The chemistry of CH2I2 on MoAl alloy thin films formed on dehydroxylated alumina: insight into methylene insertion reactions

The chemistry of diiodomethane is explored in ultrahigh vacuum on a MoAl alloy film grown on planar, dehydroxylated alumina by reaction with molybdenum hexacarbonyl. The majority of the diiodomethane forms methylene species below approximately 250 K, although a small proportion forms CH(2)I((ads)), which hydrogenates to form iodomethane. The majority ( approximately 90%) of the adsorbed methylene species thermally decomposes to carbon and hydrogen. The remainder undergoes several reactions, including partial hydrogenation to form adsorbed methyl species or total hydrogenation to form methane. The methyl species can couple forming ethane or undergo methylene insertion reactions to form alkyl species up to C(4). These form alkenes via a beta-hydride elimination reaction. This chemistry is relatively unique, only having been found previously for Ni(110) surfaces. No such chemistry is found on Ni(100) and Ni(111).     

Coupling versus surface-etching reactions of alkyl halides on GaAs(100): I. CF3CH2I reactions

We report the rich surface chemistry exhibited by the reactions of 1,1,1-trifluoroethyl iodide (CF3CH2I) adsorbed onto gallium-rich GaAs(100)-(4 x 1), studied by temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED) studies and X-ray photoelectron spectroscopy (XPS). CF3CH2I adsorbs molecularly at 150 K but dissociates, below room temperature, to form a chemisorbed monolayer of CF3CH2 and I species. Recombinative desorption of molecular CF3CH2I competes with the further reactions of the CF3CH2 and I chemisorbed species. The CF3CH2 species can either undergo beta-fluoride elimination to yield gaseous CF2=CH2 or it can undergo self-coupling to form the corresponding higher alkane, CF3CH2CH2CF3. A second coupling product, CF3CH2CH=CF2, is also evolved, and it is postulated that migratory insertion of the liberated CF2=CH2 into the surface-carbon bond of the chemisorbed CF3CH2 is responsible for its formation. The iodines, formed by C-I scission in the chemisorbed CF3CH2I, and the fluorines, derived from beta-fluoride elimination in CF3CH2, react with the surface gallium dimers, and Ga-As back-bonds to generate five etch products (GaF, AsF, GaI, AsI, and As2) that desorb in the temperature range of 420 to >600 K. XPS data reveal that the surface stoichiometry remains constant throughout the entire annealing temperature range because of the desorption of both gallium- and arsenic-containing etch products, which occur sequentially. In this article, plausible mechanisms by which all products form and the binding sites of these reactions in the (4 x 1) reconstruction are discussed. Factors that control the rate constants of etch product versus hydrocarbon product formation and in particular how they impact on the respective desorption temperatures will be discussed.     

Transition state for beta-hydride elimination in alkyl groups on Pt(111)

The transition state for beta-hydride elimination in alkyl groups on the Pt(111) surface has been probed by studying the effects of fluorine substitution on the barriers to beta-hydride elimination, DeltaE++(betaH). Four different fluoroalkyl groups have been formed on the Pt(111) surface by dissociative adsorption of four fluoroalkyl iodides: RCH(2)CH(2)-I (R = CF(3), CF(3)CH(2), and CF(3)CF(2)) and (CF(3))(2)CHCH(2)-I. In the absence of preadsorbed hydrogen, fluoroalkyl groups on the Pt(111) surface dehydrogenate via beta-hydride elimination to form unsaturated fluorocarbons and deposit hydrogen atoms onto the surface. Those hydrogen atoms then hydrogenate the remaining fluoroalkyl groups to produce fluoroalkanes that desorb rapidly from the surface. The kinetics of hydrogenation and fluoroalkane desorption are rate limited by the beta-hydride elimination step and thus serve as measures of the kinetics of beta-hydride elimination. The field effects of the fluorinated substituents increase the barriers to beta-hydride elimination with a reaction constant of rho(F) = 19 +/- 2 kJ/mol. The interpretation of this effect is that the beta-carbon atom in the transition state is cationic, [RC(delta+...)H]++, with respect to the reactant. The field effect of the fluorinated substituent energetically destabilizes the electron deficient beta-carbon atom in the transition state. This is consistent with observations made on the Cu(111) surface; however, the substituent effect is significantly smaller on the Pt(111) surface. On the Pt(111) surface, the transition state for beta-hydride elimination is less polarized with respect to the initial state alkyl group than on the Cu(111) surface.     

