Homolytic Dissociation Research Articles

Para-Substituted diphenylborylated organocobaloximes: effects of substituents on conformation and redox properties

Some new derivatives of organocobaloximes containing para-substituted diphenylboryl groups, RCo(DH) 2− n (DB( p-XPh) 2) n L (R=alkyl or aryl group, L= N-MeIm, Py or H 2O, X=OCH 3, CH 3 or Cl, n=1 or 2) have been synthesized. The X-ray structures and the 1H-NMR spectra are compared with those of the corresponding RCo(DH) 2− n (DBPh 2) n L and RCo(DBF 2) 2L complexes. The insertion of X groups in the phenyl rings does not significantly affect the equatorial CoN distances, whereas the CoPy distances increase slightly in the order (DB( p-OCH 3Ph) 2) 2<(DB( p-ClPh) 2) 2<(DBF 2) 2. 1H-NMR spectra suggest that the conformational distribution in solution is similar to that observed in the corresponding BPh 2 derivatives. Electrochemical studies on the corresponding MeCo(DB( p-XPh) 2) 2H 2O compounds show a mono-electron Co(III)/Co(II) transfer reaction followed by two parallel reactions: (a) mono-electron Co(II)/Co(I) transfer; (b) homolytic dissociation of the CoC bond with the formation of Co(I) species, the relative rates of the two processes being dependent on X. As the electron-withdrawing power of the equatorial ligand increases, the reduction potentials associated with both Co(III)/Co(II) and Co(II)/Co(I) processes shift towards more positive values, indicating a decrease of electron density on the Co atom. The effects are comparable with those observed by changing the axial ligands.

Photo-Redox Mechanism for the Reaction of “Cation-like” Zirconocene Silyl Complexes with Silanes

A kinetic study of the photochemical reaction of [Cp2Zr(μ-SiH2Ph)]22+2[BBun(C6F5)4-n]- (1) with PhCH2SiH3 to give the analogous PhCH2SiH2 complex 2 was carried out using a combination of NMR and EPR spectroscopy to follow the rates of disappearance of reactants and the appearance of intermediates and products. The paramagnetic intermediate [Cp2ZrIII]+[BBun(C6F5)4-n]- (3) is produced with a quantum efficiency of 0.96 ± 0.16 on irradiation of 1 with light having λ < 450 nm. The rate law and analysis of intermediates supports a Zr(IV)/Zr(III) redox mechanism in which the rate-determining step is photodecomposition of 1 to generate 3 and silyl radicals by homolytic dissociation. In the presence of excess PhCH2SiH3 3 reacts rapidly to give products.

Dealkylation of Coenzyme B12 and Related Organocobalamins: Ligand Structural Effects on Rates and Mechanisms of Hydrolysis

Rates and mechanisms of dealkylations of coenzyme B12, Ado-B12, and of five related organocobalamin compounds, including 2‘,5‘-dideoxyadenosyl, 3‘,5‘-dideoxyadenosyl, 2‘,3‘,5‘-trideoxyadenosyl, 1,5-dideoxyribofuranosyl, and tetrahydrofurfuryl complexes (2‘dAdo-B12, 3‘dAdo-B12, 2‘,3‘ddAdo-B12, 1dRF-B12, and THFF-B12, respectively), were determined in acidic solutions. In each case, competitive homolytic and acid-induced hydrolytic cobalt−carbon bond decomposition pathways were identified. Two mechanisms were observed for Co−C bond hydrolysis: the first, involving initial depurination followed by elimination from an organometallic intermediate, predominates for 2‘dAdo-B12 and 2‘,3‘-ddAdoB12; the second path, involving ring-opening protonation at the ribofuranosyl oxygen, analogous to hydrolyses of simple β-hydroxy and β-alkoxy complexes, predominates for the other four complexes. The rates of both hydrolysis pathways exhibited a marked dependence on the ligand functional groups. Ado-B12, the most substituted and most stable of the complexes, decomposes nearly 10 000-fold more slowly than the least stable, unsubstituted THFF-B12 complex. Systematic variation of the hydroxy and adenine substituents on the furanosyl ring afforded insights into the roles of these substituents in effecting this large stabilization toward hydrolysis. Because of the extreme hydrolytic stability of the coenzyme, biologically relevant homolytic dissociation of 5‘-deoxyadenosyl radical is competitive with hydrolysis over a wide pH range where the unprotonated, base-on form is kinetically dominant. Determination of the pH dependence of the dealkylation rates of Ado-B12, 2‘dAdo-B12, and 3‘dAdo-B12 afforded quantification of the competition between homolytic and hydrolytic paths. In contrast to hydrolysis, the limiting homolytic Co−C bond dissociation rate was found to be insensitive to hydroxy substitution. Finally, broader issues relevant to organocobalamin chemistry and also bearing on earlier observations, are considered, among them protonation equilibria and dealkylation kinetics of organocobalamins, and fitting procedures for axial base dissociation equilibria.

