Which of the following is NOT IR-active?
A vibrational mode is infrared active only if its motion changes the molecular dipole moment, creating transient charge separation that couples to infrared photons. In heteronuclear bonds like C-H, O-H bend and C=O stretch, oscillation modulates polarity producing strong absorption. Homonuclear diatomic molecule nitrogen, N≡N, comprises two identical electronegative atoms sharing electrons equally, maintaining zero dipole at equilibrium and throughout stretching. No oscillating dipole is generated, so infrared radiation cannot be absorbed despite high bond dissociation energy. Such molecules are detected by Raman spectroscopy, which depends on polarizability change rather than dipole, explaining atmospheric nitrogen transparency.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.