Density functional theory studies of negishi alkyl-alkyl cross-coupling reactions catalyzed by a methylterpyridyl-Ni(I) complex

Density functional theory calculations were done to examine the potential energy surfaces of Ni(I)-catalyzed Negishi alkyl-alkyl cross-coupling reactions by using propyl iodide and isopropyl iodide as model alkyl electrophiles and CH 3ZnI as a model alkyl nucleophile. A four-step catalytic cycle involving iodine transfer, radical addition, reductive elimination, and transmetalation steps were characterized structurally and energetically. The reaction mechanism for this catalytic cycle appears feasible based on the calculated free energy profiles for the reactions. The iodine transfer step is the rate-determining step for the Ni(tpy)-CH 3 (tpy = 2,2'6',2'-terpyridine) reactions with alkyl iodides. For secondary alkyl electrophiles, the oxidative addition intermediate, Ni(III), prefers to undergo decomposition over reductive elimination, whereas for the primary alkyl electrophiles, Ni(III) prefers to undergo reductive elimination over decomposition based on comparison of the relative reaction rates for these two types of steps. In addition, thermodynamic data were employed to help explain why the yield of the coupled product is very low from the Ni(II)-alkyl halide reactions with organozinc reagents.     

Chemical and electrical passivation of silicon (111) surfaces through functionalization with sterically hindered alkyl groups

Crystalline Si(111) surfaces have been alkylated in a two-step chlorination/alkylation process using sterically bulky alkyl groups such as (CH3)2CH- (iso-propyl), (CH3)3C- (tert-butyl), and C6H5- (phenyl) moieties. X-ray photoelectron spectroscopic (XPS) data in the C 1s region of such surfaces exhibited a low energy emission at 283.9 binding eV, consistent with carbon bonded to Si. The C 1s XPS data indicated that the alkyls were present at lower coverages than methyl groups on CH(3)-terminated Si(111) surfaces. Despite the lower alkyl group coverage, no Cl was detected after alkylation. Functionalization with the bulky alkyl groups effectively inhibited the oxidation of Si(111) surfaces in air and produced low (<100 cm s(-1)) surface recombination velocities. Transmission infrared spectroscopy indicated that the surfaces were partially H-terminated after the functionalization reaction. Application of a reducing potential, -2.5 V vs Ag+/Ag, to Cl-terminated Si(111) electrodes in tetrahydrofuran resulted in the complete elimination of Cl, as measured by XPS. The data are consistent with a mechanism in which the reaction of alkyl Grignard reagents with the Cl-terminated Si(111) surfaces involves electron transfer from the Grignard reagent to the Si, loss of chloride to solution, and subsequent reaction between the resultant silicon radical and alkyl radical to form a silicon-carbon bond. Sites sterically hindered by neighboring alkyl groups abstract a H atom to produce Si-H bonds on the surface.     

Chemisorption and reaction characteristics of methyl radicals on Cu(110)

Methyl radicals are generated by pyrolysis of azomethane, and the condition for achieving neat adsorption on Cu(110) is described for studying their chemisorption and reaction characteristics. The radical-surface system is examined by X-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, temperature-programmed desorption, low-energy electron diffraction (LEED), and high-resolution electron energy loss spectroscopy under ultrahigh vacuum conditions. It is observed that a small fraction of impinging CH3 radicals decompose into methylene possibly on surface defect sites. This type of CH2 radical has no apparent effect on CH3(ads) surface chemistry initiated by dehydrogenation to form active CH2(ads) followed by chain reactions to yield high-mass alkyl products. All thermal desorption products, such as H2, CH4, C2H4, C2H6, and C3H6, are detected with a single desorption peak near 475 K. The product yields increase with surface coverage until saturation corresponding to 0.50 monolayer of CH3(ads). The mass distribution is, however, invariant with initial CH3(ads) coverage, and all desorbed species exhibit first-order reaction kinetics. LEED measurement reveals a c(2 x 2) adsorbate structure independent of the amount of gaseous exposure. This strongly suggests that the radicals aggregate into close-packed two-dimensional islands at any exposure. The islanding behavior can be correlated with the reaction kinetics and is deemed to be essential for the chain propagation reactions. Some relevant aspects of the CH3/Cu(111) system are also presented. The new results are compared with those of prior studies employing methyl halides as radical sources. Major differences are found in the product distribution and desorption kinetics, and these are attributed to the influence of surface halogen atoms present in those earlier investigations.     

Direct observation of reductive elimination of methyl iodide from a rhodium(III) pincer complex: the importance of sterics

A rare case of directly observed alkyl halide reductive elimination from rhodium is reported. Treatment of the naphthyl-based PCP-type Rh(III) methyl complexes 2a,b [(C10H5(CH2PR2)2)Rh(CH3)(I)] (R = iPr 2a, R = tBu 2b) with CO resulted in facile reductive elimination of methyl iodide in the case of 2b, yielding the Rh(I) carbonyl complex [(C10H5(CH2PR2)2)Rh(CO)] 3b (R = tBu), while the less bulky 2a formed CO adducts and did not undergo reductive elimination, contrary to expectations based on electron density considerations. Moreover, 3b oxidatively added methyl iodide, while 3a did not. CD3I/CH3I exchange studies in the absence of CO indicate that reversible formation of (ligated) methyl iodide takes place in both systems. Subsequently, when CO is present, it displaces methyl iodide in the bulkier tBu system, whereas with the iPr system formation of the Rh(III) CO adducts is favored. Iodide dissociation followed by its attack on the rhodium-methyl group is unlikely.     

