80
2 The Interaction of Electromagnetic Waves with Water
in the molecular torque when the hydrogen atoms (H) are replaced with twice-asheavy deuterium atoms (D). The spectral bands of H 2 O and D 2 O are extremely
broad, overlap, and have a highly complicated temperature dependence. However,
the interpretation of the very complex bands of pure components can be done using
the analysis of the spectra of semi-heavy water (HDO). The main advantage of these
studies is that the three native fundamental vibrations of HDO, and their overtones
and combination bands, are widely separated [56] so that Fermi resonance does not
occur (at least for bending modes).
Maréchal [57] has measured the IR spectra of light (H 2 O), heavy (D 2 O), and
semi-heavy (HDO) water, and found that the “pure” HDO spectra, obtained from
the spectra of mixtures with different proportions of H 2 O and D 2 O, are slightly
different near the bending modes (ν 2 ). Later Max and Chapados [58] showed, in a
more detailed study, that mismatches are reproducible and consistently change with
the molar fraction.
Recent work in this regard [40] showed that there are mismatches (see Fig. 2.16)
in the IR spectra of HDO, obtained from different proportions of light and heavy
water, which correspond to fingerprints of the fluctuation-born short-living H 3 O
+ ,
DH 2 O
+ , HD 2 O
+ , and D 3 O
+ ions with concentrations of about 2% of the content
of water molecules. These ions, in which the bending-mode model is shown in
Fig. 2.17, presumably coexist with long-lived pH-active ions, thus making liquid
water an effective ionic liquid on the picosecond timescale.
The origin of two far-IR absorption bands at ν s = 200 cm
−1 = 5 THz and ν 3 =
60 cm
−1 = 1.8 THz (see also Fig. 2.4) is not clear at the moment. These modes show an
intermolecular nature, as they do not show the isotope effect (see Sect. 2.7) and have
high intensities, which assume unusually strong intermolecular interactions [59].
12
LO–TO splitting
13 of the mode near 240 cm
−1 was experimentally confirmed [60],
which means that the long-range force effect exists in ice. The detailed structure and
properties of the ν s and ν 3 modes are still poorly studied. However, time-domain
spectrometers, which appeared a decade ago, triggered the studies of the terahertz
spectrum of water and ice, which is considered in Sect. 2.6.
Summarizing, the vibrationally averaged microscopic dynamics in water do not
reduce to the oscillatory dynamics of H 2 O molecules only. The strong correlations
and the intermediate states of protonic transport affect the molecular modes and contribute to the IR spectrum. In particular, it was shown that Zundel and Eigen cations
(see Sect.1.3.2) contribute to the whole IR region [61–63]. Another important observation is that the IR spectra of ice and water are very similar. That automatically
assumes a unified interpretation, which is missing at the moment. For example, a
longitudinal and transverse dispersion resembling optical phonons has been demonstrated for ice [60], but was never discussed for water until recently [64]. Inasmuch as
12 The maximum of the IR absorption of water and ice is only an order of magnitude lower than that
for the ionic crystal of NaCl, but three orders of magnitude lower than the absorption of covalently
bounded crystalline silicon.
13 LO–TO splitting manifests itself in a frequency difference between the longitudinal optical (LO)
and transverse optical (TO) phonon modes.
2 The Interaction of Electromagnetic Waves with Water
in the molecular torque when the hydrogen atoms (H) are replaced with twice-asheavy deuterium atoms (D). The spectral bands of H 2 O and D 2 O are extremely
broad, overlap, and have a highly complicated temperature dependence. However,
the interpretation of the very complex bands of pure components can be done using
the analysis of the spectra of semi-heavy water (HDO). The main advantage of these
studies is that the three native fundamental vibrations of HDO, and their overtones
and combination bands, are widely separated [56] so that Fermi resonance does not
occur (at least for bending modes).
Maréchal [57] has measured the IR spectra of light (H 2 O), heavy (D 2 O), and
semi-heavy (HDO) water, and found that the “pure” HDO spectra, obtained from
the spectra of mixtures with different proportions of H 2 O and D 2 O, are slightly
different near the bending modes (ν 2 ). Later Max and Chapados [58] showed, in a
more detailed study, that mismatches are reproducible and consistently change with
the molar fraction.
Recent work in this regard [40] showed that there are mismatches (see Fig. 2.16)
in the IR spectra of HDO, obtained from different proportions of light and heavy
water, which correspond to fingerprints of the fluctuation-born short-living H 3 O
+ ,
DH 2 O
+ , HD 2 O
+ , and D 3 O
+ ions with concentrations of about 2% of the content
of water molecules. These ions, in which the bending-mode model is shown in
Fig. 2.17, presumably coexist with long-lived pH-active ions, thus making liquid
water an effective ionic liquid on the picosecond timescale.
The origin of two far-IR absorption bands at ν s = 200 cm
−1 = 5 THz and ν 3 =
60 cm
−1 = 1.8 THz (see also Fig. 2.4) is not clear at the moment. These modes show an
intermolecular nature, as they do not show the isotope effect (see Sect. 2.7) and have
high intensities, which assume unusually strong intermolecular interactions [59].
12
LO–TO splitting
13 of the mode near 240 cm
−1 was experimentally confirmed [60],
which means that the long-range force effect exists in ice. The detailed structure and
properties of the ν s and ν 3 modes are still poorly studied. However, time-domain
spectrometers, which appeared a decade ago, triggered the studies of the terahertz
spectrum of water and ice, which is considered in Sect. 2.6.
Summarizing, the vibrationally averaged microscopic dynamics in water do not
reduce to the oscillatory dynamics of H 2 O molecules only. The strong correlations
and the intermediate states of protonic transport affect the molecular modes and contribute to the IR spectrum. In particular, it was shown that Zundel and Eigen cations
(see Sect.1.3.2) contribute to the whole IR region [61–63]. Another important observation is that the IR spectra of ice and water are very similar. That automatically
assumes a unified interpretation, which is missing at the moment. For example, a
longitudinal and transverse dispersion resembling optical phonons has been demonstrated for ice [60], but was never discussed for water until recently [64]. Inasmuch as
12 The maximum of the IR absorption of water and ice is only an order of magnitude lower than that
for the ionic crystal of NaCl, but three orders of magnitude lower than the absorption of covalently
bounded crystalline silicon.
13 LO–TO splitting manifests itself in a frequency difference between the longitudinal optical (LO)
and transverse optical (TO) phonon modes.