Ring Strain: CH2 v. O

Last Monday (Feb 11) we spent some time discussing ring strain in two bicyclic molecules: 2-norbornene and its 7-oxa analog. While several groups contribute to ring strain in these molecules, any difference in ring strain might be due to angle strain at position 7.

Spartan’s model database contains high-level models (MP2/6-31G*) of both molecules. The C-C7-C and C-O7-C bond angles are 93.9° and 95.6°, respectively. In our discussion, we had noted that the ideal angles for carbon were larger than those for oxygen ( 109° in CH4 v. 104° in H2O). Comparing the bond angles in the bicyclics with the ideal angles, we might predict that 2-norbornene is more strained. And this might actually be true. Using total energies from the model data, along with HF/3-21G ZPE energies, I have estimated that the following reaction is exothermic by -5.6 kcal/mol. This is consistent with strain release at atom 7.

You might wonder if CH4 and H2O are the best models for estimating ideal bond angles. These molecules contain H-Z-H combinations while the bicyclics contain C-Z-C. As it happens, the experimentally measured bond angles in CH3-Z-CH3 are 112.4° (Z = CH2) and 111.7° (Z = O) (note: the MP2 angles are in nearly perfect agreement with experiment). These angles are quite similar, although the COC angle is still smaller.