Dewars

Dewar benzene is a valence isomer of benzene, to which it reverts on heating. 

These methods try to bracket the transition state from both the reactant and the product side [72, 73]. For example, in the method of Dewar etal [73], two stmctiires, one in the reactant valley and one hi the product valley, are optimized simultaneously. The lower-energy stmcture is moved to reduce the distance separating the two stmctures by a small amount, e.g. by 10%, and its stmcture is reoptimized under the constraint that the distance is fixed. This process is repeated until the distance between the two stmctures is sufficiently small. 

Dewar M J S, Mealy E F and Stewart J J P 1984 Location of transition states in reaction mechanisms J. Chem. Soc. Faraday Trans. II80 227... 

M. J. S. Dewar, The Molecular Orbited Theory of Organic Chemistry, McGraw-Hill, New York, 1969,... 

MINDO/3, MNDO, and AM 1 wxrc developed by the Dervar group at the University of i exasat Austin. This group ehose many parameters, such as heats of formation and geometries of sample molecules, to reproduce experimental quantities. The Dewar methods yield results that are closer to experiment than the CN DO and IN DO methods. 

Molecular mechanics force fields have much information built into them and can be accurate for the molecules used in their param eten/ation. For molecules outside the limited scope for 40. Dewar. J. S. Dicier, K. M../. Am. Chem. Soc. 108 807. ), 1086. 

MIXDO/3 is the earliest of the Dewar methods. It provides more accurate geometries and heats of formation than CNDO or INDO. and has been used widely. The limitations of the INDO approximation, on which MI lhO/3 is based, frequently lead to problems of accuracy wdi cri dealing w i th m olecules con tain ing h eteroatorn s. 

Hecaiise the repulsion interaction energy of two point charges is inversely proportional to the distance separating the two charges, Dewar and co-workers, for example, represent the (ssiss) two-ceri-ter two-electron integral by ... 

L sirif Lhc above asymptotic forms of the two-ceiiLer Lwo-dccLron integrals, the paramelers A. and can he derived. C. ertainly, parameter A. is different for different orbitals even though they reside on the same atom, Dewar used AM to represent the parameter A obtained via AD to represent the parameter A obtained via and AQ to represent the parameter A obtained from Hpp-... 

Bingiiam R C, M J S Dewar and D H Lo 1975a. Ground States of Molecules. XXV. MINDO/3. An improved Version of the MINDO Semi-empirical SCFMO Method. Journal of the American Chemical Society 97 1285-1293. 

Bingham R C, M J S Dewar and D H Lo 1975b. Ground States of Molecules. XXVI. MINDO/3. 

T orbital for benzene obtained from spin-coupled valence bond theory. (Figure redrawn from Gerratt ], D L oer, P B Karadakov and M Raimondi 1997. Modem valence bond theory. Chemical Society Reviews 87 100.) figure also shows the two Kekule and three Dewar benzene forms which contribute to the overall wavefunction Kekuleform contributes approximately 40.5% and each Dewar form approximately 6.4%. 

Semiempirical molecular orbital calculations have gone through many stages of refinement and elaboration since Pople s 1965 papers on CNDO. Programs like PM3, which is widely used in contemporary research, are the cumulative achievement of numerous authors including Michael Dewar (1977), Walter Thiel (1998), James Stewart (1990), and their coworkers. 

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