Comparison of calculated hydrocarbon strain energies using ab initio and composite methods

The strain energies for a small set of hydrocarbons ranging in size of thre e to six carbon atoms has been calculated using homodesmotic reactions and ab initio methods. The values calculated using the correlated methods B3LYP , MP2, QCISD(T), G1, G2, CBS-4, and CBS-Q were compared. Values determined at the QCISD(T)/6-311 + G(2df,p) level were used as the reference set, and values from the other methods compared using the mean absolute deviation (M AD) as a measure of the performance for each one. Of the non-composite meth ods, the MP2/6-311G(d,p) level gave the best overall agreement with a MAD o f only 0.36 kcal/mol. The B3LYP/6-311 + G(3df,2p) level has the largest dev iation with a MAD of 12.1 kcal/mol. The strain energy of two larger straine d hydrocarbons, quadricyclane and cubane, were calculated at the MP216-311G (d,p) level and compared with the available experimental data. (C) 1999 Els evier Science B.V. All rights reserved.