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

Abstract

The heats of formation at 0 K (ΔfH0) 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, •CH3, •NH2, •OH, •SiH3, •PH2, •SH, •N2+, •NO, •ONO, •O2-, •CN, •CO+, •CS+, •CCH, •CHO, •OOH, •CHCH2, •CH2CH3, •CH2OH, •OCH3, •SCH3 and •COCH3) and triplet biradicals (:CH2, :NH, :SiH2, :O2, and :S2). 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, •CH2CHCH2, •CH2CN, •CH2COOH, and •CH2C6H5) for which there are greater uncertainties associated with the experimental values.