Environmental Engineering Reference
DNAs. Water is essential for life, and the water molecule contains two hydro-
gen atoms and one oxygen atom, bound together in a bent geometry. The
H-O-H bond angle is 104.5° in the ground electronic state. The bent geom-
etry results in a net permanent dipole moment of the water molecule. Hydro-
gen bonding between water molecules is another unique feature of water.
The combination of hydrogen bonding and permanent dipole moment are
largely responsible for the fact that water is a liquid at room temperature and
1 atm, a property critical for life. If we count the two lone pairs of electrons,
water molecule has a near tetrahedral geometry with the oxygen atom in
the center. In its ground electronic state, the water molecule has three vibra-
tional frequencies: symmetric stretch (3657 cm −1 ), antisymmetric stretch
(3756 cm −1 ), and bending (1595 cm −1 ), based on gas phase data. The rela-
tively high vibrational frequencies, in conjunction with fast rotation, are
important for its role as a common solvent and other significant properties,
such as heat conduction.
Most organic molecules contain hydrogen atoms. Examples include satu-
rated and unsaturated hydrocarbons, aromatic compounds, lipids, alcohols,
ethers, and esters. Many small drug molecules, polymers, and petrochemi-
cals, such as gasoline, are examples of important molecules that contain
Almost all biological molecules contain hydrogen, for example, proteins
and DNAs. Of course, complex structures, such as cells, viruses, bacteria,
and tissues, all contain hydrogen as an essential element in their constituent
1.4 SAFETY AND PRECAUTIONS ABOUT HYDROGEN
Hydrogen poses several safety concerns, from potential detonation and fire
in air to being an asphyxiant in its pure, oxygen-free form. For instance, as
a cryogen, liquid hydrogen presents dangers associated with very cold
liquids. Hydrogen “dissolved” in metals can lead to cracks and explosions.
Hydrogen gas in air may spontaneously ignite, and the detonation parame-
ters, for example, critical detonation pressure and temperature, strongly
depend on the container geometry. Thus, hydrogen must be handled with
extreme care and caution in gaseous or liquid form.
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2. Kolos, W., Rychlewski, J. Unusual double-minimum potential-energy curves: The H and
G S g
3 + states of the hydrogen molecule. Journal of Molecular Spectroscopy 1990 , 143 (2),