Huckel molecular orbital analysis, extended

Huckel realized that his molecular orbital analysis of conjugated systems could be extended beyond neutral hydrocarbons He pointed out that cycloheptatrienyl cation also called tropyhum ion contained a completely conjugated closed shell six tt electron sys tern analogous to that of benzene... 

Recent evidence favors Dg symmetry and a pentagonal bipyramidal structure for the heptafluoride. Claassen et al. (2) review the earlier debate about structure of lower symmetry. They provide convincing evidence of D symmetry from the first observation of Raman spectra of the vapor state and re-examination of the infrared spectra. Their data—including five fundamentals in Raman (two polarized), five fundamentals in infrared, no coincidences between Raman and infrared, and one fundamental inactive—are consistent only with symmetry. This is confirmed by Falconer et al. (4) who interpret their electric-deflection experiments as indicating a symmetry-forbidden dipole moment. Extended Huckel-molecular-orbital calculations (5) also predict Dg symmetry. The adopted structural parameters are from our approximate analysis of the electron-diffraction data of Thompson and Bartell (6). The authors gave a radial distribution curve and suggested only a gross (unrefined) structure because of the probable presence of... 

Huckel realized that his molecular orbital analysis of conjugated systems could be extended beyond the realm of neutral hydrocarbons. He pointed out that cycloheptatrienyl cation contained a tt system with a closed-shell electron configuration similar to that of benzene (Figure 11.13). Cycloheptatrienyl cation has a set of seven tt molecular orbitals. Three of these are bonding and contain the six tt electrons of the cation. These six tt electrons are delocalized over seven carbon atoms, each of which contributes one 2p orbital to a planar, monocyclic, completely conjugated tt system. Therefore, cycloheptatrienyl cation should be aromatic. It should be appreciably more stable than expected on the basis of any Lewis structure written for it. 

Qualitative ideas of catalytic H2 activation on metals were devised in the late 1950s in connection with the formation of transition states or intermediates prior to OA of H2 to hydride complexes (see Chapter 2). Surprisingly, there was no molecular orbital analysis of this intriguing theoretical problem until Dedieu carried one out in 1979.17 Both extended Huckel and ab initio Hartree-Fock calculations were carried out on H2 addition to square-planar d8 RhCl(PH3)3, a model for the well-known Wilkinson catalyst in which the phosphine is PPh3. In this 16e complex, the H2 approaches the filled dzi metal orbital. Calculations indicate that at the beginning of the reaction, end-on (i/1) approach of H-H is preferred over side-on (tf2) approach (Eq. 4.1). 

A number of studies have compared the use of the multiple regression technique using semiempirical parameters such as tt and o-, and parameters calculated for the particular molecules from molecular orbital theory. Hermann, Culp, McMahon, and Marsh (23) studied the relationship between the maximum velocity of acetophenone substrates for a rabbit kidney reductase. These workers were interested in the reaction mechanism, and two types of quantum chemical calculations were made (1) extended Huckel treatment, and (2) complete neglect of differential overlap (CNDO/2). Hydride interaction energy and approaching transition-state energies were calculated from the CNDO/2 treatment. All these parameters plus ir and a values were then subjected to regression analysis. The best results are presented in Table II. 

Tel. 358-0-4572378, fax 358-0-4572302, e-mail leif.laaksonen csc.fi Analysis of molecular simulation trajectories from CHARMM, Discover, YASP, MUMOD, GROMOS, and AMBER. Interface to ICON8 for extended Huckel calculations and to VSS for electrostatic potentials. 2D graphics of surfaces and electron density and orbitals. Silicon Graphics. 

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