Chemistry 130 CHM 220   IR

IR/Functional Group Laboratory Experiment

Background information*

Infrared (IR) radiation is radiation in the energy range between the visible and microwave regions of the electromagnetic spectrum. The portion of the IR spectral region between 4000 and 400cm-1 is of greatest practical use to the organic chemist. An IR spectrum, as shown in Figure 1, is a plot of the percentage of IR radiation that passes through a sample versus the frequency of the radiation. The radiation that passes through the sample is measured as percent transmission. The frequency of the radiation is measured in wavelengths per centimeter, which is known as a wavenumber.
Figure 1

In IR spectroscopy, a simple molecule can produce a very complex spectrum. An organic chemist takes advantage of this complexity by comparing the spectrum of an unknown compound with that of a reference compound. It is unlikely that any two compounds would produce exactly the same IR spectrum.

Although an IR spectrum is characteristic of the entire molecule, certain groups of atoms, called functional groups, give rise to particular absorption bands, or peaks. These absorption bands occur at or near the same frequency, regardless of the structure of the rest of the molecule. The persistence of these characteristic bands permits chemists to obtain useful structural information by comparing the absorption bands for a sample to tables of functional group absorption frequencies.

Studying Vibrations and Energy - Understanding IR spectroscopy theory requires the study of vibration mechanics. In a normal mode of vibration, each atom in a molecule executes a simple harmonic oscillation (vibration) about its equilibrium position. A ball-and-spring molecular model can be used to demonstrate the effects of vibrations in stretching and bending springs, together with the motions of the balls. According to classical mechanics, the frequency of vibration v of two balls of total mass m connected by a spring with a force constant k, is shown in Equation 1.

The method of classical mechanics can be used to study the vibrational motions and frequencies of a model containing several balls (or atoms) of various masses connected by springs (or bonds) with different force constants.  The results of these studies form the basis for the interpretation of vibrational spectra. 

Classical mechanics would indicate that there is a continuum of vibration levels, and that a molecule may undergo any of numerous vibrations.  Quantum mechanics, however, places restrictions on microphysical systems.  These restrictions limit a molecule to having discrete energy levels.  The difference in energy (delta E) between the vibrational levels is given by Equation 2, in which h is Planck’s constant and k is the force constant. 

The reduced mass m, defined by Equation 3, is given for a molecule AB, where MA and MB are the atomic masses of atoms A and B.

When the frequency of infrared light applied to a compound is exactly the same as the natural vibrational frequency of an interatomic bond, the molecule absorbs the light and the amplitude of the bond vibration increases. As indicated in Equation 2, the force constant for the deformation determines the vibrational frequency. Therefore, frequency, and thus the frequency of the radiation absorbed, is related to the rigidity or strength of the bond and the masses of the bonding atoms. Specifically, the vibrational frequency is higher for stronger bonds and for lighter atoms.

Two types of vibrations, stretching and bending, are responsible for most of the important peaks used to identify organic compounds. For example, Figure 2 shows a few types of vibrations for the CH2 group. Bending motions require less energy than stretching motions, so the bending motions absorb at lower frequencies, and, therefore, have smaller wavenumbers.
The magnitude of infrared absorption bands is proportional to the change in dipole moment, or separation of charges, that a bond undergoes when it stretches. Thus, the more intense bands in an infrared spectrum are often produced by C=O and C–O stretching vibrations. In contrast, the C triple bond C stretching band for a symmetrical alkyne is almost nonexistent because the molecule undergoes no net change in dipole moment when it stretches.

Interpreting Spectra - It is usually not possible to assign specific molecular vibrations to the majority of bands in an infrared spectrum. However, it is helpful to divide an IR spectrum into two parts. The 4000 to 1500cm-1 portion is useful for identifying various functional groups. Bands in the 1500 to 600cm-1 portion, called the fingerprint region, are the result of many types of vibrations that are characteristic of the molecule as a whole. This complex fingerprint region represents a unique pattern for each organic compound. The region is useful for comparing the spectrum of an unknown compound with the spectra of a known compound for identification purposes.

Two examples, 2-methyl-2-propanol and 2-butanone, illustrate the application of IR spectroscopy to identify organic functional groups. In the IR spectrum of 2-methyl-2-propanol, shown in Figure 3(a), the intense band from 3500 to 3100cm-1 can be attributed to the OH group. Specifically this band illustrates the stretching frequency of the O–H bond. The stretching frequency of a C–H bond is near 2900cm-1, and the stretching frequency of a C–O bond is near 1150cm-1.

The stretching frequency of the C=O group in 2-butanone results in an intense band near 1700cm-1, as shown in Figure 3(b). Clearly, the fingerprint regions in Figures 3(a) and 3(b) are different, reflecting the different structures of the two compounds.

Table 1 gives a correlation of the more common structural units and their characteristic vibrational frequencies.

Measuring IR Spectra – Currently, there are two types of instruments used to record IR spectra:  dispersive double-beam and Fourier transformed (FT) spectrophotometers.  In a double-beam spectrophotometer, the IR radiation is emitted into a monochromator, which separates the wavelengths of light.  The monochromatic radiation goes into a beam splitter composed of a mirror and a prism.  The beam splitter allows equivalent beams of relatively narrow wavelength range to pass simultaneously through a sample and a reference cell.  Another prism and mirror system focuses the emergent light into a rotating-sector mirror.  An electronic bridge system detects the radiant energy transmitted by the sample and the reference as a voltage difference.  A recording of the percent transmission of the IR radiation through the sample with varying wavelengths produces an IR spectrum. 

The FT-IR method splits the electromagnetic radiation into two beams (see Figure). One beam travels over a longer path inside the spectrophotometer than the other beam. A recombination of the two beams creates an interference pattern or interferogram.
Figure 3. FTIR InstrumentFTIR

Fourier transformation, a computerized mathematical manipulation of the data from the interferogram, converts the interferogram into the usual IR spectrum. The instrument does not use a monochromator. The radiation from the entire IR spectrum passes through the sample simultaneously, saving much time. FT-IR instrument can have very high resolution (<0.001 cm-1). Moreover, because the data undergo analog-to-digital conversion, FT-IR data can be easily manipulated. Combinations of results of several scans average out random artifacts, allowing excellent spectra from very small sample.

Preparing IR samples – Infrared spectra of liquid samples are prepared by placing neat, or pure, undiluted samples between two salt plates.  Glass is opaque to IR radiation and cannot be used.  Instead, the sample is prepared using potassium bromide (KBr), sodium chloride (NaCl), or silver chloride (AgCl) plates, which are transparent to IR radiation. 

A solid sample can be mixed with solid KBr (see Figure 4).  The mixture is then pressed into a very thin pellet.  Alternatively, a solid sample can be dissolved in a solvent such as dichloromethane, trichlormethane, tetrachloromethane, or carbon disulfide.  In a third method, solid samples may be ground into a very fine powder and mixed with Nujol mineral oil to form a mixture called a Nujol mull.  Some solvents do absorb IR at certain frequencies, so those parts of a spectrum are not useable. 

Typically, neat samples for liquids and KBr pellets for solids are used because neither method produces extraneous absorptions to obscure the spectra. 

KVCC’s instrument is a Perkin Elmer Spectrum BX FTIR: bench-top FT-IR capable of routine mid-IR work (4000 cm-1 to 400 cm-1)


*Adapted from Modular Laboratory Program in Chemistry, Tech 710 by Joe Jeffers