Unimolecular reactions of chemically activated CF2BrCF2CH3 and CF2BrCF2CD3: evidence for 1,2-FBr interchange

Vibrationally excited CF2BrCF2CH3 and CF2BrCF2CD3 molecules were prepared with 96 kcal mol-1 energy at room temperature by the recombination of CF2BrCF2 and CH3 (CD3) radicals. The observed unimolecular reactions are 1,2-BrF interchange to give CF3CFBrCH3 (CD3) molecules and 2,3-FH (FD) elimination; the rate constants are 2.2 x 10(5) (1.5 x 10(5)) s(-1) and 2.0 x 105 (0.75 x 10(5)) s(-1), respectively. The CF3CFBrCH3 (CD3) molecules rapidly, relative to the reverse reaction, eliminate HBr or DBr to give the observed product CF3CF=CH2 (CD2). Density functional theory at the B3PW91/6-311+G(2d,p) level was used to obtain vibrational frequencies and moments of inertia of the molecule and transition states for subsequent calculations of statistical rate constants for CF2BrCF2CH3 and CF2BrCF2CD3. Matching experimental and calculated rate constants gave threshold energies of 62 and 66 kcal mol-1 for 1,2-BrF interchange and 2,3-FH elimination, respectively. The BrF interchange reaction is compared to ClF interchange from CF2ClCF2CH3 and CF2ClCHFCH3.     

The surface chemistry of propylene, 1-iodopropane, and 1,3-diiodopropane on MoAl alloy thin films formed on dehydroxylated alumina

The adsorption of C3 hydrocarbons propylene, 1-iodopropane, and 1,3-diiodopropane is studied in ultrahigh vacuum on a molybdenum-aluminum alloy formed by molybdenum hexacarbonyl reaction with a planar alumina film grown on a Mo(100) substrate. Carbon-iodine bond scission occurs below approximately 200 K to deposit iodine, and form propyl species from 1-iodopropane and a C3 metallacycle from 1,3-diiodopropane. Propyl species either undergo beta-hydride elimination to yield propylene or hydrogenate to form propane. Propylene adsorbs as both pi- and di-sigma-bonded species, and the di-sigma form hydrogenates to yield propane, where the addition of the first hydrogen to form propyl species is slower than the second hydrogenation step to yield propane. Propylene also thermally decomposes on the surface to desorb hydrogen and deposit carbon where the methylyne group is the most, and the methyl group the least reactive. The metallacyclic intermediate reacts to give an allylic intermediate, which forms propylene, but also decomposes by C-C bond cleavage to evolve ethylene and deposit methylene species on the surface. This is a key step in the mechanism proposed for heterogeneously catalyzed olefin metathesis and this is the first time that this chemistry has been directly identified in ultrahigh vacuum.     

Unimolecular reactions including ClF interchange of vibrationally excited CF2ClCHFCH2CH3 and CF2ClCHFCD2CD3

Vibrationally excited CF(2)ClCHFC(2)H(5)(CF(2)ClCHFC(2)D(5)) molecules were prepared in the gas phase at 300 K with approximately 93 kcal mol(-1) of energy by recombination of CF(2)ClCHF and C(2)H(5) or C(2)D(5) radicals. Three unimolecular reactions were observed. 1,2-ClF interchange converts CF(2)ClCHFC(2)H(5)(CF(2)ClCHFC(2)D(5)) into CF(3)CHClC(2)H(5)(CF(3)CHClC(2)D(5)), and subsequent 2,3-ClH (ClD) elimination gives CF(3)CH=CHCH(3) (CF(3)CH=CDCD(3)). 2,3-FH(FD) elimination gives cis- and trans-CF(2)ClCH=CHCH(3) (CF(2)ClCH=CDCD(3)), and 1,2-ClH elimination gives CF(2)=CFCH(2)CH(3) (CF(2)=CFCD(2)CD(3)). The experimental rate constants for CF(2)ClCHFC(2)H(5) (CF(2)ClCHFC(2)D(5)) were 1.3 x 10(4) (0.63 x 10(4)) s(-1) for 1,2-FCl interchange and 2.1 x 10(4) (0.61 x 10(4)) s(-1) with a trans/cis ratio of 3.7 for 2,3-FH(FD) elimination. The 1,2-ClH process was the least important with a branching fraction of only 0.08 +/- 0.04. The rate constants for 2,3-ClH (ClD) elimination from CF(3)CHClC(2)H(5) (CF(3)CHClC(2)D(5)) were 1.8 x 10(6) (0.49 x 10(6)) s(-1) with a trans/cis ratio of 2.4. Density functional theory was used to compute vibrational frequencies and structures needed to obtain rate constants from RRKM theory. Matching theoretical and experimental rate constants provides estimates of the threshold energies, E0, for the three reaction pathways; 1,2-FCl interchange has the lowest E0. The unimolecular reactions of CF(2)ClCHFC(2)H(5) are compared to those of CF(2)ClCHFCH(3). Both of these systems are compared to CH(3)CHFC(2)H(5) to illustrate the influence of a CF(2)Cl group on the E0 for FH elimination.     

