STRAIN-ENERGY OF 3-MEMBERED RINGS - A NEW ULTRADIAGONAL DEFINITION ASAPPLIED TO SILICON-CONTAINING AND CARBON-CONTAINING SPECIES

Strain energies (SEs) in the 3-membered rings (CH2)(2)X and (SiH2)(2)X , where X is CH2, NH, O and SiH2, have been calculated by ab initio me thods using the homodesmic reactions cyclo-(CH2)(2)X --> 1/2cyclo-(CH2 )(4)X(2) and cyclo-(SiH2)(2)X --> 1/2cyclo-(SiH2)(4)X(2) All species i nvolved were fully geometry optimized by HF/6-31G* calculations, and energies were calculated at the MP2//SCF level. Thermal energy contrib utions were included. For the (CH2)(2)X series, we find that the SEs d ecrease on going from X being CH2 to NH to O, but increase substantial ly when X is SiH2. For the (SiH2)(2)X series we find a slight increase from X = CH2 to X = O, but all of these values are higher than for X = SiH2. The predicted SEs are in accordance with the limited experimen tal information available.