Triatomic molecules analysis

In this section, we briefly discuss spectroscopic consequences of the R-T coupling in triatomic molecules. We shall restrict ourselves to an analysis of the vibronic and spin-orbit structure, determined by the bending vibrational quantum number o (in the usual spectroscopic notation id) and the vibronic... 

More complicated behaviors are expected for triatomic molecules (i.e., for three-body problems). In general, the analysis is facilitated by the fact that... 

The photochemical processes of triatomic molecules have been extensively studied in recent years, particularly those of water, carbon dioxide, nitrous oxide, nitrogen dioxide, ozone, and sulfur dioxide, as they are important minor constituents of the earth s atmosphere. (Probably more than 200 papers on ozone photolysis alone have been published in the last decade.) Carbon dioxide is the major component of the Mars and Venus atmospheres. The primary photofragments produced and their subsequent reactions are well understood for the above-mentioned six triatomic molecules as the photodissociation involves only two bonds to be ruptured and two fragments formed in various electronic states. The photochemical processes of these six molecules are discussed in detail in the following sections. They illustrate how the knowledge of primary products and their subsequent reactions have aided in interpreting the results obtained by the traditional end product analysis and quantum yield measurements. 

A detailed analysis of the rotational degrees of freedom and of bending motion has been carried out by Freed and Band (2) and by Morse and Freed (52-54). The former authors considered the photodissociation of a linear triatomic molecule. In that work the rotational part of the initial wavefunction is written in the form... 

This expression has been analyzed for the dissociation of ICN. The initial thermal distribution corresponds to large j. A distribution peaked around j 25 was obtained, in good agreement with experimental data. The analysis has been generalized to describe the case of a bent triatomic molecule (53,5A). Moreover, these authors consider the scalar coupling which corresponds to indirect photodissociation. 

This implies that three electrons are in the two pn orbitals with lobes perpendicular to the internuclear O-H axis which serves as the 2-axis of the body-fixed frame of the diatom (not to be confused with the body-fixed system of the triatomic molecule). Following Andresen et al. (1984) we assume that two of the pit electrons are paired and one is unpaired, the latter determining the open-shell character of the OH radical. For a more refined analysis see Alexander and Dagdigian (1984). 

For a non-linear triatomic molecule three parameters are needed to specify (the assumed) fixed geometry, but only two moments of inertia can be measured. The situation becomes rapidly worse for larger molecules. Isotopic substitution may be used to produce more data, but the situation where sufficient data are available for a unique solution is rare. In the final analysis measured spectra may well be consistent with an assumed molecular conformation, without excluding many other classical or non-classical possibilities. 

The purpose of this chapter is to review some properties of isomerizing (ABC BCA) and dissociating (ABC AB + C) prototype triatomic molecules, which are revealed by the analysis of their dynamics on precise ab initio potential energy surfaces (PESs). The systems investigated will be considered from all possible viewpoints—quanmm, classical, and semiclassical mechanics—and several techniques will be applied to extract information from the PES, such as Canonical Perturbation Theory, adiabatic separation of motions, and Periodic Orbit Theory. 

Kong FA (1989) An Alternative Periodic Table for Triatomic Molecules. In Hefferlin R Periodic Systems and their Relation to the Systematic Analysis of Molecular Data. Edwin Mellen, Lewiston, New York Kong FA (1993) unpublished communication... 

Most of the ( e and XeCOg values listed in Tables 2.1 a and 2.1b were obtained mainly by analysis of rotational-vibrational fine structures of infrared or Raman spectra in the gaseous phase [1]. Their values can also be obtained by the analysis of overtone progression in resonance Raman spectra (Sec. 1.22). Vibrational spectra of unstable (or metastable) molecules were obtained in inert gas matrices (Sec. 1.25). Anharmo-nicity corrections are limited largely to diatomic molecules because nine parameters are required even for nonlinear triatomic molecules. 

Note added in proof a theoretical analysis of the polarization of fluorescence from the diatomic fragments formed in the photodissociation of triatomic molecules has been developed by Macpherson, Simons and Zare ). 

An analysis of the stretching vibrational motion of a symmetrical linear triatomic molecule (Figure 4) reveals that in terms of mass-weighted Cartesian coordinates qi, qi, and q the normal vibrations (excluding a zero-frequency translational solution) are given by... 

The relative positions of atoms in a molecule are not fixed but instead fluctuate continuously as a consequence of a multitude of different types of vibrations and rotations about the bonds in the molecule. For a simple diiitomic or triatomic molecule, it is easy to define the number and nature of such vibrations and relate these to energies of absorption. An antily-sis of this kind becomes difficult if not impossible for molecules made up of many atoms. Not only do large molecules have a large number of vibrating centers, but also interactions among several centers can occur and must be taken into account for a complete analysis. 

Xe—F bonding character and therefore the bonding in Xep2 can be described in terms of a 3c-2e interaction. Three-centre two-electron interactions are not restricted to triatomic molecules, as we illustrate in the next section with a bonding analysis of B2Hg. 

In a polyatomic molecule, the normal modes depend on the relative masses and bond strengths. Although the analysis for large molecules is an ideal problem to leave to a computer, we will work through one simple example here the stretching modes of a Dcoh triatomic molecule such as CO2, labeling the atoms... 

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