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 (Δf 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, •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.

Computational Studies of RAFT Polymerization–Mechanistic Insights and Practical Applications

“…As a result, they are less expensive than W1 but also less reliable. They have been shown to deliver 'kcal accuracy' when assessed against a large test set of experimental thermochemical data [43,44] and to provide good agreement with W1 for a variety of radical reactions. [13,14] For the specific problem of radical addition to C S double bonds, the G3 methods provide excellent agreement with W1 for the reaction barriers: for reaction enthalpies, the errors are slightly larger (ca.…”

Rearrangements in Model Peptide‐Type Radicals via Intramolecular Hydrogen‐Atom Transfer

“…5). As a consequence, the E , see Table 6), reflecting the greater stabilization of C-radicals by adjacent N-atom lone-pair donors than by carbonyl acceptors [21] [27] [28]. The E z fwd (98.6 kJ · mol…”

“…Specifically, equilibrium structures and transition structures (Scheme 1) were located at the UMP2/6-31G(d), UB3-LYP/6-31G(d), UB3-LYP/6-31G(2df,p), and UB3-LYP/6-311+G(d,p) levels of theory, followed by energy calculations on the optimized geometries with UB3-LYP/6-311++G(3df,2p) and the URCCSD(T)/6-311+G(d,p) procedure of MOLPRO (which involves unrestricted coupled cluster calculations on RHF wave functions) (Tables 1 and 2). The performance of both lower-cost methods (e.g., BMK [17], MPWB1K [18], B3-LYP, and RMP2) and computationally more demanding methods (e.g., G3//B3-LYP [19], G3(MP2)-RAD [20] [21], and CBS-QB3 [22]) were then evaluated in relation to the prediction of radical relative energies and H-transfer barriers. Unless stated otherwise, energies within the text refer to G3(MP2)-RAD values at 0 K. We note that G3(MP2)-RAD was originally introduced [21] to give an improved description of radicals, and in particular eliminates the UMP2 aspects of standard G3(MP2).…”

“…The performance of both lower-cost methods (e.g., BMK [17], MPWB1K [18], B3-LYP, and RMP2) and computationally more demanding methods (e.g., G3//B3-LYP [19], G3(MP2)-RAD [20] [21], and CBS-QB3 [22]) were then evaluated in relation to the prediction of radical relative energies and H-transfer barriers. Unless stated otherwise, energies within the text refer to G3(MP2)-RAD values at 0 K. We note that G3(MP2)-RAD was originally introduced [21] to give an improved description of radicals, and in particular eliminates the UMP2 aspects of standard G3(MP2).…”

Alkoxyl Radicals asO-Synthons in Self-Terminating Radical Oxygenations: An Experimental and Theoretical Study

“…Since, to our knowledge, no kinetic studies have been performed on this fragmentation reaction, or on the reverse addition of an alkyl radical to the carbonyl oxygen, which could be used to assess the quality of the theoretical re- 16 the geometry of the methyl radical, for which reliable experimental data are available, 17 was taken as benchmark. The obtained electronic energies without and with zero-point vibrational energy (ZPE) correction included, <s 2 > values and geometrical data are compiled in Table 2.…”