Ring Strain in Cyclopropane, Cyclopropene, Silacyclopropane, and Silacyclopropene

The strain energy in four three-membered rings is calculated by using the appropriate isodesmic reactions. Calculations with and without d functions on the heavy atoms indicate that while these polarization functions tend to reduce ring strain, the addition of d orbitals has little effect on the predicted trends. While introduciton of unsaturation drastically increases the strain in the carbon system (cyclopropane---. cyclopropene), this is not the case for silicon (silacyclopropane--+ silacyclopropene). As a result cyclopropane and cyclopropene are predicted to be the least and most strained rings, respectively. Ring strain is a concept which is frequently used to rationalize instability and reactivity of small cyclic compounds. Recently, a novel quantitative measure of ring strain has been proposed. 1 This method makes use of isodesmic reactions in which the number of bonds of each type is formally conserved on going from reactants to products. The use of isodesmic reactions is particularly useful in quantum chemistry since the effect of correlation error is expected to be minimal in such reactions. As a result, one can use Hartree-Fock-level energy differences with some degree of confidence. In this paper the ring strain in cyclopropane, cyclopropene, silacyclopropane, and silacyclopropene is examined by using ab initio molecular orbital calculations. Cyclopropane and cyclopropene have been studied before1 and are included here for internal consistency. While semiempirical24 and ab initio5 calculations have been carried out on the silicon rings, no attempt has been made to evaluate the strain in these rings or, more importantly, their ring strain relative to the hydrocarbon analogues. In view of the apparent stability of the silicon-containing rings,6 such a comparison is clearly of interest. The methodology used in the calculations is summarized in the Methodology section, while the results of the ring strain calculations are presented and discussed in the Results and Discussion section. To aid the latter discussion approximate quadratic force fields have been generated for the four rings, and these are also presented in the Results and Discussion section.