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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
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.
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