BLBS102-c05 BLBS102-Simpson March 21, 2012 12:2 Trim: 276mm X 219mm Printer Name: Yet to Come
5 Water Chemistry and Biochemistry 87(1858–1947) thought the waves also have particle-like proper-
ties except that they have no mass. He further called the light
particlesphotons,meaning bundles of light energy. He assumed
thephoton’s energy, E, to be proportional to its frequency. The
proportional constanth(=6.62618× 10 −^34 J/s), now called the
Planck constantin his honor, is universal. The validity of this
assumption was shown by Albert Einstein’s photoelectric-effect
experiment.
Max Planck theorized that a bundle of energy converts into a
light wave. His theory implies that small systems can be only
at certain energy states calledenergy levels. Due to quantiza-
tion, they can gain or lose only specific amounts of energy.
Spectroscopy is based on these theories. Water molecules have
quantized energy levels for their rotation, vibration, and elec-
tronic transitions. Transitions between energy levels result in
the emission or absorption of photons.
The electromagnetic spectrum has been divided into several
regions. From low energy to high energy, these regions are long
radio wave, short radio wave, microwave, infrared (IR), visible,
ultraviolet (UV), X rays, and gamma rays. Visible light of various
colors is actually a very narrow region within the spectrum. On
the other hand, IR and UV regions are very large, and both are
often further divided into near and far, or A and B, regions.
Microwaves in the electromagnetic spectrum range from
300 MHz (3× 108 cycles/s) to 300 GHz (3× 1011 cycles/s).
The water molecules have many rotation modes. Their pure ro-
tation energy levels are very close together, and the transitions
between pure rotation levels correspond to microwave photons.
Microwave spectroscopystudies led to, among other valuable
information, precise bond lengths and angles.
Water molecules vibrate, and there are some fundamental vi-
bration modes. The three fundamental vibration modes of water
are symmetric stretching (for^1 H 216 O),ν 1 , 3657 cm−^1 , bending
ν 2 , 1595 cm−^1 , and asymmetric stretchingν 3 , 3756 cm−^1. These
modes are illustrated in Figure 5.3. Vibration energy levels are
represented by three integers,ν 1 ,ν 2 , andν 3 , to represent the
combination of the basic modes. The frequencies of fundamen-
tal vibration states differ in molecules of other isotopic species
(Lemus 2004).
Water molecules absorb photons in the IR region, exciting
them to the fundamental and combined overtones. As pointedout earlier, water molecules also rotate. The rotation modes
combine with any and all vibration modes. Thus, transitions
corresponding to the vibration-rotation energy levels are very
complicated, and they occur in theinfrared(frequency range
3 × 1011 –4× 1014 Hz) region of the electromagnetic spectrum.
High-resolutionIR spectrometryis powerful for the study of
water in the atmosphere and for water analyses (Bernath 2002a).
Visible lightspans a narrow range, with wavelengths between
700 nm (red) and 400 nm (violet) (frequency 4.3–7.5× 1014 Hz,
wave number 14,000–25,000 cm−^1 , photon energy 2–4 eV). It
is interesting to note that the sun surface has a temperature of
about 6000 K, and the visible region has the highest intensity of
all wavelengths. The solar emission spectrum peaks at 630 nm
(16,000 cm−^1 , 4.8× 1014 Hz), which is orange (Bernath 2002b).
Water molecules that have energy levels corresponding to
very high overtone vibrations absorb photons of visible light,
but the absorptions are very weak. Thus, visible light passes
through water vapor with little absorption, resulting in water
being transparent. On the other hand, the absorption gets pro-
gressively weaker from red to blue (Carleer et al. 1999). Thus,
large bodies of water appear slightly blue.
Because visible light is only very weakly absorbed by water
vapor, more than 90% of light passes through the atmosphere
and reaches the earth’s surface. However, the water droplets in
clouds (water aerosols) scatter, refract, and reflect visible light,
giving rainbows and colorful sunrises and sunsets.
Like the IR region, theultraviolet(UV, 7× 1014 –1× 1018 Hz)
region spans a very large range in the electromagnetic spectrum.
The photon energies are rather high, more than 4 eV, and they
are able to excite the electronic energy states of water molecules
in the gas phase.
There is no room to cover the molecular orbitals (Gray 1964)
of water here, but by analogy to electrons in atomic orbitals one
can easily imagine that molecules have molecular orbitals or
energy states. Thus, electrons can also be promoted to higher
empty molecular orbitals after absorption of light energy. Ul-
traviolet photons have sufficiently high energies to excite elec-
trons into higher molecular orbitals. Combined with vibrations
and rotations, these transitions give rise to very broad bands
in the UV spectrum. As a result, gaseous, liquid, and solid
forms of water strongly absorb UV light (Berkowitz 1979). TheV 1 V 2 V 3
Figure 5.3.The three principal vibration modes of the water molecule, H 2 O:ν 1 , symmetric stretching;ν 2 , bending; andν 3 , asymmetric
stretching.