Introduction

Infrared spectroscopy is another common tool used by chemists for the characterisation of compounds and the study of their reactivity. IR spectroscopy is covered in many A-level curricula but the coarses fail to give students a good idea of the practicalities involved with spectroscopy and the theory on which it is based. This review of the core aspects of infrared spectroscopy is designed to give the reader a better feel for what goes on in IR spectroscopy and how it is carried out.

Theory

What does IR spectrscopy actually detect? Atoms in molecules are held together by chemical bonds and these can be losely thought of as analogous to springs connecting two balls. Just as the spring vibrates and the balls move toward and away from each other, chemical bonds do a similar thing. IR spectroscopy essentially detects the vibrations of chemical bonds

What is a bond? An atom is composed to two primary parts, the positively charged nucleus and the surrounding negatively charged electrons. As two atoms are brought together, the electrons on one atoms get to "see" more of the possitive charge of the nucleus of the second atom and vice versa creating an energetic stabilisation of the electrons. This results in a decrease in energy as the two atmos approach. However, a second interation occurs where the positive charges of two atomic nuclei and negative charges of the electrons of the two atom repel one another leading to a rise in energy. When these two energetic interactions are summed, the result is that the energy initially falls making the system more stable reaching a minimum at the equilibrium distance, r_e, before rising rapidly again. Hence we have a potential energy well with maximum stability at this equilibrium distance. However, the quantum nature of molecules means that within this energy well, several energy levels exist and these are associated with the motion of the atoms in the bond vibrations. The energy gap between these vibrational levels correspond to the enrgies of photons in the IR region of the spectrum and so in IR spectroscopy, we are detecting the absorption of photons responsible for causing transitions between these energy levels.

The picture is slightly more complicated as the IR radiation only interacts with certain vibrations and these involve polar bonds. Atoms of different elements have differing abilities to pull negatively charged electrons toward their positively charged nuclei. Hence, when atoms of two different elements are bonded to each other, the electrons may not be evenly distributed between them resulting in an overall polarisation with partial charges on the atoms and is called a dipole. IR light is a form of electro-magnetic radiation and it is the electric field component that interacts with the electric dipole of a bond. More importantly, for the transition to occur, the direction or magntide or both of the bond dipole must be altered during the vibration

How an Infrared Spectromter Works

Fourier Transform Infrared (FTIR) Spectroscopy

Virtually all modern IR spectroscopy involves Fourier transformation of aquired data. By this method, all wavelength of IR light are collected at once and the spectrum computed through Fourier transformation (FT) of the raw data. A modern FTIR spectromter uses a single beam of radiation and has no means of or need to separate this light its component frequencies. This is because FTIR works by a method calle interferometry. Figure X shows a schematic of the optics of a modern FTIR spectrometer and this isa variant of the Michelson-Morely type interferometer. IR radiation from the source lamp passes through the sample and is then split into two beams by a beam splitter which is basically a half silvered mirror. One beam proceeds on and is reflected straight back by a static mirror, the other beam is reflected by a mirror which moves back and forward, increasing and decreasing the mirror to beam splitter difference. When the two beam arrive back at the beam splitter, a portion of both beams is recombined and goes to the detector which measures the total energy of the incident radiation. Because of the action of the beam splitter and their subsequent recombination, the waves of the two beams can interfere with each other. What is recorded is called an interferogram, a record of the energy recieved by the detector as a function of the passage of the moving mirror. What is found is that when the moving and static mirrors are equidistant from the beam splitter, all wavelengths of light of the two beams are in phase and constructively interfere and a large spike in energy is recorded. This is called the "centre burst". As the mirror is moved toward or away from the mirror, the two beams become rapidly out of phase and the energy drops. As the mirror continues to move, the detected beam energy rises and falls as different wavelength of the two beams come into and go out of phase. The interferogram is a series of data points and as a whole contains all the information about the intensity of light recieved by the detector at all wavelengths. No particular part of the interferogram relates to any particular part of the spectrum. Each data point contain a contribution to the information of the entire spectrum. The Fourier transform extracts this information to give the spectrum. The major advantage of this method over the older method is speed, it is much faster to record a spectrum. Also, only one beam is needed as a background spectrum can be recorded first and subtracted automatically by computer. It is also advantageous if chemical reactions are to be monitored. With traditional systems, the spectrometer had to be set at the required wavelength characteristic of the compound being studied and the intensity of this wavelength plotted as a function of time. The advanatge of FTIR is that the whole spectrum rather than a single data point of it can be recorded in a matter of second so that in a reaction where intermediate species are formed and consumed but are observable, different compounds can be folowed simultaneously rather thn having to reset the machine and repeat the experiment.

