Molecular orbital methods CNDO method

The molecular orbital methods which have been employed for such studies include extended Hiickel theory (EHT), CNDO, and ab initio LCAO-SCF. 

Lewis, D.F.V., Ioannides, C., and Parke, D.V., Prediction of chemical carcinogenicity from molecular structure a comparison of MINDO/3 and CNDO/2 molecular orbital methods, Toxicol. Lett., 45, 1-13, 1989. 

Besides these theories, which are applicable to general systems, some other theories of less general applicability have been proposed. These are outlined separately below. A group of empirical methods which has been omitted from the present review are the semi-empirical molecular orbital methods known by acronyms such as MINDO, INDO and CNDO. The reader is referred to a book by Murrell and Harget259 for a description of these methods and to articles by Chutjian and Segal260 and by MacGregor and Berry261 for examples of their use. 

All valence electron methods In contrast to ab initio methods, the semi-empirical molecular orbital methods only consider the valence electrons for the construction of the atomic orbitals. Well-known semi-empirical methods are EHT, CNDO, MNDO, PCILO, AMI, and PM3. These methods are orders of magnitude faster than ab initio calculations. 

Both 1,5-dithionane (27) and 1,4,7-trithionane (28) have been studied by extended Huckel, MINDO/3 and SCF-CNDO/2 molecular orbital methods, as well as by molecular mechanics... 

EHT, extended Huckel theory CNDO, complete neglect of differential overlap INDO, intermediate neglect of differential overlap NNDO, neglect of diatomic differential overlap SCF, self-consistent field MINDO, modified INDO ab initio, without the use of independently derived parameters. For an independent assessment of the different molecular orbital methods applied to carbocations, see Ref. 3. 

Consequently eqs. 5 and 6 are frequently employed to calculate nuclear shielding by semi-empirical methods usually in conjunction with eqs. 7 to 15. These may be evaluated by any serai-empirical molecular orbital method such as INDO. CNDO, MINOO, etc. Good agreement with experimental 0, and... 

HyperChem currently supports one first-principle method ab initio theory), one independent-electron method (extended Hiickel theory), and eight semi-empirical SCFmethods (CNDO, INDO, MINDO/3, MNDO, AMI, PM3, ZINDO/1, and ZINDO/S). This section gives sufficient details on each method to serve as an introduction to approximate molecular orbital calculations. For further details, the original papers on each method should be consulted, as well as other research literature. References appear in the following sections. 

The PPP-MO method has proved extremely successful for the prediction of a wide range of colour properties, and it is currently the most extensively used method for this purpose. It does have some deficiencies. For example, the method carries out its calculations based on rc-electrons only and therefore cannot, except in a rather empirical way, account for some of the subtle effects of a-electrons on colour. Among such effects commonly encountered are hydrogen bonding and steric hindrance. As more and more powerful computing facilities become accessible, there is clear evidence that colour chemists are turning their attention towards the use for colour prediction of more sophisticated molecular orbital techniques which take into account all valence electrons, such as the CNDO and ZINDO approaches, and in due course they may well prove to be the methods of choice. However, at the present time, it has not been established with absolute certainty that these methods will routinely provide superior colour prediction properties. 

Various theoretical methods (self-consistent field molecular orbital (SCF-MO) modified neglect of diatomic overlap (MNDO), complete neglect of differential overlap (CNDO/2), intermediate neglect of differential overlap/screened approximation (INDO/S), and STO-3G ab initio) have been used to calculate the electron distribution, structural parameters, dipole moments, ionization potentials, and data relating to ultraviolet (UV), nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), photoelectron (PE), and microwave spectra of 1,3,4-oxadiazole and its derivatives <1984CHEC(6)427, 1996CHEC-II(4)268>. 

The theoretical interpretation of the results was made (334) in terms of the molecular orbital perturbation theory, in particular, of the FMO theory (CNDO-2 method), using the model of the concerted formation of both new bonds through the cyclic transition state. In this study, the authors provided an explanation for the regioselectivity of the process and obtained a series of comparative reactivities of dipolarophiles (methyl acrylate > styrene), which is in agreement with the experimental data. However, in spite of similar tendencies, the experimental series of comparative reactivities of nitronates (249) toward methyl acrylate (250a) and styrene (250b) are not consistent with the calculated series (see Chart 3.17). This is attributed to the fact that calculation methods are insufficiently correct and the... 

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