1,3-Butadiene, excited state

Broad-line NMR, 359 6-type transition, 219 Butadiene excited states, 312-313... 

As computational methods for describing excited states have been refined, additional understanding of the structures has developed. Relatively early computational studies provided some indication of the geometries associated with the butadiene excited states.The ground state has a maximum at a twist of 90° about the C(l)-C(2) bond. This structure, which can be approximately described as a singlet methylene-allyl diradical, is found at about 2.3 eV and is more stable than a structure with 90° twist at both terminal groups (3.1 eV). There is no major pyramidalization of the methylene groups in this second structure. The spectroscopic (Franck-Condon) state is about... 

The reverse reaction, closure of butadiene to cyclobutene, has also been explored computationally, using CAS-SCF calculations. The distrotatory pathway is found to be favored, although the interpretation is somewhat more complex than the simplest Woodward-Hoffinann formulation. It is found that as disrotatory motion occurs, the singly excited state crosses the doubly excited state, which eventually leads to the ground state via a conical intersection. A conrotatory pathway also exists, but it requires an activation energy. 

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

The photochemical behavior of butadienes has been closely studied. When these compounds are exposed to light, they move from the ground state to an excited state. This excited state eventually returns to one of the ground state conformations via a process that includes a radiationless decay (i.e., without emitting a photon) from the excited state potential energy surface back to the ground state potential energy surface. 

The red line follows the progress of the reaction path. First, a butadiene compound b excited into its first excited state (either the cis or trans form may be used—we will be considering the cis conformation). What we have illustrated as the lower excited state is a singlet state, resulting from a single excitation from the HOMO to the LUMO of the n system. The second excited state is a Ag state, corresponding to a double excitation from HOMO to LUMO. The ordering of these two excited states is not completely known, but internal conversion from the By state to the Ag state i.s known to occur almost immediately (within femtoseconds). 

Problem 30.1 Look at Figure 30.1, and tell which molecular orbital is the HOMO and which is the LUMO for both ground and excited states of ethylene and 1,3-butadiene. 

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. 

TABLE 6. CASPT2 vertical excitation energies (eV) for the low-lying excited states of butadiene, hexatriene and octatetraene0... 

In the above example although the ground state orbital of cyclobutene, o, correlates with the ground state orbitals of butadiene /b the n orbital of the former does not correlate the /2 orbital of the latter, but rather it correlates with the /3 which is an excited state. So in such a case the thermal transformation by disrotatory processes will be a symmetry forbidden reaction. 

On the other hand in a photochemical transformation by a disrotatory process, one electron is promoted from jt to n orbital and so the o, n and it orbitals of cyclobutene would correlate with /. /2 and /3 orbitals of butadiene. Thus the first excited state of cyclobutene, since it correlates with the first excited state of butadiene, therefore, the process would be a photochemically symmetry allowed process. 

The first excited state of cyclobutene (o27t ) is correlated with the upper excited state ( /J /2 /3) of butadiene making it a high energy symmetry forbidden process. 

Similarly the first excited state of butadiene V1V2V3 is correlated with a high energy upper excited state G27tc of cyclobutene. Thus a photochemical conrotatory process in either direction would be a symmetry forbidden reaction. 

SCHEME 2. Excited state reaction path for butadiene... 

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