The Science Nest | @MrElement: 2017-02-26

3 Mar 2017

Chemical Wonders: Experimental Techniques (Part 5) - Electron Paramagnetic Resonance

Using the power of the microwave

What is EPR (also called electron spin resonance, ESR) spectroscopy?

EPR spectroscopy utilizes the quantum nature of electrons by exciting them with microwaves, doing so creates a transition between the magnetic energy levels of electrons changing the 'spin' and angular momentum of the electron. This is called the Zeeman electronic effect, see below:

The spin 'up' and 'down' states are also given the 𝜶 and 𝜷 notations, respectively. 𝛥E corresponds to a microwave region in the microwave region in the electromagnetic spectrum, which is applied to the analyte, allowing two spin states to occur. The EPR spectrometer records these transitions and represents them in an EPR spectra. Compare this to NMR spectroscopy - which uses the nuclear 'spin' transitions as a result of blasting different radiofrequencies at the analyte using a magnetic field.

Typical EPR spectrometers operate at the 9-10 GHz range, otherwise known as the 'X-band' range. There are other bands used, however:

'L-band' : 1-2 GHz
'S-band' : 2-4 GHz
'Q-band' : 35 GHz
'W-band' : 95 GHz

More modern machines, such as the FT-EPR spectrometers are capable of much higher resolution spectra, which has allowed us to explore even more types of chemical species, such as the paramagnetic center found in a metalloprotein (like methemoglobin). I'll just drop in an interesting article where the blood (ionized by radiation) of Chernobyl workers were analysed using EPR spectrometry, here.

The EPR spectra

The EPR spectra is recorded in the form of the derivative of the very first peak in the spectra (see above). Why? This is because this form is more sensitive to the microwave signal, improving the signal : noise ratio, On the spectra, the absorption maximum is where the derivative is equal to zero - this is also the point where the magnetic sample B_sample is recorded. We find the g-value of the sample using the following equation:

𝛥E = hv = g_sample x 𝜇B x B_sample

The g-value, is calculated experimentally as the frequency of the wave v, is precisely known in modern spectrometers and h being the Planck constant, and 𝜇B being the Bohr magneton (a constant). Older spectrometers must be calibrated based on g_sample and comparing it with B_sample. g-values provide diagnostic information about the analyte you are investigating. Paramagnetic metal centres, for instance, the g-value is characteristic of the oxidation state, coordination numbers and even the symmetry of the molecule. System with cubic symmetry (T_d, O_h, and I_h) on the other hand, the g-value will depend on the principle axis when applied to the magnetic field. Samples are placed in magnetic fields on three orthogonal planes.

For example, Anisotropic systems with axial symmetry have two axis (x,y) which are different from a principal axis (z). This results in two g-value, g_⊥ (perpendicular from the principle axis) and g∥ (parallel to the principle axis). How does this relate to the symmetry? Well: g_xy = g_xx = g_∥, while, g_zz = g_⊥. These values only arise depending on whether or not the molecular principle axis is aligned with the magnetic field or not. Other anisotropic systems, where the x, y, z values are unique, consequently giving rise to three g-values: g_zz, g_xx and g_yy.
Isotropic systems on the other hand, such as a metal complex with 6 ligands (MX₆ species,O_h symmetry), are a bit simpler, as the three g-values that arise all equal each other: gxx = gyy = gzz = giso. This is because these molecules are perfectly symmetrical.

I'm unable to show these types of symmetry in EPR, however, this source from the University of Manchester provides a good visual of the spectra produced from isotropic and anisotropic systems outlined above.

Some examples of spectra:

Other information from EPR spectra include the nuclear spin quantum number (I), which is close to the paramagnetic centers (which is what EPR highlights). Where I =/= 0, the free electrons present in the paramagnetic species interact magnetically with the nuclei, causing a hyperfine interaction.This leads to further splitting in the Zeeman electronic effect, increasing the energy differences between spin states. However, this results in far more complicated spectra than shown above.

