9.4: Exercise 1 - Estimation of the IR Spectrum of HCl and DCl
- Page ID
- 419801
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Exercise 1 - Estimation of the IR Spectrum of HCl and DCl
The goal of this first exercise is to compare your experimental FTIR data from the HCl and DCl diatomic molecules with computational predictions.
- Select New Build from the File from the menu, or click the
button.
- The right side of the screen is the model kit.
Notice that you are in the Organic area of the model kit. Here you will see several common atom types and their hybridization. You might take a moment now to familiarize yourself with the other screens of the model kit, Inorganic, Substituent, Peptide and Nucleotide.
- From within the Organic area of the model kit, click on the –Cl atom and then double-click somewhere in the working area of the main Spartan screen. You will see a colored line, with the Cl atom at the end. Select Save as from the File menu, or click the
button. Keep the suggested filename, or give the file a name of your choice. A hydrogen atom is automatically added to your molecule. To see the hydrogen, click the
button to leave the Build Mode and enter the View Mode.
Clicking the button returns you to the Edit Build Mode.
The exact view of the molecule will depend upon what “model” system you have selected. Use the Model menu to change how the molecule looks.
- Select Calculations from the Setup menu. Choose Equilibrium Geometry at Ground State in Gas, with Hartree-Fock, 3-21G. Total charge should be Neutral, and Unpaired Electrons should be 0. Select compute: IR (with Current Model). Click on Submit. If prompted, save the file (use the same file name). If everything is correct, you will see the “Job “filename” has been submitted” box. Click OK.
- After a moment a “Job “filename” completed” box appears on the screen. Click OK. Inspect the output of this job using Display, Output. The Summary tab will give you the major results of the calculation. If you expand the IR section of the Summary you will see the calculated estimate of the frequency of vibration. Record this value in your lab notebook. In the Output tab, you will see information about the chosen basis set, the number of steps for geometry optimization, and other information related to the calculation. Close the Output dialog box. Then select Spectra under the Display menu and click on the
, and select IR Calculated. The calculated spectrum appears on the screen. Click on the
and you will again see the harmonic oscillator frequency of HCl molecule.
- To animate the vibrational motion, put a check in the box next to the frequency (or click on the peak in the spectrum). If you like, click on the
again and add the Experimental spectrum to the window. How well does the Hartree-Fock calculation reproduce the experimental IR spectrum? To stop the animation, click again on its frequency, or remove the check from the checkbox next to the vibration. Close the spectrum window.
If the animated vibration is going too fast, the speed of the animation can be changed in the following way. Click on the 
icon at the left of the spectrum display window. You will two editable buttons appear.
To slow down the animation, change the number of steps to a larger value (for example, 30).
Changing the Amplitude will alter the degree of displacement of the nuclei through the course of the vibration.
- Back in the main window, under the Geometry menu choose Measure Distance, and find the H-Cl bond length by clicking on the two atoms (or by clicking on the bond between the two atoms). Look to the lower right corner of the screen to see the bond distance. Record the bond length in your lab manual and compare the result with your experimental and literature data. Click the
button to close your HCl file.
- Make a new HCl molecule. Click the
to enter the View Mode. Click once on the hydrogen atom to select it. Select Properties from the Display menu. An Atom Properties Box appears. Change the Mass Number from 1 to 2 (Deuterium). Close the Atom Properties box. Return to the Setup Calculations box and make configure the calculation to be the same HF calculation you did on HCl (do not forget to check the IR option). Click on Submit. Save the file under a new name and let the calculation run.
- Find the harmonic oscillator frequency and the bond length for DCl. Compare your results with both your HCl results and your experimental (FTIR) DCl data. Select File, Close to close your molecule when you are done.
|
Data for HCl and DCl |
Calc. \( \nu \) (cm–1) |
Exp. \( \nu \) (cm–1) (FTIR) |
Cal. Bond Length (Å) |
Exp. Bond Length (Å) (FTIR) |
|
HCl |
||||
|
DCl |
- Summary: Compare your results to those you obtained from the FTIR experiment done earlier in this course. What can you conclude about the accuracy of computing bond lengths and vibrational frequencies using this Hartree-Fock calculation? Why is the bond length identical for both HCl and DCl?
If you are interested, and since the calculations are relatively quick, you might try the same calculation with a different basis set (6.31-G* for example), and/or a completely different model (semi-empirical or density functional). Does the accuracy of the results change?
This section of your lab notebook should include:
- A table summarizing the results of the HCl/DCl calculations
- Comparison of computational results to your experimental FTIR data and literature data
- What can you conclude about the accuracy of computing bond lengths and vibrational frequencies using this Hartree-Fock calculation?
- Why is the bond length identical for both HCl and DCl?
(See your ELN template for details)