Nitration of unsaturated fatty acids by nitric oxide-derived reactive nitrogen species peroxynitrite, nitrous acid, nitrogen dioxide, and nitronium ion.

Reactive nitrogen species derived from nitric oxide are potent oxidants formed during inflammation that can oxidize membrane and lipoprotein lipids in vivo. Herein, it is demonstrated that several of these species react with unsaturated fatty acid to yield nitrated oxidation products. Using HPLC coupled with both UV detection and electrospray ionization mass spectrometry, products of reaction of ONOO- with linoleic acid displayed mass/charge (m/z) characteristics of LNO2 (at least three products at m/z 324, negative ion mode). Further analysis by MS/MS gave a major fragment at m/z 46. Addition of a NO2 group was confirmed using [15N]ONOO- which gave a product at m/z 325, fragmenting to form a daughter ion at m/z 47. Formation of nitrated lipids was inhibited by bicarbonate, superoxide dismutase (SOD), and Fe3+-EDTA, while the yield of oxidation products was decreased by bicarbonate and SOD, but not by Fe3+-EDTA. Reaction of linoleic acid with both nitrogen dioxide (*NO2) or nitronium tetrafluoroborate (NO2BF4) also yielded nitrated lipid products (m/z 324), with HPLC retention times and MS/MS fragmentation patterns identical to the m/z 324 species formed by reaction of ONOO- with linoleic acid. Finally, reaction of HPODE, but not linoleate, with nitrous acid (HONO) or isobutyl nitrite (BuiONO) yielded a product at m/z 340, or 341 upon reacting with [15N]HONO. MS/MS analysis gave an NO2- fragment, and 15N NMR indicated that the product contained a nitro (RNO2) functional group, suggesting that the product was nitroepoxylinoleic acid [L(O)NO2]. This species could form via homolytic dissociation of LOONO to LO* and *NO2 and rearrangement of LO* to an epoxyallylic radical L(O)* followed by recombination of L(O)* with *NO2. Since unsaturated lipids of membranes and lipoproteins are critical targets of reactive oxygen and nitrogen species, these pathways lend insight into mechanisms for the formation of novel nitrogen-containing lipid products in vivo and provide synthetic strategies for further structural and functional studies.

Mechanism of the Anionic Wittig Rearrangement. An ab Initio Theoretical Study

The mechanism of [1,2] methyl Wittig migration in the simple free and lithiated anionic H2COCH3 model system is investigated at the CAS-MCSCF and CCSD(T) levels of theory, providing, on one hand, a picture of the processes taking place in the gas phase and, on the other hand, two extreme descriptions of ionic association relevant to the condensed phase (absent or tight anion−cation interaction). (A) The bonding situation of the free carbanion correlates with the homolytic dissociation limit, but the heterolytic case is significantly lower in energy, and an avoided crossing takes place: dissociation becomes consequently heterolytic in character. However, the heterolytic limit is not necessarily attained, because nucleophilic attack on the carbonyl carbon by methide occurs before an oxygen−carbon distance of 3 A is reached. (B) In contrast, in the lithiated system, the O−C bond is homolytically cleaved, because the lithium counterion stays firmly bound to oxygen, thus stabilizing the incipient radical anio...

The adsorption and dissociation of ROH molecules on TiO 2(110)

First-principles calculations based on density-functional theory and the pseudopotential method are used to investigate the energetics of adsorption of the series of molecules H 2O, CH 3OH, H 2O 2 and HCO 2H on the TiO 2(110) surface. The aim of the work is to elucidate the factors that determine whether the adsorption is molecular or dissociative, including the acidity of the molecule, the geometry and electrostatic properties of the surface, and the interaction between adsorbed species. It is shown that the theoretical methods reproduce experimental values of the gas-phase heterolytic and homolytic dissociation energies to reasonable accuracy (within ∼20 kJ mol −1 for all four molecules). We find that the adsorption energy for the most favourable molecular mode of adsorption is extremely close to that for dissociative adsorption in the cases of H 2O, CH 3OH and H 2O 2. For HCO 2H, dissociative adsorption is favoured by a substantial margin, provided the dissociated geometry preserves the equivalence of the two oxygens in the formate ion. It is also shown that the geometry of the TiO 2(110) surface plays a crucial role in determining the conformations of the most stable geometries, and that hydrogen bonding between adsorbed species is also important.

