Simulation of the HCl Rotational-Vibrational Spectrum
A Physical Chemistry Laboratory
Most physical chemistry courses include in their lab curriculum the
rotational-vibrational infrared spectrum of gaseous hydrogen chloride,
HCl. The experimental spectrum may contain up to 50 transitions, 25 for
each of the two major isotopes of chlorine. The spectrum is
interesting in that it is divided into two halves,
called the P and R branches, and each branch shows a unique pattern
The following lab uses a computer program to generate a simulation
of the HCl spectrum from a list of molecular parameters input to
the program. The program accounts for both P and R branches, their
intensities and their widths and shapes. To get the program, just download
at the link below.
There are two versions of the program: Macintosh and Windows 95.
The Mac version requires a PowerPC; the windows version requires Win95
to be running for the program to execute. You cannot be running only
Both Mac and Windows 95 applications should unstuff automatically
simulation folder that containing five files.
Once you have the software on your machine the program is run easily.
All you need to do is double click the application icon (hcl_sim_ppc for
Macs, hclsim.exe for Windows95). Please note: the input file, hcl.dat,
must be in the same folder as the application, and the output file,
spectrum.dat, will be written to
this same folder. DO NOT CHANGE THE NAME OF THE INPUT FILE.
You can call the folder anything you want.
Also included below are experimental and simulated spectra that you can
look at to
see if you are interested in pursuing the laboratory. Note that the simulation is
very much like the experimental spectrum. In fact, I doubt if you can tell the
difference between the two.
If you do use the program I suggest
that you overlay your simulations onto an experimental spectrum obtained in the p-chem lab
and check the "fit" of each simulation. Once a good fit is achieved, one usually
assumes that the molecular parameters used to generate the
simulation accurately reflect those of the molecule itself.
You can easily change the characteristics of the simulation
by changing the molecular parameters in the input data file.
The program can accommodate only one of the chlorine isotopes at a
time. You can generate a simulation for each of the two isotopes,
add them together, and then overlay the sum onto the experimental spectrum.
Unfortunately the present program does not plot the simulation for you; you will have
to provide your own graphics package. You can use something as simple as
EXCEL and Quatro Pro, but you may find more sophisticated plotting
applications more useful. Hopefully future versions
will plot and overlay spectra for you within the simulation program
Things you can do in this lab:
Calculation of the P and R branches for the 0 to 1 vibrational transition
is made using the equation: (See D.P. Shoemaker,
C.W. Garland and J.W. Nibler, "Experiments in Physical Chemistry," McGraw-Hill, 6th
edition, pp. 397-404)
wavenumber = nue - 2xenue +
(2Be - 2alfae)m -alfaem2 -
nue is the vibrational wavenumber
xenue is the vibrational anharmonicity
Be is the rotational constant
alfae is the coupling between rotation and vibration
De is the centrifugal stretching constant, and
m = J"+1 for the R branch and -J" for the P branch.
- Change the value of Be, the rotational constant. The
spacing between adjacent lines in each of the branches is roughly equal to two times
- What happens if Be is set to zero? Predict what happens before you
perform the calculation.
- The spacing between adjacent lines in the R branch is noticeably
less than that in the P branch. Why? What happens to the
spectrum if you set alfae equal to zero in the program? Does the result
make sense? Look at the equation above.
- What effect does changing De have on the spectrum, based upon
the equation given above? Is this borne out in the simulation? Which lines
(i.e., what values of J") shift the most when the value of De is
- Compare the HCl spectrum with the DCl spectrum by using molecular
parameters for DCl in the input file.
- If you have already done the HCl lab in your course work you probably determined
the values for the molecular parameters by graphing.
Using your values, generate a simulation. (You will have to modify the file hcl.dat
which inputs the molecular parameters to the program.) If your
experimental spectrum can be viewed with a graphics software package do so.
Then read your simulated spectrum into the same package and try to overlay it
on the experimental spectrum. Does the simulation "fit" the experimental data?
If so, then you did a good job when determining the molecular parameters.
If not, which parameters must be changed to get a better fit. Why not try
making these changes (one at a time) in the input file, and checking the fit of each
Downloads and stuff to look at:
- Macintosh: The program should run on any PowerPC Macintosh. I
have used it on a relatively slow
6214CD Performa running at only 75 MHz (15 seconds)
and on a 300 MHz G3 (3 seconds).
Windows95: The program should run on any machine running Windows 95. I
have used it on a 233 MHz MMX (10 seconds) and a 300 MHz Pentium II (3
Both the input and output files, hcl.dat and spectrum.dat, are text files.
You should be able to use most any text editor to read and modify these files.
(Some editors may not be able to read spectrum.dat due to its large size.)
If you are a Mac user, I recommend you use BBEdit to edit text files.
If you want to use
BBEdit but don't want to pay for it or you just want to try it out first,
you can download a free "lite" version from
The program itself was written using CodeWarrior, which is a great
environment for writing C-programs.
Of course, I cannot be responsible for any damage this program does to
anyone's machine. I and my students have never had any problems, but
you must use it at your own risk.
- Thanks to fellow "CodeWarrior" Gary Wernsing for his expertise
in computer programming, and for introducing me to
the world of Macintosh.
Send me email if you have
comments, questions or problems.
W. Bryan Lynch
Assistant Professor of Chemistry
University of Evansville
1800 Lincoln Avenue
Evansville, IN 47722