CNDO calculations compounds

Carbon-13 signals of parent heteroaromatic compounds [433-464] (Table 4.67) occur between 105 and 170 ppm. CNDO calculations performed for six-membered heteroaromatic compounds such as pyridine and azines [456, 458, 465] give a linear correlation between carbon-13 shift and total electron density at the individual carbon. The experimental slope, 160 ppm per electron, is the same as that found for monocyclic aromatic compounds (Section 4,11.1). The 13C shift values of pyrrole (107-117 ppm) and pyridine... 

If we take as reference the dipole moments calculated for the predicted most stable tautomers of the lactam forms of the hydroxypurines (as discussed in Section VII) and which are generally the N(7)H tautomers, we come out with a rather high dipole moment ( 9 D) for 2-hydroxypurine and relatively low dipole moments for 6-hydroxypurine (2.6 D) and 8-hydroxypurine (1.4 D) (the numbers quoted come from CNDO calculations). Under these conditions the lactimization should correspond to a decrease in the dipole moment in 2-hydroxypurine (3-7 D) and to its increase in 6-hydroxypurine (3.6-5.7 D) and 8-hydroxypurine (3.2-4.6 D). Unfortunately no experimental data are available in this field. Calculations are also available for the directions of the moments (as well as for the transition moments and the HOMO s and LEMO s of all these compounds). 

MNDO calculations have been reported for the heterocycle (7) in order to predict its HOMO energy and ionization potential <89SM179>. Compound (7) is a derivative of tetrathiafulvalene (8) and sulfur-containing compounds related to heterocycle (8) are of intense current interest as organic metals. CNDO calculations on 3,6-dithioxo-l,2,4,5-tetrathiapentalene (9) have been reported and there is agreement between experimental and calculated 13C NMR data <84ZC418>. 

The dipole of 4-azidotriazole, a compound related to 4-aminotriazole, was found to be 2.35 D (in dioxane, 25°C) and was compared to 4.46 D for the parent 1,2,3-triazole. It was concluded, after comparative studies with other azido compounds, that the predominating tautomer was HN-2 (11). Whether the azido group is oriented mainly Z or was not experimentally determined, but CNDO calculations indicated the Z form, i.e., the one with the azido group turned toward the nearest ring-nitrogen atom (N-3) (78BSB189). For the use of H NMR to assign cis and trans structures, see Section II,D. 

The photoelectron spectra of the compounds RCS2 CF3 (R = F, Cl, or CFs-S) have been analysed with reference to the spectra of the thioketones R2CS (R = F, Cl, or alkylthio) and through CNDO calculations the CFs-S group has a substituent effect comparable to that of chlorine. i A study of the reactions of the acid derivatives XCO SCl (X = F or Cl) has shown that nucleophilic substitution at carbonyl carbon is more rapid with the dichloro-compound (X = Cl), although the S—Q bonds in both compounds are of comparable reactivity some chemical conversions of the fluoro-compound are shown in Sicheme 32. 

Scheme 2.19. The reaction pathway of a model compound based on CNDO calculations.

Photoelectron and E.S.C.A. Spectra. The diselenide structure of thiuram diselenides has been established by means of Rdntgenphotoelectron spectroscopy. The photoelectron spectra of thiocarbonyl halides have been studied and interpreted on the basis of CNDO calculations by two groups of workers. ESCA investigations have been performed on dithio-esters derived from active-methylene compounds and carbon disulphide, and on isocoumarins, thioisocoumarins, and isoquinolines containing the thiocarbonyl function." ... 

The question of symmetry in 6a-thiathiophthens, in relation to the interpretation of various physical studies previously reported and to theoretical studies, is reviewed in a paper dealing with CNDO calculations of ionization and excitation energies, for the parent compound and its 2,S-dimethyl derivative. 6a-Thiathiophthen is included in a further theoretical study which covers a range of heterocyclic sulphur compounds. 

Experimental and calculated (CNDO/S) vertical ionization energies have been measured for pyrazol-3-ine-5-thiones (108 R = H, Me). These compounds exhibit an intense low-energy band (7.55-7.60 eV) corresponding to the ionization of both a thiocarbonyl tt-electron and the sulfur n electron (78JA1275). 

