Zirconium, hafnium, and tantalum amide silyl complexes: their preparation and conversion to metallaheterocyclic complexes via gamma-hydrogen abstraction by silyl ligands

New transition metal silyl amide complexes (Me(2)N)(3)Ta[N(SiMe(3))(2)](SiPh(2)Bu(t)) (1) and (Me(2)N)M[N(SiMe(3))(2)](2)(SiPh(2)Bu(t)) (M = Zr, 2a, and Hf, 2b) were found to undergo gamma-H abstraction by the silyl ligands to give metallaheterocyclic complexes (3) and (M = Zr, 4a, and Hf, 4b), respectively. The conversion of 1 to 3 follows first-order kinetics with DeltaH() = 23.6(1.6) kcal/mol and DeltaS() = 3(5) eu between 288 and 313 K. The formation of 4a from (Me(2)N)Zr[N(SiMe(3))(2)](2)Cl (5a) and Li(THF)(2)SiPh(2)Bu(t) (6) involves the formation of the intermediate 2a, followed by gamma-H abstraction. Kinetic studies of these consecutive reactions, a second-order reaction to give 2a and then a first-order gamma-H abstraction to give 4a, were conducted by an analytical method and a numerical method. At 278 K, the rate constants k(1) and k(2) for the two consecutive reactions are 2.17(0.03) x 10(-)(3) M(-)(1) s(-)(1) and 5.80(0.15) x 10(-)(5) s(-)(1) by the analytical method. The current work is a rare kinetic study of the A + B --> C --> D (+ E) consecutive reactions. Kinetic studies of the formation of a metallaheterocyclic moiety have, to our knowledge, not been reported. In addition, gamma-H abstraction by a silyl ligand to give such a metallaheterocyclic moiety is new. Theoretical investigations of the gamma-H abstraction by silyl ligands have been conducted by density functional theory calculations at the Becke3LYP (B3LYP) level, and they revealed that the formation of the metallacyclic complexes through gamma-H abstraction is entropically driven. X-ray crystal structures of (Me(2)N)(3)Ta[N(SiMe(3))(2)](SiPh(2)Bu(t)) (1), (Me(2)N)Zr[N(SiMe(3))(2)](2)Cl (5a), and (M = Zr, 4a, and Hf, 4b) are also reported.     

Absolute Rate Constants for the Reactions of Primary Alkyl Radicals with Aromatic Amines(1)

Hydrogen abstraction from diarylamines (4-X-C(6)H(4))(2)NH [X = H, CH(3), C(8)H(17), CH(3)O, and Br] by the 2-methyl-2-phenylpropyl radical in n-dodecane solution was investigated by thermolysis of 3-methyl-3-phenylbutanoyl peroxide in the presence of various concentrations of the amines. The reaction is a non-chain process in which the 2-methyl-2-phenylpropyl radical and its rearrangement product, the 2-benzylpropan-2-yl radical, abstract hydrogen from both the solvent and the amine. Cross-disproportionation reactions of the rearranged radical led to the formation of significant amounts of beta,beta-dimethylstyrene. Rate constants for hydrogen abstraction by the unrearranged, primary alkyl radical from n-dodecane (k(373K) = 3.5 x 10(3) M(-)(1) s(-)(1)), diphenylamine (k(373K) = 1.3 x 10(6) M(-)(1) s(-)(1)), and the substituted diarylamines were determined from the product yields and the known rate constant for the radical rearrangement. From kinetic experiments with N-deuteriodiphenylamine the deuterium kinetic isotope effect,k(NH)/k(ND), was found to be 2.3 at 373 K.     

