Dewar model

The same qualitative model describes the binding of H2 to transition-metal centers in which the extent of metal to H2 antibonding orbital (back) donation describes the continuous variation of H-H distance from that of coordinated H2 to that of two individual hydride ligands (Figure A1.20). Although the Dewar model was there in the literature, it took the more recent experimental work of Kubas and those who followed him to demonstrate the realities of these so-called cr complexes of H2. 

Theory and Spectra.—Theory. H.M.O. calculations, using the theoretical Fukui and Dewar models of cyclophosphazenes, show good agreement with each other... 

The orbital interaction leading to the fission of the M-R bond and the formation of the new bonds is the typical synergetic interaction of transition metals with n-acceptor ligands. The a(M-R) and 7t(alkene) orbitals, which are occupied, interact in a four-centered transition state with the a (M-R) and 7t (alkene) orbitals, which are vacant. This interaction produces a mixing of the rr(alkene) and 7t (alkene) orbitals that polarizes the alkene Compared to the simple alkene coordination (Fig. 6.1, left), where the donor and acceptor alkene orbitals interact only with the metal orbitals (this is the classical Chatt-Dewar model), the interaction with o(M-R) and o (M-R) leading to insertion polarizes the donor alkene orbital towards C ... 

The orbital interaction scheme in which the attractive contribution of the adsorbate surface bond is estimated from the donative and respective back-donative interactions is called the Blyholder model. It is the analogous to the Chatt-Dewar model which is used to describe chemical bonds in organometallic complexes. 

The camera is equipped with the OCLl 10% bandwith filters at 8.8,9.8,10.3, r and 12.5 m, and an 8-14 ftm CVF with a resolution of 3%. Filters, op-, baffles, heater, temperature sensor, and array are mounted on a copper e plate rigidly connected to a liquid-He dewar model HD-3(8) manuiac->d by Infrared Laboratories Inc.(Tucson,USA). 

The fact that the ratios of rates were much greater in chlorination than in nitration, prompted Dewar to suggest that the actual transition state was intermediate between the Wheland model and the isolated molecule model. He accommodated this variation in the relative rates within his discussion by treating yS as a variable whose value depended on the nature of the reaction. With the notation that y ) is the... 

Dewar s treatment of transition state structure, using reactivity numbers, has the logical defect that in the intermediate kinds of transition states for which it provides evidence the electron localisation is only partial. However, in obtaining the values of the reactivity numbers (which are approximate localization energies), the process of localization is considered to be complete thus, values of parameters which strictly are relevant only to the Wheland type of transition state are incorporated into a different model. ... 

I am conscious that I have missed many sets of acronyms from my guided tour of the differential overlap models, and I will just tell you that MINDO, MINDO/1, MINDO/2 all appeared but have now been consigned to oblivion. With MINDO/3, Dewar thought that he had at last developed a reliable model for use by organic chemists. The abstract to the landmark MINDO/3 paper is terse ... 

AMI a New General Purpose Quantum Mechanical Molecular Model Michael J. S. Dewar, Eve G. Zoebisch, Eamonn E. Healey and James... 

After some experience with MNDO, it became clear that there were certain systematic en ors. For example the repulsion between two atoms which are 2-3 A apart is too high. This has as a consequence that activation energies in general are too large. The source was traced to too repulsive an interaction in the core-core potential. To remedy this, the core-core function was modified by adding Gaussian functions, and the whole model was reparameterized. The result was called Austin Model 1 (AMl) in honour of Dewar s move to the University of Austin. The core-core repulsion of AMI has the form ... 

Features (2) and (3) are explicable in terms of the Dewar-Chatt-Duncanson model for bonding in alkene complexes (Figure 3.63), which involves... 

The chemistry of the disilenes (disilaethenes) has developed very rapidly since the discovery of stable compounds. It was an obvious challenge to explore also the possibility of a n-coordination of disilenes to transition metals. According to the Dewar-Chatt-Duncason bonding model, a high stability for a disilene complex should result. 

In low oxidation states, transition metals possess filled or partly filled d shells. The Dewar-Chatt-Duncanson model envisages some of that electron density in (local) d (e.g. d., d y) orbitals being donated into the empty n orbitals of the carbon monoxide ... 

In the approach of Dewar and co-workers (34), termed the half-electron method , a physical model is considered in which an unpaired electron is replaced by two hypothetical half-electrons of opposite spin. For radicals containing one unpaired electron, the eigenvalue problem of this method is, in our opinion, identical with the method of Longuet-Higgins and Pople (27) ... 

Dewar MJS et al. (1985) Development and use of quantum mechanical molecular models. 76. AMI a new general purpose quantum mechanical molecular model. J Am Chem Soc 107(13) 3902-3909... 

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