Gouterman four-orbital model

Scheme 4.2 Frontier orbitals of the porphyrins according to the Gouterman four-orbital model for D4h point group (top). In the presence of a perturbation (substituents or ligand distortion) the degeneracy might be lifted (bottom). With permission from Wiley, ref 7a.

TPP (45, 46), as well as data for octaalkyl and octathioalkyl porphyrazines. As with the phthalocyanines and porphyrins, the electronic spectra of porphyrazines can be rationalized using Gouterman s four-orbital model, shown in Fig. 5 (47, 48). All of these macrocycles, when symmetrically substituted and with a metal ion incorporated in the central hole, for example, the M[pz(A4)] or B4, have D4h symmetry, with a doubly degenerate lowest unoccupied molecular orbital (LUMO) (eg) and two highest lying highest occupied molecular orbitals (HOMOs) that complete the four Gouterman orbitals with alM and a2u symmetry. 

Figure 5. Gouterman s four-orbital model. [Adapted from (47) and (48).]...

The four-orbital model of the porphyrin spectra as applied by Gouterman (49-50) also serves to explain the mechanism of metal-to-porphyrin backbonding in metalloporphyrins of the spectral hypso type. This backbonding has been invoked for these complexes by Williams (45) and Falk (20). 

Braterman, Davies and Williams (44) considered the possibility of configuration interaction between the porphyrin n states and the charge transfer states. According to the four-orbital model of Gouterman, discussed in Section IIB, the Soret and a— excited states are of Eu symmetry. The only allowed charge transfer transitions are from the porphyrin a u and aexcited states, and from the porphyrin a2u orbital to the metal... 

Gouterman M, Wagniere GH, Snyder LC (1963) Spectra of porphyrins Part II four orbital model. J Mol Spectrosc 11 108-127... 

Gouterman s theory is called the four-orbital model, because it considers only the two highest occupied (with electrons) molecular orbitals (HOMOs) of a porphyrin, and the two lowest unoccupied molecular orbitals... 

B and Q bands, but did provide qualitative data on how skeletal variations affect porphyrin absorption spectra. The stage was now set for the strengths of all these theories to be unified. Martin Gouterman did this by using configuration interaction, a technique vital in free electron and cyclic polyene theories, to describe the porphyrin excited states generated by Hiickel theory. He called this new synthesis the four-orbital model. ... 

The absorption spectra of cyclic tetrapyrroles are generally described by the four-orbital model first proposed by Gouterman for cyclic tetrapyrroles, resulting in four possible electronic transitions that are generally each split into two vibrational bands. In the fully unsaturated porphyrin-type pigments like... 

Fig. 3 Gouterman s four-orbital linear combination of atomic orbital model...

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