Pariser-Parr-Pople method, compared

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. 

The pyrido[l,2-a]pyrazinium cation has been compared with the quinolizinium ion and the other monoazaquinolizinium cations, using the Pariser-Parr-Pople method to calculate electronic structures and absorption spectra. The ultraviolet spectrum of pyrido[l,2-a]pyrazinium bromide in water shows maxima at 232, 276, 287, 322, and 336 nm. Ultraviolet spectra have been recorded for various substituted compounds and their iV-oxides. The bands at 1685 and 1655 cm in the infrared spectrum (KBr disc) of the oxo compound 5 (R = H) confirm its formulation as the 0X0 form rather than as the corresponding hydroxy tautomer. Similarly the dioxo derivative 6 exists as such, in preference to the hydroxy form 5 (R = OH). The presence of a two-proton singlet at 6 6.1 in the PMR spectrum (in trifluoroacetic acid) of compound 6 is strong evidence in favor of this formulation. Ultraviolet spectral data are consistent with structures 6 and 5 (R = H). ... 

Under some simplifications associated with the symmetry of fullerenes, it has been possible to perform calculations of type Hartree-Fock in which the interelec-tronic correlation has been included up to second order Mpller-Plesset (Moller et al. 1934 Purcell 1979 Cioslowski 1995), and calculations based on the density functional (Pople et al. 1976). However, given the difficulties faced by ab initio computations when all the electrons of these large molecules are taken into account, other semiempirical methods of the Hiickel type or tight-binding (Haddon 1992) models have been developed to determine the electronic structure of C60 (Cioslowski 1995 Lin and Nori 1996) and associated properties like polarizabilities (Bonin and Kresin 1997 Rubio et al. 1993) hyperpolarizabilities (Fanti et al. 1995) plasmon excitations (Bertsch et al. 1991) etc. These semiempirical models reproduce the order of monoelectronic levels close to the Fermi level. Other more sophisticated semiempirical models, like the PPP (Pariser-Parr-Pople) (Pariser and Parr 1953 Pople 1953) obtain better quantitative results when compared with photoemission experiments (Savage 1975). 

The linear acenes, benzene to pentacene, are used as examples of the CURES-EC procedure. The results obtained utilizing MINDO/3 and AMI are compared. In addition to calculating the Ea by subtracting the energies of the optimized form, the LUMO of the neutral is compared with the experimental Ea. The electron affinity of hexacene has been estimated from the electronegativity and experimental ionization potential. As a further example of the use of CURES-EC, both the ionization potential and electron affinity of heptacene are estimated. The Ea of octacene and nova-cene are calculated for comparison to values obtained by using Koopman s theorem and a semi-empirical method based on a variable-parameter modification of the Pariser Parr Pople (PPP) approximation to the Hartree Fock equation [10]. 

Among the earliest examples of the application of molecular orbital calculations to lignin chemistry is, as might be expected, the formation and reactivity of phenoxy radicals [32]. This work described the use of Pariser-Parr-Pople molecular orbital calculations, comparing various methods to determine the rr-electron spin densities for a number of model compounds. The different techniques gave similar results, with the bulk of the unpaired spin density at the phenolic oxygen, followed by the ortho and para carbons. In a cinnamaldehyde model, the P-position also exhibited considerable unpaired spin density. 

The correlation of the arylmethyl hydrogen abstraction data with the PMO-.F calculations is shown in Figure 3. The correlation coefficient is 0.911, the lowest found yet for any application of the PMO.-F or PMO.-Fet) methods. However, this value compares favorably with the results of the VBSRT calculations (37) and the HMO calculations (49) for which the correlation coefficients are 0.933 and 0.855, respectively, for the rate data versus resonance energy differences. We also find again a very good correlation of the PMO F localization energies with, in this case, open-shell Pariser-Parr-Pople SCF-tt calculations (49), correlation coefficient 0.963. In any event, the PMO F calculations can be used to quantitatively correlate and predict rates of reaction in this case as well. 

In this review we summarized our experience with the development and applications of semiempirical Pariser-Parr-Pople (PPP)-type and all-valence-electron methods to electronic spectra of radicals. After the era of PPP calculations on closed-shell molecules and the advent of semiempirical all-valence-electron methods, the electronic spectra of radicals represented a new challenge for molecular orbital (MO) theory. It was a time when progress in experimental techniques resulted in accumulation of a vast amount of data on the electronic spectra of radicals of various structural types. Compared to closed-shell molecules, the electronic spectra of some radicals exhibited peculiar features bands in the near infrared, many transitions in the whole UV/vis region, and some bands of extraordinary intensity. Clearly, without the help of MO theory, their interpretation seemed even harder than with closed-shell molecules. 

Semiempirical calculations have been carried out by an unparameterized SCF-MO method with integral approximations [5], various versions of the CNDO [37 to 42] and INDO [6, 38, 43 to 45] methods, the MNDO [46, 47] and MINDO [48] methods, the extended Hiickel method [3, 4, 49, 50] (presumably also [51 ]), a Pariser-Parr-Pople-type open-shell method [49] (presumably also [51]), and a simple MO approach [52]. Besides some other molecular properties, the charge distribution (atomic charges and/or overlap populations) [5, 38,40,41,43,49 to 51] and the spin density distribution (and thus, the hyperfine coupling constants, compare above and p. 241) [3 to 6, 46, 48] have been the subjects of many of these studies. 

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