Vibrational spectroscopy — best description: what does this class of spectroscopic techniques primarily analyze in molecules?

Introduction / Context:
Vibrational spectroscopy includes infrared (IR) and Raman techniques that reveal how atoms move within molecules. By probing vibrational modes, chemists deduce functional groups, bond strengths, and molecular environments.

Given Data / Assumptions:

• Molecules possess quantized vibrational energy levels.
• IR absorption involves changes in dipole moment; Raman scattering involves changes in polarizability.
• Typical vibrational frequencies correspond to mid-IR wavenumbers.

Concept / Approach:
These techniques measure transitions between vibrational levels. Each normal mode (stretch, bend, twist, rock) produces characteristic spectral features that serve as fingerprints for functional groups.

Step-by-Step Solution:
Identify the property probed: vibrational motions within molecules.IR selection rule: requires a change in dipole moment during the vibration.Raman selection rule: requires a change in polarizability during the vibration.Conclude that vibrational spectroscopy analyzes molecular vibrational modes rather than purely mechanical or electronic phenomena.

Verification / Alternative check:
Correlation tables mapping IR/Raman bands to functional groups (e.g., C=O stretch near 1700 cm^-1) confirm that bond vibrations are the target signal.

Why Other Options Are Wrong:
Mechanical analogies (a, b) are not spectroscopic definitions. Option d is too narrow; Raman is also vibrational. Option e refers to electronic/X-ray processes, not vibrations.

Common Pitfalls:
Assuming IR alone defines vibrational spectroscopy and overlooking complementary Raman selection rules.

Final Answer:
Molecular motions such as bond stretching and bending (vibrational modes).