Hiickel molecular orbital studies

For this class of thiazoles most of the chemical and physicochemical studies are centered around the protomeric equilibrium and its consequences. The position of this equilibrium may be determined by spectroscopic and titrimetric methods, as seen in each section. A simple HMO (Hiickel Molecular Orbitals) treatment of 2-substituted compounds however, may, exemplify general trends. This treatment considers only protomeric forms 1 and 2 evidence for the presence of form 3 has never been found. The formation energy reported in Table 1 is the energy difference in f3 units. 

The Dotz reaction mechanism has received further support from kinetic and theoretical studies. An early kinetic investigation [37] and the observation that the reaction of the metal carbene with the alkyne is supressed in the presence of external carbon monoxide [38] indicated that the rate-determining step is a reversible decarbonylation of the original carbene complex. Additional evidence for the Dotz mechanistic hyphotesis has been provided by extended Hiickel molecular orbital [23, 24] and quantum chemical calculations [25],... 

Several older theoretical studies are included in the review by Knowles <1996CHEC-II(7)489>, and little has been reported since. Nagy et al. carried out quantum calculations on several [l,2,3]triazolo[4,5-t/ pyridazines to reveal electron distributions in the ground and the excited state using the semi-empirical Hiickel molecular orbital (MO) method to explain the fluorescence of such compounds (see Section 10.13.3.2) <2002JPH83>. 

Over the past 20 years and, in particular since 1955, many theoretical studies of the electronic structure of pyrrole using the molecular orbital approach with varying degrees of refinement have been reported. The 7r-electronic structure of pyrrole has been extensively discussed in terms of both the simple Hiickel molecular orbital (LCAO) theory37- 41-55-65 and the more sophisticated self-consistent field molecular orbital method (SCFMO method).18- 66-77 Extended... 

A study of the initially formed 94 used acetonitrile solvent.145 Rapid chemical follow-up steps were proposed. Reduction in aqueous solution was shown to proceed along similar lines.146 The energies of Hiickel molecular orbitals for alkylpyridinium ions indicated that no correlation exists between reduction potential and the quaternary nitrogen electron density.147 EPR spectra of the radicals (94) were analyzed.148... 

Studies of the tautomeric state of 1,2,4-thiadiazines have been more or less confined to the biologically active 1,2,4-benzothiadiazines. Ultraviolet spectroscopic studies suggest that the 4//-tautomer of 1,2,4-benzothiadiazines, e.g. 152, is preferred in ethanol (60JOC970) in alkali, the anion dominates. Extended Hiickel molecular orbital calculations (70MI2) and 13C-NMR studies (79T2151) confirm this view (Scheme 6). 

In this section, we first discuss the bonding in two linear triatomic molecules BeH2 with only a bonds and C02 with both a and n bonds. Then we go on to treat other polyatomic molecules with the hybridization theory. Next we discuss the derivation of a self-consistent set of covalent radii for the atoms. Finally, we study the bonding and reactivity of conjugated polyenes by applying Hiickel molecular orbital theory. 

In this section, we first study the jt bonding in conjugated polyenes by means of the Hiickel molecular orbital theory. Then we will see how the wavefunctions obtained control the course of the reaction for these molecules. 

Whereas empirical force-field calculations predict a ring A boat conformation for lanost-8-en-3-one, combined empirical force-field-extended Hiickel molecular orbital calculations favour a ring A chair conformation. Europium-shift n.m.r. results indicate that the molecule adopts the latter conformation. Studies on the peracetic acid-boron trifluoride etherate Baeyer-Villiger oxidation of 4,4-dimethyl-3-keto-triterpenoids to 5-lactones (18) and their subsequent ring contraction to y-lactones have been reported. Dehydrogenation of lanost-8-en-3j8-ol with 2,3-dichloro-5,6-dicyanobenzoquinone afforded, in addition to the corresponding 7,9(1 l)-diene, the aromatic seco-lanostane derivatives (19)... 

Graphics-based Hiickel molecular orbital calculator of energies and orbitals of TT electrons. EnzymeKinetics for fitting Michaelis—Menten kinetics parameters. ESP (Experimental Section Processor) for organizing synthetic procedures in publication format. LabSystant for evaluating quantitative lab data. Diatomic Molecular Motion and Mechanics. PC-Mendeleev for studying periodic table. SynTree for creating database of reactions. TAPP (Thermodynamic and Physical Properties) database with physical and thermodynamic data on more than 10,000 compounds. PCs and Macintosh. 

Calculations have been made of the rr-electron distribution in the pyrazine ring by many methods (127-134) and the effects of protonation on the total electron densities have been calculated using the extended Hiickel theory (135). A good correlation was obtained between total carbon electron densities and both proton and carbon-13 chemical shifts. A recent molecular orbital study has been made of protonation in pyrazine (and other diazines) (135a). 

The Hiickel molecular orbital theory for non-planar conjugated organic molecules has been applied to study the electronic structure and properties of the proposed icosahedral geometry of C ). The results support the suggestion that C o may be the first example of a spherical aromatic molecule. The molecule is calculated to have a stable closed shell singlet ground electronic state. 

In order to explain the isomerization of neopentane to isopentane on platinum films, Anderson and Avery [34) proposed a mechanism involving, as precursor, an a,a,y-triadsorbed species, and, in the transition state, a 71 complex of the Dewar type, attached to the surface by two carbon-metal bonds. By simplified Hiickel molecular orbital (MO) calculations, they showed that hyperconjugative effect and partial charge transfer to the metal could account for the relative isomerization rates of the various molecules studied (neopentane > isobutane > n-butane) (Scheme 19). 

Reports on the application of molecular orbital calculations specifically to the chemistry of lignin dates back at least as far as 1966 and the work of Lindberg and coworkers [29,30,31]. In the paper by Lindberg and Tylli [30], Hiickel molecular orbital calculations were used in studies on a range of monomeric model compounds... 

Professors Kenichi Fukui (Kyoto University) and Roald Hoffmann (Cornell University) received the 1981 Nobel Prize in Chemistry for their quantum mechanical studies of chemical reactivity. Their applied theoretical chemistry research is certainly at the core of computational chemistry by today s yardstick. Professor Fukui s name is associated with frontier electrons, which govern the transition states in reactions, while that of Hoffmann is often hyphenated to R. B. Woodward s name in regard to their orbital symmetry rules. In addition, Professor Hoffmann s name is strongly identified with the extended Hiickel molecular orbital method. Not only was he a pioneer in the development of the method, he has continued to use it in almost all of his over 300 papers. 

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