CNDO/S method

The CNDO and CNDO/S methods apply the ZDO approximation to all integrals, regardless of whether the orbitals are loeated on the same atom or not. In the INDO method, whieh was designed to improve the treatment of spin densities at nuelear eenters and to handle singlet-triplet energy differenees for open-shell speeies, exehange integrals... 

In the CNDO/2 approach as originally implemented, ab initio (orbital-level) calculated values of the energies mentioned below are used in determining the requisite parameters. In the later CNDO/S method, experimental values of these energies are employed. Briefly, in any CNDO method ... 

Practically all CNDO calculations are actually performed using the CNDO/ 2 method, which is an improved parameterization over the original CNDO/1 method. There is a CNDO/S method that is parameterized to reproduce electronic spectra. The CNDO/S method does yield improved prediction of excitation energies, but at the expense of the poorer prediction of molecular geometry. There have also been extensions of the CNDO/2 method to include elements with occupied d orbitals. These techniques have not seen widespread use due to the limited accuracy of results. 

Table 1. Dipole moments, charge transfers, and molecular hyperpolarizabiUties for azobenzene chromophores, calculated using CNDO/S method ...

Larsson and co-workers have used relation (18) to calculate Tjb for organic molecules in which two centers are bridged by saturated groups [65,66], and for mixed valence systems [67]. The stationary states /i and /2 are determined by a CNDO/S method, with extensive configuration interaction and use of semi-empirical parameters. The nuclear configuration Q where relation (18) is valid is adjusted so as to satisfy the delocalization property expressed by (17). These... 

Ellis, Squire, and Jaffe have carried out a detailed study of the spin-orbit coupling in formaldehyde and azulene. The wave functions were calculated using the CNDO/S method, which is more general than the PPP method discussed earlier and also allows ff-orbitals to be included. Let us review their findings on formaldehyde ... 

Racemic l,ll-dimethyl-5,7-dihydrodibenzo[c,< ]thiepine A-oxide 24 and A,A-dioxide 25 and 10,12-dihydro-4/7,6/7-[2]benzothiepino[6,5,4- i/][2]benzothiepine 30, based on bridged biphenyl structures, were resolved into enantiomers by chromatography on swollen microcrystalline TAC <2000HCA479>. The first-eluted enantiomers were assigned to have the (A)-configuration by comparison of the experimental CD spectra with spectra calculated by the CNDO/S method (see also Sections 13.03.3.1 and 13.03.3.3). 

CNDO calculations show that the MO energies are not affected by the inclusion of the d orbitals <1983JST(105)375> and a modified version of the CNDO/S method show there to be only one cr-level which is strongly localized on the sulfur atom and is thus an n-type <1983ACH97>. Calculations were obtained indicating that the d orbitals on sulfur do not affect the orbital energies to any significant extent as they do some of the other molecular properties. 

The CNDO/S method ( )el Bene and Jaff6, I%8) has also been used successfully to calculate excitation energies. Determinations of transi-... 

The optimum geometry of tri-t-butylphosphine has been determined by CNDO/2 calculations,234 and the CNDO/S method has been extended to include second-row elements.235 The results for phosphorin (15) satisfactorily explain the observed u.v. transitions, dipole moment, and ionization potentials. An X-ray structure of 1-benzylphosphole (16) points to the presence of a non-planar ring with a mean P—C(ring) distance of 1.783 A 236 the shortening of the latter over the sum of the single-bond radii is consistent with some delocalization of the phosphorus lone pair. A number of new cyclic phosphides of carboxylic acids such as (17) can be... 

M. Adachi and S. Nakamura, Comparison of the INDO/S and the CNDO/S method for the absorption wavelength calculation of organic dyes, Dyes and Pigments 17, 287-296 (1991). 

This ICT must lead to a decrease of electron density on the 0(1) atom. For 277-chromenes, charge transfer from the 0 atom during the S0 - S3 transition was found to be 0.060 e [CNDO(S) method].30 A decrease of the charge on the O atom is accomplished at the expense of a electrons of the Cspiro—O bond, which provides the driving force for further polarization. As was shown above, this type of redistribution of electron density leads to an increase of the [N(r)]- T (cspiro-o) interaction in SP molecules and a further contraction of the CspitoN(l ) bond and weakening and elongation of the Cspiro-0 bond in the excited state. 

Shevchenko, S. M., and J. Jakobsons. 1990. Vertical ionization potentials of the compounds related to lignin. 6. Calculation of the potentials using CNDO/S method. Khim. Drev. 1990(6) 33-35. 

The CNDO methods became popular very quickly, but nearly as quickly were replaced by INDO models described below. The CNDO/S method is still widely used and yields useful results for the spectra of rather large systems. Modern versions of CNDO/S use the Mataga-Nishimoto form for the two-center, two-electron integrals, rather than the Pariser-Parr form... 

Only the INDO/S and CNDO/S methods have been parameterized for electronic spectra, and the strength of these methods has been discussed earlier in this review. 

Electronic absorptions determined for N2O2 (the ds-dimer, present in condensed phases of NO), N2O3, and N2O4 agree reasonably well with transitions predicted by the CNDO/S method. ... 

The UV absorption spectra of 1,3,5-triazine derivatives which contain 7c-electron donors (Cl, O, OH, and OMe) have been studied <91 MI 612-02). Together with calculations of the electronic excitation energies and their intensities by CNDO/S method, the structure of cyanuryl chloride hydrolysis products have been determined. Conjugation of substituents with the 1,3,5-triazine ring leads to a bathochromic shift <6IRTCI58>. Cyanuric acid absorbs in the far UV with a weak band at 280 nm and a hypsochromic shift was reported as the pH was lowered <64SA397>. 

In a calculation applied to saturated systems, the CNDO/S method has been used to investigate the influence of orbital interactions on the conformational behavior of 1,3,2- and 1,3,5-dioxaphos-phinanes <84ZOB2532>. 

An investigation of the oxidative addition of ChUand CD4[57a], as well as ethane [57b], to a bare palladium atom has demonstrated that quantum tunneling plays a very important role in the process. The barrier of insertion of different transition metal atoms into a C-C bond has been found to be 14-20 kcal mol higher than the barrier for insertion into a C-H bond [57c], Calculations for the activation of the C-H bond in ethylene by second row transition metal atoms showed that the oxidative addition barrier is lowest for the atoms to the right (for rhodium there is no barrier and for palladium the barrier is almost zero) [57d], The activation energy for B2 insertion into methane has been predicted to be 4.1 kcal mol while this value increases to 16.2 kcal mol for insertion of B [58], Two mechanisms have been considered by the SCF CNDO/S method for the oxidative addition of methane to the palladium cluster Pd2 [59a], In the first possible reaction, the C-H bond oxidatively adds to different palladium atoms ... 

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