Assignment CompChem3: Frequency Calculations

Use WebMO to complete the following exercises. You are encouraged to comment on the additional results provided by WebMO in your answers, if appropriate.

1. Vinyl amine stationary points (exercise 4.2).

Build and optimize planar and pyramidal vinyl amine. For planar vinyl amine, adjust the hybridization of N to sp² before cleaning up the geometry. Be sure to adjust the dihedral of the amine group appropriately for each molecule. Perform an Optimize+Vib Freq, Hartree-Fock 6‑31G(d) calculation on each molecule. Report the energy of each conformation in a table. Characterize each stationary point using the frequency results. Use your chemical intuition to explain why this conformation is a transition state. Hint: Why is the other conformation more stable?

2. Carbonyl stretching frequencies (exercise 4.4).

Calculate the C=O stretching frequency for acetaldehyde and acetyl chloride using an Optimize+Vib Freq, Hartree-Fock 3‑21G calculation for each molecule. Make a table with columns for molecule, CO stretch frequency, scaled CO stretch frequency, and experimental frequency. Hint: Scaling factors are listed on p. 64 of ECESM.

Insert figures of each C=O vibration. Comment on the accuracy of vibrational frequency calculations.

3. Transition state for 1,3 hydrogen shift of fluoropropene (exercise 4.6).

Build the fluoropropene transition state. Do not do a comprehensive cleanup, as the Add Hydrogens function uses rules that are applicable to stable molecules, not transition states. Instead, manually add the hydrogens, including a hydrogen that is bonded to both C1 and C3. Adjust the hybridization of C2 to sp². Clean up the geometry only, and then adjust the H-C2 bond angle appropriately. Perform an Optimize Transition State, PM3 calculation on the molecule. View the result and then create a new job using the same geometry. Perform a Frequency, PM3 calculation on the previously optimized geometry. View the transition state.

Insert a picture of the transition state and reaction coordinate. How do you know that it is a transition state? What is the energy of the transition state? State what additional calculations would have to be performed to determine the reaction barrier for 1,3 hydrogen shift of fluoropropene.

4. Thermochemistry of CH₃CHO ® CO + CH₄ in the gas phase.

Build and perform Optimize+Vib Freq Hartree-Fock, 6‑31G(d) calculations on acetaldehyde, carbon monoxide, and methane. To speed up your calculation, start your acetaldeyhde calculation from the same geometry as a previous lower level optimization.

Make a table with columns for molecule, C_v, S, E₂₉₈, H₂₉₈, and G₂₉₈.

Visit the NIST webbook (http://webbook.nist.gov/chemistry) and make a table with columns for molecule, C_p, S, and D_fH (kcal/mol).

Comment on the agreement between Gaussian and NIST by comparing C_p, S, and D_rxnH in a table. Hint: Use the relationship that C_p = C_v + R for an ideal gas. Hint: 1 Hartree = 627.5095 kcal/mol.

5. How could WebMO be improved to assist you with your calculations? Please be as imaginative and thorough as possible with your suggestions and constructive criticism!