INTRO, IR Spectroscopy

Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by a sample positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is to determine the chemical functional groups in the sample. Different functional groups absorb characteristic frequencies of IR radiation. Hence it is used to identify chemical compounds or monitor changes occurring in the course of a chemical reaction. For example, the spectra of two related molecules, 1-propanol (an alcohol), and propanoic acid (a carboxylic acid) are given below. Their spectra are considerably different and tell about important features of a molecule. Using various sampling accessories, IR spectrometers can accept a wide range of sample types such as gases, liquids, and solids. Thus, IR spectroscopy is an important and popular tool for structural elucidation and compound identification.

Infrared refers to that part of the electromagnetic spectrum between the visible and microwave regions. Electromagnetic spectrum refers to the seemingly diverse collection of radiant energy, from cosmic rays to X-rays to visible light to microwaves, each of which can be considered as a wave or particle traveling at the speed of light.  

Infrared radiation spans a section of the electromagnetic spectrum having wave numbers from roughly 13,000 to 10 cm^–1 , or wavelengths from 0.78 to 1000 μm. It is bound by the red end of the visible region at high frequencies and the microwave region at low frequencies.

Of greatest practical use to the organic chemist is the limited portion of this region, the mid IR range 4000–400 cm^–1. An increase in wave number corresponds to an increase in energy. This is a convenient relationship for the organic chemist 

Infrared radiation in the range from about 10000-100 cm^-1 is absorbed by organic molecules and converted into energy of molecular vibration. This absorption is quantized but vibrational spectra appear as bands rather than as lines because a single vibrational energy change is accompanied by a number of rotational energy changes. it is with these vibrational-rotational bands, particularly those occurring between 4000-400 cm^-1 that an organic chemist concerned. In IR spectroscopy, an organic molecule is exposed to infrared radiation. When the radiant energy matches the energy of a specific molecular vibration, absorption occurs. The frequency of absorption depends on the-

1. Relative masses of atoms 
2. Force constants of the bonds and
3. Geometry of the molecule

In a typical IR spectrum the band positions are presented as wave numbers. The wave number, plotted on the X-axis, is proportional to energy; therefore, the highest energy vibrations are on the left. Band intensities can be expressed either as transmittance (T) or absorbance (A). Transmittance, T, is the ratio of radiant power transmitted by the sample (I) to the radiant power incident on the sample (I₀). Absorbance (A) is the logarithm to the base 10 of the reciprocal of the transmittance (T). The percent transmittance (%T) is plotted on the Y-axis. Absorption of radiant energy is therefore represented by a “trough” in the curve. Zero transmittance corresponds to 100% absorption of light at that wavelength. Organic chemist usually report intensity in semi-quantitative terms (s = strong, m = medium, w = weak).

Except at very, very low temperatures, all molecules are in motion in some manner. Molecules translate (move from place to place), they rotate in space, and, importantly for this experiment, they vibrate. At temperatures above absolute zero, all the atoms in molecules are in continuous vibration with respect to each other. When the frequency of a specific vibration is equal to the frequency of the IR radiation directed on the molecule, the molecule absorbs the radiation.