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Short answer
The infrared spectrum of liquid water is dominated by a broad O-H stretching band between about 3000 and 3800 cm⁻¹, a bending band near 1650 cm⁻¹, and a set of low-wavenumber librational bands between 400 and 1000 cm⁻¹. The exact shape of the stretching band reports on hydrogen bonding, so the same molecule gives a different spectrum as its network changes.
What is IR spectroscopy, and how does it work for water?
Infrared (IR) spectroscopy measures which infrared wavelengths a sample absorbs. A molecule absorbs when the light matches one of its vibrational motions, and the spectrum plots that absorption against wavenumber (cm⁻¹). The general method, the instrument and the reading routine are covered in the IR spectroscopy functional group frequency table and the FTIR spectrophotometer guide; this page concentrates on what is specific to water.
Water has three internal vibrations that matter for the bands below: the O-H stretches, the H-O-H bend, and, in the liquid, motions of whole molecules held by hydrogen bonds (libration and hindered translation). Each of those appears in a different part of the spectrum, which is what makes liquid water a useful, if unusually broad, test case for reading a spectrum.
How to read the IR spectrum of water
Read a water spectrum from high to low wavenumber and attach each feature to a motion. The table below lists only assignments stated in the two research papers cited under Sources.
| Region (cm⁻¹) | Motion | What the papers state |
|---|---|---|
| 3000-3800 | Intramolecular O-H stretching | Begusic and Blake describe the stretching modes as spanning 3000-3800 cm⁻¹. Ishikawa and co-workers assign 3210 cm⁻¹ to strongly hydrogen-bonded water and 3374 cm⁻¹ to weakly hydrogen-bonded water. |
| About 2150 | Combination (bend plus libration) | Begusic and Blake call this the “association” band and describe it as a combination of the bend and a libration. |
| About 1650 | Intramolecular H-O-H bending | Begusic and Blake give 1650 cm⁻¹. |
| 400-1000 | Libration (hindered rotation) | Both papers place libration in this region. Ishikawa and co-workers report the L2 band maximum at 670 cm⁻¹ and state that it is sensitive to water structure. |
| Below 300 | Hindered translation | Ishikawa and co-workers place hindered translation below 300 cm⁻¹. |
Why the O-H band is broad and shifts
The O-H stretching band of liquid water is wide because the molecules sit in a spread of hydrogen-bonding environments rather than one. Ishikawa and co-workers separate that envelope into a strongly hydrogen-bonded component near 3210 cm⁻¹ and a weakly hydrogen-bonded component near 3374 cm⁻¹. Stronger hydrogen bonding moves intensity to lower wavenumber.
Begusic and Blake report the same trend from the structural side: as the tetrahedral order of the liquid increases, the hydrogen-bond stretch frequency rises while the O-H stretch frequency falls. They also note that the libration-bend feature shifts to lower wavenumber as temperature rises, matching the linear IR spectrum. In practice, a spectrum of water is not a fixed fingerprint; temperature and structure move the bands.
The low-wavenumber libration region carries a similar signal. In the Ishikawa study, bands at 673 and 403 cm⁻¹ were read as ordered and distorted hydrogen-bond structures, with intensity as the measured quantity.
Water as the sample versus water as an artefact
A water-like band in a spectrum does not always mean the sample is water. Three cases look similar on a first read and are worth separating before interpreting anything else:
- Liquid water as the analyte or solvent. A broad O-H band across 3000-3800 cm⁻¹ with the bend near 1650 cm⁻¹ and, if the range is recorded, libration below 1000 cm⁻¹.
- Moisture in a pressed halide disc. The frequency-table guide describes damp KBr as a broad band at roughly 3200-3550 cm⁻¹ with a bend near 1630 cm⁻¹, appearing on top of the real sample bands.
- Atmospheric water vapour. The same guide places vapour lines at about 3550-3900 and 1300-1900 cm⁻¹, as many sharp, jagged lines rather than one smooth band, and describes purging the instrument to reduce them.
For samples that contain water, the choice of sampling mode matters. The ATR versus transmission FTIR guide notes that attenuated total reflection suits aqueous samples. The Ishikawa study used single-reflection ATR with a correction for the wavenumber-dependent penetration depth, 4 cm⁻¹ resolution, 256 scans and 25 °C, a useful example of the parameters worth recording when reporting a water spectrum. For a comparison with a related vibrational technique, see Raman versus FTIR spectroscopy.
What the NIST reference spectrum is, and is not
The NIST Chemistry WebBook lists one IR spectrum for water, from the Coblentz Society evaluated collection (Coblentz No. 10116, contributed by the Dow Chemical Company, dated 21 October 1969). It is a neat liquid held in a capillary between AgCl plates, measured by transmission on a KBr fore-prism grating instrument, and digitised by NIST from hard copy. Three consequences for anyone using it as a library match:
- It is a dispersive scan, and NIST notes that spectra in this collection may differ from FTIR spectra.
- NIST states that concentration information is not available for it, so molar absorptivity values cannot be derived.
- The WebBook page gives no peak list or assignments, so band positions should be taken from the research literature, not attributed to NIST.
Common misunderstandings
- “Water has one O-H peak.” The O-H stretch is a broad envelope built from many hydrogen-bonding environments, not a single line.
- “The bend sits at one fixed value.” Band positions move with temperature and structure, so name the source and state its value.
- “Any broad band near 3400 is water in the sample.” Damp KBr and room air can contribute, so check for them first.
- “The NIST water spectrum gives band positions and absorptivities.” It provides a historical neat-liquid reference trace only.
FAQ
What is IR spectroscopy?
IR spectroscopy records how strongly a sample absorbs infrared light at each wavenumber. Absorption occurs where the light matches a vibration of the molecule, so the pattern of bands characterises the bonds present. For water, the relevant vibrations are O-H stretching, H-O-H bending and hydrogen-bond-related librations.
How does IR spectroscopy work?
An instrument passes infrared light through or against the sample and measures what is absorbed. In ATR, the beam probes a thin layer at the crystal surface, which is why a correction for wavenumber-dependent penetration depth is applied in quantitative work such as the Ishikawa study. The detailed instrument description is in the FTIR spectrophotometer guide.
How to read IR spectroscopy?
Work from high to low wavenumber, assign each feature to a motion, and rule out artefacts before interpreting. For water: the broad stretch at 3000-3800 cm⁻¹, the bend near 1650 cm⁻¹, then libration at 400-1000 cm⁻¹. Then compare against moisture and vapour signatures, as listed above.
Sources
Sources checked on 2026-10-29.
- NIST Chemistry WebBook, water, infrared spectrum: webbook.nist.gov (checked 2026-10-29).
- Ishikawa, Shichishima, Shinohara, Yang and Fujii, J. Phys. Chem. A 2025, 129(28), 6179-6185, doi:10.1021/acs.jpca.4c08369: PMC12278207 (checked 2026-10-29).
- Begusic and Blake, “Two-dimensional infrared-Raman spectroscopy as a probe of water’s tetrahedrality”, Nature Communications 2023, 14:1950: PMC10082090 (checked 2026-10-29).