Rates and mechanisms for the reactions of chlorine atoms with iodoethane and 2-iodopropane

The reaction of Cl atoms with iodoethane has been studied via a combination of laser flash photolysis/resonance fluorescence (LFP-RF), environmental chamber/Fourier transform (FT)IR, and quantum chemical techniques. Above 330 K, the flash photolysis data indicate that the reaction proceeds predominantly via hydrogen abstraction. The following Arrhenius expressions (in units of cm3 molecule(-1) s(-1)) apply over the temperature range 334-434 K for reaction of Cl with CH3CH2I (k4(H)) and CD3CD2I (k4(D)): k4(H) = (6.53 +/- 3.40) x 10(-11) exp[-(428 +/- 206)/T] and k4(D) = (2.21 +/- 0.44) x 10(-11) exp[-(317 +/- 76)/T]. At room temperature and below, the reaction proceeds both via hydrogen abstraction and via reversible formation of an iodoethane/Cl adduct. Analysis of the LFP-RF data yields a binding enthalpy (0 K) for CD3CD2I x Cl of 57 +/- 10 kJ mol(-1). Calculations using density functional theory show that the adduct is characterized by a C-I-Cl bond angle of 84.5 degrees; theoretical binding enthalpies of 38.2 kJ/mol, G2'[ECP(S)], and 59.0 kJ mol(-1), B3LYP/ECP, are reasonably consistent with the experimentally derived result. Product studies conducted in the environmental chamber show that hydrogen abstraction from both the -CH2I and -CH3 groups occur to a significant extent and also provide evidence for a reaction of the CH3CH2I x Cl adduct with CH3CH2I, leading to CH3CH2Cl formation. Complementary environmental chamber studies of the reaction of Cl atoms with 2-iodopropane, CH3CHICH3, are also presented. As determined by relative rate methods, the reaction proceeds with an effective rate coefficient, k6, of (5.0 +/- 0.6) x 10(-11) cm3 molecule(-1) s(-1) at 298 K. Product studies indicate that this reaction also occurs via two abstraction channels (from the CH3 groups and from the -CHI- group) and via reversible adduct formation.     

Ab initio and kinetic study of the reaction of ketones with OH for T = 500-2000 K. Part I: hydrogen-abstraction from H3CC(O)CH(3-x)(CH3)x, x = 0 ↦ 2

A theoretical study is presented of the mechanism and kinetics of the reactions of the hydroxyl radical with three ketones: dimethyl (DMK), ethylmethyl (EMK) and iso-propylmethyl (iPMK) ketones. CCSD(T) values extrapolated to the basis set limit are used to benchmark the computationally less expensive methods G3 and G3MP2BH&H, for the DMK + OH reaction system. These latter methods are then used in computations involving the reactions of the larger ketones. All possible abstraction channels have been modeled. A stepwise mechanism involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel has been recognized in part of the abstracting processes. High-pressure limit rate constants of the title reactions have been calculated in the temperature range of 500-2000 K using the Variflex code including Eckart tunneling corrections. Variable reaction coordinate transition state theory (VRC-TST) has been used for the rate constants of the barrier-less entrance channel. Calculated total rate constants (cm(3) mol(-1) s(-1)) are reported as follows: k(DMK) = 1.32 × 10(2)×T(3.30)exp(503/T), k(EMK) = 3.84 × 10(1)×T(3.51)exp(1515/T), k(iPMK) = 2.08 × 10(1)×T(3.58)exp(2161/T). Group rate constants (on a per H atom basis) for different carbon sites in title reactions have also been provided.     

Analysis of quantum yields for the photolysis of formaldehyde at lambda > 310 nm

Experimental quantum yields of the photolysis of formaldehyde at lambda > 310 nm are combined with absolute and relative rate calculations for the molecular elimination H2CO --> H2 + CO (1), the bond fission H2CO --> H + HCO (2), and the intramolecular hydrogen abstraction H2CO --> H ... HCO --> H2 + CO (3) taking place in the electronic ground state. Temperature and pressure dependencies of the quantum yields are analyzed with the goal to achieve consistency between experiment and modeling. Two wavelength ranges with considerably different properties are considered: 340-360 nm, where channel 1 competes with collisional deactivation of excited molecules, and 310-340 nm, which is dominated by the competition between the formation of radical and molecular products. The close relation between photolysis and pyrolysis of formaldehyde, such as analyzed for the pyrolysis in the companion paper, is documented and an internally consistent treatment of the two reaction systems is provided. The quantum yields are modeled and represented in analytical form such that values outside the available experimental range can be predicted to some extent.     

