Although ab initio HF calculations fail in predicting molecular atomization energies, one can still use HF energies
Question:
The bond-separation reaction for benzene is C6H6 + 6CH4S 3C2H6 + 3C2H4. (If the energy of a molecule could be represented as the sum of bond energies that were invariant from molecule to molecule, then the energy change for any isodesmic reaction would be zero. The energy change for a bond-separation reaction measures the interactions between the bonds in the molecule.) If an SCF MO calculation could fairly accurately predict the energy change for the bond-separation reaction of a large molecule, then we could use the known energies of the small product molecules like C2H6 to get a reasonably good estimate for the energy of the large molecule from an SCF calculation, without having to use expensive correlation methods.
(a) Write the bond-separation reaction for cyclopropane. Do the same for CH3CHO.
(b) Do HF/6-31G* energy and vibrational frequency calculations on the molecules in the CH3 CHO bond-separation reaction. Scale the vibrational frequencies (Section 15.12) to find the zero-point energies. Find the ∆H0 value predicted by the HF/6-31G* method for the gas-phase CH3CHO bond-separation reaction. Compare with the experimental value of 11.5kcal/mol.
(c) Repeat (b) using MP2/6-31G* calculations. (Although isodesmic-reaction calculations work reasonably well for small molecules, the errors become too large when used with large molecules.)
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