Hexatrienes excited states

Figure 30.2 The six n molecular orbitals of 1,3,5-hexatriene. In the ground state, the three bonding MOs are filled. In the excited state,

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. 

Fig. 7.33. The symmetry of the highest occupied MO in the groundstate and the excited-state hexatriene molecules...

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

In a recent comprehensive study at the CASSCF level of ab initio theory, Cave and lohnson have carried out calculations for all six rotamers of the hexatriene radical cation. In agreement with experiment they found that the first excited state is hardly affected by the additional interactions which prevail in partially cA-configurated rotamers, whereas the energy of the second excited states decreases as the number of those cA-interactions increases. On this basis, they were able to confirm some of the original assignments of the observed spectra305 but proposed revisions for some of the others. 

On the other hand, the 1,3,5-hexatriene was postulated to be formed from the singlet excited state. On the basis of electron-scavenging experiments it was concluded that the... 

Figure 27 (a) The cyclohexadiene (CHD)/cZc-hexatriene (cZc-HT) photoconversion problem involves the formation of a common excited state intermediate (cZc-HTF )) and its decay via a conical intersection point (CICHD). (b) Because CICHD has a tetraradical electronic nature, relaxation on S0 may occur along three different routes (a, b, and c) associated with different bond formation modes and different recouplings. (From Ref. 82.)... 

Myers, A.B., Mathies, R.A., Tannor, D.J., and Heller, E.J. (1982). Excited state geometry changes from preresonance Raman intensities Isoprene and hexatriene, J. Chem. Phys. 77, 3857-3866. 

M. Olivucci, F. Bernardi, P. Celani, I. Ragazos, M. A. Robb, J. Am. Chem. Soc. 116, 1077 (1994). Excited-State Cis-Trans Isomerization of cA-Hexatriene. A CAS-SCF Computational Study. 

FIGURE 7.10 Covalent resonance structures for hexatriene. (b) Generation of symmetry adapted combinations of R2 and R3. (c) A VB mixing diagram of the symmetry adapted VB combinations yielding the covalent ground and excited states. 

Fig. 7. Disrotatory electrocyclic ring opening of 1,3-cyclohexadiene to 1,3,5-hexatriene. The excited state process, forbidden in the disrotatory mode, as indicated on the right, is allowed in the eonrotatory mode (not shown).

Highest occupied molecular orbital of the first excited state of hexatriene... 

As described in the discussion of UV-visible spectroscopy in Section 15.1, the absorption of a photon of light by a compound causes an electron to be excited from an occupied MO to an unoccupied MO. The lowest-energy excitation occurs when an electron in the HOMO is excited to the LUMO. In the case of 1,3,5-hexatriene an electron in 773 is promoted to 774 to form the lowest-energy excited state (see Figure 22.3). in the excited state, the HOMO is 774 and the LUMO is 775. It is this excited state that reacts in a photochemical reaction. 

Energies and nodal properties of the pi MOs for the ground state and the excited state of 1,3,5-hexatriene. 

So far we carried out only a very preliminary study of the excited states [62] for four typical 7r-electron systems CBT, benzene, hexatriene and naphthalene. In all cases we found that the PPP-VB method can successfully describe the low lying excited states using only a small subset of VB structures. For states of a predominantly covalent character, the same OEAO basis as for the ground state can be used. However, for higher lying states with significant ionic character, a re-optimized OEAO basis is preferable. 

However, on irradiation (i.e. photochemically) an electron is promoted from i 3 to vj 4 in hexatriene. Now all the orbitals of hexatriene (in the excited state) correlate with the orbitals of cyclobutadiene. 

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