β-Fragmentation of alkoxyl radicals: Natural bond orbital analysis (original) (raw)
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β-Fragmentation of Tertiary Alkoxyl Radicals: G3(MP2)-RAD and Natural Bond Orbital Investigations
ChemPhysChem, 2012
Tertiary alkoxyl radicals are routinely used in radical chemistry, both in polymerization (initiation, curing, cross-linking, grafting reactions) or chemical modification, and in organic synthesis. [2] They are generated either by reduction of t-alkyl hydroperoxides by metals or by thermal or photochemical decomposition of di t-alkyl peroxides, perketals, and peroxyesters. This family of radicals has been used for decades for addition onto olefins or for H-abstraction from various substrates. [2] The efficiency of these reactions depends on the occurrence of the b-fragmentation reactions which provide new radicals exhibiting quite different reactivities. [2] The reactivity of alkoxyl radicals has been recently investigated computationally by quantum chemical calculations and DFT methods. The thermodynamics and kinetics of the b-fragmentation of some alkoxyl radical carrying nitrogen atoms in the b-position have been investigated at a high level of theory. However, their orbital interactions both in the starting materials and at transition state (TS) have not been investigated although it is wellknown that stereoelectronic effects may play a major in the reactivity of organic radicals. It is known that the fragmentation requires the TS geometry to exhibit good overlapping between the SOMO and the bonding s CÀY or the antibonding s* CÀY orbitals of the cleaved CÀY bond . Such molecular orbital interactions lead to the weakening of the bond, which is observed through both its lengthening and the diminution of its electron population. In a recent work on the application of natural bond orbital (NBO) analysis to the b-fragmentation of alkoxyl radicals, we observed in starting materials 1 c that the expected interactions between the SOMO and the s and/or the s* orbitals did not occur with the experimentally observed CÀOOEt bond cleavage despite that an early TS is observed for such a reaction (reactant-like from the Hammond's postulate). This puzzling result led us to investigate, using the recommended G3(MP2)-RAD method, the b-fragmentation of the CÀY (left pathway 1, Scheme 1) and CÀMe (right pathway 2, Scheme 1) bonds for alkoxyl radicals 1 a-d, both in its thermodynamical aspects (reaction entalphies DH r , and reaction free Gibbs energies, DG r ) and kinetic aspects (activation barriers DG ¼ 6 ), and some of our results are summarised in . For comparison, thermodynamic data were calculated using G3 MP2B3 methods. To perform the NBO analysis at TS, kinetic parameters were calculated using the UB3LYP/6-31 + G(d,p) DFT method. The DH r 1 , DH r 2 , DG r 1 , and DG r 2 computed by the G3 MP2B3 method differed by less than 1 kJ mol À1 from those computed by the recommended G3 (MP2)-RAD method, and followed the experimental trend. DG 1 ¼ 6 and DG 2 ¼ 6 calculated by DFT method for radical reaction were very close (less than 5 kJ mol À1 ) to those computed by the recommended G3(MP2)-RAD method, except for the loss of EtOC (DDG ¼ 6 % 25 kJ mol À1 ) of 1 b and the loss of t-Bu· (DDG ¼ 6 % 11 kJ mol À1 ) of 1 d. For the latter, the DFT value was even closer to the one given by the experiment whereas both methods afforded values very close to the experimental one for 1 a. Whatever the method, calculations afforded the reactivity trends expected from the experimental observations: EtOC < MeC < EtOOC < t-BuC Because 3 b and 3 c were strongly stabilized (very low DG r 2 for ester and perester, respectively), lower values of DG 2 ¼ 6 were observed for the CÀMe cleavage in 1 b and 1 c than in 1 a. The low DG r 1 for 1 c was due to the release of the stabilized EtOOC radicals (3-electron-2-orbital interaction) in sharp contrast with EtOC, and, hence, favouring the CÀOOEt bond cleavage for 1 c. As expected, whatever the pathways, at TS (Table 2, ) the CÀOC bonds were shortened by 0.04 to [a] Prof.
