CNDO/S-CI calculation

The CD signs of the ICD of P-CDx complexes with mono-, di-, and triazanaphthal-enes in aqueous solutions depend on the polarization directions of these azanaphthal-enes. The polarization assignments come from the CNDO/S-CI calculation 165) coupled with the MCD data of these azanaphthalenes 165). In the first place, the origin of the coordinates was choosen in the center of a ring of P-CDx that is assumed to have a 7-fold symmetry axis. Next, ep which is put at the origin of the p-CDx-fixed Cartesian coordinates, is expressed by the spherical polar coordinates (Fig. 23). 

The CD induced in 2-(2,4,6-cycloheptatrien-l-ylidine)-4-cyclopentene-l,3-dione upon its inclusion in 6-cyclodextrin was used to obtain full assignment of the observed absorption bands [49]. It was concluded that the low energy band system was composed of three electronic transitions, two of which were polarized perpendicular, and one parallel, with respect to the long molecular axis of the solute. The higher energy band system was found to also consist of three allowed electronic transitions, but two of these were polarized parallel, and one perpendicular, to the long axis of the solute molecule. These deductions were supported by a series of CNDO/S-CI calculations performed on the system. A similar approach was taken to assign the polarizations of the various transitions associated with 2-thioxo-l,3-benzodithiole and 2-selenoxo-1,3-benzodithiole [50]. 

CNDO/s-CI calculations have been carried out on a seresi of compounds containing the —NO2 moiety NOg, nitramide, ethyl nitrate, HNO3,... 

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). 

The characteristic electronic and chiroptical properties of hexahydro-pyrido[l,2-a]pyrimidin-4-ones 29 were qualitatively characterized by quantum-chemical calculations in the CNDO/S-CI approximation on some simplified models having the geometries of the molecules studied (80MI2 85JOC2918). 

The induced circular dichroism (ICD) spectra of the (/ -CDr) complexes of pyrido[2,3-6]pyrazines have been measured. The polarization analysis and spectral assignments were made by comparing the theoretical results calculated using the CNDO/S-CI approximation with the observed ICD spectra <82JA7059>. 

For a series of carbonyl systems, one of which was a benzodifuran derivative (2), the energies of the first five singlet and triplet excited states were calculated by CNDO/S-CI methods. For compound (2), three quasi degenerate low-lying mr states were proposed, and the effects of NH2, OH, and CN substituents were determined <85ZN(A)859>. 

Cyclic dipeptides, especially when N-alkylated, undergo extremely fast epimerization (79JA1885). For example, cyclo(L-Pro-L-Phe) is rapidly converted to its diastereomer, cyclo(D-Pro-L-Phe) (80% conversion), by treatment with 0.5 N NaOH at 25°C for 15 min. This diastereomer is the one in which the proline residue has epimerized and not the more activated phenylalanine. CNDO/2 calculations seem to provide a rationale for this. It is not yet completely clear why such base-catalyzed epimerizations of piperazinediones are so easy the conformation of the molecule may play a role in this (79MI1). It is also worth noting that even in linear peptides, rm-amides of N-alkyl-amino acids, which consist of s-trans and s-cis rotamers of almost equal energy, are more prone to racemization than the sec-amides, which exist only in the s-trans configuration. Of course, the amide functions of piperazine-2,5-diones are obliged to assume the s-cis conformation. 

Ab initio MO calculations on 3-formyl-(5 2-isoxazoline-BF3-complex suggests that s-trans conformation is preferred over s-cis conformation. Nucleophiles attack the stable s-trans conformer from the opposite side of the C-4 substituent, giving the respective anti adducts <93T7637>. The regioselectivity of the 1,3-dipolar cycloadditions of mesitonitrile oxide with benzo[b]thiophene S-oxide and S,S-dioxide yielding only 2,3-dihydrobenzo[b]thieno[2,3-d]isoxazolines, is discussed in terms of frontier MO interactions on the basis of photoelectron spectra and CNDO/S calculations <82JOC2461>. 

These UV determinations have been extended to jS-alkylamino and ) -dialkylamino acrylamides and to a-substituted enaminones , and CNDO/S calculations have been reported. These give only fair agreement with the experimental data of Table 19, i.e, for j -aminoacrolein 224.3 nm calculated vs 252 nm observed for the rrans-form (226a) and 239.7 nm calculated vs 287 nm observed for the cis-form (227a), However, the differences of calculated transitions which are related to experimentally derived spectral increments could be predicted better than absolute transitions. 

Several attempts have been made to include Rydberg states within the INDO/S scheme, but they have not proven generally useful. See for example, D. R. Salahub and C. Sandorfy, Theor. Chim. Acta, 20, 111 (1971). CNDO, INDO, and RCNDO-CI Calculations on the Electronic Spectra of Saturated Hydrocarbons. W. Haque, /. Chon. Phys., 67, 3629 (1977). Use of the RINDO Method in Spectroscopy. Molecular Rydberg S -S and T<-T Transitions. 

The CIS approximation is used in a majority of semiempirical SCF methods to calculate energies and oscillator strengths of electronic transitions (CNDO/S, INDO/S, CINDO-E/S, NDDO/MC, NDDO-G). 

Our CNDO/S calculations on E-and Z-3-hexen--l,5-di3mes show that the Z-isomer should absorb at a longer wavelength than the E-isomer, which means that cis-trans isomerisation should have a similar effect in the absorption properties of the enyne-and the polyene systems, as well. The prediction and the observation are consistent in the case of 1,4-diphenylbutenynes. Both isomer may have a stable fully conjugated planar conformation, since no repulsion is expected between the phenyl group and the triple bond in these compounds. 

Until recently, most excited state calculations on large open-shell molecules made use of special versions of semiempirical methods such as PPP-Cl for planar n systems, " " " and CNDO/S or INDO/S-CP" " for molecules in which a electrons must be accounted for if excited states are to be adequately described. These semiempirical methods were parameterized for a Cl that includes only single excitations (CIS), which turns out to be sufficient for many applications. Although these methods have proven useful, their quantitative accuracy is limited, especially when it comes to modeling higher excited states, for which doubly excited configurations are frequently important. 

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