Explanation semiempirical

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. 

In his PGSE study of polyethylene oxide and polydimethylsiloxane in QHg and CHC13, Tanner 39) also measured the diffusion of a fixed fraction of solvent in the polymers. He concluded that their diffusion rate in polymers of molecular weight lower than their own was approximately equal to that of the polymers. As the polymer molecular weight exceeded that of the solvent, the solvent diffusion rate approached a constant value, independent of polymer molecular weight. Tanner offered semiempirical explanations for this effect. 

For explanation of experimental results and for correlation of charge densities with NMR data, semiempirical quantum-chemical calculations of benzo[c]pyrylium cation have been employed. Interestingly, the first calculation of 1,3-dimethyl-benzo[r]pyrylium cation by the simple linear combination of atomic orbitals/molecular orbital (LCAO/MO) method (70KGS1308) revealed a preference for the resonance from a in which the value of the charge density at C was three times as much as at C3. 

The semiempirical quantum chemical consideration led to the conclusion that the discrepancy can be a consequence of ion-pair formation (Zuilhof Lodder 1995). In the case of cyclo-octatetraene, the ion-pairing phenomenon deserves more detailed explanation. While the dianion of cyclo-octatetraene is completely planar and meets all the requirements of aromaticity, the anion radical of cyclo-octatetraene is a nonaromatic species and is not completely planar. In the equilibria just considered, both the anion radical and the dianion had alkali cations as counterparts. The dialkali salt of the dianion has two cations symmetrically located over and beneath the octagonal plane. Distortions from ion pairing between the dianion plane and these two alkali cations are reciprocally compensated. With the... 

Abstract We review the potential contribution of single fullerenes and buckyonions to interstellar extinction. Photoabsorption spectra of these molecules are compared with some of the most relevant features of interstellar extinction, the UV bump, the far UV rise and the diffuse interstellar bands. According to semiempirical models, photoabsorption by fullerenes (single and multishell) could explain the shape, width and peak energy of the most prominent feature of the interstellar absorption, the UV bump at 2,175 A. Other weaker transitions are predicted in the optical and near-infrared providing a potential explanation for diffuse interstellar bands. In particular, several fullerenes could contribute to the well known strong DIB at 4,430 A comparing cross sections and available data for this DIB and the UV bump we estimate a density of fullerenes in the diffuse interstellar medium of 0.1-0.2 ppm. These molecules could then be a major reservoir for interstellar carbon. 

The reactivities of the hydroxyl groups of D-glucal and D-galactal are in the order 0-4 > 0-3 > 0-6 for methylation and benzylation conducted in Ar,Ar-dimethy 1 form am ide via the sodium or lithium alkoxides or electrochemically.86 These observations are corroborated by semiempirical calculations87 and are in interesting contrast to the selectivities just given. A possible explanation is that while the last-reported data pertain to reactions of oxyions, the others may refer to those of the undissociated alcohols. 

The MO-type calculation has been employed by Bramanti et al. (4) to explain the absorption spectrum of Tl+ in KC1. The calculated positions of the TT energy levels in KC1 explained changes in the spectrum on going from free ions to the solid state. Similarly, in the case of hydrogen impurity in LiF, Hayns (5) has shown that the excitation energy predicted by calculation is in accord with experimental results. These results have inspired confidence in the semiempirical method as a means for providing qualitative explanations for several electronic phenomena. 

The very concept of resonance originated in VB theory and was extensively exploited in qualitative explanations of stability and other properties of tt-electron systems [78], surviving vitriolic attacks by some Soviet philosophers of the time. Recent re-examination of these ideas [61,65,79-82], based on new semiempirical and ab initio results, shows that the concept of resonance or 7r-electron delocalization is very subtle and prone to misinterpretation when improperly isolated. 

Because this chapter is a follow-up of previous work in the field it is not necessary to repeat the basics of ab initio methods. This has been done in detail by Basch and Hoz, who also discuss the most important atomic properties of Ge, Sn and Pb. We also recommend the theoretical section in the chapter by Apeloig about organosilicon compounds in this series who gave an excellent overview about the most important aspects of ab initio, semiempirical and force-field methods. The reader will find there an explanation of the most common standard methods which will be mentioned in this review without further explanation. We will focus in the following on those theoretical and computational aspects of methods which are particularly important for heavy-atom molecules that have been advanced in the last decade, i.e. ECPs and DFT. We also briefly discuss relativistic effects. We point out that semiempirical methods" and force field parameters are available for the elements Ge, Sn and Pb. However, the application of the two methods has not gained much popularity and not many papers have been published in the field. Most reports are restricted to special problems. ... 

A nonempirical approach to the chemical reactivity may of course be made along the same lines as has been practised for years in treatments by semiempirical all-valence electron methods. Typically, the results of such treatments provide qualitative explanation of the observed facts and give guidance for further experiments. Here we shall deal only with what may be taken as the ultimate goal of ab initio calculations in the field of chemical reactivity - the predictions of absolute values of equilibrium and rate constants. 

All MO calculations, both semiempirical and ab initio, have demonstrated that the barrier to aZ>3h-C4v-D3h interconversion, the Berry pseudorotation process, is small for acyclic phosphoranes (see also Section 2.1.). Estimates of 1.4 (extended Hiickel MO34 ), 4.8 (ab initio35 ) and 3.5 kcal/mol (CNDO/231 ) have been obtained for PFS. The barrier computed from the ab initio calculation increases to 8.5 kcal/mol if the 3c -orbitals are not included. The turnstile rotation mechanism traverses a much higher energy barrier 10.0 (extended Hiickel MO), 18.1 (ab initio) and 9.1 kcal/mol (CNDO/2). On the basis of these calculations, the Berry pseudorotation mechanism must be the preferred explanation of pentatopal isomerisation in acyclic phosphoranes (see Section 2.1.). 

Jeffry L. Ramsey holds a B.A. and an M.S. in chemistry and a Ph.D. in the conceptual foundations of science from the University of Chicago. He was assistant professor of philosophy at Oregon State University, Corvallis, and is now at Smith College. His research focuses on how scientists view questions about theory construction, explanation, and reduction when they are faced with problems that are insoluble in practice or in principle (e.g., in the area of semiempirical models). He is also interested in questions of conceptual analysis as they arise in the chemical sciences (e.g., the case of shape). His essays have appeared in Philosophy of Science, Studies in History and Philosophy of Science, and Synthese, among other journals. 

The first step in quantitative description of pure polyamorphic fluid is a selection of the model that can qualitatively describe a possible multiplicity of critical points in wide range of temperatures and pressures. A great many of explanations of multicriticality in monocomponent fluids (perturbation theory models semiempirical models lattice models, two-state models, field theoretic models, two-order-parameter models, and parametric crossover model has been disseminated after the pioneering work by Hemmer and Stell Here we test more extensively the modified van der Waals equation of state (MVDW) proposed in work and refine this model by introducing instead of the classical van der Waals repulsive term a very accurate hard sphere equation of state over the entire stable and metastable regions... 

---