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

(b) How would you account for bonding in the following fluxional molecules based on ¹H NMR spectral studies at variable temperatures?
(i) (C₅H₅

Question

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

(b) How would you account for bonding in the following fluxional molecules based on ¹H NMR spectral studies at variable temperatures?
(i) (C₅H₅)₄ Ti
(ii) C₃(CH₃)₄Fe(CO)₄ (20 marks)

(c) Radiation of wavelength 2500 Å was passed through a cell containing 10 ml of a solution which was 0·05 molar in oxalic acid and 0·01 molar in uranyl sulphate. After absorption of 80 joules of radiation energy, the concentration of oxalic acid was reduced to 0·04 molar. Calculate the quantum yield for the photochemical decomposition of oxalic acid at the given wavelength.
(Given : N = 6·022×10²³ mol⁻¹, h = 6·626×10⁻³⁴ J s and c = 3×10⁸ m s⁻¹) (20 marks)

UPSC Answer Check · Accepted Answer

Begin with a brief introduction acknowledging the diverse nature of the three parts covering bioinorganic, organometallic, and photochemistry domains. Allocate approximately 20% time/space to part (a) on iron-sulphur proteins, 40% to part (b) on fluxional molecules with detailed NMR analysis for both compounds, and 40% to part (c) with systematic calculation showing all steps. For (b), explicitly state the directive 'How would you account for bonding' requires explaining the dynamic processes and temperature-dependent NMR coalescence phenomena. Conclude with a brief synthesis if time permits, though not mandatory.
- Part (a): Structures of [2Fe-2S], [4Fe-4S], and [3Fe-4S] clusters with correct oxidation states (Fe²⁺/Fe³⁺) and redox couples (e.g., [2Fe-2S]²⁺/⁺, [4Fe-4S]²⁺/⁺)
- Part (a): Recognition that ferredoxins and high-potential iron proteins (HiPIPs) represent different redox families with distinct cluster types
- Part (b)(i): Explanation of ring-whizzing/ring rotation in (C₅H₅)₄Ti with η¹↔η⁵ hapticity interchange, showing single ¹H NMR signal at room temperature due to rapid exchange
- Part (b)(ii): Analysis of C₃(CH₃)₄Fe(CO)₄ as a trimethylenemethane (TMM) complex with η⁴-bonding, explaining fluxionality via Berry pseudorotation or TMM rotation, and temperature-dependent NMR showing methyl equivalence
- Part (c): Correct calculation of photon energy E = hc/λ = (6.626×10⁻³⁴ × 3×10⁸)/(2500×10⁻¹⁰) = 7.95×10⁻¹⁹ J per photon
- Part (c): Moles of oxalic acid decomposed = (0.05-0.04) × 0.01 = 10⁻⁴ mol; moles of photons absorbed = 80/(7.95×10⁻¹⁹ × 6.022×10²³) = 1.67×10⁻⁴ mol; quantum yield Φ = 0.6
- Part (c): Recognition that uranyl sulphate acts as photosensitizer in the uranyl-oxalate actinometer system, a classic photochemical application

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	25%	12.5	Demonstrates precise understanding of iron-sulphur cluster electronic structures, distinguishes ferredoxin vs HiPIP redox couples, correctly identifies η¹/η⁵ hapticity interchange in titanocene and η⁴-TMM bonding in iron carbonyl, understands photosensitization mechanism in uranyl-oxalate system	Identifies basic cluster types and redox states but confuses [2Fe-2S] and [4Fe-4S] redox potentials; recognizes fluxional behavior but misassigns hapticity modes; understands quantum yield concept but unclear on photosensitizer role	Confuses iron-sulphur cluster types with haem proteins; describes static structures only without addressing fluxionality; fundamental errors in defining quantum yield or ignores photosensitizer completely
Mechanism / equation	20%	10	Clearly writes NMR coalescence equation (Arrhenius/Eyring) for fluxional processes, explains low-temperature frozen vs high-temperature averaged spectra, states photochemical mechanism: UO₂²⁺* + (COOH)₂ → UO₂⁺ + 2CO₂ + 2H⁺ with proper electron transfer steps	Describes temperature-dependent NMR changes qualitatively without rate equations; mentions oxalate oxidation products but mechanism incomplete or unbalanced	No mention of NMR timescale or coalescence temperature; completely wrong photochemical mechanism or missing redox steps
Numerical accuracy	20%	10	All calculations correct: wavelength conversion (2500 Å = 250 nm), photon energy, moles of photons, moles reacted, final quantum yield Φ ≈ 0.6 with proper significant figures and units throughout	Correct method but arithmetic errors in final steps; or correct final answer with missing intermediate steps; unit conversion errors (Å to m)	Major errors in formula application (e.g., using λ in Å directly); order of magnitude errors; no calculation or completely wrong answer
Diagram / structure	20%	10	Clear labeled diagrams of [2Fe-2S] rhombic and [4Fe-4S] cubane clusters with cysteinyl bridges; proper orbital diagrams showing Fe-S bonding; well-drawn structures of (C₅H₅)₄Ti showing η¹ and η⁵ modes; TMM-Fe(CO)₄ with correct η⁴ coordination	Basic sketches of clusters without proper geometry; generic metallocene drawing without showing hapticity change; TMM structure but CO arrangement unclear	No diagrams despite explicit 'draw' instruction; incorrect connectivity in clusters; structures missing or chemically implausible
Application context	15%	7.5	Cites Indian relevance: iron-sulphur proteins in nitrogen-fixing bacteria (Rhizobium-legume symbiosis in Indian agriculture); mentions Indian research in bioinorganic chemistry (IITs, BARC); connects uranyl photochemistry to nuclear chemistry applications at DAE facilities; notes NMR spectroscopy facilities at NMR Research Centres in India	General mention of biological importance of Fe-S proteins in electron transport; standard textbook applications without Indian context	No application context provided; completely irrelevant examples or confusion with unrelated systems

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