The electronic spectra of mixtures of CH₂Cl₂ solutions (each 0.993 mmol dm⁻³) of I₂ and the donor D shown in the diagram after the table were recorded for different volume ratios of the two solutions. Values of the absorbance for the absorption at λ_max = 308 nm are as follows:

(a) Suggest how compound D might interact with I₂.

(b) Use the data in the table to establish the stoichiometry of the complex formed between D and I₂. Why can the absorbance data be used for this purpose?

(c) In the Raman spectrum of the complex, a band at 162 cm⁻¹ is assigned to the I₂ stretching mode. Explain why this value is shifted from that of 215 cm⁻¹ for I₂ itself.

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Volume ratio I₂: D 0:10 1:9 2:8 3:7 4:6 5:5 6:4 7:3 8:2 9:1 10:0 Absorbance 0.000 0.056 0.097 0.129 0.150 0.164 0.142 0.130 0.103 0.070 0.000