Clustered chromophores

Marcus AH, Payer MD (1991) Electronic excitation transfer in clustered chromophore systems—calculation of time-resolved observables for intercluster transfer. J Chem Phys 94(8) 5622-5630. dol l0.1063/1.460498... 

Once the alcohol or at least the cluster contains a soft ionization or fluorescence chromophore, a wide range of experimental tools opens up. Experimental methods for hydrogen-bonded aromatic clusters have been reviewed before [3, 19, 175]. Fluorescence can sometimes behave erratically with cluster size [176], and short lifetimes may require ultrafast detection techniques [177]. However, the techniques are very powerful and versatile in the study of alcohol clusters. Aromatic homologs of ethanol and propanol have been studied in this way [35, 120, 121, 178, 179]. By comparison to the corresponding nonaromatic systems [69], the O—H - n interaction can be unraveled and contrasted to that of O—H F contacts [30]. Attachment of nonfunctional aromatic molecules to nonaromatic alcohols and their clusters can induce characteristic switches in hydrogen bond topology [180], like aromatic side chains [36]. Nevertheless, it is a powerful tool for the size-selected study of alcohol clusters. 

Aromatic alcohol clusters have been well-studied, also for methodical reasons. The UV chromophore can be exploited for sensitive detection of the IR spectrum [35, 36, 120, 179]. Time-domain experiments become possible [21], which show that the initial energy flow out of the O—H stretching mode occurs primarily via C—H stretching and bending doorway states. Like in the case of carboxylic acid dimers [245], the role of the hydrogen bond is to shift the O—H stretching mode closer to these doorway states and thus to accelerate the initial energy flow. 

We wanted to extend this approach to include dynamical effects on line shapes. As discussed earlier, for this approach one needs a trajectory co t) for the transition frequency for a single chromophore. One could extract a water cluster around the HOD molecule at every time step in an MD simulation and then perform an ab initio calculation, but this would entail millions of such calculations, which is not feasible. Within the Born Oppenheimer approximation the OH stretch potential is a functional of the nuclear coordinates of all the bath atoms, as is the OH transition frequency. Of course we do not know the functional. Suppose that the transition frequency is (approximately) a function of a one or more collective coordinates of these nuclear positions. A priori we do not know which collective coordinates to choose, or what the function is. We explored several such possibilities, and one collective coordinate that worked reasonably well was simply the electric field from all the bath atoms (assuming the point charges as assigned in the simulation potential) on the H atom of the HOD molecule, in the direction of the OH bond. 

The isolated enantiomers S (M ) and R (Mr) of a chiral molecule M exhibit the same spectral features since their physical properties are identical. However, their aggregation with a chiral chromophore of defined configuration (Cr/s) leads to the formation of two diastereomeric complexes with different spectral properties, i.e., and [C /yM ]. The lcR2PI spectroscopy is able to discriminate between Mj and by measuring the spectral shift of the diastereomeric [C /yM ] and [Cj5/5-Mj ] complexes with respect to that of the bare chromophore Cr/s- It is convenient to define the diastereomeric clusters as homochiral when the chromophore and the solvent have the same configuration, and heterochiral in the opposite case. 

D = -0.7 cm", E/D = 0.26 for the S = 3/2 center in dithionite-reduced AvV (B. J. Hales, unpublished results). MCD magnetization data for dithionite-reduced Avl and Avl (8) indicate that the EPR-detectable S = 3/2 paramagnets are also responsible for the temperature-dependent MCD transitions. Therefore, by analogy with the MoFe protein it seems probable that the S = 3/2 paramagnetic chromophore in dithionite-reduced Avl is a V-Fe-S cluster. However, the EPR and MCD studies indicate that this cluster has magnetic and electronic properties distinct from that of the Mo-Fe-S clusters in conventional Mo nitrogenases. 

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