CHEM 408 – Sp05 4/12/2006 Worksheet #10.B – Vibrations & Thermochemistry Calculations Team #: _______ Members: _____________________________________ Due: 4/17/06 _____________________________________ Resources: Exploring Chem excerpt _____________________________________ Worksheet 10 materials _____________________________________ _____________________________________________________________________________________ I have placed three input files into the 408 directory (/home2/chem408) for your use. They are named “benzene_vib.com”, “HF_scan.com”, and “HF_opt.com”. The latter two files perform calculations using the HF/cc-pVQZ model chemistry. The results obtained using this basis set should be close to those at the Hartree-Fock limit. _____________________________________________________________________________________ 1. Use the input file “benzene_vib.com” in the /home2/chem408 directory to run a frequency calculation on benzene. Then use the /Results/Vibrations menu to view the vibrational modes and answer the following questions: (a) How many vibrational modes are calculated by G03. Is this the number you expect? (b) Some of the vibrational modes come in pairs. Why? (c) Describe and sketch the nature of the modes responsible for the 3 most intense peaks in the IR and Raman spectra according to these calculations. _____________________________________________________________________________________ 2. First run the HF_scan.com job. (a) Print and annotate the HF_scan.com file, explaining the meaning of all components. (Remember that you can look up the meaning of keywords in the on-line G03 manual.) (b) Use the results of the job to make a plot of the computed energy as a function of bond length. (c) How is the curve in part (b) related to how a molecular mechanics force field might model the stretch in the HF molecule. (d) Using a quadratic fit of the four lowest energy points, determine the (harmonic) force constant (in units of N m-1) min22rrelrdrEdk=⎟⎟⎠⎞⎜⎜⎝⎛= and the vibrational frequency μπνrk21= where FHFHmmmm+=μ in units of cm-1 (i.e. c/νν= ). Show your work.CHEM 408 – Sp06 4/12/2006 2 To save time, use the following data and conversion factors: 1 Eh Å-2 = 435.975 N m-1 μ = 1.589 × 10-27 kg (e) Based on the vibrational frequency you calculated, determine the contribution o the zero-point energy to the internal energy of a mole of HF gas at 298 K. Express your result in both kJ/mol and kcal/mol. (f) Also determine contribution to the internal energy (U) of a mole of HF gas at 298 K from translational, rotational, and vibrational degrees of freedom. Again express your results in both kJ/mol and kcal/mol. _____________________________________________________________________________________ 3. Run the G03 job specified by the HF_opt.com file. (a) Print out and annotate the HF_opt.com file as in #1(a). (b) How can De, the equilibrium dissociation energy, be determined from the data in the output (log) file? (c) Below I’ve compiled experimental data for the HF molecule. Compare the results obtained from the HF/cc-pVQZ model chemistry to these experimental data. Include %errors as shown. Property Expt HF/cc-pVQZ %error MP2/cc-pVQZ %error Re / Å 0.9168 μ / D 1.82 ν / cm-1 4138.2 De / kJ mol-1 591 Note that I’ve chosen the cc-pVQZ basis set for these calculations and the results obtained should be close to those at the Hartree-Fock limit. (d) Modify the input file in order to run the same calculations using the lowest order perturbation correction for the effect of electron correlation, MP2. Rerun the calculations and fill in the remainder of the table. Comment on the importance of electron correlation for different types of properties. (e) Finally, cut and paste the Thermochemistry part of the output of the HF calculations and compare the internal energy that Gaussian calculates to the values from #2(e) and (f).
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