Atmospheric chemistry of ethyl propionate

Ethyl propionate is a model for fatty acid ethyl esters used as first-generation biodiesel. The atmospheric chemistry of ethyl propionate was investigated at 980 mbar total pressure. Relative rate measurements in 980 mbar N(2) at 293 ± 0.5 K were used to determine rate constants of k(C(2)H(5)C(O)OC(2)H(5) + Cl) = (3.11 ± 0.35) × 10(-11), k(CH(3)CHClC(O)OC(2)H(5) + Cl) = (7.43 ± 0.83) × 10(-12), and k(C(2)H(5)C(O)OC(2)H(5) + OH) = (2.14 ± 0.21) × 10(-12) cm(3) molecule(-1) s(-1). At 273-313 K, a negative Arrhenius activation energy of -3 kJ mol(-1) is observed.. The chlorine atom-initiated oxidation of ethyl propionate in 980 mbar N(2) gave the following products (stoichiometric yields): ClCH(2)CH(2)C(O)OC(2)H(5) (0.204 ± 0.031), CH(3)CHClC(O)OC(2)H(5) (0.251 ± 0.040), and C(2)H(5)C(O)OCHClCH(3) (0.481 ± 0.088). The chlorine atom-initiated oxidation of ethyl propionate in 980 mbar of N(2)/O(2) (with and without NO(x)) gave the following products: ethyl pyruvate (CH(3)C(O)C(O)OC(2)H(5)), propionic acid (C(2)H(5)C(O)OH), formaldehyde (HCHO), and, in the presence of NO(x), PAN (CH(3)C(O)OONO(2)). The lack of acetaldehyde as a product suggests that the CH(3)CH(O)C(O)OC(2)H(5) radical favors isomerization over decomposition. From the observed product yields, we conclude that H-abstraction by chlorine atoms from ethyl propionate occurs 20.4 ± 3.1%, 25.1 ± 4.0%, and 48.1 ± 8.8% from the CH(3)-, -CH(2)-, and -OCH(2)- groups, respectively. The rate constant and branching ratios for the reaction between ethyl propionate and the OH radical were investigated theoretically using quantum mechanical calculations and transition state theory. The stationary points along the reaction path were optimized using the CCSD(T)-F12/VDZ-F12//BH&HLYP/aug-cc-pVTZ level of theory; this model showed that OH radicals abstract hydrogen atoms primarily from the -OCH(2)- group (80%).     

Hydrogen atom abstraction reactions from tertiary amines by benzyloxyl and cumyloxyl radicals: influence of structure on the rate-determining formation of a hydrogen-bonded prereaction complex

A time-resolved kinetic study on the hydrogen atom abstraction reactions from a series of tertiary amines by the cumyloxyl (CumO(?)) and benzyloxyl (BnO(?)) radicals was carried out. With the sterically hindered triisobutylamine, comparable hydrogen atom abstraction rate constants (k(H)) were measured for the two radicals (k(H)(BnO(?))/k(H)(CumO(?)) = 2.8), and the reactions were described as direct hydrogen atom abstractions. With the other amines, increases in k(H)(BnO(?))/k(H)(CumO(?)) ratios of 13 to 2027 times were observed. k(H) approaches the diffusion limit in the reactions between BnO(?) and unhindered cyclic and bicyiclic amines, whereas a decrease in reactivity is observed with acyclic amines and with the hindered cyclic amine 1,2,2,6,6-pentamethylpiperidine. These results provide additional support to our hypothesis that the reaction proceeds through the rate-determining formation of a C-H/N hydrogen-bonded prereaction complex between the benzyloxyl α-C-H and the nitrogen lone pair wherein hydrogen atom abstraction occurs, and demonstrate the important role of amine structure on the overall reaction mechanism. Additional mechanistic information in support of this picture is obtained from the study of the reactions of the amines with a deuterated benzyloxyl radical (PhCD(2)O(?), BnO(?)-d(2)) and the 3,5-di-tert-butylbenzyloxyl radical.     

