Axial excited state

Another well-defined configuration of the classical three body Coulomb problem with unambiguous quantum correspondence is the collinear antisymmetric stretch configuration, where the electrons are located on opposite sides of the nucleus. In contrast to the frozen planet orbit, the antisymmetric stretch is unstable in the axial direction (G.S. Ezra et.al., 1991 P. Schlagheck et.al., 2003), with the two electrons colliding with the nucleus in a perfectly alternating way (Fig. 3 (left)). Hence, already the one dimensional treatment accounts for the dominant classical decay channel of this configuration. As for the frozen planet, there are doubly excited states of helium associated to the periodic orbit of the ASC as illustrated in Fig. 3 (left). 

Asymmetry in the ligand environment, either geometric or in charge distribution (or both), affect the asymmetry parameter, tp An r = 0 value corresponds to complete axial symmetry, whereas r = 1 corresponds to pure rhombic symmetry. Electric monopole interactions between the nuclear charge distributions and the electrons at the nucleus cause a shift of the nuclear ground and excited states. These interactions are known as the isomer shift, 8. Both the Mossbauer source and the absorber (the sample of interest) experience an isomer shift, and it is customary to quote 8 relative to a standard, usually Fe metal or Na2[Fe(CN)5NO] 2H2O at... 

The low-lying excited states of the hydrogen molecule conhned in the harmonic potential were studied using the configuration interaction method and large basis sets. Axially symmetric harmonic oscillator potentials were used. The effect of the confinement on the geometry and spectroscopic constants was analyzed. Detailed analysis of the effect of confinement on the composition of the wavefunction was performed. 

While it is apparent that there are several pathways for relaxation of the excited state of iron porphyrins, the exact steps involved in these different pathways are still to a great extent unknown. It is the purpose of this study to explore the role of spin state and axial ligands in the decay process of the excited states of iron porphyrins. 

The relaxation of Fe(II) protoporphyrin dimethyl ester in benzene is seen in Figure 1. It can be seen tnat in this complex which has no axial ligands and a high spin state, S=2 (14), there is an excited state absorption band which extends from 520 nm into the blue. The peak is not seen because of experimental cut-off in the spectrum at 450 nm. This difference spectrum also indicates an absorption band extending to the blue from 620 nm and interupted by the strong ground state bleaching at 570 nm. The peak of the excited state absorption band seems to be centered at about 585-590 nm. 

Dissociation of axial ligands has been followed by picosecond spectroscopy for a number of metalloporphyrins. For the well-known photodissociation of O2 and CO from hemoglobin and myoglobin the photoproducts appear very early < 10 psec. Dissociation of basic axial ligands such as pyridine and piperadine occurs within the lifetime of the excited state for Ni(II), Co(III) as well as for Fe(II) porphyrins. Whether the ejected species is "hot" with energy from the electronic deactivation of the porphyrin is not known, but the dissociation process does not appear to be dependent upon the wavelength of the excitation pulse (30,32). 

The ease of formation of porphyrin radicals and the existence of ground state charge transfer into the axial ligands (3 ) makes it necessary to keep in mind the possibility of ligand ejection accompanied by oxidation-reduction during the excited states of metal1oporphyri ns. 

Excited State Axial Ligation Processes in Coordinating Solvents. 

We have previously investigated ligand release in the 6-coordinate piperidine complexes of Ni octaethylporphyrin (10). For Ni(OEP), formation of the 6-coordinate complex is not complete, and so, the picture is complicated by the presence of both 4- and 6-coordinate species in the initial sample. However, upon excitation with the pulsed laser the relative proportions of the two Ni(OEP) species change as determined by changes in the relative intensities of the 4- and 6-coordinate sets of Raman marker lines. Thus, axial ligand release is observed in the excited state generated during the... 

A for the 5-coordinate species, and 2.01 A for the unligated B. transient excited state. In 5-coordinate complexes the axial ligand-Ni stretching mode has been identified. The mode can also be used to follow coordination changes. Five-coordinate Ni-porphyrin complexes have been observed, so far, only in the Ni-reconstituted proteins. 

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