Assignment CompChem4: Model Chemistries

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. Effect of varying basis set on calculation of hydrogen fluoride (exercise 5.1).

Perform Optimize+Vib Freq calculations at the MP2 level for hydrogen fluoride (HF) using the following basis sets: STO-3G, 3-21G, 6-31G(d), and 6-311+G(d,p). Make a tabl;e with columns for basis set, bond length, dipole moment, vibrational frequency, energy, and CPU time. Also include the experimental values (0.9171Å, 1.91 D, and 4139 cm^-1) in the table. Comment on the effect of basis set on the calculation.

2. Effect of varying theoretical method on calculation of hydrogen fluoride (example 6.3).

Perform Optimize+Vib Freq calculations using the 6-311+G(d,p) basis set for hydrogen fluoride (HF) using the following methods: HF, MP2, MP4, B3LYP. (Note that you already did the MP2 calculation in the previous problem.) Make a table with columns for method, bond length, dipole moment, vibrational frequency, energy, and CPU time. Also include the experimental values (0.9171Å, 1.91 D, and 4139 cm^-1) in the table. Comment on the effect of theoretical method on the calculation.

3. Ozone destruction by atomic chlorine using compound methods (exercises 6.9 and 7.2).

Perform CBS-4M calculations on atomic Cl (doublet), O₃ (bent geometry, singlet), OCl (doublet), and O₂ (triplet). [I have observed that OCl must be built with oxygen as the first atom; otherwise, the CBS-4M portion of the calculation fails.] When configuring Gaussian job options, set Calculation = “Other(CBS-4M)”, Theory = “Other()”, and Basis Set = “Other()”. Make a table with columns for molecule, energy, and enthalpy. Use your results to calculate the enthalpy change of the reaction

Cl + O₃ ® ClO + O₂

Compare your results to the experimental value of DH = -39.1 kcal/mol. Is the difference between calculation and experiment in line with what can be expected from compound methods? How does your result compare to Hartree-Fock calculations (such as the decomposition of CH₃CHO into CO and CH₄ that you calculated last week)?

4. The CBS-4M compound method (pp. 154-156).

View the raw output of a CBS-4M calculation for a molecule (not an atom). Copy all the route lines in the output file and paste them into your homework. Use these lines to explain how the CBS-4M compound method actually works.

5. How could WebMO be improved to assist you with your calculations? Please be more imaginative and thorough with your suggestions and constructive criticism than you have been in the past. Thank you in advance!