Diffusion controlled hydrogen atom abstraction from tertiary amines by the benzyloxyl radical. The importance of C-H/N hydrogen bonding

The rate constants for H-atom abstraction (k(H)) from 1,4-cyclohexadiene (CHD), triethylamine (TEA), triisobutylamine (TIBA), and DABCO by the cumyloxyl (CumO(?)) and benzyloxyl (BnO(?)) radicals were measured. Comparable k(H) values for the two radicals were obtained in their reactions with CHD and TIBA whereas large increases in k(H) for TEA and DABCO were found on going from CumO(?) to BnO(?). These differences are attributed to the rate-determining formation of BnO(?) C-H/amine N lone-pair H-bonded complexes.     

Switch-over in photochemical reaction mechanism from hydrogen abstraction to exciplex-induced quenching: interaction of triplet-excited versus singlet-excited acetone versus cumyloxyl radicals with amines

The fluorescence and phosphorescence quenching of acetone by 13 aliphatic amines has been investigated. The bimolecular rate constants lie in the range of 10(8)-10(9) M(-1) s(-1) for singlet-excited acetone and 10(6)-10(8) M(-1) s(-1) for the triplet case. The rate data indicate that a direct hydrogen abstraction process dominates for triplet acetone, while a charge-transfer mechanism, namely, exciplex-induced quenching, becomes important for singlet-excited acetone. Pronounced stereoelectronic effects toward H abstraction, e.g., for 1,4-diazabicyclo[2.2.2]octane (DABCO), and significant steric hindrance effects, e.g., for N,N-diisopropyl-3-pentylamine, are observed. A negative activation energy (E(a) = -0.9 +/- 0.2 kcal mol(-1) for triethylamine and DABCO) and the absence of a significant solvent effect on the fluorescence quenching of acetone are indicative of the involvement of exciplexes. Full electron transfer can be ruled out on the basis of the low reduction potential of acetone, which was found to lie below -3.0 V versus SCE. The participation of H abstraction for triplet acetone is corroborated by the respective quenching rate constants, which resemble the reaction rate constants for cumyloxyl radicals. The latter were measured for all 13 amines and showed also a dependence on the electron donor properties of the amines. It is suggested that the H abstraction proceeds directly and not through an exciplex or ion pair. Further, abstraction from N-H bonds in addition to alpha C-H bonds has been corroborated as a significant pathway for excited acetone. Product studies and quantum yields for photoreduction of singlet- and triplet-excited acetone by triethylamine (8% for S(1) versus 24% for T(1)) are in line with the suggested mechanisms of quenching through an exciplex and photoreduction through direct H abstraction.     

Atmospheric chemistry of two biodiesel model compounds: methyl propionate and ethyl acetate

The atmospheric chemistry of two C(4)H(8)O(2) isomers (methyl propionate and ethyl acetate) was investigated. With relative rate techniques in 980 mbar of air at 293 K the following rate constants were determined: k(C(2)H(5)C(O)OCH(3) + Cl) = (1.57 ± 0.23) × 10(-11), k(C(2)H(5)C(O)OCH(3) + OH) = (9.25 ± 1.27) × 10(-13), k(CH(3)C(O)OC(2)H(5) + Cl) = (1.76 ± 0.22) × 10(-11), and k(CH(3)C(O)OC(2)H(5) + OH) = (1.54 ± 0.22) × 10(-12) cm(3) molecule(-1) s(-1). The chlorine atom initiated oxidation of methyl propionate in 930 mbar of N(2)/O(2) diluent (with, and without, NO(x)) gave methyl pyruvate, propionic acid, acetaldehyde, formic acid, and formaldehyde as products. In experiments conducted in N(2) diluent the formation of CH(3)CHClC(O)OCH(3) and CH(3)CCl(2)C(O)OCH(3) was observed. From the observed product yields we conclude that the branching ratios for reaction of chlorine atoms with the CH(3)-, -CH(2)-, and -OCH(3) groups are <49 ± 9%, 42 ± 7%, and >9 ± 2%, respectively. The chlorine atom initiated oxidation of ethyl acetate in N(2)/O(2) diluent gave acetic acid, acetic acid anhydride, acetic formic anhydride, formaldehyde, and, in the presence of NO(x), PAN. From the yield of these products we conclude that at least 41 ± 6% of the reaction of chlorine atoms with ethyl acetate occurs at the -CH(2)- group. The rate constants and branching ratios for reactions of OH radicals with methyl propionate and ethyl acetate were investigated theoretically using transition state theory. The stationary points along the oxidation pathways were optimized at the CCSD(T)/cc-pVTZ//BHandHLYP/aug-cc-pVTZ level of theory. The reaction of OH radicals with ethyl acetate was computed to occur essentially exclusively (～99%) at the -CH(2)- group. In contrast, both methyl groups and the -CH(2)- group contribute appreciably in the reaction of OH with methyl propionate. Decomposition via the α-ester rearrangement (to give C(2)H(5)C(O)OH and a HCO radical) and reaction with O(2) (to give CH(3)CH(2)C(O)OC(O)H) are competing atmospheric fates of the alkoxy radical CH(3)CH(2)C(O)OCH(2)O. Chemical activation of CH(3)CH(2)C(O)OCH(2)O radicals formed in the reaction of the corresponding peroxy radical with NO favors the α-ester rearrangement.     

