Molecular orbitals cyclopropane

Fig. 3.26 Frontier molecular orbital analysis for the Simmons-Smith cyclopropanation. [Dargel, T.K. Koch, W.J. Chem. Soc., Perkin Trans. 1996, 2, 877. Reproduced by pemiission of The Royal Society of Chemistry]...

CC and CH bond orbitals but also for the CTL, ami CH3 group orbitals. If the local symmetry elements are preserved in the full molecule, the 7r (or a) local orbitals can combine to give v (or o) molecular orbitals. The reader should, therefore, not be surprised to find, for instance, tt type molecular orbitals in cyclopropane which are delocalized over the CH2 groups. 

In the interaction of the local 2pv orbitals, two more bonding molecular orbitals are formed against one less bonding. In all previous cases the opposite occurred. This is due to the negative overlap between adjacent 2py orbitals—whether, by convention, all positive lobes point in the clockwise direction, or whether all positive lobes point in the anticlockwise direction. The two bonding 2pv combinations in fact fall below the two antibonding (hybrid 2s, 2px) combinations. The former each have two electrons while the latter are empty. The six electrons of the three C—C bonds are nicely accounted for. The method creates simultaneously the acc and or c molecular orbitals of cyclopropane (note that the latter three lie relatively close in energy). 

The relevant orbitals to consider are the molecular orbitals 5Aj and 6A, of cyclopropene (III.46) which correlate with the lowest acc orbital of cyclopropane (3A, III.56) and with one of the higher degenerate <7CC orbitals (3E, III.56). The cyclopropene orbitals are very similar to their cyclopropane counterparts. In particular the 6Aj orbital is a linear combination of all three crcc bond orbitals and extends over the entire cyclopropene molecule. Its amplitude, shown in Fig. 46, is fairly accurately represented by the... 

Cyclopropane, C.H, is a hydrocarbon composed of a three-membered ring of carbon atoms, (a) Determine the hybridization of the carbon atoms, (b) Predict the CCC and HCH bond angles at each carbon atom on the basis of your answer to part (a), (c) What must the real CCC bond angles in cyclopropane be (d) What is the defining characteristic of a cr-bond compared with a ir-bond, for example (e) How do the C—C cr-bonds in cyclopropane extend the definition of conventional o-bonds (f) Draw a picture depicting the molecular orbitals to illustrate your answer. 

The reported proposed sequence also offers two additional alternative mechanisms for the cyclodimerization of BCP (3), involving either intermediate 463 or 464 [6a, 13b]. However, they appear less likely, requiring successive three-membered ring fissions and formations. Alternatively, a thermally allowed concerted [jt2s + rt2a -I- pericyclic reaction involving the Walsh type molecular orbital of cyclopropane [124] has been proposed (Fig. 4) [13b]. 

Fig. 3. 2-4. Radial (bottom) and tangential (top) bonding molecular orbitals for cyclopropane [12e] (left) and hexa-borane(6) dianion (right) illustrating the planar and three dimensional a aromaticity, respectively. Tangential orbitals of [B6H6]2 are triply degenerate.

Problem 6-8. Cyclopropane, C3H3, has D3/1 symmetry. What is the symmetry species of a molecular orbital approximately equal to 2sa + 2sb -I- 2sc (2sa is a Is orbital centered on atom A, etc.) What is the symmetry species of a molecular orbital of the form 2p A + 2p,rB +... 

In diastereomeric bi- and tricyclic ring systems containing three-membered rings, the 13C chemical shifts can vary considerably due to molecular orbital interactions with the cyclopropane electrons (see review s on this topic430,431). 

No discussion about strained-ring radical cations would be complete without the valence isomers quadricyclane (15 +) and norbornadiene, (16 +) 15 features two adjacent rigidly held cyclopropane rings, whereas 16 contains two ethene n systems well suited to probe through-space interactions.Molecular orbital considerations suggest the antisymmetric combination of the ethene n orbitals (16) or cyclopropane Walsh orbitals (15) as respective HOMOs of the two parent molecules. The radical ions have different state symmetries and their SOMOs have different orbital symmetries. 

These interactions may usefully be described as an acid-base type interaction, in which the cyclopropane ring acts as a base (electron donor) and the proton or cationic center acts as the acid (electron acceptor). One of the factors that controls the basicity of a hydrocarbon is the energy of the highest occupied molecular orbital (HOMO).60 The 6-31 G HOMO energies of some cycloalkanes and cycloalkenes are given in Table 4.61... 

The key to the understanding of physical properties and chemical reactivity of 1 is found in the electronic structure of the molecule, which can be described in terms of molecular orbitals (MOs), valence bond (VB) orbitals or its electron density distribution p(r). Numerous investigations have considered the MOs of 12311 and, therefore, one could expect that a review article on cyclopropane appearing in the year 1995 can skip this part by just referring to one of the previous review articles 1-14 20. However, there is considerable confusion among chemists with regard to the appropriate MOs of cyclopropane, which needs clarification. 

TABLE 13. Distortion of the cyclopropane ring upon substitution at Cl by substituent X. Predictions by the molecular orbital (MO) and electron density (ED) model0 33 97... 

Exercise 23-6 Explain why the configuration of the nitrogen in 1-ethylazacyclo-propane, 1, is more stable than in triethylamine. Why is the configuration of oxaza-cyclopropanes, such as 2, exceptionally stable (Consider the tt molecular orbitals of an ethene bond, Figure 21-3, as a model for orbitals of the adjacent O and N atoms in the planar transition state for inversion in 2.)... 

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