The performance of a variety of theoretical methods in computing stabilization energies of the substituted methyl and vinyl radicals ˙CH2F, ˙CH2CN, ˙CH2CHCH2, ˙CH2CHO, CH2C˙F and CH2C˙CN is examined. The influence of electron correlation (UHF, UMP2, PMP2, RMP2, UB3-LYP, UQCISD, UQCISD(T), UCCSD(T), URCCSD(T) and RRCCSD(T)) and basis set size (from 6-31G(d) to 6-311++G(3df,3pd)) on stabilization energies is evaluated, as well as the performance of compound methods such as G2, G3, CBS-Q and CBS-APNO and their variants. The results indicate that generally reliable radical stabilization energies can be obtained at modest cost using RMP2/6-311+G(2df,p)//RMP2/6-31G(d) energies. A slightly less accurate but more economical procedure is RMP2/6-311+G(d)//B3-LYP/6-31G(d). UMP2 and PMP2 are unsuitable for obtaining radical stabilization energies for spin-contaminated radicals, while UB3-LYP appears generally to overestimate stabilization energies.

An assessment of theoretical procedures for the calculation of reliable radical stabilization energies

High-level quantum-chemical methods have been used to study the scope and physical origin of the significant long-range stabilizing interactions between nonmutually conjugated anion and radical moieties in SOMO–HOMO converted distonic radical anions. In such species, deprotonation of the acid fragment can stabilize the remote radical by tens of kilojoules, or, analogously, formation of a stable radical (by abstraction or homolytic cleavage reactions) increases the acidity of a remote acid by several pKa units. This stabilization can be broadly classified as a new type of polar effect that originates in Coloumbic interactions but, in contrast to standard polar effects, persists in radicals with no charge-separated (i.e., dipole) resonance contributors, is nondirectional, and hence of extremely broad scope. The stabilization upon deprotonation is largest when a highly delocalized radical is combined with an initially less stable anion (i.e., the conjugate base of a weaker acid), and is negligible for highly...

Origin and scope of long-range stabilizing interactions and associated SOMO-HOMO conversion in distonic radical anions.

The bond dissociation energies (BDEs) and radical stabilization energies (RSEs) which result from 166 reactions that lead to carbon-centered radicals of the type ˙CH2X, ˙CHXY and ˙CXYZ, where X, Y and Z are any of the fourteen substituents H, F, Cl, NH2, OH, SH, CHCH2, CCH, BH2, CHO, COOH, CN, CH3, and CF3, were calculated using spin-restricted and -unrestricted variants of the double-hybrid B2-PLYP method with the 6-311+G(3df,2p) basis set. The interactions of substituents X, Y, and Z in both the radicals (˙CXYZ) and in the precursor closed-shell molecules (CHXYZ), as well as the extent of additivity of such interactions, were investigated by calculating radical interaction energies (RIEs), molecule interaction energies (MIEs), and deviations from additivity of RSEs (DARSEs) for a set of 152 reactions that lead to di- (˙CHXY) and tri- (˙CXYZ) substituted carbon-centered radicals. The pairwise quantities describing the effects of pairs of substituents in trisubstituted systems, namely pairwise MIEs (PMIEs), pairwise RIEs (PRIEs) and deviations from pairwise additivity of RSEs (DPARSEs), were also calculated for the set of 61 reactions that lead to trisubstituted radicals (˙CXYZ). Both ROB2-PLYP and UB2-PLYP were found to perform quite well in predicting the quantities related to the stabilities of carbon-centered radicals when compared with available experimental data and with the results obtained from the high-level composite method G3X(MP2)-RAD. Particular selections of substituents or combinations of substituents from the current test set were found to lead to specially stable radicals, increasing the RSEs to a maximum of +68.2 kJ mol−1 for monosubstituted radicals ˙CH2X (X = CHCH2), +131.7 kJ mol−1 for disubstituted radicals ˙CHXY (X = NH2, Y = CHO), and +177.1 kJ mol−1 for trisubstituted radicals ˙CXYZ (X = NH2, Y = Z = CHO).

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Effect of substituents on the stabilities of multiply-substituted carbon-centered radicals.

