Electromagnetic Absorption By Water - Vibrational Spectrum

Vibrational Spectrum

The water molecule has three fundamental molecular vibrations. The O-H stretching vibrations give rise to absorption bands with band origins at 3657 cm−1 (ν₁, 2.734 μm) and 3756 cm−1 (ν₃, 2.662 μm) in the gas phase. The asymmetric stretching vibration, of B₂ symmetry in the point group C_2v is a normal vibration. The H-O-H bending mode origin is at 1595 cm−1 (ν₃, 6.269 μm). Both symmetric stretching and bending vibrations have A₁ symmetry, but the frequency difference between them is so large that mixing is effectively zero. In the gas phase all three bands show extensive rotational fine structure. ν₃ has a series of overtones at wavenumbers somewhat less than n ν₃, n=2,3,4,5... Combination bands, such as ν₂ + ν₃ are also easily observed in the near infrared region. The presence of water vapor in the atmosphere is important for atmospheric chemistry especially as the infrared and near infrared spectra are easy to observe. Standard (atmospheric optical) codes are assigned to absorption bands as follows. 0.718 μm (visible): α, 0.810 μm: μ, 0.935 μm: ρστ, 1.13 μm: φ, 1.38 μm: ψ, 1.88 μm: Ω, 2.68 μm: X. The gaps between the bands define the infrared window in the Earth's atmosphere.

The infrared spectrum of liquid water is dominated by the intense absorption due to the fundamental O-H stretching vibrations. Because of the high intensity, very short path lengths, usually less than 50 μm, are needed to record the spectra of aqueous solutions. There is no rotational fine structure, but the absorption band are broader than might be expected, because of hydrogen bonding. Peak maxima for liquid water are observed at 3450 cm−1 (2.898 μm), 3615 cm−1 (2.766 μm) and 1640 cm −1 (6.097 μm). Direct measurement of the infrared spectra of aqueous solutions requires that the cuvette windows be made of substances such as calcium fluoride which are water insoluble. This difficulty can be overcome by using an Attenuated total reflectance (ATR) device.

In the near-infrared range liquid water has absorption bands around 1950 nm (5128 cm−1), 1450 nm (6896 cm−1), 1200 nm (8333 cm−1) and 970 nm, (10300 cm−1). The regions between these bands can be used in near-infrared spectroscopy to measure the spectra of aqueous solutions, with the advantage that glass is transparent in this region, so glass cuvettes can be used. The absorption intensity is weaker than for the fundamental vibrations, but this is not important as longer path-length cuvettes are used. The absorption band at 698 nm (14300 cm−1) is a 3rd overtone (n=4). It tails off onto the visible region and is responsible for the intrinsic blue color of water. This can be observed with a standard UV/vis spectrophotometer, using a 10 cm path-length. The colour can be seen by eye by looking through a column of water about 10m in length; the water must be passed through an ultrafilter to eliminate color due to Rayleigh scattering which also can make water appear blue. In both liquid water and ice cluster vibrations occur, which involve the stretching (TS) or bending (TB) of intermolecular hydrogen bonds (O–H . . . O). Bands at wavelengths λ = 50-55 μm (44 μm in ice) have been attributed to TS, intermolecular stretch, and 200 μm (166 μm in ice), to TB, intermolecular bend

The spectrum of ice is similar to that of liquid water, with peak maxima at 3400 cm−1 (2.941 μm), 3220 −1 (3.105 μm) and 1620 −1 (6.17 μm)