Of course, this is just an extremely brief overview of EPR spectra - however, this type of spectroscopy has wide applications from inorganic chemistry, identifying compounds in organic chemistry. It even has applications in biochemical systems, such as metal complexes found in proteins (Cu²⁺, Fe²⁺ etc.).

2 Mar 2017

What makes water so special?

Water is essential for life. Why?

Water is so normal for many of us, and we don't question why it is an essential part of life. Around 71 percent of our planet's surface is water, from the vast salt-water seas, the streams that flow from mountains and into lakes and to the frigid polar ice caps. It is also abundant in the universe, comprising of the most abundant elements: hydrogen and oxygen. It has been detected all across the milky way. In fact there is a "cloud" of water with over 140 million times the water on earth surrounding an area around a black hole called a quasar, 12 billion light years from here.

About everywhere water exists on earth, life does too. The answer lies in its chemistry - from its physical properties to how it interacts with the stuff around it

The physical properties of water

Water exists on earth in three main states: solid (ice), liquid, and gas (clouds). Water is most useful to life in liquid form at standard temperatures (25℃) and normal atmospheric pressure (1 atm). Water is also able to exist as a liquid at a broad range of temperatures, from 0℃ to 100℃ and compared to other similar compounds its boiling point is much much higher. See below:

Its solid state is also different from similar compounds: it is less dense than the liquid form, which is something almost unique to water. For example, liquid water's max density is 1,000 kg/m3 and solid water has a density of 917 kg/m3. Water expands in its solid state by about 9% compared to its liquid form.

Why does water have these properties?

Water, Hydrogen bonding and its importance to life

Water is what we call a polar molecule, one part of the molecule is much more negatively charged than other parts of the molecule. What this forms is an electrical dipole moment, which is the separation of more negatively charged parts of a molecule, see the diagram below:

It is the separation of slight negative and positive charge that gives water its special properties. The more negative areas attract the positive areas of other molecules, when this occurs a weak bond between the water molecules form. Up to four of these "hydrogen bonds" can be formed (two on the oxygen as it has two lone pairs of electrons),

These bonds are individually quite weak and are easily broken, but in large quantities has a dramatic effect on the substance's boiling and melting points, as well as the structural properties in solid form. As discussed above, ice is more dense and expands up to 9% more than the maximum density of liquid water. This is because the orientation of hydrogen bonds in water causes the molecules in solid form to push further apart which reduces the density of the solid. Below, "1" shows where hydrogen bonds occur.

Water's polarity also enables it to dissolve other polar substances, and enables an environment for many organic compounds to react with each other, that would otherwise be impossible in solid form and quite difficult in gaseous form. Hydrogen bonding isn't just found in water present in lifeforms, but also in many organic compounds such as DNA and every protein in your body. These compounds could only exist in your body in aqueous form (mixed with water) because of water's properties as a solvent.

Water is also involved in many biological reactions in many of which occur in metabolic pathways. In anabolic pathways, water is a product of forming large and complex molecules. While in catabolic pathways, water is always one of the reactants, that "add water" to the large molecules, breaking it down.

It is also an important to the pH balance of life forms, as the hydrogen ion (H+, a proton) acts as a "Lewis acid," which donates itself to a "Lewis base," often a hydroxide ion (-OH) to form water. It is this interaction (with other acids and bases too) that has enabled us to create the pH scale, which determines the concentrations of H+ ions and -OH ions present in a solution.

The many uses of water

Water has its many uses for us humans, besides for drinking. We use it in our agricultural industries, for cleaning ourselves, moving around the world, to generate power, to extinguish fires and, to have fun and relax in. It is even used to exchange heat because of its high capacity of heat absorption. This capacity, especially for vaporization (into gaseous form), is called the enthalpy of vaporization, which is the required heat needed to convert a quantity of substance into vapor form. This capacity makes it useful in many power stations, nuclear power plants or in old machine guns (like the Vickers machine gun). Its usefulness outside of needing it to stay hydrated makes it one of the most versatile chemicals in the universe.