Unsupported Ti−Co and Zr−Co Bonds in Heterobimetallic Complexes: A Theoretical Description of Metal−Metal Bond Polarity

The synthesis, structural, and theoretical characterization of heterobimetallic complexes CH3Si-{SiMe2N(4-CH3C6H4)}3M-Co(CO)3(L) (M = Ti, Zr; L = CO, PPh3, PTol3) with unsupported metal-metal bonds between cobalt atoms and titanium or zirconium atoms is being reported. The synthesis of the dinuclear compounds was achieved by salt metathesis of the chlorotitanium and zirconium complexes and the alkalimetal carbonylates. X- ray crystal structure analyses of four of these heterobimetallic complexes established the unsupported metal-metal bonds M = Ti, L = CO (3): 2.554(1) A ; M = Ti, L = PTol3 (4b): 2.473(4) A ; M = Zr, L = CO (5): 2.705(1) A ; M = Zr, L = PPh3 (6a): 2.617(1) A as well as the 3-fold molecular symmetries. Upon axial phosphine substitution, a metal-metal bond contraction of ca. 0.08 A is observed, which also results in the quantum chemical structure optimizations performed on the model compounds (H2N)3-Ti- Co(CO)4 (3x) and (H2N)3Ti-Co(CO)3(PH3) (4x) using gradient-corrected and hybrid density functionals. A theoretical study of the homolytic dissociation of the metal-metal bonds focuses on the relaxation energies of the complex fragments and indicates that the geometrical constraints imposed by the tripod ligand lead to a major thermodynamic contribution to the stability of the experimentally investigated complexes. The central question of the polarity of the metal-metal bond is addressed by detailed analysis of the calculated electron charge distribution using natural population analysis (NPA), charge decomposition analysis (CDA), Bader's atoms in molecules (AIM) theory, and the electron localization function (ELF). Both the orbital-based NPA and CDA schemes and the essentially orbital-independent AIM and ELF analysis suggest a description of the Ti-Co bond as being a highly polar covalent single bond. The combination of AIM and ELF is employed for the first time to analyze metal-metal bond polarity and appears to be a powerful theoretical tool for the description of bond polarity in potentially ambiguous situations.

Photodissociation of N-(Triphenylmethyl)anilines: A Laser Flash Photolysis, ESR, and Product Analysis Study1

N−(Triphenylmethyl)anilines (Ph3C-NHPh-R, R = H, o-Me, m-Me, p-Me, p-CPh3), upon photolysis with 248- and 308-nm laser light, as well as by lamp irradiation with 254-nm light in acetonitrile and hexane solutions, undergo exclusively C−N bond homolysis to give the triphenylmethyl radical in a monophotonic process. Product analysis gives as main products Ph3CH, 9-Ph-fluorene, and PhNH2. The quantum yields for the formation of Ph3C• are high (0.6−0.8, 248-nm excitation) and independent of the solvent. This effective homolytic dissociation results from the low electronegativity difference between the carbon and nitrogen atoms constituting the bond to be broken, the low bond dissociation enthalpy of the C−N bond, the high excitation energy of the local chromophore (aniline), and probably from a favorable alignment of the C−N bond in a plane perpendicular to the anilino chromophore (due to the large steric requirements of the trityl group), thus enabling an effective hyperconjugative interaction with it. The above dissociation competes effectively with heterolytic cleavage, which is the pathway dominating, e.g., in the photolysis of Ph3C−Cl in MeCN under the same conditions. At high pulse intensities the trityl radicals formed above are excited by a second photon leading to either electrocyclization to 4a,4b-dihydro-9-phenylfluorenyl radical (DHPF•), or photoionization to Ph3C+, the latter only in MeCN and only on 248-nm photolysis. A new intermediate (9-Ph-4aH-fluorene) on the way to the final product 9-Ph-fluorene is identified resulting from electrocyclization, supporting a mechanism proposed earlier. The optical measurements are supported by ESR studies (irradiation with 254-nm light).