Transition metal ion-assisted photochemical generation of alkyl halides and hydrocarbons from carboxylic acids

Near-UV photolysis of aqueous solutions of propionic acid and aqueous Fe(3+) in the absence of oxygen generates a mixture of hydrocarbons (ethane, ethylene and butane), carbon dioxide, and Fe(2+). The reaction becomes mildly catalytic (about five turnovers) in the presence of oxygen which converts a portion of alkyl radicals to oxidizing intermediates that reoxidize Fe(2+). The photochemistry in the presence of halide ions (X(-) = Cl(-), Br(-)) generates ethyl halides via halogen atom abstraction from FeX(n)(3-n) by ethyl radicals. Near-quantitative yields of C(2)H(5)X are obtained at ≥0.05 M X(-). Competition experiments with Co(NH(3))(5)Br(2+) provided kinetic data for the reaction of ethyl radicals with FeCl(2+) (k = (4.0 ± 0.5) × 10(6) M(-1) s(-1)) and with FeBr(2+) (k = (3.0 ± 0.5) × 10(7) M(-1) s(-1)). Photochemical decarboxylation of propionic acid in the presence of Cu(2+) generates ethylene and Cu(+). Longer-chain acids also yield alpha olefins as exclusive products. These reactions become catalytic under constant purge with oxygen which plays a dual role. It reoxidizes Cu(+) to Cu(2+), and removes gaseous olefins to prevent accumulation of Cu(+)(olefin) complexes and depletion of Cu(2+). The results underscore the profound effect that the choice of metal ions, the medium, and reaction conditions exert on the photochemistry of carboxylic acids.     

Experimental and computational studies of the phenyl radical reaction with propyne.

The kinetics for the gas-phase reaction of phenyl radical with propyne has been measured by cavity ring-down spectrometry (CRDS), and the mechanism and initial product branching have been elucidated with the help of quantum chemical calculations. Absolute rate constants measured by the CRDS technique can be expressed by the following Arrhenius equation: (k/cm(3) mol(-1) s(-1)): k(propyne)(T=301-428 K)=(3.68+/-0.92) x 10(11)exp[-(1685+/-80)/T]. The experiment is unable to distinguish between the possible reactive channels, but theory indicates that phenyl radicals preferably add to the unsaturated terminal carbon atom in propyne under our experimental conditions. Theoretical kinetic calculations, employing high-level G2M(RCC, RMP2) and G3 energetic and IRCMax(RCCSD(T)//B3LYP-DFT) molecular parameters, reproduce the total experimental rate constants within a factor of three. Calculated total and branching rate constants are provided for high-T kinetic modeling. Addition reactions of phenyl to C3H4 are estimated to be less important molecular-growth pathways in high-T conditions (T>1000 K) in comparison to the C6H5 + C2H2 reaction.     

