Non-polar SCF

Lithium ester enolates are extremely important in polymer chemistry as initiators and active centers of the anionic polymerization of acrylic and methacrylic monomers in polar solvents. Thus, HF-SCF studies, comparable to those mentioned above, were undertaken on monomeric methyl isobutyrate (MIB) enolate210,211. The overall conclusions on the aggregation and solvation trends are exactly the same, the bent rj3-0,C mode being preferred over the rj1-O planar one by ca 3.3 kcalmol-1. While the dimeric MIB enolate solvated by four molecules of THF was found to be the enthalpically most stable aggregate, the prismatic S6 unsolvated MIB hexamer was computed as the preferred structure in non-polar solvents (Scheme 55)212. In the latter case, the supplementary oxygen of the ester acting as a side-chain ligand for the lithium seems to explain this remarkable stability. 

The ab initio calculations at the SCF/3-21G level predicted the predominance of the thiol form of unsubstituted 2- and 4-mercaptopyrimidines in the gas phase in accordance with the experimental IR spectra (91JPC2404, 91SA(A)339). Ab initio SCRF calculations have been carried out for tautomerism of 2-mercaptopyrimidine in the gas phase and in solution. In the gas phase and non-polar solvents, the thiol tautomer was found to be the most stable (AE — 8.16kcal/mol in the gas phase and 5.91kcal/mol in cyclohexene), whereas in polar solvents the thione tautomer becomes more stabilized (AE — —5.51 kcal/mol in DMSO and —5.83 kcal/mol in water) (95CPL(232)61). 

Liquid carbon dioxide is a non-polar (non-ionizing) solvent, in many ways like hexane. However, in the supercritical phase the dielectric constant of CO2 increases with increasing pressure. ( ) The dielectric constant is an indicator of the polarity of the SCFCO2 solvent [Figure 4]. Thus, by controlling pressure (or the addition of a polar cosolvent), the selectivity of SCF can be fine tuned for the preferential extraction of the compounds of interest based on their polarity. 

These cluster results are in remarkably good agreement with the periodic calculations of Jaffe and Hess [130] for the adsorption of CO on the non-polar ZnO(lO-lO) surface,both at the SCF and the correlated levels (Table 5). The Zn atoms at this surface are threefold coordinated as are those on the terraces of the ZnO(OOOl) surface plane. Therefore, the adsorption of CO at the Zn Q atom of the free Zn404 cluster can be taken as a model for the adsorption on the Zn atoms in both surfaces. The good agreement is quite surprising since completely different methods have been used periodic approach vs free cluster at the SCF level, perturbation theory (MP2) vs approximate coupled cluster approach (MC-CEPA) at the correlated level. This is also an indication that both treatments will converge to similar results, provided that the calculations are performed correctly. 

Inclusion complexes. Solubility of poorly soluble drugs, such as piroxicam, can be enhanced by forming inclusion complexes with cyclodextrins. For many non-polar drugs, previously established inclusion complex preparation methods involved the use of organic solvents that were associated with high residual solvent concentration in the inclusion complexes ]. Cyclodextrins had previously been used for the entrapment of volatile aromatic compounds after supercritical extraction ]. On the basis of this principle. Van Hees employed SCFs for producing piroxicam and j8-cyclodexhin inclusion... 

The majority of SCFs also show a sharp increase in the dielectric constant (e) with increasing pressure in the compressible region (around the critical point). This behavior reflects, to some extent, the change in density. The magnitude of the increase depends on the nature of the SCF whereas the dielectric constant varies little with pressure for non-polar substances such as SCCO2, dramatic increases are observed for more polar SCFs such as water or fluoroform (Figure 4.4). " ... 

To derive the MM picture we must consider the question of transfer-ability of the ESPs. Above we defined two groups of parameters entering the expression for the energy Eq. (17). Let us consider the first group of parameters. In the case of lone pairs they are perfectly transferable. The SCF approximation for non-polar bonds gives the geometry independent density matrix elements ... 

In a previous work [1,2], we were interested in the calculation of second order hyperpolarizabilities of eonjugated systems including substituted benzenes, pyridine N-oxydes and vinyl oligomers, in relation with non linear optical activity [3]. We showed that MNDO ealeulations were in good agreement with SCF ab initio results obtained using a double zeta basis set plus polarization and diffuse orbitals. 

Non-additive terms effects of polarization in the potential function. Polarization effects induced by the ionic presence on the ion-molecule system have been investigated. We use the expression by Lybrand and Kollman24 that includes in the potential function a self-consistent field (SCF) polarization energy Upoi based on classical electrostatics, given by ... 

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