Before we start off on this application of MO theory, let us recap by asking ourselves two questions: Is N₂ diamagnetic or paramagnetic? Is O₂ diamagnetic or paramagnetic?

Now, to help you answer this question, a molecule is considered to be diamagnetic if all its electrons are paired, whereas it is paramagnetic if there are unpaired electrons in its molecular orbitals. Essentially, diamagnetic molecules are not attracted by external magnetic fields, whereas paramagnetic molecules are. Hence, we can easily answer these two questions by drawing out the molecular orbital diagrams of N₂ and O₂ and filling up the molecular orbitals with electrons, as follows:

As you can see, all the electrons in diatomic nitrogen molecules (N₂) are paired, but on the other hand, there are two unpaired electrons in diatomic oxygen molecules (O₂). This means that nitrogen gas is diamagnetic, whereas oxygen gas is paramagnetic. Hopefully you arrived at the same answer as us.

These properties of nitrogen and oxygen are extremely useful in oxygen level analysers, which is the application of MO theory that we will be discussing in this post. An oxygen level analyser, as the name suggests, is a device that measures the concentration of oxygen in the air around it. They look like this:

(Source: http://img.directindustry.com/images_di/photo-g/oxygen-analyzer-zirconia-27141-2635413.jpg)

These devices are extremely useful in safety-related and industrial applications. For example, they can be used to monitor oxygen levels in environments where oxygen levels may drop to hazardous levels without warning. Such places include underground mines, where circulation of fresh air from the surface is limited, and chemical plants, where a gas leak may quickly displace oxygen in the atmosphere. At any point in time when the device detects that the oxygen level is below a safe level, it can sound an alarm warning all workers in the vicinity to evacuate to a safe area, and potentially save many lives in the process.

Other applications include detecting oxygen purity (or impurity) levels in the products of a reaction, or to check if oxygen has contaminated a gaseous stream of another gas.

So, how do these oxygen analysers work? There are many designs, but for this post we shall explain a simple design that makes use of the fact that oxygen is paramagnetic – a fact explained by MO theory. A schematic of such a design is shown below:

(Modified from: http://www.systechillinois.com/media/679c73a6/paramag_98%20figure%201.jpg)

The design contains two nitrogen-filled glass spheres suspended in a “dumbbell” layout in the middle of the device. The “dumbbell” is light and free to rotate about the central axis, where there is a mirror. When the device is turned on, the light source shines a beam of light onto the mirror, and the light beam is captured by a light sensor. Notice that if the “dumbbell” rotates, the amount of light captured by the light sensor will change, and this change is detected by the device. This means that the device can sense the amount of rotation on the “dumbbell” (due to a force acting on it) by measuring the change in light detected by the light sensor. In addition, the device is also programmed such that any rotation sensed by the device also causes it to apply a force on the “dumbbell” to restore it to its original position.

A strong magnetic field is also produced when the device is turned on, but as the spheres are filled with nitrogen (recall that it is diamagnetic), no force will act on the “dumbbell” due to the magnetic field on the nitrogen. However, this strong magnetic field will attract oxygen (recall that it is paramagnetic) from the outside environment, which enters via the gas inlet and moves towards the gas outlet. As oxygen moves from the inlet to the outlet, it will exert a force on the “dumbbells” to make them rotate. Therefore, the greater the oxygen level in the environment, the greater the rate at which it is attracted by the magnetic field in the device, and the greater the force exerted by oxygen on the “dumbbells” to make them rotate more. In other words, the oxygen level in the environment is proportional to the rotation sensed by the light sensor, so a simple calibration makes the device able to measure oxygen levels in the surrounding environment.

We hope you had fun learning about this important application of MO theory, and if you ever visit a mine or industrial plant, remember that MO theory keeps you safe!

Reference: http://www.systechillinois.com/en/paramagnetic-cells_54.html

Earlier this year, we heard about the world record on fastest train being broken by the Japanese bullet train. And yes, as you could have guessed from the title, it’s a maglev train! The train reached a record speed of 590 kilometers per hour on a test track which don’t use metal tracks. They float nearly 10 cm above special guideways, allowing for frictionless movement and work by using magnets to push the train away from the tracks and drive the train forward.

But how does magnetic levitation work? And what is its link to MO theory?

We have learnt that by drawing MO diagrams, we can determine the magnetism of a material – paramagnetic or diamagnetic. Apparently, diamagnetic materials are imperative in magnetic levitation.