Scientifically, it plays another role for us chemists. Namely, we use it to base many standards of measurement. This includes the degrees Kelvin (K), in which the base of the measurement was the triple point of water (0.01 ℃), in which water can exist as a solid, liquid or gaseous form at the same temperature. This point of measurement is exactly 273.16 K and is what we call and absolute temperature scale, this unit of measurement is used quite often in scientific equations. The measurement of degrees Celsius on the other hand relies on the melting point (0 ℃) and the boiling point of water (100 ℃). These units of measurement have been instrumental in experimentation and improving the reproducibility of experiments because units exist in easily replicated conditions.

. . .

Water is an interesting molecule - it doesn't fit in with molecules of similar size and type, which makes it have all sorts of interesting properties not found in the same way in other molecules. It is the most important molecules that has enabled life here on earth to exist the way it does. Without it, it'd be hard to say we'd be here at all.

Thanks for reading!

You can also check this same post out on minds, right here.

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27 Feb 2017

"Green Chemistry" - Moving toward a sustainable future

Protecting our environment...with chemistry

Green chemistry, as defined by the EPA, is: "the design of chemical products and processes that reduce or eliminate the generation of hazardous substances." This type of research is at the forefront of developing sustainable technologies. Some of the goals of green chemistry include:

Using new solvents to replace volatile and hazardous substances, such as chlorinated and volatile organic substances.
Reducing the energy consumption in commercial and industrial processes; as well as reducing the amount of hazardous waste produced in industries.
Using renewable feedstocks, which is biological matter that can be used directly as a fuel. Or it can be converted into another fuel. Also called biomass feedstocks.
And using less hazardous chemicals in industrial processes.

At the beginning of this century, the journal Green Chemistry, published by the Royal Society of Chemistry provides a forum for developments in green chemistry. In the US, the American Chemistry Society Green Chemistry Institute (ACS Green Chemistry Institute) was established in order to promote the ideas and benefits green chemistry and "green engineering" to industries and the global chemical enterprise.

The challenges of building a sustainable future

The development of "green chemistry" techniques has been largely modeled after studies conducted in the late 20th century. Namely, Anastas and Warner (1998) developed principles of green chemistry, which can be summarized in 12 points:

Wherever it is practical to do so, synthetic methods should be used to ensure that the products of the method possess little or no toxicity to humans or the environment.
The synthetic methods should be designed in a way that maximizes the total amount of reactants used in the process to produce a final product. Ensuring very little goes to waste.
The products should preferably have reduced toxicity while keeping the efficacy of function of the replaced substance.
It is preferable to prevent waste before it is formed, as opposed to treating or cleaning it after it is formed.
"Auxiliary substances" such as solvents and separating agents should be used only when necessary or preferably are innocuous (harmless) when used.
Feedstock used for energy production should preferably be renewable whenever economically viable rather than depleting feedstocks.
Whenever feasible, synthetic methods should be done at ambient temperatures and pressures. The environmental and economic impacts (in the long and short term) of energy requirements should be recognized.
Unnecessary protection, deprotection steps or any derivatisation in the synthetic method should be minimized or completely avoided in organic synthesis.
Catalysts, as opposed to stoichiometric reagents are far superior for chemical synthesis. And, wherever possible, the catalysts should be as selective as possible.
Chemical products should not persist in the environment after their function has come to an end. Instead, they should degrade into innocuous products or be reused in some other process.
Substances used in chemical synthesis should be chosen as such that they reduce chemical accidents such as explosions, and fires.
Further developing analytical methods that allow real-time monitoring and control during a process as to predict and minimize the formation of potentially hazardous substances.

With those 12 principles in mind and developments in other fields, it's possible to build a better future. In fact, the Presidential Green Chemistry Challenge Awards was established in 1995 to challenge chemists and encourage the development of green technologies at the academic and commercial levels. You can find a list of all years' winners since then here.