Deuterium enrichment of interstellar methanol explained by atom tunneling

We calculate, down to low temperature and for different isotopes, the reaction rate constants for the hydrogen abstraction reaction H + H(3)COH → H(2) + CH(2)OH/CH(3)O. These explain the known abundances of deuterated forms of methanol in interstellar clouds, where CH(2)DOH can be almost as abundant as CH(3)OH. For abstraction from both the C- and the O-end of methanol, the barrier-crossing motion involves the movement of light hydrogen atoms. Consequently, tunneling plays a dominant role already at relatively high temperature. Our implementation of harmonic quantum transition state theory with on the fly calculation of forces and energies accounts for these tunneling effects. The results are in good agreement with previous semiclassical and quantum dynamics calculations (down to 200 K) and experimental studies (down to 295 K). Here we extend the rate calculations down to lower temperature: 30 K for abstraction from the C-end of methanol and 80 K for abstraction from the OH-group. At all temperatures, abstraction from the C-end is preferred over abstraction from the O-end, more strongly so at lower temperature. Furthermore, the tunneling behavior strongly affects the kinetic isotope effects (KIEs). D + H(3)COH → HD + CH(2)OH has a lower vibrationally adiabatic barrier than H + H(3)COH → H(2) + CH(2)OH, giving rise to an inverse KIE (k(H)/k(D) < 1) at high temperature, in accordance with previous experiments and calculations. However, since tunneling is more facile for the light H atom, abstraction by H is favored over abstraction by D below ~135 K, with a KIE k(H)/k(D) of 11.2 at 30 K. The H + D(3)COD → HD + CD(2)OD reaction is calculated to be much slower than the D + H(3)COH → HD + CH(2)OH, in agreement with low-temperature solid-state experiments, which suggests the preference for H (as opposed to D) abstraction from the C-end of methanol to be the mechanism by which interstellar methanol is deuterium-enriched.     

2+2] Photocycloaddition reaction dynamics of triplet pyrimidines

Taking the 266 nm excited pyrimidine (uracil or thymine) with cyclopentene as model reaction systems, we have examined the photoproduct formation dynamics from the [2 + 2] photocycloaddition reactions of triplet pyrimidines in solution and provided mechanistic insights into this important DNA photodamage reaction. By combining two compliment methods of nanosecond time-resolved transient IR and UV-vis laser flash-photolysis spectroscopy, the photoproduct formation dynamics as well as the triplet quenching kinetics are measured. Characteristic IR absorption bands due to photoproduct formation have been observed and product quantum yields are determined to be ～0.91% for uracil and ～0.41% for thymine. Compared to the measured large quenching rate constants of triplet uracil (1.5 × 10(9) M(-1)s(-1)) or thymine (0.6 × 10(9) M(-1)s(-1)) by cyclopentene, the inefficiency in formation of photoproducts indicates competitive physical quenching processes may exist on the route leading to photoproducts, resulting in very small product yields eventually. Such an energy wasting process is found to be resulted from T(1)/S(0) surface crossings by the hybrid density functional calculations, which compliments the experiments and reveals the reaction mechanism.     

Relative rate and product studies of the OH-acetone reaction

The rate constants for the reaction of acetone (kH) and d6-acetone (kD) with OH radicals have been measured at atmospheric pressure over a range of temperatures by a relative rate method by using on-line mass spectrometry. The following Arrhenius expressions have been determined for these reactions (in units of cm(3) molecule(-1) s(-1)): k(H)(T) = (9.8 x 10(-13)) exp[-(484 +/- 44)/T] between 253 and 373 K, and kD(T) = (4.0 x 10(-13)) exp[-(755 +/- 89)/T] between 293 and 373 K. This is the first study to investigate the temperature dependence of kH and kD by using a relative rate method and confirms previous rate constants determined by absolute methods. Agreement of our rate constants with those determined in the absence of water suggests that the presence of water vapor has a minimal effect on the kinetics of this reaction under the conditions of our study. The observed kinetic isotope effect (kH/kD = 5.6 +/- 0.4 at 293 K) is evidence that H-atom abstraction occurs in the mechanism. The acetic acid yields of the reaction of OH with acetone and d6-acetone were also investigated by on-line mass spectrometry. Acetic acid yields show a negative temperature dependence that decreases from 0.12 at 273 K to 0.05 at 353 K. The yields of d3-acetic acid decrease from 0.20 at 283 K to 0.13 at 323 K. Kinetic modeling of our data suggests that 50-70% of the observed acetic acid in our system may be due to secondary reactions involving acetonoxy and HOx radical reactions. However, secondary chemistry cannot easily explain the observed formation of d3-acetic acid in the deuterated system, where about 90% of the observed d3-acetic acid is likely due to an OH-addition mechanism.     

