Pariser-Parr type calculation

From among Sandorfy s subsequent theoretical projects, two in particular should be mentioned the first treatment of the excited states and electronic spectra of saturated hydrocarbons9 by a new rc-electron method of the Pariser-Parr type, and the first attempt to introduce Rydberg orbitals into molecular orbital calculations.10... 

S. Katagiri and C. Sandorfy, Theor. Chim. Acta, 4, 203 (1966). Pariser and Parr Type Calculations on Saturated Hydrocarbons. I. 

Figure 15 Topologically chiral interlocked molecules that were studied by CD. (a) The structures of the ahnost-symmetrical supermolecules that exhibited pronounced CD signals, (b) The absolute configuration of knot-type molecule 28 was determined by TDPPP (time-dependent Pariser-Parr-Pople) calculation of the CD spectrum of a fully optimized AMI geometry.

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. 

The molecular orbital (MO) approach developed more slowly but was ultimately much more successful than the other approaches. Dewar (1950) was able to predict absorption maxima for various types of cyanine dyes in excellent agreement with experiment. A major advance was made in 1953 when a self-consistent molecular orbital method specifically taking into account antisymmetrisation and electron repulsion effects was developed by Pariser, Parr and Pople. The PPP-MO method established itself over the next two decades as the most useful and versatile technique for colour prediction, especially when the microelectronics revolution provided facilities for overcoming the complexity of the necessary calculations. 

Much work has been reported on type B azapentalenes, and this can be classified according to the degree of sophistication of the calculations. Simple Hiickel linear combination of atomic orbitals (LCAO) methods have been widely used, though more recently more sophisticated Pariser-Parr-Pople (PPP) methods (using only re-electrons) and all-electron CNDO calculations have been reported. 

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). 

Although in the course of parameterizing the Pariser-Parr-Pople type of calculations, various methods have been developed to reproduce such a curve, not all of these have been employed in the parameterization of all valence electrons calculations. We shall now discuss the most common of these methods. 

O-H bond. Among such properties a prominent one is the ultraviolet absorption spectrum and the theory may therefore be used for the examination of some of the spectroscopic shifts which accompany the lactam-lactim tautomerization. Much caution must, however, be exercised in this respect. Thus, in a recent paper Kwiatkowski135,137 performed Pariser-Parr-Pople-type calculations on the electronic structure of hydroxypurines, essentially to interpret their ultraviolet spectra. In these calculations he assumed that these compounds exist predominantly in their lactim form, and the results of his calculations, at least for 6- and 8-hydroxypurine, did not seem to contradict this assumption. It is only in the case of the 2-hydroxy isomer that a particularly striking disagreement between theory and experiment led him to admit that this last compound may exist in the lactam form. Calculations carried out for this form gave, in fact, a more satisfactory agreement with experiment.138 As we have seen, unambiguous infrared spectroscopy evidence clearly show s that all three isomers exist essentially in the lactam form. This shows that ultraviolet absorption may provide only very uncertain evidence about the lactam-lactim tautomerism in hydroxypurines and related compounds. 

In the N NMR spectrum of quinazoline in dibromomethane only one broad resonance signal (half height 950 + 50 Hz) was observed at 90 + 3 ppm (relative to internal nitromethane) due to overlapping. The N NMR chemical shifts depend almost linearly on the n-charge density at the nitrogen atom obtained from SCF-MO calculations of the Pariser-Parr-Pople type. NNQR parameters for quinazoline at 77 K showed reasonable agreement with ab initio MO calculations. ... 

The Pariser-Parr-Pople Hamiltonian for the description of the 7i-electrons in trans-polyacetylene is reparametrized using ab initio Coupled Cluster Doubles calculations based on a Restricted Hartree Fock reference on trans-butadiene. To avoid the spin contaminations inherent in Unrestricted Hartree Fock (UHF) type calculations on polymethine chains in the doublet state the Annihilated Unrestricted Hartree Fock (AUHF) model is applied in our PPP calculations (tPA (CH) , polyenes H-(CH)2N-H, polymethines H-(CH)2N+1-H). In geometry optimizations on polymethine chains it is shown that in contrast to results from Hiickel type models the width of neutral solitons is strongly... 

Two models are particularly well suited for the calculation of the electronic spectra of conjugated ir-electronic systems. Both are based on the principle of differential overlap and involve no multicenter repulsion integrals. The Pariser-Parr-Pople (PPP) model (7), whose basis set contains only atomic orbitals of it symmetry, is particularly good for irir transitions in hydrocarbons and other molecules with a limited number of heteroatoms. Because of the unreliability of the PPP method in the presence of heteroatoms, we chose a second model which is available in two versions (CNDO/S (8) and INDO/S (9) for the calculations on 1 and 2. This model utilizes a basis set which contains all valence shell orbitals and leads to less uncertainty in parameterization even when a large number of heteroatoms are present. This model works well for irir transitions but often fails when applied to nir, air and ira transitions. This should not be a serious limitation in the present application since the only transitions of sufficient intensity to be of interest are of the wir type. 

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