Molecular orbitals cyclopropenyl

Draw an energy diagram for the three molecular orbitals of the cyclopropenyl system (C l I3). How ate these three molecular orbitals occupied in the cyclopropenyl anion, cation, and radical Which of the three substances is aromatic according to Hiickel s rule ... 

Fig. 20. (a) One component of the bonding et metal-benzene molecular orbital set. Diagrams (b) and (c) illustrate the effect of tilting the p orbitals of the benzene and cyclopropenyl rings. 

Figure 5.50 shows three related molecules, the 7-methyl substituted (the visual orbital progression explained here is not quite as smooth for the unsubstituted molecules) derivatives of the 7-norbomyl cation (a), the neutral alkene norbomene (b), and the 7-norbomenyl cation (c). For each species an orbital is shown as a 3D region of space, rather than mapping it onto a surface as was done in Fig. 5.49. In (a) we see the LUMO, which is as expected essentially an empty p atomic orbital on C7, and in (b) the HOMO, which is, as expected, largely the n molecular orbital of the double bond. The interesting conclusion from (c) is that in this ion the HOMO of the double bond has donated electron density into the vacant orbital on C7 forming a three-center, two-electron bond. Two n electrons may be cyclically delocalized, making the cation a bishomo (meaning expansion by two carbons) analogue of the aromatic cyclopropenyl cation [326], This delocalized bishomocyclopropenyl structure for 7-norbomenyl cations has been controversial, but is supported by NMR studies [327].

Fig. 2. Representations of the -molecular orbitals of the cyclopropenyl (left) and allyl (right) groups...

In the cyclopropenyl radical with three Ti-electrons, the third electron has to be accommodated in one of the two degenerate molecular orbitals causing the Jahn-Teller distortion, which results in a change in symmetry from Dj, to C2,.. The ab initio (STO-3G 4-31G), as well as semi-empirical (CNDO/2), molecular orbital calculations demonstrate that the odd electron is incorporated in the pyramidalized rr-type orbital giving rise to loss of resonance stabilization in the three-membered ring. Thus, the cyclopropenyl radical is considered to be a highly unstable, short-lived species. 

Following the instructions for drawing the tt molecular orbital energy levels of the compounds shown in Figure 15.2, draw the tt molecular orbital energy levels for the cyclohep-tatrienyl cation, the cycloheptatrienyl anion, and the cyclopropenyl cation. For each compound, show the distribution of the tt electrons. Which of the compounds are aromatic Which are andaromatic ... 

Figure 4.26 shows the FfMOs for cyclopropenyl carbocation, square planar cyclobutadiene, cyclopentadienyl anion, benzene, cycloheptatrienyl carbocation, and planar (Ds/,) cyclooctatetraene. If we place electrons into the molecular orbitals of each species according to the aufbau principle, we notice an important relationship between the stability of the systems and the... 

Synthetic and dynamic NMR studies of the ri3-triphenyl and Ti3-trimethylcyclopropenyl complexes [(ti -C5R5)Ru(ti3-C3R 3)X2] (R = H, Me R = Me, Ph X = Cl, Br, I) have been reported and the barrier to rotation of the ri3-cyclopropenyl ligands in isoelectronic molydenum and ruthenium complexes has been investigated using Extended Huckel molecular orbital calculations. 

Cyclopropene has the correct number of it electrons to be aromatic, namely 4(0) + 2 = 2, but it does not have a continuous closed loop of 2p orbitals. If, however, the CH2 group becomes a CH+ group in which the carbon atom is sp hybridized and has a vacant 2p orbital, thus still containing only two electrons, then the overlap of orbitals is continuous, and according to molecular orbital theory, the cyclopropenyl cation should be aromatic. The cyclopropenyl cation can be drawn as a resonance hybrid of three equivalent contributing structures. The fact that we can draw three equivalent contributing structures is not the key to the aromaticity of this cation the key is that it meets the Hiickel criteria of aromaticity. 

Hiickel pointed out that, on the basis of molecular orbital theory, monocyclic conjugated polymethines have filled shells of tt-electrons when the number of TT-electrons is An + 2, where n is an integer. These systems may be expected to be stable. The rule may be illustrated by reference to Fig. 2.1. If = 0, then a system with 27r-electrons should be stable. Such a situation is found in the cyclopropenyl positive ion, which has been isolated as the hexachloroanti-monate. For n = the prediction is that the cyclopentadienyl anion, benzene and the cycloheptatrienyl (tropylium) cation are stable. This is certainly in accord with experience. The stability of benzene is well known, the cydo-pentadienyl anion is readily formed by the action of potassium metal on cyclopentadiene, and the cycloheptatrienyl cation is one of the most stable carbonium ions known. Huckel s rule also predicts that some of the larger cyclic conjugated systems are stable, e.g. those with 10,14 and 18 rr-electrons. However, the situation is complicated by steric problems (see for example Garratt, 1971) and need not be considered further here. 

All of the examples used so far in this chapter have employed orbitals constructed from a single s orbital on each center. However, our arguments can be carried over without change directly to the case of planar organic tt systems. For example, the three pn orbitals of cyclopropenyl (5.12) may be represented as in 5.13 which shows the view of the upper lobes of the p orbitals projected onto the molecular plane. Thus, the level structure shown in Figure 5.1 for triangular H3 is identical... 

Because symmetry orbitals depend on symmetry and not on finer details of molecular structure, they recur again and again in molecules of similar symmetry. For instance, the a and e combinations of hydrogen coefficients discussed above for ammonia will be found for hydrogen AOs in staggered or eclipsed ethane (see the drawings in Fig. 13-14, for instance) and for carbon AOs in MOs for cyclopropenyl (Chapter 8). Because symmetry requirements transcend the differences between various approximate methods... 

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