Infrared (IR) spectroscopy fundamentals: Which of the following is NOT a recognized molecular vibrational mode detected in IR analysis?

Introduction / Context:
Infrared (IR) spectroscopy identifies functional groups by measuring molecular vibrations that absorb IR radiation. Typical vibrational modes include stretching and several types of bending motions. Knowing the standard vocabulary for these modes helps analysts interpret spectra accurately and avoid confusing nonstandard terms with real physical motions.

Given Data / Assumptions:

• The question asks for the option that is not a standard IR vibrational mode.
• Commonly taught modes: stretching (symmetric/asymmetric), and bending subtypes such as scissoring, rocking, wagging, and twisting.
• We assume a basic, gas-phase or condensed-phase IR context using conventional terminology.

Concept / Approach:
IR-active vibrations involve periodic changes in bond length (stretching) or bond angle (bending). Bending can be categorized into in-plane (scissoring, rocking) and out-of-plane (wagging, twisting). Terms like “rolling” are not part of the established set describing normal modes for polyatomic molecules in IR spectroscopy.

Step-by-Step Solution:
List recognized modes: stretching, scissoring, rocking, wagging, twisting.Compare with options: three are standard (stretching, scissoring, rocking).Identify any term outside the standard set: “rolling.”Conclude that “rolling” is not a recognized IR vibrational mode.

Verification / Alternative check:
Any IR textbook or spectral interpretation guide enumerates these canonical modes; “rolling” does not appear among accepted normal mode descriptors.

Why Other Options Are Wrong:

• Stretching: Fundamental IR mode involving bond length change.
• Scissoring: In-plane bending where two atoms move toward/away like scissors.
• Rocking: In-plane motion of a group rocking together.
• Wagging: Out-of-plane bending of two atoms moving together.

Common Pitfalls:
Confusing informal words with formal normal-mode names; ensure familiarity with the five standard bending terms.

Final Answer:
Rolling.