Molecular Structure and Orbitals

If we consider methane (CH4), we find that not only does the central carbon atom possess four hydrogen substituents, these four hydrogens are equally spaced in a tetrahedral 

---

Molecular structures and orbitals can be classified into various point groups according to the symmetry elements they contain. Symmetry elements are lines, planes or points with respect to which various symmetry operations such as rotation can be performed without changing the structure. The symmetry operations that concern us here are as follows ... 

The simplest molecular orbital method to use, and the one involving the most drastic approximations and assumptions, is the Huckel method. One str ength of the Huckel method is that it provides a semiquantitative theoretical treatment of ground-state energies, bond orders, electron densities, and free valences that appeals to the pictorial sense of molecular structure and reactive affinity that most chemists use in their everyday work. Although one rarely sees Huckel calculations in the resear ch literature anymore, they introduce the reader to many of the concepts and much of the nomenclature used in more rigorous molecular orbital calculations. 

Hartree-Fock (HF), molecular orbital theory satisfies most of the criteria, but qualitative failures and quantitative discrepancies with experiment often render it useless. Methods that systematically account for electron correlation, employed in pursuit of more accurate predictions, often lack a consistent, interpretive apparatus. Among these methods, electron propagator theory [1] is distinguished by its retention of many conceptual advantages that facilitate interpretation of molecular structure and spectra [2, 3, 4, 5, 6, 7, 8, 9]. 

Figure 1 Molecular structures and singly occupied molecular orbitals for radicals 1-7...

L. Salem, The Molecular Orbital Theory of Conjugated Systems, W. A. Benjamin, New York, 1966 B. M. Gimarc, Molecular Structure and Bonding The Qualitative Molecular Orbital Approach, Academic Press, New York, 1979 T. A. Albright, J. K. Burdet and M. H. Whangbo, Orbital Interactions in Chemistry, Wiley, New York, 1985. 

Quantum mechanics provide many approaches to the description of molecular structure, namely valence bond (VB) theory (8-10), molecular orbital (MO) theory (11,12), and density functional theory (DFT) (13). The former two theories were developed at about the same time, but diverged as competing methods for describing the electronic structure of chemical systems (14). The MO-based methods of calculation have enjoyed great popularity, mainly due to the availability of efficient computer codes. Together with geometry optimization routines for minima and transition states, the MO methods (DFT included) have become prevalent in applications to molecular structure and reactivity. 

Figure 7.4 (a) The molecular structure and the numbering of the carbon atoms for bicyclooctatriene. ( >) A sketch showing the orientation of the pn orbitals (frs) used in the MO treatment. 

In this section, solvent effect on the symmetry and the absorption spectrum of CV was examined using molecular orbital (MO) calculation to find what molecular structure and absorption spectrum are possible for CV when solvent molecules interact with CV molecules. For simplicity only one methanol molecule was considered in the current calculation. 

B. M. Gimarc, Molecular Structure and Bonding The Quantitative Molecular Orbital Approach, Academic Press, New York, 1979. 

Molecular structure and shape are related to orbital angular momentum and chemical change is shown to be dictated by the quantum potential. The empirical parameters used in computer simulations such as molecular mechanics and dynamics are shown to derive in a fundamental way from the relationship between covalence and the golden ratio. 

---