References: Carey, pp 518-523 and Pavia et al., pp A14-A31
For starters, consider the possible motions
available to a molecule containing n atoms. Since each atom is capable of moving
in three dimensions [components along the orthogonal or mutually perpendicular
x-, y-, and z- coordinate axes, as defined by us for our own convenience], there
are a total of 3n possible ways for the molecule to move.
Of this total, three modes are translational -- the molecule as a whole moves in the x-, y- or z- direction without change in any of its bond angles or distances. The available translational kinetic energy states form a continuum for all practical purposes.
An additional three modes are rotational -- the molecule as a whole can rotate around its axes parallel to the x-, y-, or z- directions we have defined without change in any of its bond angles or distances. Rotational kinetic energy is quantized; the spacing of the allowable rotational kinetic energy levels is such that transitions between them can be produced by absorption of electromagnetic radiation in the microwave portion of the electromagnetic spectrum.
This leaves a total of 3n-6 vibrational modes of motion, in which bond lengths and/or angles may change. [For completely linear molecules, there are 3n-5 possible vibrational modes, since one of the rotational modes is unavailable.] Vibrational kinetic energy is also quantized, and the spacing of the allowable vibrational kinetic energy levels is such that transitions between them can be produced by absorption of electromagnetic radiation in the infrared or IR portion of the electromagnetic spectrum.
If one models a covalent bond as a spring connecting two point masses, the fundamental vibrational frequency of the resultant harmonic oscillator will depend on the masses of the atomic nuclei and the "force constant" of the spring [strength of the bond]. As a result, each type of covalent bond has its own characteristic frequency ranges of infrared light absorbed during a transition between levels. There are additional frequencies absorbed when transitions involving more than two atoms are considered. The main value of IR spectroscopy is that it permits the identification of functional groups present in a molecule. See the "IR Primary Clues" for more on this.
Because the number of allowable vibrational modes will be much larger than the number of functional groups present, there will be additional IR absorptions unique to each molecule. Thus, by comparing an IR spectrum to that of a known sample -- if it is available -- we may even be able to identify a specific compound.
Rather than reinvent the wheel and try to do what others have already done better, let me now direct you to the following sites [new window for each, close when done], where you will find some very useful introductions to infrared spectroscopy:
|"What Is an Infrared Spectrum?"
by Jim Clark, retired Head of Chemistry and Head of Science at Truro School in Cornwall, UK:
|IR Basic Tutorial by Jim Byrd at
|IR Tutor 1.1 by Len Fine at
Columbia University from John Wiley and Sons
more details available at
REALLY worth the $29.95 list price!
|and from Patty Feist at the University of Colorado, for
IR and NMR there's a
great intro at
and the IR spectroscopy tutorial at
|"The Tools of the Second Chemical Revolution:
Instrumental History and Design"
by Christopher J. Brubaker and David L. Powell at The College of Wooster, OH
A senior independent study project, this site provides an excellent introduction to the history, instrumentation, and basic theory of IR, UV/VIS, and NMR spectroscopy as well as pH measurement.
|"What Are Vibrations?" by Roma Oakes at Queen's
University of Belfast
seems to have become rather dysfunctional
|Vibrational Modes of Small Molecules
by John Nash at Purdue University
has neat Chime animations of different molecular vibrations
|Molecular Vibrations by Mol4D at U
also has Chime animations of vibrations
|Chime-based interactive IR spectral site
by Robert J. Lancashire at the University of the West Indies, Mona, Jamaica
I have it on good authority [RJL] that
clicking on a peak in the IR will load a Chime animation file of the corresponding vibrational mode.
An alternate version of this site at the University of Massachusetts
by Paul Lahti, Allison Rafuse, Eric Motyka at U. Mass and Robert Lancashire at UWI Mona is at
The U. Mass version works great -- I particularly recommend seeing p-xylene doing its sexy little shimmy!
|Infrared Spectroscopy by ??? At the University of Missouri,
prints out at 25 pages of single-spaced text with illustrations.
Very thorough, may encourage you to read your own texts! [Carey, pp 518-523 and Pavia et al., pp A14-A31]
|Organic Compound Identification Using Infrared Spectroscopy
by Dr. Walt Volland, Bellevue Community College
|Webspectra Problems in NMR and IR Spectroscopy
Want some practice before the next exam? Here's the place to go to put it all together!
Elucidation by Bradley D Smith, Bill Boggess, and Jaroslav Zajicek
at the University of Notre Dame has 64 problems with IR, 1H NMR, 13C NMR, and mass spec
|Spectroscopic Tools von Steffen
Thomas bei der Universität Potsdam
Has "wizards" for IR, 1H-NMR, MS, and a 13C-NMR database
|ComSpec3D von Frank
Oellien and Wolf-D. Ihlenfeldt bei der Universität
IR/Raman spectra prediction from structures you create,
plus VRML animations of the vibrations you select from the predicted spectrum
|David Sullivan's FT-IR Library at
the University of Texas
Hey, they look upside-down because they're absorbance, not transmittance, spectra --
but the basic info's the same!
|Infrared Spectroscopy by Neil
Glagovich at Central Connecticut State University
http://www.chemistry.ccsu.edu/glagovich/teaching/316/index.html > IR
| a long list of IR web references by João Aires de Souza
|additional IR tutorials are listed at
|Carey PowerPoint slides
for chapter 13 from Columbia University
[13.19, infrared spectroscopy]
|Characteristic Infrared Absorption
Frequencies by Christine K.F. Hermann at Radford University
in case you think the "Primary Clues" page has too much information....
|IR Basic Instruction by Bette Kreuz
at the University of Michigan-Dearborn
And, of course, there's always my "Spectral City"
page, which has the sites above and much, much more!
Close this window to return to class schedule or click here
© Ronald W. Rinehart, 2002-2007