Rotational spectroscopy — Why are pure rotational transitions generally of limited use for routine spectroscopy of liquids and solids?

Introduction / Context:
Rotational spectroscopy is powerful for gas-phase molecules, yielding precise molecular constants. However, in condensed phases (liquids/solids) it is far less informative. Understanding why helps select the appropriate spectroscopic technique for a given sample state.

Given Data / Assumptions:

• Rotational transitions occur in the microwave region.
• Liquids and solids feature frequent intermolecular interactions and collisions.
• Spectral resolution depends on narrow, well-defined transitions.

Concept / Approach:
In gases, molecules rotate relatively freely, yielding sharp rotational lines. In liquids/solids, rotational freedom is hindered and lifetimes are shortened by collisions, hydrogen bonding, lattice interactions, and viscosity. These effects cause severe line broadening or even suppression of discrete rotational spectra, making pure rotational analysis impractical in condensed phases.

Step-by-Step Solution:

Verification / Alternative check:
Microwave spectroscopy is typically performed on gas-phase samples or molecular beams; IR rotational fine structure is often lost in condensed phases for the same reasons.

Why Other Options Are Wrong:

• Energy is measurable with modern instruments; it is not “too small to measure.”
• Rotational transitions are not rare; they are ubiquitous in suitable phases.
• “All of the above” is invalid because the first two statements are incorrect.

Common Pitfalls:
Assuming instrument sensitivity is the limitation; the real issue is physical line broadening and hindered rotation in condensed matter.

Final Answer:
In liquids and solids the rotational lines are broadened by collisions and interactions, obscuring resolution