The CNDO method has been modified by substitution of semiempirical Coulomb integrals similar to those used in the Pariser-Parr-Pople method, and by the introduction of a new empirical parameter to differentiate resonance integrals between a orbitals and tt orbitals. The CNDO method with this change in parameterization is extended to the calculation of electronic spectra and applied to the isoelectronic compounds benzene, pyridine, pyri-dazine, pyrimidine and pyrazine. The results obtained were refined by a limited Cl calculation, and compared with the best available experimental data. It was found that the agreement was quite satisfactory for both the n TT and n tt singlet transitions. The relative energies of the tt and the lone pair orbitals in pyridine and the diazines are compared and an explanation proposed for the observed orders. Also, the nature of the lone pairs in these compounds is discussed. 

Bagal (1974) studied the influence of substituents on the ground and first excited states of arenediazonium ions. With regard to compounds in which mesomeric structures such as 4.1b are important, these authors are skeptical about the validity of the PP method. Later, Bagal et al. (1982) used CNDO/2. The calculated 7r-electron densities at all the nitrogen and carbon atoms were similar to those in the earlier PP results. 

Another type of interaction is the association of radical ions with the parent compounds. Recently (118), a theoretical study was reported on the interaction of butadiene ions with butadiene. Assuming a sandwich structure for the complex, the potential curve based on an extended Hiickel calculation for two approaching butadienes (B + B) revealed only repulsion, as expected, while the curves for B + and B + B" interactions exhibit shallow minima (.068 and. 048 eV) at an interplanar distance of about 3.4 A. From CNDO/2 calculations, adopting the parameter set of Wiberg (161), the dimer cation radical, BJ, appears to be. 132 eV more stable than the separate B and B species, whereas the separate B and B species are favored by. 116eV over the dimer anion radical, BJ. This finding is consistent with experimental results formation of the dimer cation radical was proved in a convincing manner (162) while the attempts to detect the dimer anion radical have been unsuccessful. With other hydrocarbons, the reported formation of benzene dimer anion radical (163) represents an exceptional case, while the dimeric cation radical was observed... 

These compounds have been the subject of several theoretical [7,11,13,20)] and experimental[21] studies. Ward and Elliott [20] measured the dynamic y hyperpolarizability of butadiene and hexatriene in the vapour phase by means of the dc-SHG technique. Waite and Papadopoulos[7,ll] computed static y values, using a Mac Weeny type Coupled Hartree-Fock Perturbation Theory (CHFPT) in the CNDO approximation, and an extended basis set. Kurtz [15] evaluated by means of a finite perturbation technique at the MNDO level [17] and using the AMI [22] and PM3[23] parametrizations, the mean y values of a series of polyenes containing from 2 to 11 unit cells. At the ab initio level, Hurst et al. [13] and Chopra et al. [20] studied basis sets effects on and y. It appeared that diffuse orbitals must be included in the basis set in order to describe correctly the external part of the molecules which is the most sensitive to the electrical perturbation and to ensure the obtention of accurate values of the calculated properties. 

The geminal coupling of some azo1ooxaphospho1enes is reported to correlate with A.. 9 1 Vicinal couplings have been used in the conformational analysis of a variety of compounds such as dioxaphosphorinanes,92 phosphorinanes (in combination uith CNDO/2 calculations),93 and bromophosphorinanes (in combination uith dipole moment measurements).99 The various couplings for 1,1-viny1idene-diphosphonic acid and its salts have also been measured.95... 

The parent heterocycle of pyrido[2,3-e][l,2,4]triazine and its phenyl derivative 39 were prepared (89JHC475) by cyclization with polyphos-phoric acid of 3-acyIhydrazino-2-aminopyridines 36, obtained by reduction of the corresponding 3-acylhydrazino-2-nitropyridines 35. Compounds 35 were obtained from 3-fluoro-2-nitropyridine 34 either by reaction with benzoylhydrazine or by reaction with hydrazine and subsequent for-mylation of the initially formed 3-hydrazino-2-nitropyridine 38. Attempts to prepare 38 from 3-chloro-2-nitropyridine gave 2-hydrazino-3-chloropyridine 37. These results could be explained by semiempirical calculations (CNDO and MNDO calculations). 

The relative importance of the intrinsic (twist-related) and external contributions (e.g. stemming from the axial allylic groups) was evaluated for the first time by Rosenfield and Charney23 who carried out CNDO/S calculations on the model compounds 16-18. 

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