An Assessment of the Performance of High-Level Theoretical Procedures in the Computation of the Heats of Formation of Small Open-Shell Molecules

The heats of formation at 0 K (Δ f H 0 ) of 29 small (containing up to 3 heavy atoms) open-shell molecules, with accurately known experimental values, have been calculated using a number of high-level theoretical procedures. The theoretical methods examined include variants of Gaussian-n (G2, G2-RAD(RMP2), G2-RAD(B3-LYP), G2-RAD(QCISD), G3, G3-RAD, G3X, G3X-RAD, G3(MP2), G3(MP2)-RAD, G3X(MP2) and G3X(MP2)-RAD), CBS (CBS-APNO, CBS-Q, CBS-RAD and CBS-QB3), and Martin extrapolation (Martin-2, Martin-3, W1, W1‘, W1h, W2h and W2) procedures. The open-shell systems include doublet radicals (•BeH, •CH, •CH 3, •NH 2, •OH, •SiH 3, •PH 2, •SH, •N 2 +, •NO, •ONO, •O 2 -, •CN, •CO +, •CS +, •CCH, •CHO, •OOH, •CHCH 2, •CH 2 CH 3, •CH 2 OH, •OCH 3, •SCH 3 and •COCH 3 ) and triplet biradicals (: CH 2,: NH,: SiH 2,: O 2, and: S 2 ). The results for these systems are used to assess the performance of the various theoretical methods. The smallest mean absolute deviations (MADs) from experiment are found with the G3-RAD, G3X-RAD, G3X, W1h, W2, W1, and W2h procedures with MADs lying in the range 2.0−2.5 kJ mol -1 . The smallest values for the largest deviation (LD) from experiment are found with the G3X, G3X-RAD, W2, and W2h procedures and are ±6.4 kJ mol -1 . A selection of the most accurate theoretical procedures (G3-RAD, G3X-RAD, G3X, W1h, W2, W1, and W2h) is used to predict the heats of formation for several radicals (•OF, •CH 2 CHCH 2, •CH 2 CN, •CH 2 COOH, and •CH 2 C 6 H 5 ) for which there are greater uncertainties associated with the experimental values.