Molecular-orbital nomenclature

TABLE 13.1 Molecular-Orbital Nomenclature for Homonuclear Diatomic Molecules ... 

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. 

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

Organic Polymers, Natural and Synthetic 610 Appendix 1 Units, Constants, and Reference Data 635 Appendix 2 Properties of the Elements 641 Appendix 3 Exponents and Logarithms 643 Appendix 4 Nomenclature of Complex Ions 648 Appendix 5 Molecular Orbitals 650... 

In order to understand this process, the nature of the Fermi level must be considered. Within the nomenclature of physics, the Fermi level is defined as the energy at which there is a 50% probability of finding an electron in a metal— what a chemist would call the highest filled molecular orbital (i.e., the top of the valence band). In an intrinsic semiconductor, a material having a completely filled valence band and a completely empty conduction band, this energy level... 

Cycloadditions that involve two unsymmetric reactants can lead to regioisomers. The regioselectivity of these adducts can be predicted with a high degree of success through the use of frontier molecular orbital theory.22 25 The ortho product (this nomenclature follows the analogy of disubstituted aromatic systems) is usually the preferred isomer from 1-substituted dienes, whereas 2-substituted dienes provide the para isomer as the major adduct. However, when a Lewis acid is used as a catalyst in the reaction, the ratio of these isomers can alter dramatically and, occasionally, can be reversed.22... 

Molecular orbitals 02 and (73 become degenerate. According to the nomenclature of group theory, they are named c functions (doubly degenerate) whereas the non-degenerate 7i m.o. is classified as o . 

---