Food Biochemistry and Food Processing

5 Water Chemistry and Biochemistry 107

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 rotation 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 fun-
damental vibration modes. The three fundamental
vibration modes of water are symmetric stretching
(for^1 H 216 O),  1 , 3657 cm-1, bending  2 , 1595 cm-1,
and asymmetric stretching  3 , 3756 cm-1. These
modes are illustrated in Figure 5.3. Vibration energy
levels are represented by three integers,  1 ,  2 , and
 3 , to represent the combination of the basic modes.
The frequencies of fundamental vibration states dif-
fer 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 pointed out earlier, water molecules
also rotate. The rotation modes combine with any
and all vibration modes. Thus, transitions corre-
sponding to the vibration-rotation energy levels are
very complicated, and they occur in the infrared
(frequency range 3  1011 to 4  1014 Hz) region of
the electromagnetic spectrum. High-resolution IR
spectrometryis powerful for the study of water in
the atmosphere and for water analyses (Bernath
2002a).
Visible lightspans a narrow range, with wave-
lengths 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 temper-

ature of about 6000 K, and the visible region has the

highest intensity of all wavelengths. The solar emis-

sion spectrum peaks at 630 nm (16,000 cm-1, 4.8 

1014 Hz), which is orange (Bernath 2002b).

Water molecules that have energy levels corre-

sponding 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 trans-

parent. 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, the ultraviolet(UV, 7  1014

to 1  1018 Hz) region spans a very large range in

the electromagnetic spectrum. The photon energies

are rather high, 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 elec-

trons in atomic orbitals one can easily imagine that

molecules have molecular orbitals or energy states.

Thus, electrons can also be promoted to higher empty

Figure 5.3.The three principle vibration modes of the water molecule, H 2 O:  1 , symmetric stretching;  2 , bending;
and  3 , asymmetric stretching.