Walsh model cyclopropane

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 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.

A. The Coulson-Moffit and Walsh Models of the Molecular Orbitals of Cyclopropane 220... 

The Coulson-Moffit and Walsh models of the cyclopropane orbitals have been used since the 1940s but there has been some argument about which, if either, provides an adequate description of the facts. Walsh s model has been the more popular because it is perhaps easier in concept and also because it was believed that the models are equivalents, although it was pointed out that they were not at the very outset more recent studies have particularized the differencesS S. ... 

The underlying reason could be that the Walsh model of cyclopropane has a quasi-cyclopropenyl ring system which could be more effective at accepting an electron pair than the external p system, which is so adept at electron pair donation. Delocalization into the former one would require the planar conformation observed for the radical anions 127 and 128 . 

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. 

The research on bicydopropylderivatives has led to two corollarys (1) The purely tangential Walsh model works only if the dihedral angle, 6, between both cyclopropane rings does not deviate much from zero degree. (2) The interaction integral Pvvw is found to be —1.73 eV, a value close to tliat found for vinylcyclopropane. 

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). 

Of the two, the Walsh model (scheme 74) has become by far the preferred one, especially if one wants to explain the conjugative properties of the cyclopropyl group in terms of overlap, e.g. between and a neighbouring p-type AO or group orbital, attached to the cyclopropane ring in position 1. For such a purpose, the model shown... 

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... 

There are two classical approaches to bonding in cyclopropane namely the Walsh molecular orbital approach and the bent bonds or banana bonds of the Coulson-Mofiit model. 

Several hypotheses concerning the electronic structure of cyclopropane have been suggested. This topic has been interestingly pinpointed by Bemett (I). A cyclopropane model proposed by Walsh (2) indicates that C-C bonds of the rings are caused by an overlap of one of the sp- hybridized orbitals of each carbon atom and of each orbital (Walsh proposed, in fact, an sp2 hybridization state for each cyclopropane carbon). 

Two models have been advanced to describe the bonding in the cyclopropane ring the bent bond model advocated by Coulson, Moffitt and Forster and the model proposed by Walsh Both descriptions are natural extensions to three centers of theoretical descriptions of the ethylenic two center double bond. 

In Fig. 10a no admixture of radial contributions is assumed (x = 0), in Fig. 10b we allow 20% admixture. The main difference between the ZDO result of Fig. 10b and the result of an extended Hiickel calculation (Fig. 10c) is the asymmetry of the plot in the latter case. Different orbital energy patterns are predicted for the s-cis (9 = 0°) and the s-trans (6 = 180°) conformation. This asymmetry can be traced back to the interaction of the central C-C a-bond with the Walsh orbitals of the two cyclopropane units in bicyclopropyl. It turns out that only the linear combinations Wa (S) and Wb (S) will be influenced by such an interaction This can be represented in the ZDO model by raising the energy of the Wg (S) orbital and lowering the energy of the Wb (S) orbital by a roughly similar amount. This refinement predicts that the difference in energy between the second and third level from the top should be smaller for the s-cis than for the s-trans conformation. 

The electronic effects of substituents on the structure of the cyclopropane ring continue to attract attention. The X-ray structure of 1,1,2,2-tetracyanocyclopropane shows the C-1 —C-2 bond to be lengthened (1.563 A), whereas the remote ring bonds in the cyclopropane derivatives (1), (2), (3), and (4) are shortened in accord with the Walsh orbital model. The microwave spectrum of (1) shows that the molecule adopts the bisected conformation depicted with the chlorine atom cis with respect to the C-1 proton. A bisected conformation is also observed in the dione (2) where the carbonyl groups are each cis with respect to the adjacent cyclopropane ring, but trans with respect to each other. The n.m.r. spectra of partially oriented chloro-, bromo-, and cy ano-cyclopropane provide some indirect evidence in support of the orbital theory... 

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