Spectra porphyrins

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). 

As judged by visual observation, the widest bands in the porphyrins were broken into fairly uniformly spaced narrow bands. The other substances failed to show this phenomenon. Photographs of the porphyrin spectra at room temperature and at liquid air temperatures brought out the effect even more clearly. (Fig. 1.)... 

This article is not intended to present an exhaustive review of the extensive literature on haemoprotein spectra, which dates back almost 90 years. Our aim is to discuss the origin of the absorption bands of ferric haems and haemoproteins in the light of the most recent physico-chemical and theoretical studies. We consider that the spectra of ferrous haems and haemoproteins are reasonably well understood in terms of the theory of porphyrin spectra those of the ferric compounds are much more complex, and the number of distinct assignments and interpretations which have been offered in the literature in the last twelve years is approaching double figures. A critical summary of the relevant experimental facts and theoretical interpretations seems timely. 

All recent theoretical treatments of porphyrin spectra regard the lowest-lying absorption bands as intra-porphyrin re re transitions. Simpson (29) proposed a model in which the highest-energy filled re-orbitals had an angular momentum about the fourfold axis of L — 4. The lowest empty re -orbitals had L = 5, so that the lowest-lying re—re transitions corresponded to AL = 1 and 9. The former is strongly allowed and... 

Comparison of the uv-visible absorption spectra of these fractions (Figure 2) with the spectra of known vanadium chlorin, phylloerythrin and DPEP complexes (7) suggested that two (samples 422 and 4422) with absorption maxima at 600 nm were probably vanadium phylloerythrin complexes. These two fractions do not appear to be identical to the rhodo-type petroporphyrins with an absorption maximum at 590 nm isolated by Millson et. al. (8) and discussed in detail by Baker et. al. (9) because of the significant difference in the absorption spectra. One fraction (sample 630) may be a vanadium chlorin derivative, while the absorption spectra of the remaining four fractions (samples 3, 61, 421 and 441) were typical vanadium porphyrin spectra with maxima at 412, 538 and 575 nm. For these samples the ratios of the intensities of the 575 and 538 nm bands were in the range 1.15-1.20 which are consistent with the ratios... 

FIGURE 12.2 Porphyrin spectra. The spectrophotometric characteristics of iron-complexed TPPS4, like those of other porphyrins, are sensitive to changes in hydrophobicity, ionic strength, and pH. The first panel shows the absorbance spectrum of Fe-TPPS (0.8 pM) in 50 mM NaPi pH 7 (1) and those of the porphyrin in 50 mM NaPi pH 7 with 50% ethanol (2), 50% methanol (3), or 50% DMSO (4). The second panel shows the impact of ionic strength on the absorbance characteristics comparing spectra collected in 50 mM NaPi pH 7 (3) and 500 mM NaPi pH 7 (1). Also shown are absorbance spectra collected in 50 mM NaPi for pH 4.8 (2) and pH 9.2 (4). 

The pictorial classification of porphyrin spectra relates the number and relative intensity of the Q bands, in the Ccise of an unmetallated porphyrin, to the substituents on the p)UTole B- and meso-positions, or, in the case of a... 

Fig. 3.23 (a) Q band porphyrin spectra for metal-free porphyrins and a chlorin. (b) Q band spectra for porphyrin dications, monocation, and some metal complexes. In the latter, the smaller the ratio between the a and p band intensities the less stable the metal complex. 

Origins of metal-free spectra Owing to the central importance of porphyrins and related compounds in biology, there have been many attempts over the years to derive from theory the main facts of porphyrin UV-visible spectra, i.e. the positions and multiplicities of the B and Q bands. Foremost in this search has been the American theoretical chemist, Martin Gouter-man. He developed a theory of metal-free porphyrin spectra that allows an intuitive appreciation of the light-induced electronic movements that occur inside porphyrins and related molecules. Ultimately, this begins to answer questions such as why chlorins are used as sensitisers of photosynthesis. 

So, to summarise, FET and CPT produced surprisingly accurate but qualitative accounts of porphyrin spectra which could not, however, be generalised to consider variations in the porphyrin skeleton. On the other hand, simple Hiickel theory, which took account of the detailed shape of the porphyrin macrocycle, wrongly predicted the relative intensities of the... 

Figure 11.9. Fluorescence spectra of ZnTPPS-ZnTMPyP and the ZnTPPs-ZnTMPyP ion pair in aqueous solution (a). As for the absorption spectra in Figure 11.8, the fluorescence spectrum of the ion pair cannot be interpreted in term of a linear combination of the individual porphyrin spectra (b).

It is fair to note that the oscillator strengths of both the TDDFT and CASPT2 results still leave much room for improvement. The deficiency for the TDDFT case for this molecule is not shared by other choices for the xc functionals, and also does not show up in similar studies on metal-porphyrin spectra, where excellent agreement with experiment is usually obtained. A calculation on a system of this size can nowadays be performed in a few minutes. So-called linear scaling techniques make TDDFT calculations on molecules with hundreds of atoms feasible. 

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