The energies of silicon–oxygen hydrides SiOHn (n=0–4) and their cations SiOHn+ (n=0–5) have been studied using Gaussian‐2 (G2) theory. Atomization energies, enthalpies of formation, bond dissociation energies, and ionization energies have been evaluated. There is little experimental thermochemical data available for these species, although many of them have been detected. In silanol, the O–H bond dissociation energy is 119.3 kcal/mol and the first Si–H bond dissociation energy is 92.6 kcal/mol. The O–H bond is significantly stronger than the same bond in methanol (105.0 kcal/mol) and the S–H bond in methylmercapton (86.0 kcal/mol). The silicon–oxygen systems tend to show a preference for isomers containing O–H bonds as opposed to H–Si bonds because of the strong O–H bond. The G2 proton affinity of SiO (191.1 kcal/mol at 298 K) is in good agreement with a recently measured value (189.3±2.6 kcal/mol at 295 K). The G2 ionization energy of SiO is 11.58 eV, in agreement with the observed value (11.61 eV) from ...

Theoretical study of the silicon–oxygen hydrides SiOHn (n=0–4) and SiOH+n (n=0–5): Dissociation energies, ionization energies, enthalpies of formation, and proton affinities

The thermochemistry and kinetics of the pyrolysis of 2-butanol have been conducted using ab initio methods (CBS-QB3 and CCSD(T)) and density functional theory (DFT). The enthalpies of formation and bond dissociation energies of some alcohols including 2-butanol and its derived radicals have been calculated. A variety of simple and complex dissociations have been examined. The results indicated that dehydration to 1- and 2-butene through four-center transition states is the most dominant channel at low to moderate temperatures (T ≤ 700 K), where formation of butenes is kinetically and thermodynamically more favorable than other complex and simple bond scission reactions. Although the C−C bond fission channels require more energy than needed for some complex decomposition reactions, the former pathways predominate at higher temperatures (T ≥ 800 K) due to the higher values of the pre-exponential factors. The progress of the complex decomposition reactions has been followed through intrinsic reaction coordin...

Thermal Decomposition of 2-Butanol as a Potential Nonfossil Fuel: A Computational Study

Stability of silicon-based distonic radical cations

Electronic interactions in disubstituted radicals CHXY $(X, Y is F, OH or NH_2)$ have been probed by examining the energetics of numerous conformations at the UMP3/6-31G (d)//ROHF/4-31G level. Negative hyperconjugative inter- actions in these radicals are indicated to be comparable in magnitude to $\pi$ conjugation. Most of these disubstituted radicals are predicted to adopt unsymmetrical geometries in order to benefit from both types of stabilizing interactions. These results have interesting implications for reactions involving carbohydrate radicals, for which the simple systems serve as models. Thus, the structures and energetics of the dihydroxymethyl radical have been used to explain the observed stereoselectivity of a few reactions involving sugar-based radicals. The non-additivities of substituent effects in the disubstituted radicals have been compared with the magnitude of the anomeric effect in the corresponding disubstituted methanes. The inter-substituent interactions in the radicals should not be overlooked when using these systems as reference species in estimating other non-additive effects, such as the captodative effect.

Ab initio study of conjugative and negative hyperconjugative interactions in disubstituted radicals

Ab initio molecular orbital calculations have been carried out to obtain the structures and relative isomer energies of organometallic radical anions of the types CH3X–˙ and H2C˙–XH–(X = Li, BeH, BH2, Na, MgH, or AlH2). Comparison with results for the corresponding neutral molecules indicates a dramatic reduction in the energy gap between classical and hydrogen-shifted isomers on addition of an electron. However, the classical isomers of the radical anions generally represent the preferred form. The only exception is H2C˙–BH3–, which is calculated to be virtually isoenergetic with H3CBH2–˙. Organoborane radical anions are thus the most likely systems to adopt distonic structures with the formal charge and radical centres on different atoms. Further separation of the charge and the radical sites through homologation is predicted to be ineffective in stablising the distonic structures. MNDO calculations on larger models point to experimentally accessible distonic radical anions. The present study leads to a better understanding of the origin of distonic stablisation in radical ions.

Towards stable distonic radical anions: a theoretical study

Radical ions are calculated to have large nonadditive captodative stabilisation, in contrast to their neutral analogues

Kuruvilla Pius

Papers

Stability of silicon-based distonic radical cations

Ab initio study of conjugative and negative hyperconjugative interactions in disubstituted radicals

Towards stable distonic radical anions: a theoretical study

Remarkably large captodative stabilisation in radical ions

Towards Stable Distonic Radical Anions: A Theoretical Study.