Stable levitation with permanent magnets was proven impossible by British mathematician Samuel Earnshaw in 1842 in his Earnshaw’s Theorem which states that a collection of point charges cannot be maintained in a stable stationary equilibrium configuration solely by the electrostatic interaction of the charges. Imagine putting permanent magnets on top of each other with their like poles meeting. They do indeed repel but will not float – the top magnet will simply slip aside and fall. Unless we put walls over the magnets to prevent slipping, the magnets won’t float.

In the picture above, the stick acts as the axis of stabilization to prevent the ring magnet from slipping. Without the stick the magnet will not float.

That is why diamagnetism is the key to magnetic levitation, as they provide the extra stabilizing force needed. Diamagnetic materials are materials which induce a weak magnetic field in the opposite direction when exposed to a magnetic field. They thus repel, and are repelled by a strong magnetic field. Generally, however, this repulsive force is not strong enough to overcome the force of gravity on the Earth’s surface.  To cause diamagnetic levitation, both the diamagnetic material and magnetic material must produce a combined repulsive force to overcome the force of gravity.

One way to do this is by placing a diamagnetic material in a strong magnetic field. A thin piece of pyrolytic graphite (a diamagnet) is placed over a strong rare-earth magnet. The pyrolytic graphite is levitated above the magnet.

Another way to do this is by placing a magnetic material in diamagnetic fields with a biasing magnet. A permanent magnet is placed in the field of an electromagnet, and stabilized by diamagnetic plates put above and below the magnet. The magnet is levitated at a point far below the electromagnet where it is stable horizontally, but vertically unstable. The diamagnetic plates above and below stabilize the vertical motion.

The figure above shows the setup with the magnet levitated 2.5 m below an unseen 11 T superconducting solenoid stabilized by the feeble diamagnetism of fingers (x’1025). The magnet here is Ni-Fe-B which is a very strong rare-earth magnet. If a stronger diamagnetic material such as graphite is used for vertical stabilization, the levitation can be accomplished with common permanent magnets.

So why be a magician and do the trick by illusion when you can be a chemist and accomplish this for real? ;p

As you have read from the “Concepts” section, MO Theory can be used to calculate the bond order and thus the bond strength of a molecule. Generally speaking, a higher bond order would mean that a molecule possesses higher bond strength. Bond strength, on the other hand, is related to the frequency of bond vibration via Hooke’s Law. This is shown in the following equation (click to enlarge):

Credits: The McGraw-Hill Companies, Inc.

From the equation, we can see that the greater the bond strength, the greater the frequency of infrared radiation (IR) absorbed.

So what does all this have to do with global warming?

We know that the Sun emits radiation in the form of Ultraviolet (UV) and Infrared Radiation, in addition to radiation with wavelengths falling in the spectrum of visible light. Greenhouse gases such as CO2 and CH4 absorb heat from the Sun, therefore creating the greenhouse effect whereby the Earth is able to maintain a stable average temperature of ~16ºC. It is this effect which enables living things to survive on Earth.

In recent years, however, the percentage of greenhouse gases in our atmosphere has increased, arguably due to human activity. The Industrial Revolution has caused unprecedented levels of CO2 to be released in the atmosphere, and the concentration of methane, CH4, has also increased due to increased cattle-rearing. This has led to the “enhanced greenhouse effect”, which means that more infrared radiation is trapped in the Earth’s atmosphere than before, instead of being reflected out into space. Take a look at the picture below.

Credit: Will Elder, National Park Service

I am sure that you more-or-less have an idea of what this entails, so I won’t go into the details of that here.

Our purpose now, then, is to explain: How and why do greenhouse gases such as CO2 and CH4 trap heat?

For a simple explanation, take a look at the video below from 10:16-11:50.

If you’re too lazy to fast-forward the video till the time interval stated above, here’s the words version of the explanation:

Greenhouse gases absorb IR of a certain frequency because the bonds between the atoms in their molecules absorb IR of that frequency in order to vibrate and become excited. When IR from the Sun reaches the Earth, CO2 and CH4 are some examples of molecules which can absorb IR. They will absorb around 5-10% of IR, and the rest will be reflected out by the Earth into space.

When these molecules come back down to their lower energy states, they will release the energy in the form of photons again. When this energy is emitted, it will radiate out in all directions. Some of the energy will be radiated out into space, while the rest is radiated back to the Earth’s surface, therefore causing the Earth to become hotter.

–

There! Now you know how MO Theory can be linked to Global Warming!

🙂