The oxidative degradation of dibenzoazepine derivatives by cerium(IV) complexes in acidic sulfate media

The kinetics of the oxidation of imipramine and desipramine using cerium(IV) complexes were studied in the presence of a large excess of azepine derivative (TCA) in acidic sulfate media using UV-Vis spectroscopy. The reaction proceeds via dibenzoazepine radical formation, identified by EPR measurements. The kinetics of the first degradation step were studied independently of the further slower degradation reactions. Linear dependences, with zero intercept, of the pseudo-first-order rate constants (k(obs)) on [TCA] were established for both dibenzoazepine radical formation processes. Rates of reactions decreased with increasing concentration of the H(+) ion indicating that cerium(IV) as well as both reductants exist in an equilibrium with their protolytic forms. The activation parameters for the degradation of dibenzoazepine derivatives in the first oxidation stage were as follows: ΔH(≠) = 39 ± 2 kJ mol(-1), ΔS(≠) = -28 ± 8 J K(-1) mol(-1) for imipramine and ΔH(≠) = 39 ± 2 kJ mol(-1), ΔS(≠) = -28 ± 6 J K(-1) mol(-1) for desipramine, respectively. Imipramine and desipramine radicals dimerized leading to an intermediate radical dimer, which decayed in a first-order consecutive decay process. These two further reactions proceed with rates which are characterized by non-linear dependences of the pseudo-first-order rate constants (k(obs)) on [TCA]. The degradation reaction of the intermediate radical dimer leads to an uncharged dimer as a final product. Mechanistic consequences of all the results are discussed.     

Kinetics and mechanism of hydrogen-atom abstraction from rhodium hydrides by alkyl radicals in aqueous solutions

The kinetics of the reaction of benzyl radicals with [L(1)(H(2)O)RhH{D}](2+) (L(1)=1,4,8,11-tetraazacyclotetradecane) were studied directly by laser-flash photolysis. The rate constants for the two isotopologues, k=(9.3±0.6) × 10(7) M(-1) s(-1) (H) and (6.2±0.3) × 10(7) M(-1) s(-1) (D), lead to a kinetic isotope effect k(H)/k(D)=1.5±0.1. The same value was obtained from the relative yields of PhCH(3) and PhCH(2)D in a reaction of benzyl radicals with a mixture of rhodium hydride and deuteride. Similarly, the reaction of methyl radicals with {[L(1)(H(2)O)RhH](2+) + [L(1)(H(2)O)RhD](2+)} produced a mixture of CH(4) and CH(3)D that yielded k(H)/k(D)=1.42±0.07. The observed small normal isotope effects in both reactions are consistent with reduced sensitivity to isotopic substitution in very fast hydrogen-atom abstraction reactions. These data disprove a literature report claiming much slower kinetics and an inverse kinetic isotope effect for the reaction of methyl radicals with hydrides of L(1)Rh.     

Catalytic and direct oxidation of cysteine by octacyanomolybdate(V)

