(a) Draw the structures of various iron-sulphur proteins and their corresponding redox states. (10 marks) (b) How would you…

(a) Iron–sulphur proteins and their redox states Fe–S proteins have iron bound to cysteine thiolate and inorganic sulphide (S²⁻), and act as electron carriers. Rubredoxin: one Fe tetrahedrally bound to four Cys-S; Fe³⁺/Fe²⁺ (no inorganic sulphur). [2Fe–2S] ferredoxin: two Fe bridged by two S²⁻ (a planar Fe₂S₂ rhombus), each Fe also bound to two Cys-S; oxidised Fe³⁺Fe³⁺, reduced Fe³⁺Fe²⁺ (one electron added; the electron is localised on one Fe). Rieske proteins bind two His on one Fe. [4Fe–4S]: a cubane Fe₄S₄ core with each Fe bound to one Cys-S. Ferredoxins cycle between [4Fe–4S]²⁺ (2Fe³⁺ + 2Fe²⁺) and [4Fe–4S]⁺ (1Fe³⁺ + 3Fe²⁺); high-potential iron proteins (HiPIP) cycle between [4Fe–4S]³⁺ and [4Fe–4S]²⁺. [3Fe–4S]: a cubane lacking one Fe (as in inactive aconitase), with 1+/0 states. Sulphur stays as S²⁻, so only iron changes oxidation state.

(b) Fluxional molecules Fluxional molecules show rapid intramolecular rearrangements, so that their NMR spectrum depends on temperature: at the slow-exchange limit (low T) each environment gives a separate signal, and at the fast-exchange limit (high T) a single time-averaged signal appears, with broadening and coalescence in between. (i) (C₅H₅)₄Ti is (η⁵-C₅H₅)₂Ti(η¹-C₅H₅)₂. Two rings are π-bonded (pentahapto) and two are σ-bonded (monohapto, via one C), giving a 16-electron complex (Ti(IV) d⁰ plus 2 × 6 + 2 × 2 electrons). At room temperature a single sharp ¹H singlet appears (all 20 H equivalent) because the σ-rings undergo 1,5 shifts (“ring whizzing”) and η⁵/η¹ exchange. On cooling, the spectrum splits into two singlets in the ratio 1:1 (10 H of η⁵ rings and 10 H of the η¹ rings, themselves averaged), confirming the mixed bonding. (ii) C₃(CH₃)₄ is tetramethylallene, Me₂C=C=CMe₂, bound to Fe(CO)₄ through one C=C double bond: an 18-electron, trigonal-bipyramidal η²-allene complex. At low temperature the ¹H NMR shows two methyl signals (coordinated CMe₂ and free CMe₂). On warming, Fe(CO)₄ migrates between the two C=C bonds, the signals broaden, coalesce, and a single methyl singlet results at high temperature, showing the iron is equally shared.

(c) Quantum yield Energy of one photon = hc/λ = (6.626 × 10⁻³⁴ × 3 × 10⁸)/(2.5 × 10⁻⁷) = 7.95 × 10⁻¹⁹ J. Photons absorbed = 80/7.95 × 10⁻¹⁹ = 1.006 × 10²⁰; in moles (einsteins) = 1.006 × 10²⁰/6.022 × 10²³ = 1.67 × 10⁻⁴. Oxalic acid decomposed = (0.05 − 0.04) mol L⁻¹ × 0.010 L = 1.0 × 10⁻⁴ mol. Quantum yield Φ = moles reacted/einsteins absorbed = 1.0 × 10⁻⁴/1.67 × 10⁻⁴ = 0.60. Uranyl sulphate acts as photosensitiser (actinometer); Φ < 1 means that not every absorbed photon causes decomposition.