Infrared Spectroscopy As Level Chemistry

[Janne Desir]

Covalentbonds in a molecule can absorb specific amounts of energy, making them vibrate in a certain way. There are two ways a covalent bond can be made to vibrate: stretching and bending. At A-level, the differences between the two are not important, but the idea that energy can be absorbed by a covalent bond to make it behave in a certain way is crucial to understanding infrared spectroscopy.

The levels of energy absorbed by bonds in organic molecules can be found in the infrared region of the electromagnetic spectrum. The electromagnetic spectrum is a range of wavelengths of electromagnetic radiation that fall into different categories.

At the right of an IR spectra (between approximately 400 to 1500 wavenumbers) there is always a pattern of absorbances that are too complicated to (sensibly) analyse individually. This region is unique for every sample and is called the fingerprint region.

Similar bonds will naturally have similar infrared absorptions. Carbonyl (C=O) bonds all have absorbances around the 1600 to 1750 region as they all contain a carbon and oxygen double bond. However, C=O bonds can belong to a range of functional groups. As a result, other data in the IR can help to further identify the functional group the C=O belongs to.

Due to the nature of bond stretching and bending (see above), different bonds will not only absorb at different wavenumbers but some will be able to absorb over wider range of wavenumbers. This means absorbance peaks can have different appearances.

Due to the large number of molecules present in a sample, there will be lots of hydrogen bonding occurring between the OH groups of the molecules. Hydrogen bonding will change the nature of the OH bond, altering the energies required to vibrate the bond. The extent of hydrogen bonding will vary depending on the orientation of molecules and their physical closeness, meaning a large range of energies are required to vibrate all the OH bonds in the sample.

It shows you what radiation frequencies the organic molecule under investigation absorbs and hence the type of bonds present. This information can be used to identify the functional groups present in the molecule.

The bonds within molecules of these greenhouse gases absorb infrared radiation. This increases their kinetic energy causing the gases to heat up. As a result the temperature of the atmosphere increases, which in turn warms the earth.

However, the rapid increase in carbon dioxide levels over the last hundred or so years, due to increased combustion of fossil fuels, has led to the largest increase in temperature the planet has known in the shortest time.

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Understanding the origins of structure and bonding at the molecular level in complex chemical systems spanning magnitudes in length and time is of paramount interest in physical chemistry. We have coupled vibrational spectroscopy and X-ray based techniques with a series of microreactors and aerosol beams to tease out intricate and sometimes subtle interactions, such as hydrogen bonding, proton transfer, and noncovalent interactions. This allows for unraveling the self-assembly of arginine-oleic acid complexes in an aqueous solution and growth processes in a metal-organic framework. Terahertz and infrared spectroscopy provide an intimate view of the hydrogen-bond network and associated phase changes with temperature in neopentyl glycol. The hydrogen-bond network in aqueous glycerol aerosols and levels of protonation of nicotine in aqueous aerosols are visualized. Future directions in probing the hydrogen-bond networks in deep eutectic solvents and organic frameworks are described, and we suggest how X-ray scattering coupled to X-ray spectroscopy can offer insight into the reactivity of organic aerosols.

William King, professor of mechanical science and engineering, left; Rohit Bhargava, professor of bioengineering; and Keunhan Park, postdoctoral research associate, have demonstrated a method for simultaneous structural and chemical characterization of samples at the femtogram level (a femtogram is one quadrillionth of a gram) and below.

CHAMPAIGN, Ill. - Finding a simple and convenient technique that combines nanoscale structural measurements and chemical identification has been an elusive goal. With current analytical instruments, spatial resolution is too low, signal-to-noise ratio too poor, sample preparation too complex or sample size too large to be of good service.

Now, researchers at the University of Illinois have demonstrated a method for simultaneous structural and chemical characterization of samples at the femtogram level (a femtogram is one quadrillionth of a gram) and below.

"We demonstrated that imaging, extraction and chemical analysis of femtogram samples can be achieved using a heated cantilever probe in an atomic force microscope," said William P. King, a Kritzer Faculty Scholar and professor of mechanical science and engineering.

The new technique hinges upon a special silicon cantilever probe with an integrated heater-thermometer. The cantilever tip temperature can be precisely controlled over a temperature range of 25 to 1,000 degrees Celsius.

Using the cantilever probe, researchers can selectively image and extract a very small sample of the material to be analyzed. The mass of the sample can be determined by a cantilever resonance technique.

To analyze the sample, the heater temperature is raised to slightly above the melting point of the sample material. The material is then analyzed by complementary Raman or Fourier transform infrared spectroscopic imaging, which provides a molecular characterization of samples down to femtogram level in minutes.

"Fourier transform infrared and Raman spectroscopic imaging have become commonplace in the last five to ten years," said Rohit Bhargava, a professor of bioengineering. "Our method combines atomic force microscopy with spectroscopic imaging to provide data that can be rapidly used for spectral analyses for exceptionally small sample sizes."

As a demonstration of the technique, the researchers scanned a piece of paraffin with their probe, and removed a sample for analysis. They then used Raman and Fourier transform infrared spectroscopy to chemically analyze the sample. After analysis, the paraffin was removed by thermal decomposition, allowing reuse of the probe.

"We anticipate this approach will help bridge the gap between nanoscale structural analysis and conventional molecular spectroscopy," King said, "and in a manner widely useful to most analytical laboratories."

With King and Bhargava, co-authors of the paper are postdoctoral researcher and lead author Keunhan Park and postdoctoral research associate Jung Chul Lee. All four researchers are affiliated with the university's Beckman Institute.

Analysis of trace evidence involved in sexual assault investigations holds considerable potential as a newer avenue of identification when bulk, larger evidence is not found or unreliable. Trace analysis of forensic materials involves common findings such as strands of hair, residues left on clothing, shards of paint or glass, etc. In recent research focused on the analysis of trace materials found as evidence in a sexual assault, there has been promise in condom and bottled lubricant classification based on their chemical profiles that can provide an associative link in an investigation. Few studies have considered the examination of lubricant evidence at a trace level as it may be found on a crime scene or a victim. In this study, a new protocol will be tested and established to analyze trace lubricant evidence recovered from a fabric substrate, such as underwear, after sexual assaults using Fourier transform infrared (FTIR) spectroscopy. An experiment is proposed to examine the comparison of the spectra resulting from FTIR spectroscopic analysis of bulk and trace level lubricants recovered from a cotton substrate. The resulting spectra will be compared for their similarities using multivariate statistical techniques to test the viability of the approach.

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In addition, molecules can only absorb (or emit) energy equal to the spacing between two levels and, for a harmonic oscillation, this can only occur between adjacent levels. However, bonds in real molecules do not vibrate harmonically.

When atoms approach each other closely, they exert a force of repulsion, and beyond a certain separation distance, a bond breaks. Quantisation produces unequal separations of energy levels which add complications to spectra. Finally, molecules will not absorb infrared radiation unless they possess a dipole, thus H2 is transparent to infrared whilst HCl absorbs.

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