The oxidation of cysteine by [Mo(CN)(8)](3-) in deoxygenated aqueous solution at a moderate pH is strongly catalyzed by Cu(2+), to the degree that impurity levels of Cu(2+) are sufficient to dominate the reaction. Dipicolinic acid (dipic) is a very effective inhibitor of this catalysis, such that with 1 mM dipic, the direct oxidation can be studied. UV-vis spectra and electrochemistry show that [Mo(CN)(8)](4-) is the Mo-containing product. Cystine and cysteinesulfinate are the predominant cysteine oxidation products. The stoichiometric ratio (Deltan(Mo(V))/Deltan(cysteine)) of 1.4 at pH 10.8 is consistent with this product distribution. At pH 1.5, the reaction is quite slow and yields intractable kinetics. At pH 4.5, the rates are much faster and deviate only slightly from pseudo-first-order behavior. With 2 mM PBN (N-phenyl-tert-butyl nitrone) present at pH 4.5, the reaction rate is about 20% less and shows excellent pseudo-first-order behavior, but the stoichiometric ratio is not significantly changed. The rates also display a significant specific cation effect. In the presence of spin-trap PBN, the kinetics were studied over the pH range 3.48-12.28, with [Na(+)] maintained at 0.09-0.10 M. The rate law is -d[Mo(V)]/dt = k[cysteine](tot)[Mo(V)], with k = {2(k(b)K(a1)K(a2)[H(+)] + k(c)K(a1)K(a2)K(a3))}/([H(+)](3) + K(a1)[H(+)](2) + K(a1)K(a2)[H(+)] + K(a1)K(a2)K(a3)), where K(a1), K(a2), and K(a3) are the successive acid dissociation constants of HSCH(2)CH(NH(3)(+))CO(2)H. Least-squares fitting yields k(b) = (7.1 +/- 0.4) x 10(4) M(-1) s(-1) and k(c) = (2.3 +/-0.2) x 10(4) M(-1) s(-1) at mu = 0.1 M (NaCF(3)SO(3)) and 25 degrees C. A mechanism is inferred in which k(b) and k(c) correspond to electron transfer to Mo(V) from the thiolate forms of anionic and dianionic cysteine.     

Reversible intramolecular hydrogen transfer between cysteine thiyl radicals and glycine and alanine in model peptides: absolute rate constants derived from pulse radiolysis and laser flash photolysis

The intramolecular reaction of cysteine thiyl radicals with peptide and protein alphaC-H bonds represents a potential mechanism for irreversible protein oxidation. Here, we have measured absolute rate constants for these reversible hydrogen transfer reactions by means of pulse radiolysis and laser flash photolysis of model peptides. For N-Ac-CysGly6 and N-Ac-CysGly2AspGly3, Cys thiyl radicals abstract hydrogen atoms from Gly with k(f) = (1.0-1.1 x 10(5) s(-1), generating carbon-centered radicals, while the reverse reaction proceeds with k(r) = (8.0-8.9) x 10(5) s(-1). The forward reaction shows a normal kinetic isotope effect of k(H)/k(D) = 6.9, while the reverse reaction shows a significantly higher normal kinetic isotope effect of 17.6, suggesting a contribution of tunneling. For N-Ac-CysAla2AspAla3, cysteine thiyl radicals abstract hydrogen atoms from Ala with k(f) = (0.9-1.0) x 10(4) s(-1), while the reverse reaction proceeds with k(r) = 1.0 x 10(5) s(-1). The order of reactivity, Gly > Ala, is in accord with previous studies on intermolecular reactions of thiyl radicals with these amino acids. The fact that k(f) < k(r) suggests some secondary structure of the model peptides, which prevents the adoption of extended conformations, for which calculations of homolytic bond dissociation energies would have predicted k(f) > k(r). Despite k(f) < k(r), model calculations show that intramolecular hydrogen abstraction by Cys thiyl radicals can lead to significant oxidation of other amino acids in the presence of physiologic oxygen concentrations.     

State-to-state quantum dynamics of the H + HBr reaction: competition between the abstraction and exchange reactions

Quantum state-to-state dynamics for the H + HBr(υ(i) = 0, j(i) =0) reaction was studied on an accurate ab intio potential energy surface for the electronic ground state of BrH(2). Both the H + HBr → H(2) + Br abstraction reaction and the H' + HBr → H'Br + H exchange reaction were investigated up to a collision energy of 2.0 eV. It was found that the abstraction channel is dominant at lower collision energies, while the exchange channel becomes dominant at higher collision energies. The total integral cross section of the abstraction reaction at a collision energy of 1.6 eV was found to be 1.37 A?(2), which is larger than a recent quantum mechanical result (1.06 A?(2)) and still significantly smaller than the experimental value (3 ± 1 A?(2)). Meanwhile, similar to the previous theoretical study, our calculations also predicted much hotter product rotational state distributions than those from the experimental study. This suggests that further experimental investigations are highly desirable to elucidate the dynamic properties of the title reactions.