A theoretical study on the modes of homolytic bond fragmentation in H nXYH m, HSXY and HS(O)XY systems

Recent studies in the authors' laboratories have involved studies on the formation and reactions of substituted alkoxy radicals derived by the photo-induced homolytic cleavage of the SO bond of substituted alkyl 4-nitrobenzenesulfenates ( 1), and the alkyl radicals formed by the β-scission of the alkoxy radicals. An interesting extension of these studies would be consideration of the homolytic cleavage of the SX and XY bonds in ArSXY and ArS(O)XY systems where X and Y are various combinations of nitrogen, oxygen and sulfur. Ab initio theoretical calculations have been carried out on various parent H-S-X-Y-H and HS(O)XYH systems and on the radical species formed by the homolytic dissociation of the SX and XY bonds using the G2MP2 method in order to determine the relative magnitudes of the homolytic bond dissociation energies (taken as the difference in the sum of the total energies of the product radicals minus the total energy of the parent system, E rexs). For calibration purposes, similar calculations have been carried out on hydrogen peroxide and hydrazine for which experimental homolytic bond dissociation energies have been measured. It is found that, in the latter calculations, the results obtained at the HF/UHF level give very poor correspondence with the experimental values, but the results obtained at the UMP2 and QCISD(T) levels nicely bracket the experimental values. The results of the calculations have given an indication of whether the SX or XY bond prefers to undergo homolytic cleavage. The results of these studies could very well be of significant importance in the areas of both physical and synthetic organic chemistry. In these homolytic cleavage reactions some very interesting radical species are formed, only a few of which have been studied previously experimentally or theoretically. The results of these studies indicate that the preferred mode of homolytic cleavage of the SX and XY bonds is essentially dependent only on the relative stability of the radicals formed in the two modes of homolytic bond cleavage.

Structures and Energies of Various Isomers of Dithionous Acid, H2S2O4, and of Its Anion HS2O4- 1

Ab initio molecular orbital calculations at various, correlated levels of theory have been performed on eight isomers of the [H,O4,S2]- anion and six forms of [H2,O4,S2]. For the former species, the hypothetical O3SSOH- (9) ion is identified as the lowest energy isomer using second-order Møller−Plesset perturbation theory or the CCSD(T) method, with the protonated dithionite ion, O2SSO2H- (6), being only marginally (less than 5 kJ mol-1 at 25 °C) higher in energy. In contrast, the B3LYP density functional predicts a rather different order of relative stabilities, with O2SSO2H- (6) as the by far most stable ion. 6 is stable with respect to dissociation into SO2 and HOSO- or into SO2•- and HOSO• (which is more stable than HSO2•). For the species of composition H2S2O4 the cagelike dithionous acid molecule (HO)O2SSO2(OH) (14) represents the global minimum at all four levels of theory. However, also for this species, the relative stabilities predicted by the B3LYP approach differ considerably from results obtained from the conventional wave function-based methods. 14 is stabilized by two very strong intramolecular hydrogen bonds (symmetry C2). The homolytic dissociation of 14 at the SS bond is strongly endothermic, but the molecule is unstable with respect to dissociation into H2SO2 and SO2. The harmonic wavenumbers and infrared intensities of the fundamental vibrations of 6 and 14 are given.

Effects of electron correlation and spin projection on rotational barriers of trithiocarbenium ion [C(SH)3]+ and Radical Dication [C(SH)3]?,2+

Theoretical level dependencies are discussed of relative isomer stabilities and rotational barriers of trithiomethyl cation [C(SH)3]+ (a) and of radical dication [C(SH3)]⋅,2+ (b). Spin polarization and dynamic electron correlation are very important for the radical dictation. Removal of an electron from one of the degenerate π-HOMOs of C3h symmetric [C(SH)3]+ stabilizes the remaining π electron to such an extent that the unpaired electron is not in the HOMO of the dictation. The radial π MO's “diving below the Fermi level” facilitates strong spin polarization because of its energetic proximity to σ MOs. Projection of the first three higher spin states eliminates spin contaminations, and the terms E(PUHF(s+3))-E(UHF) and E(PMP4(s+3))-E(MP4) are discussed. The combination of annihilation of spin contamination and electron correlation is essential for the determination of relative energies and rotational barriers of the radical dication. The results obtained at this level match the results of high level QCISD(T) calculations in a near-quantitative fashion. Perturbation theory alone does not correct for spin contamination even if it is carried to full fourth order and includes triple excitations; the E(PMP4(s+3))-E(MP4) values are all negative and can exceed 5 kcal/mol in magnitude. Previous studies showed that annihilation of spin contaminations is important in regions of potential energy surfaces where σ bonds are broken (homolytic dissociation), formed (radical addition), or both (H abstraction by radical). Our findings stress that the annihilation of spin contaminations can be just as important for any process that greatly alters spin polarization and even if that process proceeds without breaking or forming of σ bonds. For comparison, density functional theory also was employed in the potential energy surface analyses. The results obtained with the B3LYP formalism were found to be less susceptible to spin contamination and resulted in rather good agreement with the best pertubation and configuration interaction results. © 1997 John Wiley & Sons, Inc. J Comput Chem 18:1023–1035, 1997

