Walsh model bonding

Later there was an attempt by ab initio calculation to fit the electron structure of diazirine into the Walsh model of cyclopropane (69MI50800). According to these SCF-LCAO-MO calculations three MOs add to the description of the lone electron pairs, all of which also contribute to some extent to ring bonding. As to strain, 7r-character and conjugative effect, the term pseudo-rr-character was used. 

The problems of why certain theoretical models seem to work, henceforth designated the justification problem, is a constant preoccupation of theoretical chemists. The papers written on justification are numerous. We note here the most recent attempt by Buenker and Peyerimhoff to summarize arguments and data relevant to the justification of the old Mulliken-Walsh model of molecular structure which proved particularly successful in the interpretation of the dependence of bond angle and bond length distortions on the number of electrons (i.e. MO occupancy) in small molecules26S ... 

The effectiveness of the second type of overlap presumably determines the extent of oMnic character of tho epoxide. That ethylene oxides are less unaaturated in character than the corresponding cyclopropane derivatives is then attributable to a less favorable oxygen atomic-orbital orientation. The Walsh model is a satisfactory one in that it predicts accurately the C—H bond force constants... 

Fig. 3. The highest occupied molecular orbital capable of 7r-bonding in the Walsh model (73) for cyclopropane.

FIGURE 3. Alternative hybridization schemes in the Coulson-Mofiit model the hybridization state is between sp and sp and the maxima of the orbitals in the plane of the ring are directed at 22° to the C-C bonds. In the Walsh model there are sp hybrid bisecting the C-C-C angles and, perpendicular to these, unhybridized C(2p) orbitals all these orbitals being in-plane... 

FIGURE 4. Showing how molecular orbitals of the Coulson-Moffit and the Walsh models are made up by the linear combination of atomic orbitals of Figure 3. For each model the upper trio of symmetry d and a l is antibonding and the lower is bonding... 

The bent bond picture, later restated in terms of localized molecular orbitals, extends the model presented by Pauling and Slater for ethylene to a stem with three centers. Walsh s model parallels that of Mulliken for the a, w model of the double bond widely used in Hiickel theory. Since the canonical orbitals are good models for the interpretation of photoelectron spectra (see Introduction) we will discuss the Walsh model briefly. 

According to the Walsh model discussed before, one linear combination of the 3e orbital of cyclopropane is capable of interaction with a tt system in the so called bisected conformation a, the most stable conformation (s-trans) of vinylcyclo-propane The same model predicts for the perpendicular conformation b no interaction between the three-membered ring and the double bond. 

As mentioned earlier, oxidative complexation (pattern 3) and reductive decomplexa-tion (pattern 13) are nothing but alternate representations of 7r-complexation (pattern 2) and TT-decomplexation (pattern 12), respectively. It might be useful to note that the -TT-complex formalism and palladacyclopropane formalism correspond to the Walsh model and more commonly used bent o--bond representation of cyclopropanes, respectively. The palladacyclopropane formalism is useful in discussing the formation of larger palladacycles in terms of carbopalladation of palladacyclopropanes and paUadcyclo-propenes (Part IV). 

Furthermore, both of them have been used to describe bonding in cyclopropane (sp hybrids in the Moffitt-Coulson model and spVp pair in the Walsh model). 

The electron effects of substituents on the structure of the cyclopropane ring have received considerable attention during the year. Microwave studies on 1-cyano- and 1,1-dicyano-cyclopropane have shown that the remote C-2—C-3 bonds are reduced in length (1.500 and 1.485 A, respectively) when compared with cyclopropane (1.510 A). These observations are in accord with the simple Walsh model for electron-accepting substituents. The prediction of a lengthened C-1—C-2 bond was borne out by 1-cyanocyclopropane (1.529 A), but this bond length was not determined for... 

The protons are regarded as being embedded in the tt orbital of the double bond. This model accounts for the hindrance of internal rotation which occurs in this molecule (see later), but the B— B distance is greater, and the force constant lower, than is to be anticipated for a double bond, even if allowance is made for some weakening of the bond by the protons. From this formula acidic properties are to be expected, but they are not observed. Another alternative structure, Xy has been suggested by Walsh ... 

A second model is that of Walsh Here, the zeroth order model used sp hybrid orbitals to form the CH bonds, with the C-C bonds formed using p orbitals along with the sp orbitals pointing toward the center of the three-carbon unit. Again, the model predicts the... 

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