Pople method

Indazoles have been subjected to certain theoretical calculations. Kamiya (70BCJ3344) has used the semiempirical Pariser-Parr-Pople method with configuration interaction for calculation of the electronic spectrum, ionization energy, tt-electron distribution and total 7T-energy of indazole (36) and isoindazole (37). The tt-densities and bond orders are collected in Figure 5 the molecular diagrams for the lowest (77,77 ) singlet and (77,77 ) triplet states have also been calculated they show that the isomerization (36) -> (37) is easier in the excited state. 

Simple HMO calculations (68JCS(B)725) satisfactorily account for the UV spectra of a great number of pyrazoles substituted by methyl and phenyl groups. The spectra of pyrazolium and indazolium salts (free bases in IN HCl) have been compared with calculated transitions (Pariser-Parr-Pople method) (74MI40403). 

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. 

A method which is similar to the Pariser-Parr-Pople method for the n electron system and is applicable to common, saturated molecules has been proposed by Pople 28>. This method is called the CNDO complete neglect of differential overlap) SCF calculation. Katagiri and Sandorfy 29> and Imamura et al. °) have used hybridized orbitals as basis of the Pariser-Parr-Pople type semiempirical SCF calculation. 

One example showing a serious discrepancy of the frontier electron method was reported by Dewar H8,ii9). This is 10,9-borazaphenanthrene, and the value of / -B) was reported to have been calculated by the Pople method, but the parameters usyd were not indicated. Fujimoto s calculation by the Pariser-Parr-Pople method 120>, in perfect disagreement with Dewar s, gives the most reactive position as 8, which parallels experiment. The ambiguity involved in the integral values adopted seems to be serious, so that the establishment of parametrization for boron heterocycles is desirable. 

The dipole moment calculated for 111 is somewhat higher than for 96, as expected. The total it charge in the cyclopentadiene ring is calculated to be 0.54 electrons. A somewhat smaller polarization, 0.40 tt electrons, is calculated for 7,8-diphenylcalicene (124), and 0.34 tt electrons for hexaphenylcalicene 115, (122) by the PPP (Pariser-Parr-Pople) method. Thus it seems as if the tt polarization roughly follows the same trend as the bond lengths and the C=C torsional barriers. 

The calculations were carried out by J. J. C. Mulder, Leiden, according to the Pariser-Parr Pople method and were rounded off by taking into account configuration-interaction of all singly excited states. See Ref. i 2) fQj- results. 

The symmetry of the quinolizinium ion is such that only four monosubstitution products are possible for any given substituent. On the basis of a computation using the Pariser-Parr-Pople method, including all singlet mono-excited electronic configurations, Galasso (68MI21000) concluded The carbon atoms at 3 and 7 are expected to be most reactive towards electrophilic substitution. ... 

Calculations by the Pariser-Parr-Pople method on the quinoneimine obtained by Fremy s salt oxidation of dibenz[6,/]azepine indicate that the carbon adjacent to the carbonyl group is the most nucleophilic center. This is confirmed by nitration [Cu(N03)2-Ac0H] and bromination [NBS-(PhC0)202] studies, in which the 1-substituted derivatives are obtained. However, acylation by Vilsmeier or Friedel-Crafts reaction fails, extensive decomposition of the ring system taking place. 

Fig. 8. Absorption spectra (AB upper curves) and circular dichroism (CD lower curves) of calycanthine (formula in the inset of the figure). The solid lines refer to the experimental spectra and the broken lines to the theoretical curves for the configuration illustrated, calculated by the Pariser-Parr-Pople method using the dipole velocity procedure21,22)...

The electronic spectra of quinoxaline and its 2-chloro, 2-methoxy, and 2-amino derivatives have been calculated by the Pariser-Parr-Pople method. The calculated and the observed spectra agree well.247... 

Electron densities and bond orders for pyran-2-one, pyran-4-one and eight benzologues or dibenzologues have been calculated using the simple Hiickel, the Hiickel autocoherent and the semi-empirical Pople methods (74BSF538). Similar properties have been obtained for flavone and the electronic absorption spectra of some substituted flavones have been examined in detail (74CHE1218). Excitation is accompanied by a transfer of charge from the... 

The available spectral and calculated (Pariser-Parr-Pople method) data constitute evidence for a decrease in electron density in the benzene ring and activation of the 5-position to electrophilic attack in the 4-hydroxythiazinone (203) (79KGS44). Nitration was carried out in organic solvents at -60°C. The best results were obtained in the case of nitration with nitric acid (specific gravity = 1.42) in glacial acetic acid containing catalytic amounts of acetic anhydride at 40-50C (Scheme 93). The mono-... 

Here the parameter is determined by a least squares fit to the heats of formation of selected bonds in a group of standard molecules chosen to include all types of hydrocarbons. As in the Pople methods, St) is determined from Slater orbitals. 

Fig. 3. Distribution of formal charge in the anionic intermediate of the Birch reduction of benzene calculated (a) by the Huckel method, (b) by the Pople method.

Fig. 6. Localization energies (eV) calculated for 10,9-borazarophen-anthrene52 by the Pople method.

We have obtained very similar results in treating borazaro-phenanthrene by the Pople method and we have found similar vagaries in the localization energies given by it for other heteroaromatic compounds. The variations in the energies are too large, and the method, as used by us, also fails to predict correctly the relative reactivities of different ring systems. More work needs to be done to overcome these difficulties before the SCF method can be used to treat problems of this kind. 

Ten unsaturated carbonyl compounds representing a variety of structural types were selected for study. These are given in Table II, together with their pKa values and dipole moments, where available the non-alternant systems are indicated in italics. The fourth column contains the it dipole moments of the neutral molecules as obtained by the Pariser-Parr-Pople method using the parameters of Table I. 

We begin our examination of specific SCF-type semiempirical methods with the simplest, the Pariser-Parr-Pople method. 

The third fundamental hypothesis of the CNDO approximation concerns the core matrix elements H. These correspond to the and parameters of the Pariser-Parr-Pople method, the core including here only the Is electrons and the nuclei. 

Aminothiazoie absorbs near 270 nm (e = 14,000) corresponding to the ir4 — tt electronic transition calculated by the Pariser-Parr-Pople method at 199 nm (63). The amino group in the 2-position introduces a bathochromic shift (90,91). Localization of the transition is shown in Fig. VI-2. This figure suggests that, in the tt4— excited state, protonation... 

M. Hida, unpublished results. b HMO, Hiickel molecular orbital theory. PPP, Pariser-Parr-Pople method. 

K. Ohno, Theor. Chim. Acta, 2, 219 (1964). Some Remarks on the Pariser-Parr-Pople Method. 

The connection between electronic structure and u.v. spectra of pyrazine and protonated pyrazine has been examined the electronic distributions were calculated using a Pariser-Parr-Pople method employing an electrostatic model (proton bounded to the unshared electron pair of the nitrogen atom by electrostatic forces) (1493). The electronic absorption of a number of alkyl-substituted pyrazines in neutral and in acid solvents have also been measured (1476). 

The Pariser-Parr-Pople method provides a satisfactory interpretation of the absorptions in the UV and visible regions. ° Transition... 

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