PCILO

The syn-anti conformational problem of a- and /3-pyrazofurins (756 one of the rare naturally occurring pyrazole compounds, see Section 4.04.4.4.3), which involves a rotation around a pyrazolic sp carbon atom and a sugar sp carbon atom, has been studied theoretically using the PCILO method (81MI40403). In agreement with the experimental observations, the /3 anomer is energetically more favourable than the a anomer, the preferred conformations being anti and syn, respectively. 

CS INDO [10] (as well as the parent C INDO [9]) shares the same basic idea as the PCILO scheme [29,30] to exploit the conceptual and computational advantages of using a basis set of hybrid atomic orbitals (AOs) directed along, or nearly, the chemical bonds. 

The results of the above cited applications [18-28,45] have clearly shown that CS INDO method is fairly successful in combining equally satisfactory predictions of electronic spectra and potential surfaces (especially along internal rotation pathways) of conjugated molecules, a goal never reached by other NDO-type procedures. CS INDO shares, at least partly, the interpretative advantages of the CIPSI-PCILO-CNDO procedure [32,33,36,37], coming from using the same hybrid AO basis sets, but improves its predictive capabilities as far as spectroscopic and photochemical properties are concerned. 

For calculations of electronic structures of azolides (CNDO, PCILO, EHMO, INDO and Molecular Mechanics Calculation) the interested reader should consult the relevant papers. 481... 

Molecular Energetics. Molecular energies can be computed in a variety of ways including empirical fixed valence potentials, full force field potentials, and semi-empirical molecular orbital techniques (CNDO-2, INDO, MINDO-3, MNDO, PCILO). 

Laurence, P. R., and C. Thomson. 1981. A Comparison of the Results of PCILO and Ab Initio SCF Calculations for the Molecules Glycine, Cysteine and N-Acetyl-Glycine. Theoret. Chim. Acta (Berl.) 58, 121-124. 

QCFF/PI and PCILO calculations106, as well as a combined MM3, NMR X-ray crystallography and structure comparison study107-109, were used to identify biologically active... 

Figure 1. The variation of bond angles C5-05-C1 and 05-C1-01 with the torsion angle < ) for 2-methoxytetrahydropyran. The curves with squares (C5-05-C1) and triangles (05-C1-01) are for the axial form and the rhombuses (C5-05-C1) and stars (05-C1-01) are for the equatorial form. These curves were calculated with PCILO, with full optimization of geometry at each increment.

Several different molecular orbital methods have been used in SAR investigations. These include simple Huckel theory, HT,(38) extended Huckel theory, EHT,(39) CND0,( ) NDD0,(41) MINDO/3,(42) and PCILO,(43)... 

The most extensive and systematic study of the chemistry of lignin with theoretical methods has been performed by Remko and co-workers. Their work has involved the nature of intramolecular (40-43) and inter-molecular (44-48) hydrogen bonding of lignin model compounds, spectral transitions (49-52), and conformational analysis (53). The methods used have included CNDO/2 and PCILO (Perturbative Configuration Interaction using Localized Orbitals) (54). 

Compound Energy (koal/mole), with respect to the individual global minimum, from PCILO calculation. Angle of rotation, (C4-N8), in degrees Barrier heights6 to rotation (kcal/mole) - from CNDO/2 calcu-lation (Rao189,197)... 

The PCILO method has been applied237,238 to calculate the dipole moments of several conformers of uridine and deoxyuridine (see footnote a to Table XXII). However, at present, it is difficult to make a detailed comparison of the calculated dipole moments of nucleosides with the experimental data. In solution, the nucleosides exist as a mixture of several conformers in unknown proportions. 

Fig. 31. Conformation of the disaccharides forming repeating unit of the backbone of mureine from PCILO calculations...

Pulmann B PCILO, QCPE program 220, Chemistry Department, Indiana University, Bloomington, IN... 

Conformational studies of diastereoisomeric alcohols of the 6,14-endo-ethenotetrahydrothebaine series, using a PCILO semi-empirical quantum-mechanical method, have been made in relation to the different agonist/ antagonist properties of the isomers.170 [N-Methyl-11 C]- and [N-methyl-14C]-morphine have been prepared.171,172... 

The foregoing relationship was utilized in conformational analysis of methyl 4-thio-a-maltoside in solution.153 The VC H values of 2.95 and 5.15 Hz measured151 for H-4 —C-4 —S—C-l—H-l differ from those predicted from Eq. 45 37c 4. h-i = 3.1 Hz and 37C-i,h-4 = 3.8 Hz for the conformation in the crystal154 as defined by H = 25.6° and PH = 3.6°. The observed VC H values were not reproduced by the calculated values for any of the stable conformations of methyl 4-thio-a-maltoside, but are in good agreement with the ensemble average (37c.h) values 2.67 and 5.24 Hz calculated for the compound in water solution, using PCILO-calculated potential surfaces. 

Torsion angles are calculated using PCILO quantum chemical method or Quanta. Taken from reference 151 and 152. 

An early example implementing the general approach to take into account first the intrabond correlation, is presented by the PCILO - perturbational configuration interaction of localized orbitals method [121,122], As one of its authors, J.-P. Malrieu mentions in [122], the PCILO method opposes the majority of the QC methods in all the fundamental concepts. In contrast to the majority of the methods based on the variational principle, the PCILO method is based on estimating the energy by perturbation theory. Also, the majority of the QC methods use one-electron HFR approximation, at least as an intermediate construct, whereas the PCILO is claimed to addresses directly the V-electron wave function and takes into account all surviving matrix elements of the electron-electron interactions. In contrast with other QC... 

All these statements, although correct in principle, are not precise from the technical point of view. For example, the zero approximate wave function in the PCILO method is a one-electron approximate function constructed from the bond wave functions determined by an a posteriori localization procedure from an HFR function. Thus the bond orbitals appear after a unitary transformation of the canonical MOs, which correspond to some more or less arbitrary localization criteria [123-125]. 

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