Environmental Engineering Reference
In-Depth Information
2 Nitrous Oxide: Environmental Effects
and Atmospheric Chemistry
The colorless, slightly sweet-smelling gas nitrous oxide was first described by
Joseph Priestley in 1774 as 'dephlogisticated nitrous air' in his “
Experiments and
Observations on Different Kinds of Air”
that also encompassed the first description
of O
2
,NH
3
, and NO [
9
]. It was found to be a useful anesthetic and also to cause a
certain light-headedness that was soon popularized through laughing gas parties
and traveling shows [
10
]. Today the gas is still in use as a mild sedative in dental
surgery and in obstetrics, where the application in medical-grade purity does no
longer give rise to fits of uncontrolled happiness. For major surgical procedures,
nitrous oxide is used as a carrier gas for more potent agents such as halothane.
2.1 Chemical Properties of Dinitrogen Monoxide
The N
2
O molecule is isoelectronic to CO
2
and shares several of its physicochemical
properties. From rotational spectroscopy measurements, the bond lengths in N
2
O
gas were determined to be 1.1282
for the N-O
bond. These bonds are notably shorter than the respective average values of 1.25
Å
for the N-N bond and 1.1842
Å
Å
Å
and 1.21
for the corresponding double bonds, and this can be ascribed to the
existence of two resonance structures (Figure
2
). The central nitrogen carries a
positive partial charge in both structures, resulting in an orbital contraction that
helps to rationalize the shortened bond lengths.
Figure 2 The N
2
O molecule. The linear molecule shows N-N and N-O bond lengths that are
shorter than those expected for double bonds. This is due to resonance stabilization that also
conveys a slightly positive partial charge to the central nitrogen atom, while both terminal atoms
carry a slightly negative partial charge. The resulting small dipole moment of N
2
O substantially
contributes to its solubility in polar solvents such as water.
The two resonance structures carry opposing charges on their terminal atoms,
resulting in a low dipole moment of 0.161 D in spite of the terminal oxygen
atom. N
2
O has an electronic configuration of (1
)
2
(2
)
2
(3
)
2
(4
)
2
(5
)
2
(6
)
2
σ
σ
σ
σ
σ
σ
)
4
(7
)
2
(2
)
4
, so that the highest occupied
(1
, each are a
degenerate pair of allyl-type orbitals. Calculations have suggested the 2
ˀ
orbital
to be slightly N-N bonding and N-O antibonding, in agreement with the shorter
ˀ
σ
ˀ
ˀ
orbitals, 1
ˀ
and 2
ˀ
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