A theoretical study of the electronic structure of .beta.-substituted alkyl radicals
Journal of the American Chemical Society, 1976
The conformational preferences of &substituted alkyl radicals are examined theoretically by means of molecular orbital calculations. It is found that conformations of these species depend on a delicate balance of hyperconjugative effects and nonbonded interactions. Thus, hyperconjugation tends to favor the planar eclipsed or eclipsed anti (for nonplanar radical centers) conformations for radicals of the type CR2CH2Xo (where X@ is a substituent from the second or succeeding rows of the periodic table). An increased preference for the staggered conformation is obtained for nonplanar radicals with bulky XB substituents, such as Br or I, due to steric effects. The extent of p-p and d-p homoconjugation relative to hyperconjugation is also discussed. No p-p homoconjugation is found for very electronegative substituents such as Xi3 = For OR, but both p-p and d-p homoconjugation appears to be present for XP substituents such as Cl. A possible mechanism for the racemization of optically active C R I R~C H~X P radicals is presented and the rate of racemization is traced to the bulkiness of the XB substituent.
Decomposition of Tertiary Alkoxy Radicals
Zeitschrift für Physikalische Chemie, 2005
Rate coefficients of β-scission reactions in tertiary alkoxy radicals, R(CH3)2CO (R = methyl, ethyl, tert-butyl and neo-pentyl) have been estimated via density functional theory (DFT) calculations in conjunction with statistical unimolecular rate theory. For tert-butoxy, results obtained by employing different basis sets are compared with experimental values, indicating that UB3LYP/6-31G(d,p) excellently predicts kinetic data. Rate coefficients for inter- and intramolecular hydrogen abstraction are also reported. Depending on R, the β-scission rate may vary by orders of magnitude. The predicted temperature dependence of the alcohol-to-ketone product ratios for alkoxy radical decomposition in a hydrocarbon environment is in remarkably close agreement with the corresponding ratios measured on the product mixtures from decomposition of tert-butyl and tert-amyl peroxyacetates in solution of n-heptane.
Journal of chemical theory and computation, 2016
Predicting the singlet-triplet splittings of divalent radicals is a challenging task for electronic structure theory. In the present work, we investigate the performance of multiconfiguration pair-density functional theory (MC-PDFT) for computing the singlet-triplet splitting for small main-group divalent radicals for which accurate experimental data are available. In order to define theoretical model chemistries that can be assessed consistently, we define three correlated participating orbitals (CPO) schemes (nominal, moderate, and extended, abbreviated as nom, mod, and ext) to define the constitution of complete active spaces, and we test them systematically. Broken-symmetry Kohn-Sham DFT calculations have also been carried out for comparison. We found that the extended CPO-PDFT scheme with translated on-top pair-density functionals have smaller mean unsigned errors than weighted-average broken-symmetry Kohn-Sham DFT with the corresponding exchange-correlation functional. The acc...
Ab Initio MO Study of the Unimolecular Decomposition of the Phenyl Radical
The Journal of Physical Chemistry A, 1997
The unimolecular decomposition of the C 6 H 5 radical has been studied by ab initio molecular orbital and statistical-theory calculations. Three low-energy decomposition channels, including the commonly assumed decyclization/fragmentation process yielding n-C 4 H 3 + C 2 H 2 , have been identified. With a modified Gaussian-2 method of Mebel et al. (ref 17), the energy barrier for the decyclization of C 6 H 5 was calculated to be 66.5 kcal/mol with the corresponding recyclization energy of 5.6 kcal/mol. The two open-chain 1-dehydrohexa-1,3-dien-5-yne radicals (with HCĊ H cis and trans structures) may undergo further fragmentation reactions producing n-C 4 H 3 + C 2 H 2 and l-C 6 H 4 (1,5-hexadiyn-3-ene) + H with the predicted barriers of 44.0 and 36.1 kcal/mol, respectively. The dominant decomposition channel of C 6 H 5 was found to take place barrierlessly by C-H breaking, producing o-C 6 H 4 (o-benzyne) + H with the predicted endothermicity of 76.0 kcal/mol.
