Molecular orbital calculations CNDO methods

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 latter three, (4)—(6), are utilized for qualitative elucidation of the observed ICD bands in signs, and the former three, (1)—(3), are successfully applicable for quantitative calculations of the observed ICD bands in both signs and magnitudes. The ICD of P-cyclodextrin complexes with benzene derivatives or azanaphthalenes162) has been analyzed by a molecular orbital calculation, using an approximation of PPP-type, which has been compared with the theoretical spectra calculated by using the CNDO/S-CI method on the basis of the MCD spectra 16S). 

True Hiickel molecular orbital calculations, among other limitations, only address TT-electrons. All atom, semiempirical calculations that include both n- and a-electrons were developed originally by Pople and coworkers [17] implementing the method of complete neglect of differential overlap (CNDO), followed by the increasingly rigorous method of intermediate neglect of differential overlap (INDO) [18]. 

Another example of the application of molecular orbital theory to drug design has been given by Andrews. Molecular orbital calculations by the EHT and CNDO/2 methods were made on a number of anticonvulsant drugs and related compounds. 

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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