyatomic molecules is very similar, in principle, to that of their infrared vibrational spectra. For linear, symmetric rotor, spherical rotor and asymmetric rotor molecules the selection mles are the same as those discussed in Sections 6.2.4.1 to 6.2.4.4. The major difference, in practice, is that, as for diatomics, there is likely to be a much larger change of geometry, and therefore of rotational constants, from one electronic state to another than from one vibrational state to another. 

In the Lewis structure of a molecule or polyatomic ion, valence electrons ordinarily occur in pairs. There are two kinds of electron pairs. 

These examples illustrate the principle that atoms in covalently bonded species tend to have noble-gas electronic structures. This generalization is often referred to as the octet rule. Nonmetals, except for hydrogen, achieve a noble-gas structure by sharing in an octet of electrons (eight). Hydrogen atoms, in molecules or polyatomic ions, are surrounded by a duet of electrons (two). 

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