Effects of electron correlation and spin projection on rotational barriers of trithiocarbenium ion [C(SH)3] and Radical Dication [C(SH)3],2

Theoretical level dependencies are discussed of relative isomer stabilities and rotational barriers of trithiomethyl cation [C(SH)3]+ (a) and of radical dication [C(SH3)]⋅,2+ (b). Spin polarization and dynamic electron correlation are very important for the radical dictation. Removal of an electron from one of the degenerate π-HOMOs of C3h symmetric [C(SH)3]+ stabilizes the remaining π electron to such an extent that the unpaired electron is not in the HOMO of the dictation. The radial π MO's “diving below the Fermi level” facilitates strong spin polarization because of its energetic proximity to σ MOs. Projection of the first three higher spin states eliminates spin contaminations, and the terms E(PUHF(s+3))-E(UHF) and E(PMP4(s+3))-E(MP4) are discussed. The combination of annihilation of spin contamination and electron correlation is essential for the determination of relative energies and rotational barriers of the radical dication. The results obtained at this level match the results of high level QCISD(T) calculations in a near-quantitative fashion. Perturbation theory alone does not correct for spin contamination even if it is carried to full fourth order and includes triple excitations; the E(PMP4(s+3))-E(MP4) values are all negative and can exceed 5 kcal/mol in magnitude. Previous studies showed that annihilation of spin contaminations is important in regions of potential energy surfaces where σ bonds are broken (homolytic dissociation), formed (radical addition), or both (H abstraction by radical). Our findings stress that the annihilation of spin contaminations can be just as important for any process that greatly alters spin polarization and even if that process proceeds without breaking or forming of σ bonds. For comparison, density functional theory also was employed in the potential energy surface analyses. The results obtained with the B3LYP formalism were found to be less susceptible to spin contamination and resulted in rather good agreement with the best pertubation and configuration interaction results. © 1997 John Wiley & Sons, Inc. J Comput Chem 18:1023–1035, 1997

FT-IR study of the gas phase thermolysis of trimethylsilyl acetate

Thermolysis of neat trimethylsilyl acetate has been investigated in the temperature range 723–818 K under static conditions using Fourier transform infrared spectroscopy. The principal gaseous products formed during the thermal decomposition are methane, hexamethyldisiloxane, cyclohexamethyltrisiloxane, carbon dioxide, and carbon monoxide, the relative amounts of which vary with the temperature. Small amounts of water and ethanoic acid are also formed. Loss of trimethylsilyl acetate is first-order over the whole temperature range; the first-order rate constants vary from 1.66 × 10 −5 s −1 at 723 K to 15.7 × 10 −5 s −1 at 818 K, leading to an activation energy E a of 104(5)kJ mol −1 . In the presence of methyl iodide the decomposition is substantially faster and the products are methane, hexamethyldisiloxane and ethanoic acid. Decomposition is also faster in the presence of nitric oxide and oxygen, although complete identification of the product mixtures was not possible in these cases. The mechanisms of the reactions are interpreted in terms of principally radical reactions involving initial homolytic dissociation of the Me 3 SiO-C(O)CH 3 bond.

Diastereomeric Control in the Formation of Carbon-Cobalt Bonds in Organocobalt Corrinoids: Reactions of Cobalt(II) Corrinoids with Organic Hydroperoxides(1).

