Geometries of Molecules

Electronic structure theory describes the motions of the electrons and produces energy surfaces and wavefiinctions. The shapes and geometries of molecules, their electronic, vibrational and rotational energy levels, as well as the interactions of these states with electromagnetic fields lie within the realm of quantum stnicture theory. 

ZINDO/1 and ZINDO/S are Dr. Michael Zerner s INDO versions and used for molecular systems with transition metals. ZINDO/1 is expected to give geometries of molecules, and ZINDO/S is parametrized to give UV spectra. 

You can use the information obtained from semi-empirical calculations to investigate many thermodynamic and kinetic aspects of chemical processes. Energies and geometries of molecules have clear relation ships to chemical ph en om ena. 0ther quan tities, like atomic charges and Frontier Orbitals, are less defined but provide useful qualitative results. 

G. Herzberg (Ottawa) contributions to the knowledge of electronic structure and geometry of molecules, particularly free radicals. 

The molecular mechanics calculations discussed so far have been concerned with predictions of the possible equilibrium geometries of molecules in vacuo and at OK. Because of the classical treatment, there is no zero-point energy (which is a pure quantum-mechanical effect), and so the molecules are completely at rest at 0 K. There are therefore two problems that I have carefully avoided. First of all, I have not treated dynamical processes. Neither have I mentioned the effect of temperature, and for that matter, how do molecules know the temperature Secondly, very few scientists are interested in isolated molecules in the gas phase. Chemical reactions usually take place in solution and so we should ask how to tackle the solvent. We will pick up these problems in future chapters. 

Geometries of molecules such as these can be predicted by the VSEPR model The results are shown in Figure 7.8 (page 181). The structures listed include those of all types of molecules having five or six electron pairs around the central atom, one or more of which may be unshared. Note that—... 

C09-0098. Draw Lewis structures and ball-and-stick structures showing the geometries of molecules of the... 

A comprehensive discussion of the geometry of molecules with a central sulfur atom in which considerable use is made of the VSEPR model. 

Chapters 8 and 9 are devoted to a discussion of applications of the VSEPR and LCP models, the analysis of electron density distributions to the understanding of the bonding and geometry of molecules of the main group elements, and on the relationship of these models and theories to orbital models. Chapter 8 deals with molecules of the elements of period 2 and Chapter 9 with the molecules of the main group elements of period 3 and beyond. 

Determining the Geometries of Molecules and Ions in Excited States by Using Resonance Raman Spectroscopy... 

Geometry of molecules and ions, structural isomerism of simple organic molecules relation of properties to structure... 

The VSEPR theory is only one way in which the molecular geometry of molecules may be determined. Another way involves the valence bond theory. The valence bond theory describes covalent bonding as the mixing of atomic orbitals to form a new kind of orbital, a hybrid orbital. Hybrid orbitals are atomic orbitals formed as a result of mixing the atomic orbitals of the atoms involved in the covalent bond. The number of hybrid orbitals formed is the same as the number of atomic orbitals mixed, and the type of hybrid orbital formed depends on the types of atomic orbital mixed. Figure 11.7 shows the hybrid orbitals resulting from the mixing of s, p, and d orbitals. 

During the last decade MO-theory became by far the most well developed quantum mechanical method for numerical calculations on molecules. Small molecules, mainly diatomics, or highly symmetric structures were treated most accurately. Now applicability and limitations of the independent particle, or Hartree-Fock (H. F.), approximation in calculations of molecular properties are well understood. An impressive number of molecular calculations including electron correlation is available today. Around the equilibrium geometries of molecules, electron-pair theories were found to be the most economical for actual calculations of correlation effects ). Unfortunately, accurate calculations as mentioned above are beyond the present computational possibilities for larger molecular structures. Therefore approximations have to be introduced in the investigation of problems of chemical interest. Consequently the reliability of calculated results has to be checked carefully for every kind of application. Three types of approximations are of interest in connection with this article. 

The ability of the MM method to calculate accurately potential energies and geometries of molecules is best suited for quantitative conformational analysis (44). Numerical values can readily be given to such terms as strain and stability (45). The topic is extensively covered in Allinger s review (lOd), therefore only some recent developments are mentioned. 

A number of methods have been proposed for calculations of the geometries of molecules in excited states. These include CIS (Configuration Interaction Singles) and variations on CIS to account for the effect of double substitutions, as well as so-called time dependent density functional models. Except for CIS (the simplest of the methods) there is very little practical experience. There is also very little solid experimental data on the geometries of excited-state molecules. 

The difference in repulsive forces between electron pairs means that when lone pairs are present the geometries change. Let s examine two common substances to see how the presence of lone pairs affects the geometries of molecules. Ammonia, NH3, contains three bonding pairs and one lone pair surrounding the nitrogen atom ... 

VSEPR Model valence shell electron pair repulsion model, model used to predict the geometry of molecule based on distribution of shared and unshared electron pairs distributed around central atom of a molecule... 

Relation between electronic absorption spectra and geometry of molecules H. Suzuki, Electronic Absorption Spectra and Geometry of Organic Molecules. Academic Press, New York, 1967. 

K.K. Innes, Geometries of molecules in excited electronic states , Reference Y, 2, Ch. 1. 

Femtosecond laser excitation makes it possible to produce in a synchronous manner accurate to within a few femtoseconds an ensemble of molecules in an excited state and observe thereafter the evolution of this ensemble in the subsequent processes of decay, relaxation, and so on, by means of other femtosecond pulses. Another femtosecond pulse is usually used as a probe pulse [1]. However, one can directly observe changes in the geometry of molecules, specifically in molecular vibrations, by the method of electron diffraction using ultrashort electron pulses. This was successfully demonstrated in Ref. 2. Whereas the production of synchronous probe laser pulses is a standard technique, the situation with femtosecond electron pulses is more complicated. I would like to call attention to the possibility of using intense femtosecond laser pulses to control electron beams, specifically to obtain femtosecond electron pulses and to focus and reflect them, and so on [3, 4]. 

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