Practice question

Molecular vibrational frequency is governed by Hooke's law, proportional to square root of force constant divided by reduced mass. Stretching vibrations involve change in bond length and require large force constants because covalent bonds strongly resist elongation, whereas bending, torsion and rotational motions involve angular changes with weaker restoring forces. Consequently, stretching modes absorb at considerably higher wavenumbers, typically 4000-1500 cm-1 for O-H, N-H, C-H and carbonyl stretches, while bending appears below 1500 cm-1 in the fingerprint region. This energetic separation enables functional group diagnosis in proteins and nucleic acids.