Organic thermochemistry at high ab initio levels. 3. A G3 study of cyclic saturated and unsaturated hydrocarbons (including aromatics)

With the purpose of exploring the reliability of the enthalpies of formatio n calculated using the G3 method, we have examined a series of saturated an d unsaturated alicyclic hydrocarbons varying the size and the number of for mal double bonds in the molecule. Heats of formation have been calculated a t the G3 level through both atomization reactions and bond separation isode smic reactions, and comparisons with experimental values and with values pr eviously calculated at the G2(MP2) and G2 levels have been made. The qualit y of the GS-calculated enthalpies of formation using atomization reactions is comparable to that obtained at the G2 level using bond separation reacti ons, whereas G3 calculations are two to three times faster than G2 calculat ions.