1,1-Dimethylpropyl hydroperoxide reacts with cobalt(II) cobinamide via a Fenton-like reaction to produce a mixture of the diastereomeric alpha- and beta-ethylcobinamides (CH(3)CH(2)Cbi(+)'s, in which the organic ligand is in the "lower" or "upper" axial ligand position, respectively), the composition of which depends on the ratio of starting materials. When the hydroperoxide reagent is in excess, <2% of the CH(3)CH(2)Cbi(+) product is the alpha diastereomer, a product distribution which agrees with previous observations of the equilibrium mixture. This distribution apparently arises because the previously demonstrated CH(3)CH(2)(*)-promoted isomerization of CH(3)CH(2)Cbi(+) diastereomers leads to equilibration. However, when cob(II)inamide is in excess over hydroperoxide, the CH(3)CH(2)Cbi(+) product contains 87% alpha diastereomer and 13% beta diastereomer. Evidently, under these conditions, trapping of the CH(3)CH(2)(*) radical is sufficiently rapid to prevent the radical-induced isomerization of diastereomers and a kinetically controlled product distribution results. This condition has been used to study the relative energetics of kinetically controlled alpha and beta alkylation of cob(II)inamide by CH(3)CH(2)(*). For cob(II)inamide itself, the alpha diastereomer is enthalpically stabilized relative to the beta diastereomer in the transition state for carbon-cobalt bond formation, but an even larger entropic stabilization of the beta diastereomer causes the latter to be the predominant product. The entropic preference for the beta diastereomer is shown to be the result of the differential side chain configurations at the alpha and beta faces of the cobalt corrinoids by experiments with side-chain-altered analogs. When the downwardly projecting f side chain is enlarged by esterification of the N-methylated nucleotide 1-alpha-D-ribofuranosyl-3,5,6-trimethylbenzimidazole 3'-phosphate, the proportion of the alpha diastereomer in the alkylated product drops to 74% and the effect is due solely to an increased entropic preference for the beta diastereomer. Similarly, when the normally downwardly projecting e propionamide side chain is epimerized to the upward (beta) face of the corrin, the proportion of the alpha diastereomer in the product is increased to 95% and the effect is entirely entropic again. Taken together with previous work, the results lead to a general picture of the energetics of alkyl radical + cobalt(II) corrinoid combination reactions and their microscopic reverse, the homolytic dissociation of carbon-cobalt bonds, a rare instance in which the subtleties of such diastereomeric control can be understood at a very fundamental level.

Thermochemical Stability of α-Amino-α-carbonylmethyl Radicals and Their Resonance As Measured by ESR

ESR spectra of the captodative alpha-amino-alpha-carbonylmethyl radicals 8 have been recorded. No coalescence temperature for the rotation of the two NMe groups was found at temperatures below the decomposition temperature of the radicals. From known coalescence temperatures and rotational barriers of substituted methyl radicals the rotational barrier of >/=17 kcal mol(-)(1) was estimated for the (*)C-N bond in the radicals 8. Enthalpies DeltaH(diss) and entropies DeltaS(diss) of the homolytic dissociation of 7a,c,d into 8a,c,d have been obtained from equilibrium measurements by ESR. By correcting for substituent interaction enthalpies in 7 (steric and geminal), a radical stabilization enthalpy RSE = -20.7 +/- 1.0 kcal mol(-)(1) was obtained for 8. By addition of the known RSEs of dialkylamino- and carbonyl groups, a RSE = -9.9 kcal mol(-)(1) is predicted for 8. The difference between the experimental and predicted values of 10.8 kcal mol(-)(1) is attributed to a synergistic captodative substituent effect. A linear correlation between the radical stabilization enthalpies of the radicals 8 and of other mono- and disubstituted alkyl radicals and their ESR aH(alpha) coupling constants was found. According to this correlation the reduction of aH(alpha) by 1 G corresponds to an increase in RSE of 1.57 kcal mol(-)(1). The large resonance of the captodative alpha-amino-alpha-carbonylmethyl radicals 3, expressed by their high RSE, their small aH(alpha) coupling constant, and their high rotational barrier, can be rationalized by a strong interaction between the alpha-amino and the alpha-carbonyl groups similar to that in amides and expressed in the resonance structures 6.

Initiation of free radical polymerization by thermal cleavage of carbon‐carbon bonds

AbstractFree carbon radicals for initiation of polymerization processes can be generated either by light‐induced fragmentation or by homolytic thermal dissociation of suitable compounds. For thermal initiation of polymerization processes beside azo, peroxy and some other thermolabile compounds also alkanes with highly substituted carbon‐carbon bonds are capable. The reason why this interesting class of initiators, even more than 100 years after their discovery, has found only limited applications in radical polymerization is caused by their special mechanism of chain reaction initiation. The formed and in most cases highly resonance‐stabilized substituted alkyl radicals are not very reactive against monomers like methyl methacrylate or styrene but more against growing macroradicals. Therefore, during the initiation step at rather high primary radical concentrations only low‐molecular oligomers with two alkane end groups are formed. Under special conditions this process is reversible, whereas in some other cases the reaction can be described as a “dead end” polymerization. The oligomers which were formed e.g. in the case of methyl methacrylate polymerization with tetraaryl butane dinitriles are able to re‐initiate the polymerization process similar as in a living polymerization which can be used for the synthesis of block copolymers. Mechanism, kinetics and possible applications of such alkane‐initiated polymerizations are discussed.

