Analytical Chemistry: Three Methods of Compound Determination

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Analytical chemistry is the science of determining what matter is and how it exists through instruments and techniques which separate, identify and quantify matter, overall determining the structure of the compound. There are three techniques that can be used to determine a compound. These techniques being infrared spectroscopy, mass spectrometry and nuclear magnetic resonance spectroscopy, or NMR. 

Infrared Spectroscopy

To begin, infrared spectroscopy analyses the interaction between infrared light and a molecule using what’s called infrared radiation. This radiation allows the molecules quick movement, generating an infrared spectrum of this energy. To create an IR spectra, the molecules go through three different phases of absorption, stretching and bending. A molecule first absorbs infrared radiation, causing its chemical bonds to vibrate. These bonds then stretch, contract and bend. With this, different molecules vibrate at different frequencies, displaying different stretches and peaks, overall proving the structure of the molecule). Now for some important information about the IR spectra. The vertical axis shown on the infrared spectra given represents transmittance while the horizontal axis represents the wavelength range. 

There are four main regions within the IR spectra, which spread across the wavelength range depending on their relation to the visible spectrum. These regions being near, middle, far and the fingerprint region. The near region is between 12800 and 4000, the middle region between 4000 and 2000, the far region between 200 and 10 and the fingerprint region between 1500 and 400, with the near and far region not being shown on the given spectra. When working with more complex compounds, the fingerprint region is required to further prove the compound’s bond, however due to these complexities it is not required for this task. I explained infrared spectroscopy first as this technique is one of the most commonly used spectroscopic techniques when determining a compound’s bonds. From the given spectra and its corresponding table, it can be observed that between the wavenumbers 3200 and 3600, there is a strong oxygen to hydrogen single bond, telling us that this compound is in the alcohol functional group. It is unknown at this point whether the compound is a primary, secondary or tertiary alcohol or its carbons count, thus further analysis of the observations gained from the other techniques is required. 

Mass Spectrometry

The second technique is mass spectrometry. Mass spectrometry is an analytical technique which accurately measures the mass of different molecules within a sample, overall determining the number of certain elements in the compound. The chemistry behind mass spectrometry is four processes which work together to determine the specific compound being represented on the graph. The processes that occur are ionization, acceleration, deflection and detection. Basically what happens is the molecules in the compound are converted to vaporised gas with heat. An electrical beam then intrudes these vapours, converting them to ions. Next, the ions are sorted according to their mass through acceleration and deflection. The positive ions created in the ionization stage accelerate towards negative plates at a speed dependent on its mass. The ions are then deflected by a magnetic field, the extent of deflection being dependent on its mass. 

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The ions then eventually reach the detector, which provides the certain peaks in the mass spectrum as shown in the given spectra. As it can be observed in the spectra, the final peak is shown on the x-axis at 32. This tells us that the atomic mass of the given compound is 32. Let’s say that my compound was methanol, containing one carbon. The equation shown on the top left-hand side of the board shows the masses of each element of methanol, added together to get an overall charge of 32. This would prove that my compound is methanol, as two carbons would get too high of a mass. To explain the mass spectra a little more, look at the line at the atomic mass 33. This small line is not used to determine the overall mass of the compound as it only displays the extra neutron that has been ignored since we are working with carbon 13 and not carbon 12.

NMR

Nuclear Magnetic Resonance Spectroscopy is a technique used to determine the contents of a compound in regard to its molecular structure, providing support for the first two techniques explained. The principle of this technique includes the spin on nuclei and its energy. All nuclei are electrically charged and many have a spin, causing them to react similarly to a magnet. With this in mind, when an external magnetic field is applied, an energy transfer may occur between the base energy and a higher energy level. This transfer of energy occurs at a corresponding wavelength to its radio frequencies so when the spin returns to its base level, the energy is emitted at the same frequency, forming the NMR spectra.

H-NMR

This technique is the section of NMR which focuses on the hydrogens in the compound. An important aspect of H-NMR spectroscopy is the different hydrogen types and their different environments, with different types of hydrogen giving rise to different signals. (Dr Hunt I, NA) In the case of the H-NMR spectrum given, it can be observed that there are two doublet peaks of different heights displayed in between 3 and 4 parts per million. This tells us that there are two different types of hydrogens, as two different signals have been given off. The larger peak representing the alcohol OH bond and the shorter peak representing the carbon to hydrogen CH bond. 

Some more background on this technique includes chemical shifts or shielding. This is a crucial topic in NMR as it provides vital information about the structure surrounding the nucleus. As electrons orbit the nucleus, the magnetic field slightly alters the difference between the energy levels, giving the resulting spectrum. With this, hydrogens can be deshielded by electron withdrawing groups, with the more powerful groups having a greater chemical shift. The protons in methanol are more deshielded as in methanol an electron withdrawing oxygen atom is directly bonded to the carbon.

To prove this compound is in the alcohol functional group, this spectra can work with a chemical shift graph shown. It can be observed that in between 3 and 4 parts per million in the chemical shift graph is carbon and hydrogen bonded to nitrogen, oxygen and a halogen. However, it can be proved that the compound is not a halogen or amide, as it is already known that the compound is an alcohol from the IR spectra. Instead it can be proven that the compound is part of the alcohol functional group as the top shaded box showing an alcohol is displayed between 3 and 4 parts per million, proving it’s an alcohol. Chemical shifts can be associated with the frequency of a nuclear spin in relation to its chemical environment. In this case, as explained before the chemical shift for an alcohol is between 1 and 5 ppm which in this case is correct.

C-13-NMR

The second section of the NMR technique focuses of the carbons in the compound. With this technique, the carbon count is determined depending on the number of peaks it shows. A carbon-13 nucleus we know behaves like a magnet, meaning that it can also be aligned with an external magnetic field or in contrast, be opposed to it. It is possible to cause this field to flip from a more stable alignment to the less stable one. The energy required to produce this flip depends on the overall strength of the magnetic field used. It is possible to detect this flip using the radio waves of just the right frequency and the carbon-13 nucleus. This flip of the carbon-13 nucleus from one magnetic alignment to the other by the radio waves is known as the resonance conditions, shown on the C-NMR spectra as a peak. In this case only one peak can be observed on the spectra, so it can easily be concluded that this compound only consists of one carbon and one environment the carbon is in, supporting the conclusion made by mass spectrometry. 

To summarise this speech, the three main techniques used to determine a compound and its structure is infrared spectroscopy, mass spectrometry and nuclear magnetic resonance spectroscopy, or NMR. Through the use of these techniques, I had the ability to determine the compound shown in the spectra’s given and cancel out compounds that it was definitely not. I did this by using the observation of the OH bonded stretch shown in the IR spectra, and through the base peaks and the calculated compound’s mass using mass spectrometry. Supporting these two facts was the NMR spectroscopy, which proved that the compound was an alcohol and that it only consisted of one carbon. With this being said it can be concluded that this compound is methanol.  

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