Methods of Sample Preparation

• Thin film

This is a sample technique for liquid samples with fairly high boiling points. A drop of the liquid is placed onto a disc of sodium chloride about an inch wide and a quarter of an inch thick (in grnaulated form better known as table salt, NaCl) and then sandwhiched as a thin film with a second disc. After aquiring a background spectrum, the two disc are put into a sample holder in the path of the IR beam and scanned. The disc itself has no or negligable IR absorption and so the sample is all that contributes to the spectrum.

• Nujol mull

This is a technique for solid samples and is carried out in an analagous way to a thin film. The solid sample (ca handful of milligrams) is ground with a small pestle amd mortar. A couple of drop of a hydrocarbon termed "nujol" is added and the ground sample suspended in this oil. A portion of the oil is then smeared and sandwiched between to NaCl disc and the spectrum recorded. The nujol does have its own IR signature but the peak are narrow and do not obscure the spectrum of the sample very much. The trick is toi use as little nujol as possible to limit it's impact on the spectrum. A thin film spectrum of nujol on it's own can be subtracted from nujol mull spectra but the substraction is not always perfect.

• KBr disc

This is another solid sample technique and is quite different to that above. The sample is again ground but a spatula full of anhydrous (dry) postassium bromide (KBr) powder is added. This is further ground to disperse the sample in a fine powder of KBr. A portion of this powder is then tranferred to a special press in which the powder is sandwiched between two highly polished metal discs and put under the equivalent of 10 tonnes pressure. After about 5 minutes, the material (if you're lucky) has formed a solid transluscent disc about a centimetre across and a millimetre of less thick. This is placed in a special sample holder in the IR beam of the spectromter and scanned after a background spectrum of the air has been taken. Similarly to the techniques using NaCl discs, the KBr has negligable IR absorption and so cointributes nothing to the spectrum.

• Solution cell

This is a technique mostly used for solid samples which typically less than 10 mg of sample is dissolved in about 1 mL of solvent. Typcial solvents are dichloromethane of pentane. The solution cell consists of a metal frame that clamps two windows made of an IR transpareant material, usually calcium fluoride (CaF). Between the windows is a teflon spacer layer about 0.1 to 0.5 mm thick with a window cut into the middle. This creates a thin space between the windows into which the sample can be injected through stoppered holes in one of the CaF windows. In order to take a spectrum, a background spectrum of the pure solvent must first be recorded. Using the same cell, the spectrum of the solution is then recorded and solvent signals are removed. This technique does have limitation as solvent will strongly absorb in certain parts of the spectrum often to saturation. Thus, a suitable solvent must be chosen whose IR signature has absorption "windows". This technque however well suited to the study of the rates of chemical reactions.

• Attenuated total reflection (ATR)

This is less common but is gaining in popularity for routine spectroscopy. In this technque, the IR beam from the spectrometer is diverted by mirrors and shone into an IR transparent crystal whose shape is designed to induce total internal reflection much like a fibre optic cable. If a sample is in contact with the surface of the ATR crystal, a small amount of the radiation can penetrate and be absorbed and is suitable for a wide variety of both solid and liquid sample with little sample preparation required. The beam then exits the ATR crystal and is diverted back into the spectrometer for detection.