Carbonylation-decarbonylation reactions of the carbomethoxymethylcobalt complexes CH 3O 2CCHRCo(CO) 3L (R [sbnd] H, CH 2CO 2CH 3; L [sbnd] CO, PPh 3) and their acyl derivatives

The primary and secondary alkyl- and acylcobalt carbonyl complexes CH 3O 2CCHRCo(CO) 3L and CH 3O 2CCHRC(O)Co(CO) 3L (R H or CH 2CO 2CH 3; L CO, PPh 3) ( 1–8) have been prepared and studied by a variety of IR and 1H, 13C 1H, 31P 1H and 17O 1H NMR spectroscopic methods. Unlike their primary analogues, all the secondary complexes readily undergo a reversible carbonylation-decarbonylation reaction. The new alkyl complex trans-CH 3O 2CCH 2Co(CO) 3PPh 3 ( 3) was characterized in solution, and exhibited remarkably different properties from the well-known homologue trans-CH 3CH 2O 2CCH 2Co(CO) 3PPh 3, i.e. it readily decomposes to [Co(CO) 3PPh 3] 2, suggesting homolytic dissociation of the carbon-cobalt bond, and its IR spectrum exhibits an organic ν(CO) absorbance at an unusually low frequency, pointing to an oxygen-cobalt interaction. The corresponding acyl complex trans-CH 3O 2CCH 2C(O)Co(CO) 3PPh 3 ( 4) was isolated as a stable solid, but smoothly decarbonylated in solution to 3. Multinuclear NMR spectroscopic monitoring of this reaction suggested the intermediate formation of cis-CH 3O 2CCH 2Co(CO) 3PPh 3, providing the first direct evidence for alkyl migration in cobalt carbonyl complexes.

A valence bond description of bond dissociation energy curves

Bond dissociation energy curves have been calculated ab initio using the valence bond self-consistent field (VBSCF) method for CH3-Y molecules, with Y = F, OH, NH2, CH3, BH2, CN and NO. Basis sets of at least double-zeta polarization quality for first row atoms have been used. The VB wavefunction is expanded in terms of one covalent (CH3:Y) and two ionic (CH3 +Y- and CH3 -Y+) structures. All results were referenced to the perfect pairing generalized valence bond method. The VB curves for CH3-F were calculated using both the same orbitals for the different structures (SODS) and tailored (different) orbitals for the three different structures (TODS). Each of these model levels was examined further using purely localized fragment (CH3 and Y) orbitals (L-SODS and L-TODS) and allowing delocalization between the CH3 and Y fragments among the passive orbitals (D-SODS and D-TODS), where only the electron pair bond undergoing homolytic dissociation uses purely localized fragment orbitals. Only the two SODS model levels were used for the other CH3-Y molecules. The lower energy covalent structure curve for CH3-F is found to be flat and essentially repulsive at all theory levels. The close but higher energy CH3 +F- ionic structure curve has a deep minimum (R ,m)at 0⋅3-0≐4 Å larger interfragment distance R than the corresponding ground state equilibrium bond length R e. The resultant normal Morse-type shape ground state dissociation curve results from the steeply rising covalent-ionic off-diagonal (resonance) interactions dominating the widening covalent-ionic energy gap as R decreases from R m. Within the SODS model, passive electron delocalization contributes a surprisingly large ~ 11 kcal mol-1 to the calculated bond dissociation energy, mainly by stabilizing the CH3:F curve. TODS adds ~ 16 kcal mol-1 to De mainly by stabilizing the CH3 +F- structure energy. In the series, Y = F → OH NH2 CH3, (a) the covalent curve develops an ever deeper energy minimum, approaching its ground state R e, (b) the contributing weight of the CH3:Y structure to the ground state wavefunction, and the ionic-covalent energy gap steadily increase, and (c) R m for the CH3 +Y- structure moves out to larger values. CH3-BH2 fits into these trends where the lower energy ionic structure is CH3-BH2 +. CH3-CN has a 70% covalent weight at its R e. CH3-NO is ~ 67% CH3:NO and 24% CH3 +NO-. CH3-CN has a Morse-type covalent structure energy curve while CH3:NO is unbound. The last three molecular species have empty low-lying valence orbitals and their explicit effect on the electronic structure description and energy dissociation curves within the VBSCF framework remains to be examined. The results obtained here have direct relevance to the VBSCF study of reaction paths.

