Q.31 Infrared (IR) spectroscopy is used for determining certain aspects of the structure of organic
compounds. Which of the following statement(s) is/are FALSE?
(A) IR radiation induces electronic transitions
(B) IR peak intensities are related to molecular mass
(C) Most organic functional groups absorb in a characteristic region of the IR spectrum
(D) Each element absorbs at a characteristic wavelengthIR spectroscopy identifies organic compound structures by detecting vibrational transitions in functional groups. The false statements in the query are (A), (B), and (D).
Correct Answer
Statements (A), (B), and (D) are false. Only (C) holds true, as IR excels at pinpointing functional group vibrations.
Option Breakdown
(A) IR radiation induces electronic transitions
IR radiation causes vibrational and rotational transitions, not electronic ones, which need UV-Vis energies. Electronic shifts involve higher-energy orbitals, mismatched to IR’s lower range (400-4000 cm⁻¹).
(B) IR peak intensities are related to molecular mass
Peak intensities depend on dipole moment changes and concentration during vibrations, unrelated to molecular mass. Mass affects frequency via reduced mass in vibration equations but not absorbance strength.
(C) Most organic functional groups absorb in a characteristic region
Functional groups like C=O (1700 cm⁻¹), O-H (3200-3600 cm⁻¹), and C-H show distinct bands for reliable identification. This group frequency region (above 1500 cm⁻¹) enables structure elucidation.
(D) Each element absorbs at a characteristic wavelength
IR absorption reflects molecular vibrations, not atomic/element-specific lines seen in atomic spectroscopy. Elemental analysis uses AAS or emission, not IR’s molecular focus.
