Combination Bands
Definition and meaning of Combination Bands in chemistry.
Combination bands are absorption features in vibrational (infrared or Raman) spectra that arise when a single photon simultaneously excites two or more different vibrational modes of a molecule, producing a peak near the sum (or, less commonly, the difference) of the fundamental frequencies involved.
In more detail
In the ideal harmonic-oscillator model, only transitions between adjacent vibrational levels of a single mode are allowed, so combination bands should not exist. Real molecules are anharmonic, and this mechanical and electrical anharmonicity mixes vibrational states enough to make combination transitions weakly allowed. Because the coupling is small, combination bands are typically 10 to 100 times weaker than fundamental bands and cluster in the near-infrared region, where they are widely used to fingerprint O-H, N-H, and C-H containing materials.
Key facts
| Field | Physical Chemistry |
|---|---|
| Typical position | ≈ ν1 + ν2 (sum) or ν1 − ν2 (difference) of two fundamentals |
| Spectral region | Often near-infrared, 4000-10000 cm⁻¹ |
| Physical origin | Anharmonicity of molecular vibrations |
Water vapor shows a combination band near 5200 cm⁻¹ (about 1.9 μm) arising from simultaneous excitation of the symmetric O-H stretch (~3652 cm⁻¹) and the H-O-H bending mode (~1595 cm⁻¹); this band is exploited in near-infrared moisture sensors.
Frequently asked questions
How do combination bands differ from overtones?
Overtones arise from multiple quanta of excitation in a single vibrational mode (e.g., 2ν1), while combination bands arise from simultaneous excitation of two or more different modes (e.g., ν1 + ν2).