MO analysis

The 260 nm band of chiral thiiranes is optically active and a Cotton effect is observed R) (+)-methylthiirane shows a negative Cotton effect at ca. 250 nm followed by a positive effect below 200 nm. An MO analysis indicates that charge transfer contributions are most important in determining the optical activity of the transition (81JCS(F2)503). The... 

Linear triatomic anions The pioneering crystallographic studies of Odd Hassel140 on trihalides and related donor-acceptor species led to a far-reaching analysis of such Hassel compounds by Henry Bent.141 The triiodide ion (I3-, stable in aqueous solution) and other known linear trihalide XYZ- species also served as the prototype for Pimentel s incisive three-center MO analysis of hypervalency. We shall therefore begin with NBO/NRT investigation of a series of hypervalent and non-hypervalent triatomic anions in order to make contact with these classic studies. While ab initio studies add many quantitative details to the understanding of these species, the basic picture sketched by Bent and Pimentel is found to be essentially preserved. 

It seems not entirely coincidental that the most effective catalysts are those with a pre-existing rhenium-oxygen bond indeed, in our studies none that lack this feature are active. According to MO analysis, the... 

Butene exists as an equilibrium mixture of two conformations, Me-skew and Me-syn (21). The most reliable composition to date is 83 17% according to combined ED, microwave (MW), and ab initio MO analysis (133). This study includes the MM (CFF)-ED-MW analysis of this molecule for comparison, which gave a final skew/syn ratio of 80 20. The molecular orbital constrained electron diffraction (MOCED) results appear to agree better with the observed data than does the MM constrained analysis, the R value of the least-squares analysis of the latter being 20% higher than that of MOCED. However, one may ask whether such a small difference in R values justifies the enormous difference in computer time between the ab initio (about 200 hr on an IBM 370/155) and MM (less than a minute) methods used in this work. 

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

In the following paragraphs we present a generalized MO analysis of some of the compounds whose chemistry and structures were described in Section 2. Where possible, the results of simple HMO methods are compared with the findings of all electron calculations. The latter approach refines the former without contradicting its basic conclusions. 

D The MO Analysis of Reactivity in 1,2-DiarylethyIene — 4a,4b-Dihydro-phenanthrene Systems... 

The MO analysis of reactivity in processes A, B and C by the explicit computation of energy profiles for the reaction path as described above for the parent system could be undoubtedly carried out also for the other members of this series. However, considering the large size of these systems and the relative lack of perfection of the computations still practical in such cases there are important advantages in resorting to approximate reactivity analyses which depend on the application of Perturbation Theory. 

On the other hand, the MO procedure, while difficult to apply in many cases, provides information which is inaccessible with a simple VB approach. Specifically, stereochemical questions and problems of reactivity which require the use of symmetry arguments depend on an MO analysis since the MO formalism rests heavily on symmetry considerations. 

Details of the MO analysis of [Mo3(/a3-S)2(ja2-S)3(PMe3)6] have been reported (125). The cluster unit Mo3S5 is the first member of the Chevrel cluster Mo3nS3n+2, and development of the chemistiy of this cluster complex is anticipated (Fig. 32). 

On the other hand, the results of an MO analysis might seem at odds with the obvious fact that in molecules such as CH4 or SFb all of the a bonds are equivalent. Actually, there is no inconsistency. If the electron density in the molecule is computed from the wave functions for all of the filled MOs (e.g., the A i and T2 MOs in CH4) the equivalence of all the bonds will be evident. At the same time, the fact that these equivalent bonds are a result of the presence of electrons in nonequivalent MOs is also experimentally verifiable by the technique of photoelectron spectroscopy. 

The type of symmetry-based MO analysis employing extended Hiickel calculations which R. Hoffmann and his students employ is well illustrated in the following two important papers ... 

At first sight, it is difficult to understand why enolates should undergo O-meth-ylations with diazomethane.20 The reason is that the CN bond of diazomethane is stronger than the 73 kcalmoR1 shown in Table 5.1, because it has a partial double bond character (as we can see from an MO analysis or the resonance hybrids below). Consequently, o CN is relatively high-lying, so O-methylation occurs ... 

This method can be successfully applied to the case of a solvation effect on the proton chemical shift. However, the effect cannot always be explained by this method. The quantity is very sensitive to the solute-solvent interaction and a serious drawback inherent in the classical-quantum hybrid approach is revealed. The result of ab initio MO analysis for small clusters suggests that electron exchange between solute and solvent is crucial to compute correct values of the chemical shift. A few attempts have been made to overcome this deficiency [18]. 

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