Lecture 4: Oxygen and Nitrogen

Read sections Read: section 1.15, 1.16, 14-2-14.4 (partial), 15.2-15.4 (partial) from your textbook. There is a quiz (Quiz2) this and the previous reading assignment due by 10 PM, Tuesday, January 23.

Formation of O2

Living things created much of the atmosphere that now exists on Earth. Cyanobacteria were responsible for the rise in the atmospheric concentration of oxygen beginning 2.3 billion years ago. These bacteria, algae, and other plants produce oxygen by photosynthesis. Although most of this oxygen is used in respiration (biological oxidation) or in the atmospheric oxidation of the carbon-containing products, approximately 0.1 % of the organic matter is sequestered in sediments and that quantity of oxygen is added to the atmosphere. Over time, the excess oxygen has built up so that it is now makes up nearly 20% of the gases close to Earth.



Oxygen and Ozone

Atomic oxygen has three isotopes, 16O (99.759 %), 17O (0.0374 %), 18O (0.2039 %). 17O NMR spectroscopy requires isotopically enriched samples. The most common oxygen species in the atmosphere is triplet oxygen, a diradical (boiling point = -183 deg C). Singlet oxygen is diamagnetic with two electrons in one of the pi antibonding orbitals. This is higher in energy than triplet oxygen and acts as an electrophile in its chemical reactions.

The singlet/triplet designations arise from the number of transitions between spin states when the molecule is placed in an external magnetic field. Where S is the sum of all electron spins, the spin state = (2S + 1):
    0 unpaired electrons, singlet state

    1 unpaired electron, doublet state

    2 unpaired electrons, triplet state

Molecules with no upaired spins are diamagnetic and are weakly repelled by magnetic fields. Molecules with one or more unpaired spins are paramagnetic and are attracted by magnetic fields.

Ozone or O3 is the other allotrope of oxygen (boiling point = -112 deg C). This is a bent, diamagnetic species. Ozone is a reactive molecule and plays an important role in the chemistry and photochemistry of the upper atmosphere. Its structure, electronic states, and interactions with other molecules have been studied extensively. A recent ab initio study by Tachekawa and Abe indicates that ozone forms a dipole-dipole complex with water vapor. Photolysis of ozone in the stratosphere helps to protect life on Earth from highly energetic UV radiation and its depletion in the ozone layer by reactions with halogenated compounds is a serious concern. The reactivity of ozone makes it toxic to humans. The reactions of nitrogen oxides, hydrocarbons, and oxygen near the Earth's surface produce ozone and the concentration of this compound is frequently used as a measure of air pollution.

Bonding in O2

Lewis structures give us an approximation of the bonding in molecules. This approximation isn't very good for molecular oxygen. The best Lewis structure doesn't indicate the ground state properties of the molecule: its diradical nature.



    We can better predict the properties of triplet dioxygen by examining the molecular orbital diagram than with the Lewis structure.

    The molecular orbital diagrams for all homonuclear diatomic molecules is similar. The 2s atomic orbitals combine to form bonding and antibonding molecular orbitals (just like the 1s orbitals on hydrogen). Similarly, 2pz orbitals on both atoms combine to form sigma bonding and antibonding orbitals.

    The 2px orbitals combine to form bonding and antibonding orbitals with pi rotational symmetry about the bond axis. The 2py orbitals combine to form another set of pi orbitals at 90 degrees to the first set. The two pi bonding orbitals and the two pi antibonding orbitals form degenerate sets.
Let's look at the sigma bonding orbitals first. The 2s orbitals from each oxygen atom combine to form bonding (2s + 2s) and antibonding (2s - 2s) molecular orbitals. The 2pz orbitals on each oxygen can overlap one of their lobes, just like the s orbitals, to form bonding (2pz + 2pz) and antibonding (2pz - 2pz) molecular orbitals. These are all sigma symmetry orbitals because rotation of either atomic orbital around the bond axis doesn't effect the overlap.



Next, let's look at the 2px orbital on each oxygen. These can overlap side-to-side. Rotation of one of the atomic orbitals about the bond axis would eliminate this overlap making this interaction pi in symmetry. The overlap of the 2py orbitals is identical to this but rotated 90 degrees. The 2 pi bonding orbitals have the same energy, as do the 2 pi antibonding orbitals. Pi bonds are weaker bonds than sigma bonds because the overlap of the orbitals is poorer. Because of this, there is less stabilization for pi bonding molecular orbitals and less destabilization of the pi antibonding molecular orbitals than for sigma orbitals.



Nitrogen

Nitrogen is present in the atmosphere mainly as N2. There are two primary isotopes of atomic nitrogen with a ratio of 14N/ 15N = 272. Dinitrogen is extremely robust with a very high heat of dissociation (944 kJ mol-1 ). It is a colorless gas with a boiling point of -196 deg C.

The Haber Process is a method for producing ammonia developed by Germany during World War I. The Germans used the ammonia as a source of nitrogen for making explosives. The process is still used by industrial chemists.

N2 + 3 H2 2 NH3


Because raising the temperature will increase the speed of both the forward and reverse reactions, a high temperature should bring the reaction to equilibrium rapidly. However, raising the temperature favors the endothermic reaction, shifting the equilibrium in this case to the left, lowering the yield of ammonia, and increasing the time needed to obtain a given quantity of ammonia. In fact, at 500 degrees C, only 0.1 % of the mass at equilibrium will be ammonia if the reaction is done at 1 atmosphere (101.3 kPa) of pressure. (The other 99.9 % is, of course, a mixture of nitrogen and hydrogen.) But by increasing the pressure, the equilibrium favors the reaction in which fewer gas molecules are produced. Such a stress shifts the equilibrium to the right and produced a mixture richer in ammonia. Today pressures of up to 1000 atm are used, and the temperature is kept at about 500 degrees C. The catalyst is a mixture of iron, potassium oxide, and aluminum oxide. Under these conditions, the yield of ammonia is 40 % to 60 %.

Bonding in Molecular Nitrogen

Across the row in the periodic table from Li to Ne, the energy gap between the 2s and 2p orbitals increases. The 2s and the 2pz orbitals have the same sigma symmetry for diatomic molecules. In O2, the orbitals are well separated and there is no interaction. However, begining with N2, we can observe an orbital interaction between these sigma orbitals.

If the 2s and 2pz orbitals from the 2 nitrogen atoms had the same energy, they would mix completely and the resulting molecular orbitals would be evenly spaced. If the 2s and 2pz orbitals are not at the same energy but are close enough in energy to have some mixing, we see 4 molecular orbitals that have some s and some p character but are not evenly spaced in energy. A simple way of thinking of this is "orbital repulsion" where the sigma bonding orbital primarily resulting from 2pz overlap moves up in energy and the sigma bonding orbital primarily resulting from 2s overlap goes down an equal amount in energy. The average energy is the same.



Nitrogen has sigma bonds involving the overlap of 2s and 2pz orbitals and pi bonds from the overlap of the 2px and 2py orbitals on the two nitrogen atoms.