Molecular orbitals ethene

Fig. 1.17. Ethene molecular orbital energy levels. Energies are in atomic units. Erom W. L. Jorgensen and L. Salem, JTie Organic Chemists Book of Orbitals, Academic Press, New York, 1973.

Hence we have two molecular orbitals, one along the line of centres, the other as two sausage-like clouds, called the n orbital or n bond (and the two electrons in it, the n electrons). The double bond is shorter than a single C—C bond because of the double overlap but the n electron cloud is easily attacked by other atoms, hence the reactivity of ethene compared with methane or ethane. 

This will result in the display of ethene s highest-occupied molecular orbital as a solid. It is a tu orbital, equally concentrated above and below the plane of the molecule. The colors ( red and blue ) give the sign of the orbital. 

Serrano-Andres, L., Merchan, M., Nebot-Gil, I., Lindh, R., Roos, B. O., 1993, Towards an Accurate Molecular Orbital Theory for Excited States Ethene, Butadiene, and Hexatriene , J. Chem. Phys., 98, 3151. 

With both (la) and (2a) above, lateral overlap of the p atomic orbitals on adjacent carbon atoms could lead to the formation of two localised n bonds as shown, and the compounds would thus be expected to resemble ethene, only twice as it were This is indeed found to be the case with (2), but (1) is found to behave differently in terms of its slightly greater stability (referred to above), in spectroscopic behaviour (see below), and in undergoing addition reactions more readily than does an isolated diene (p. 194). On looking more closely, however, it is seen that with (la), but not with (2a), lateral overlap could take place between all four p atomic orbitals on adjacent carbon atoms. Such overlap will result in the formation of four molecular orbitals (Fig. 1.2), two bonding ( and 2) and two anti-bonding (i//3 and 4)—the overlap of n atomic orbitals always gives rise to n molecular orbitals ... 

Molecular orbitals are obtained by the linear combination of atomic orbitals, and the question of phase will, of course, arise with them too. Thus we can write the two MOs (it and it, cf. p. 12) arising from the two p atomic orbitals in ethene,... 

Figure 7. Molecular orbitals diagram of the mixing process involved in the insertion of ethene into a Mt-C(alkyl) bond for a generic d° neutral group 3 or cationic group 4 catalyst.

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. 

Problem 8.25 Apply the MO theory to 1,3-butadiene and compare the relative energies of its molecular orbitals with those of ethene (Problem 8.24). 

Four p AO s (see Fig. 8-2) give four molecular orbitals, as shown in Fig. 8-3. Wherever there is a switch from -I- to -, there is a node, as indicated by a heavy dot. Note that tt, of the diene has a lower energy than it of ethene. 

The molecular orbitals directly involved in the reaction are the four ic orbitals of the two ethene molecules ... 

Figure 21-6 Energies and schematic representations of the -u molecular orbitals of localized 1,3.5-cyclohexatriene. The molecular orbitals are the w orbitals of three localized ethene bonds and the total r-electron energy is 6(a + 0) = (6a + 6/3).

Figure 21-8 Energies and schematic representations of the n molecular orbitals of localized 1,3-butadiene. The orbitals are the n orbitals of two isolated ethene bonds and the total u-electron energy is 4(a + /3) = 4 a + 4/3.

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.)... 

Figure 4 Molecular orbital correlation diagram for the ortho addition of benzene to ethene. A plane of symmetry (a) is retained during the transition from benzene plus ethene to bicyclo[4.2.0]octa-2,4-diene.

---