User`s guide

7. Exercises
21. Compound Method Calculation of Ozone Destruction by Atomic
Chlorine
Use Gaussian to perform CBS-4M calculations on atomic Cl (doublet), O
3
(bent geometry,
singlet), OCl (doublet), and O
2
(triplet). [It has been 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 + O
3
→ ClO + O
2
Compare your results to the experimental value of ∆H = −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
previously calculated decomposition of CH
3
CHO into CO and CH
4
)?
22. CBS-4M Compound Method
The CBS-4M compound method represents a good compromise between computational accuracy
and speed.
View the raw output of a CBS-4M calculation for a molecule (not an atom). Copy all the route
lines from the output file. Use these lines to explain how the CBS-4M compound method actually
works.
G. Partial Charge
23. Partial Charge Analysis of Electrophilic Aromatic Substitution
Reactions
The observed isomeric product distribution of electrophilic attack by NO
2
+
on nitrobenzene and
chlorobenzene can be analyzed by examining the partial charges on the otho, meta, and para
carbons of substituted benzene reactant.
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