Em. Maes et al., Ligand conformational effects on the resonance Raman signature of [Fe4S4(SAryl)(4)](2-) clusters, J PHYS CH B, 104(46), 2000, pp. 10878-10884
Low-temperature.(77 K) resonance Raman (RR) spectra are reported for a seri
es of analogues of [4Fe-4S] (4) (tp = thiophenol and dmtp = dimethylthiophe
nol). Structural influences of these different terminal thiolates on the RR
signature of the [4Fe-4S](2+) core are presented. Both the cluster and ter
minal ligand Fe-S vibrational modes are identified and assigned through S-3
4 isotope substitution of analogues 1 and 4 at the cluster sulfur position.
The characteristic RR frequencies of the four compounds provide new insigh
ts into the vibrational and electronic properties of the iron-sulfur cluste
r due to the ligand conformation changes. The RR spectra demonstrate a sign
ificant D-2d distortion of each cluster, and an energy crossing is observed
for the T-2 terminal (t) and bridging (b) Fe-S stretching modes above 350
cm(-1) of 1 and 4 in response to changes in the ShFe-S-t bond angles impose
d by different geometries about the Fe-S-t bonds, The results affirm the ab
solute necessity of isotope substitution for an exact assignment of the Fe-
S-t and Fe-S-b vibrational modes in [4Fe-4S] clusters when the [4Fe-4S] pro
teins are analyzed and compared. The blue-shifted to 404 nm electronic abso
rption band of 4 is identified as a predominantly S(Aryl) --> Fe charge-tra
nsfer transition by excitation profiles for two prominent Fe-S RR bands, th
e A(1) bridging stretch at 336 cm(-1) and the T-2 terminal stretch at 351 c
m(-1).