Dewar properties

The most notable studies are those of Ingold, on the orienting and activating properties of substituents in the benzene nucleus, and of Dewar on the reactivities of an extensive series of polynuclear aromatic and related compounds ( 5.3.2). The former work was seminal in the foundation of the qualitative electronic theory of the relationship between structure and reactivity, and the latter is the most celebrated example of the more quantitative approaches to the same relationship ( 7.2.3). Both of the series of investigations employed the competitive method, and were not concerned with the kinetics of reaction. 

The one-center two-electron integrals in the MNDO method are derived from experimental data on isolated atoms. Most were obtained from Oleari s work [L. Oleari, L. DiSipio, and G. DeMich-ells. Mol. Phys., 10, 97(1977)], but a few were obtained by Dewar using fits to molecular properties. 

The lobes of electron density outside the C-O vector thus offer cr-donor lone-pair character. Surprisingly, carbon monoxide does not form particularly stable complexes with BF3 or with main group metals such as potassium or magnesium. Yet transition-metal complexes with carbon monoxide are known by the thousand. In all cases, the CO ligands are bound to the metal through the carbon atom and the complexes are called carbonyls. Furthermore, the metals occur most usually in low formal oxidation states. Dewar, Chatt and Duncanson have described a bonding scheme for the metal - CO interaction that successfully accounts for the formation and properties of these transition-metal carbonyls. 

In his first published paper on the structure of benzene in 1922, Ingold stated his aim to unify aliphatic and aromatic chemistry by studies of ring formation of unsaturated systems. He argued that chemists must accept the Dewar formulation, using a bridged phase of benzene, in order to relate the properties of aromatic compounds to the facts of aliphatic chemistry. 31... 

Semiempirical methods, on the other hand, utilize minimum basis sets to speed up computations, and the loss in rigor is compensated by the use of experimental data to reproduce important chemical properties, such as the heats of formation, molecular geometries, dipole moments, and ionization potentials (Dewar, 1976 Stewart, 1989a). As a result of their computational simplicity and their chemically useful accuracy, semiempirical methods are widely used, especially when large molecules are involved (see, for example, Stewart, 1989b Dewar et al., 1985 Dewar, 1975). 

Acosta-Vigil A., London D., Morgan G.B. VI, and Dewars T.A. (2006) Dissolution of quartz, albite, and orthoclase in H20-saturated haplogranitic melt at 800°C and 200 MPa diffusive transport properties of granitic melts at crustal anatectic conditions. /. Petrol. 47, 231-254. 

Finally, if we abandon Hiickel s topological approach altogether and consider more elaborate quantum-mechanical approaches, the concept of aromaticity derived purely from a consideration of -electrons becomes blurred and tends to disappear completely. In fact, allelectron methods allow the calculation of aromatic properties (Section V,B) of a given substance without introducing explicitly the concept of aromaticity. Certain authors, notably Dewar,19 have published resonance energies derived from self-consistent field molecular-orbital (SCF-MO) calculations, and these could be used as a measure of aromaticity. 

Chemical and physical properties of metal alkene complexes have been rationalized on the basis of the Dewar-Chatt-Duncanson bonding model167,1 8 and a number of semiempirical and ab initio MO calculations have been performed for a more quantitative description of the bonding.16 "177... 

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