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 of intensities.

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 in DOS.

Both Mac and Windows 95 applications should unstuff automatically yielding a 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 itself.

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 - 4Dem3

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.

  1. Change the value of Be, the rotational constant. The spacing between adjacent lines in each of the branches is roughly equal to two times Be.

  2. What happens if Be is set to zero? Predict what happens before you perform the calculation.

  3. 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.

  4. 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 changed?

  5. Compare the HCl spectrum with the DCl spectrum by using molecular parameters for DCl in the input file.

  6. 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 revised simulation.

Downloads and stuff to look at:

System Requirements:


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