Sistemas de gestión de la seguridad de la información (SGSI)

Spectroscopy is the study of the absorption, emis-

sion, or interaction of electromagnetic radiation by molecules in solid, liquid, and gaseous phases. The spectroscopic studies of vapor, in which the H2O

molecules are far apart from each other, reveal a wealth of information about individual H2O mole-

cules.

Electromagnetic radiation (light) is the trans-

mission of energy through space via no medium by the oscillation of mutually perpendicular electric and magnetic fields. The oscillating electromagnet-

ic waves move in a direction perpendicular to both

fields at the speed of light (c _{ 2.997925  10}8

m/s). Max Planck (1858–1947) thought the waves also have particle-like properties except that they have no mass. He further called the light particles

photons, meaning bundles of light energy. He

assumed the photon’s energy, E, to be proportional to its frequency. The proportional constant h ( 6.62618  10-34 _{J/s), now called the Planck con-} stant in his honor, is universal. The validity of this

assumption was shown by Albert Einstein’s photo- electric-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 called energy levels. Due to quantization, they can gain or lose only specific amounts of energy. Spec- troscopy is based on these theories. Water molecules have quantized energy levels for their rotation, vi- bration, and electronic transitions. Transitions be- tween energy levels result in the emission or absorp- tion of photons.

The electromagnetic spectrum has been divided into several regions. From low energy to high ener- gy, these regions are long radio wave, short radio wave, microwave, infrared (IR), visible, ultraviolet (UV), X rays, and gamma rays. Visible light of vari- ous 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 rotation energy levels are very close together, and the transitions between pure rotation levels correspond to microwave photons.

Microwave spectroscopy studies 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

216O), 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  10}14_{Hz) region of}

the electromagnetic spectrum. High-resolution IR

spectrometry is powerful for the study of water in

the atmosphere and for water analyses (Bernath 2002a).

Visible light spans 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 _{ 10}18_{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, H2O: 1, symmetric stretching; 2, bending; and 3, asymmetric stretching.

molecular orbitals after absorption of light energy. Ultraviolet photons have sufficiently high energies to excite electrons into higher molecular orbitals. Combined with vibrations and rotations, these tran- sitions give rise to very broad bands in the UV spec- trum. As a result, gaseous, liquid, and solid forms of water strongly absorb UV light (Berkowitz 1979). The absorption intensities and regions of water va- por are different from those of ozone, but both are responsible for UV absorption in the atmosphere. Incidentally, both triatomic water and ozone mole- cules are bent.

HYDROGENBONDING ANDPOLYMERICWATER INVAPOR

Attraction between the lone pairs and hydrogen among water molecules is much stronger than any dipole-dipole interactions. This type of attraction is known as the hydrogen bond (O–H⎯O), a very prominent feature of water. Hydrogen bonds are directional and are more like covalent bonds than strong dipole-dipole interactions. Each water mole- cule has the capacity to form four hydrogen bonds,

two by donating its own H atoms and two by accept- ing H atoms from other molecules. In the structure of ice, to be described later, all water molecules, except those on the surface, have four hydrogen bonds.

Attractions and strong hydrogen bonds among molecules form water dimers and polymeric water

clusters in water vapor. Microwave spectroscopy

has revealed their existence in the atmosphere (Gold- man et al. 2001, Huisken et al. 1996).

As water dimers collide with other water mole- cules, trimer and higher polymers form. The direc- tional nature of the hydrogen bond led to the belief that water clusters are linear, ring, or cage-like rather than aggregates of molecules in clusters (see Fig. 5.4). Water dimers, chains, and rings have one and two hydrogen-bonded neighbors. There are three neighbors per molecule in cage-like polymers. Because molecules are free to move in the gas and liquid state, the number of nearest neighbors is between four and six. Thus, water dimers and clus- ters are entities between water vapor and condensed water (Bjorneholm et al. 1999).

By analogy, when a few water molecules are inti- mately associated with biomolecules and food mole-

Figure 5.4. Hydrogen bonding in water dimers and cyclic forms of trimer and tetramer. Linear and transitional forms are also possible for trimers, tetramers, and polymers.

cules, their properties would be similar to those of clusters.