Infrared Spectroscopy

Overview:

Infrared Spectroscopy addresses an overview of the electromagnetic spectrum, particularly that portion utilized in most IRs. It includes a study of molecular vibrations, areas of the spectrum, and techniques used in reading an InfraRed spectrum.

For most beginning organic chemistry students, the IR is most useful for identifying ordinary functional groups such as acids, alcohols, esters, aldehydes, ketones, some amines, nitriles, alkynes, some aromatics, and some other combinations of groups. Obscure combinations or absolute identification of an unknown compound is usually beyond the range of this beginning student unless computer aided identification programs are made available. The purpose of this unit it to address the general principles on which IR is based, examine select molecular vibrations, identify six areas of the spectrum the student should particularly notice, and provide some practice in reading the major signals present in several spectra.

After a brief review of the electromagnetic spectrum and the relationships that exist among wavelength, frequency, and energy, the student is introduced to the effects of infrared energy on molecules. The concept of the oscillating dipole is presented briefly and the study is directed to descriptions of molecules undergoing several vibrational modes. Animation is used to demonstrate such modes as symmetric stretching, asymmetric stretching, and scissoring. Wagging, rocking, and twisting modes are mentioned in this discussion. The idea is that the student expand what is often a limited view of the dynamics of a molecule consisting of more than two atoms.

The spectrum is broken into six areas, a classic way of introducing organic students to the complexity of signals they may be viewing. This programs identifies area A as that area where N-H bond stretch is expected, as well as O-H bond stretch, and the stretch expected in terminal alkynes. Area B is divided into two parts to help the student differentiate between saturated and unsaturated C-H stretch. Area C is developed as that area in which triple bond may be suggested. Area D is identified as the area in which carbonyl stretching and stretching in alkenes and dienes may be expected to appear. Area E actually encompasses two major sections: that having to do with the fingerprint area and that area in which aromatic characteristics may be noted.

Signal combinations are studied and characterized so that students may differentiate such things as position of carbonyl stretching in aldehydes and ketones as opposed to that in esters, amides, acids, etc. Several practice spectra are introduced and evaluated.