A researcher obtains both an infrared (IR) spectrum and a UV–visible spectrum for the…
CHEMISTRY · Multi-part · A Level · Chemistry · Organic analysis and spectroscopy · Organic analysis and spectroscopy
Problem
A researcher obtains both an infrared (IR) spectrum and a UV–visible spectrum for the organic dye β-carotene, a conjugated hydrocarbon that contains 11 carbon–carbon double bonds in a linear chain.
(a) Identify the types of molecular energy level transitions responsible for (i) the many narrow peaks observed in the IR spectrum between 400cm−1 and 1600cm−1 and (ii) the single broad absorption band centred at 460nm in the UV–visible spectrum.
(b) Explain why the IR spectrum of β-carotene contains numerous discrete peaks whereas its UV–visible spectrum contains only one broad band.
(c) Predict how the wavelength of the UV–visible absorption maximum would change if two additional C=C double bonds were added to the conjugated chain of β-carotene. Provide a rationale that relates conjugation length to the energy gap between the relevant electronic states.