A Valence bond description of bond dissociation energy curves

Bond dissociation energy curves have been calculated ab initio using the valence bond self-consistent field (VBSCF) method for CH3-Y molecules, with Y = F, OH, NH2, CH3, BH2, CN and NO. Basis sets of at least double-zeta polarization quality for first row atoms have been used. The VB wavefunction is expanded in terms of one covalent (CH3:Y) and two ionic (CH3 +Y- and CH3 -Y+) structures. All results were referenced to the perfect pairing generalized valence bond method. The VB curves for CH3-F were calculated using both the same orbitals for the different structures (SODS) and tailored (different) orbitals for the three different structures (TODS). Each of these model levels was examined further using purely localized fragment (CH3 and Y) orbitals (L-SODS and L-TODS) and allowing delocalization between the CH3 and Y fragments among the passive orbitals (D-SODS and D-TODS), where only the electron pair bond undergoing homolytic dissociation uses purely localized fragment orbitals. Only the two SODS model levels were used for the other CH3-Y molecules. The lower energy covalent structure curve for CH3-F is found to be flat and essentially repulsive at all theory levels. The close but higher energy CH3 +F- ionic structure curve has a deep minimum (R ,m)at 0⋅3-0≐4 Å larger interfragment distance R than the corresponding ground state equilibrium bond length R e. The resultant normal Morse-type shape ground state dissociation curve results from the steeply rising covalent-ionic off-diagonal (resonance) interactions dominating the widening covalent-ionic energy gap as R decreases from R m. Within the SODS model, passive electron delocalization contributes a surprisingly large ~ 11 kcal mol-1 to the calculated bond dissociation energy, mainly by stabilizing the CH3:F curve. TODS adds ~ 16 kcal mol-1 to De mainly by stabilizing the CH3 +F- structure energy. In the series, Y = F → OH NH2 CH3, (a) the covalent curve develops an ever deeper energy minimum, approaching its ground state R e, (b) the contributing weight of the CH3:Y structure to the ground state wavefunction, and the ionic-covalent energy gap steadily increase, and (c) R m for the CH3 +Y- structure moves out to larger values. CH3-BH2 fits into these trends where the lower energy ionic structure is CH3-BH2 +. CH3-CN has a 70% covalent weight at its R e. CH3-NO is ~ 67% CH3:NO and 24% CH3 +NO-. CH3-CN has a Morse-type covalent structure energy curve while CH3:NO is unbound. The last three molecular species have empty low-lying valence orbitals and their explicit effect on the electronic structure description and energy dissociation curves within the VBSCF framework remains to be examined. The results obtained here have direct relevance to the VBSCF study of reaction paths.

A theoretical investigation on the electron-acceptor properties of polar CS bonds of CH3−nFnSH (n = 0–3) thiols and CH3−nFnSCH3−nFn (n = 0–3) sulfides

Abstract The structures and stabilities of fluoro-substituted thiols and sulfides of general formula CH 3− n F n SH and CH 3− n F n SCH 3− n F n ( n = 0–3), have been calculated at the MP2/6-311G ∗∗ and MP2/6-311+ +G ∗∗ levels. The role of F-substitution in molecules and fragments resulting from CS and SH bond cleavage has been studied. In molecules, F-substitution does not bring about important changes of the CS bond lengths, and, accordingly, no important change is observed in the energy of the vertical anion species which remain much less stable than the neutral ground state. The same type of conclusion holds for the homolytic dissociation of neutral molecules (CS and SH bonds), yielding radical moieties, which exhibit only little variation upon F-substitution. When dealing with anion fragments, the situation is more striking. It is noteworthy that the well-documented effect of methyl anions stabilization by halogen substitution is appreciably increased in the new class of compounds of formula CH 3− n F n S. For the latter, the EA rises from 1.47 eV in CH 3 S to 3.08 eV in CF 3 S. On the ground of energy balance between {A 0 + B − ∞ } and {A − + B 0 ∞ } it is proposed that CF 3 SH and CF 3 SCH 3 are good candidates for the adiabatic formation of stable 3-electron bonds.