Journal of the American Chemical Society, 1996
A product analysis and kinetic study of the one-electron oxidation of a number of 1-arylpropanols, 1,2diarylethanols, and some of their methyl ethers by potassium 12-tungstocobaltate(m) (abbreviated as Co(IIf;W) in aqueous acetic acid wag carried out and complemented by pulse radiolysis experiments. The oxidations occur via radical cations which undergo side-chain fragmentation involving the Co-H and/or Co-Cp bond. With 1-(4methoxyphenyl)-2-methoxypropane (1). only deprotonation ofthe radical cation is observed. In contrast, removing the ring methoxy groirp leads to exclusive C-C bond cleavage in the radical cation. Replacing the side-chain p-OMe by B-OH, the radical cation undergoes both C-C and C-H bond cleavage, with both pathways being base catalyzed. C-C bond breaking in the radical cation is also enhanced by an cr-OH group, as shown by 1-(4.methoxyphenyl)l 2,2-dimethyl-1-propanol , where this pathway, which is also base catalyzed, is the only one observed. Interestingly, a-and B-OH groups exhibit a very similar efficiency in assisting the C-C bond cleavage route in the radical cations, as is evident from the kinetic and products study of the oxidation of 1-phenyl-2-(4-methoxyphenyl)ethanol (5) 3nd 1-(4-methoxyphenyl)-2-phenylethanol (6) by Co(Itr)W, and from pulse radiolysis experiments on 5 and 6. C-C bond cleavage is the main reaction for bsth radical cations which exhibit a very similar rate of fragmentation (ft : 2.0 and 3.2 x l}a s-1, respectively). In both fragmentation reactions a small solvent isotope effect, /<(HzO)lk(D2O) (1.4 for 5'+ and 1.2 îor 6'+) and negative activation entropies are observed. These data suggest that a key role in the assistance by o.-or B-OH groups to C-C bond cleavage is played by hydrogen bonding or specific solvation of these groups. The kinetic study of the oxidations promoted by Co(Itr)W has also shown that when only one group, OH or OMe, is present in the side chain (either on Co or Cp), the fragmentation step or both the electron transfer and fragmentation steps contributé to the overall oxidation rate. However, with an OH group on both carbons of the scissile ond, as in 1-(4-methoxyphenyl)-1,2-propanediol (9), the rate of C-C bond cleavage is so fast that the electron transfer step becomes rate deternining.
Theoretical Study of Chloroalkenylperoxy Radicals
The Journal of Physical Chemistry A, 2002
DFT and ab initio molecular orbital calculations have been performed to investigate the structures and energetics of the Cl-O 2 -isoprene peroxy radicals arising from the Cl-initiated oxidation of isoprene. Geometry optimizations of the chloroalkenylperoxy radicals were performed using density function theory (B3LYP), and the energies were computed with the single-point calculation using different levels of theory for electron correlation and basis set effects. At the CCSD(T)/6-31G(d) level of theory corrected with zero-point energy (ZPE), the chloroalkenylperoxy radicals are about 39 to 43 kcal mol -1 more stable than the separated reactants (i.e., O 2 + Cl + isoprene). We find no evidence for an energetic barrier to O 2 addition and have calculated rate constants for the O 2 addition step using canonical variational transition state theory (CVTST) based on Morse potentials to describe the reaction coordinate. The results provide the isomeric branching between the six Cl-O 2 -isoprene peroxy radicals, indicating that the two -chloroalkenylperoxy radicals with initial Cl addition at C1 and C4 positions and subsequent O 2 addition at C2 and C3 positions,respectively, play an important role in determining the reaction pathways and final product distributions of the Cl-isoprene reaction system.
International Journal of Quantum Chemistry, 2010
In this article, we have assessed the performances of some recently proposed density functionals for the prediction of reaction energies involving radicals, notably bond dissociations of small organic molecules or of TEMPO-based ones, and bscissions, focusing on our TCA family and on range-separated hybrids. It is found that no functional belonging to these two families is able to compete with the M0x one. We have tried to improve the performances of the range-separated hybrids by the optimization of the attenuation parameter, but the improvements for one dataset lead to an unavoidable deterioration for the others. Furthermore, the differences between two different approaches to the long-range/short-range separation are discussed in terms of average enhancement factors, emphasizing the crucial choice of the approximate scheme used for the short-range part. Finally, the influence of the geometries has been considered and found to be negligible for this kind of molecular sets, validating the usual single point energies strategies developed in such benchmarking assessments.
Organic Letters, 2004
The temperature dependence of the dissociation of dimers formed from highly stabilized carbon-centered radicals has been examined. Analysis of the data yields the bond dissociation energy (BDE) for the central head-to-head C−C bond in these compounds. For example, for the dimer derived from 3-phenyl-2-coumaranone, BDE is 23.6 kcal/mol and the C−C bond length 1.596 Å, a rather long value for a σ bond.