Tt orbital energy levels

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

Finally, there are tig and tau rb and tt ligand orbital combinations which do not interact with metal orbitals. The molecular orbital energy level scheme expected for the bonding situation described above is shown in Fig. 2. 

FIGURE 3.41 The molecular orbital energy-level diagram for the TT-orbitals of benzene. In the ground state of the molecule, only the net bonding orbitals are occupied. 

Another intriguing ion, hexachlorobenzene dication (46), a four tt electron system, has been observed by Wasserman and his collaborators. As predicted by the simple molecular orbital energy-level pattern, the ion has two unpaired electrons.25... 

Figure 11.5 Molecular orbital energy level diagram for pyrite, FeS2 (from Bums Vaughan, 1970). Note the increase of A as a result of Tt-bond formation facilitated in low-spin Fe2+.

Fig. 5.13 Correct moJecutor orbital energy levels for early elements of the first long row. Some mixing (hybridization) has occurred between the 2r and Ip orbitals. Note that it is somewhat more difficult to keep books" and determine the bond order here ihan in Fig. 5.J2 3o-g and Itt, are clearly bonding (they lie below the atomic orbitals contributing lO them) 4(7,. and 5cr are essentially nonbonding since they lie between the atomic orbitals contributing to them and roughly symmetrically spaced about the center of gravity." The maximum net bond order is therefore one cr bond plus two tt bonds. The electronic configuration shown is for the B2 molecule. Note the unpaired pi electrons.

Fig. 4. Orbital energy levels of azines. Solid lines tt orbitals broken lines lone-pair orbitals (72HCA255 72HCA274). 

So once again we have three p orbitals to combine. This is the same situation as before. We have the same atoms, the same orbitals, and so the same energy levels. In fact, the molecular orbital energy level diagram for this compound is almost the same as the one for the allyl cation the only difference is the number of electrons in the Tt system. Whereas in the allyl cation 7t system we only had two electrons, here we have three (two from the tc bond plus the single one). Where does this extra electron go Answer in the next lowest molecular orbital— the nonbonding molecular orbital... 

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 ... 

In terms of this model, two effective pairs around an atom will always require sp hybridization of that atom. The sp orbitals of carbon in carbon dioxide can be seen in Fig. 9.15, and the corresponding orbital energy-level diagram for their formation is given in Fig. 9.16. These sp hybrid orbitals are used to form the a bonds between the carbon and the oxygen atoms. Note that two 2p orbitals remain unchanged on the sp hybridized carbon. These are used to form the tt bonds with the oxygen atoms. 

The larger the tt-system, the smaller the distance between the orbital energy levels and the smaller excitation energies. Benzene, naphthalene, anthracene, and letra-cene, with one, two, three, and four 6-cycles merged along the bonds, respectively, absorb in the UV region, Tetracene though also absorbs in the visible (blue) and is pale yellow. 

The An + 2) tt electron standard follows from the pattern of orbital energies m monocyclic completely conjugated polyenes The tt energy levels were shown for ben zene earlier m Figure 114 and are repeated m Figure Figure 11 13a and 11 13c... 

Fig. 10.1. TT-Molecular orbitals and energy levels for the pentadienyl cation.

---