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.     

Thermal chemistry of C4 hydrocarbons on Pt(111): Mechanism for double-bond isomerization

The thermal chemistry of a number of C4 hydrocarbons (1,3-butadiene, 1-bromo-3-butene, 1-bromo-2-butene, trans-2-butene, cis-2-butene, 1-butene, 2-iodobutane, 1-iodobutane, and butane) was investigated on clean and hydrogen- and deuterium-predosed Pt(111) single-crystal surfaces by temperature-programmed desorption and reflection-absorption infrared spectroscopy. A combination of rapid beta-hydride eliminations from alkyls to olefins and the reverse insertions of those olefins into metal-hydrogen bonds explains the hydrogenation, dehydrogenation, and H-D exchange products that desorb from the surface. A preference for hydrogenation at the end carbons and dehydrogenation from the inner carbons also explains the extent of the isotope exchange and the preferential isomerization of 1-butene to 2-butene observed on this Pt(111) surface. The reactions of more dehydrogenated C4 species is also discussed.     

A [4+2]-like cycloaddition of methyl methacrylate on Si(100)-2 x 1

The attachment of methyl methacrylate (MMA) on Si(100)-2x1 was investigated using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. The HREELS spectra of chemisorbed MMA show the disappearance of characteristic vibrations of C=O (1725 cm(-1)) and C(sp(2))-H (3110, 1400, and 962 cm(-1)) coupled with the blue shift of the C=C stretching mode by 34 cm(-1) compared to those of physisorbed molecules. These results clearly demonstrate that both C=C and C=O in MMA directly participate in the interaction with the surface to form a SiCH(2)C(CH(3))=C(OCH(3))OSi species via a [4+2]-like cycloaddition. This binding configuration was further supported by XPS, UPS, and DFT studies.     

V2O5/SiO2催化剂的异丁烷选择氧化反应性能

 采用溶胶－凝胶法和浸渍法制备了V2O5／SiO2催化剂,并用XRD,IR,TPD和活性评价等手段对催化剂的表面构造、化学吸附性能和异丁烷选择氧化反应性能进行了研究．结果表明：催化剂表面由Lewis碱位V＝O双键的端氧和Lewis酸位V5＋构成,异丁烷分子主要通过甲基中的H双位吸附在催化剂表面的Lewis碱位上,异丁烯分子可通过甲基的H吸附在催化剂表面的Lewis碱位,也可通过C＝C双键吸附在催化剂表面的Lewis酸位上．在常压条件下,异丁烷选择氧化产物主要有异丁烯、甲基丙烯醛和甲基丙烯酸,其中深度氧化产物CO2主要由通过C＝C吸附的异丁烯继续反应生成．     

Synthesis,spectroscopy, and catalytic properties of cationic organozirconium adsorbates on "super acidic" sulfated alumina. "Single-site" heterogeneous catalysts with virtually 100 active sites

Sulfated alumina (AlS), a highly Br?nsted acidic sulfated metal oxide, is prepared by the impregnation of gamma-alumina with 1.6 M H(2)SO(4), followed by calcination at 550 degrees C for 3 h. (13)C CPMAS NMR spectroscopy of the chemisorbed (13)C(alpha)-enriched organozirconium hydrocarbyl Cp'(2)Zr((13)CH(3))(2) (2)/AlS (Cp' = eta(5)-(CH(3))(5)C(5)) reveals that the chemisorption process involves M[bond]C sigma-bond protonolysis at the strong surface Br?nsted acid surface sites to yield a "cation-like" highly reactive zirconocenium electrophile, Cp'(2)Zr(13)CH(3)(+). In contrast, chemisorption of 2 on dehydroxylated alumina (DA) yields a similar cation via methide transfer to surface Lewis acid sites, while chemisorption onto dehydroxylated silica yields a mu-oxo Cp'(2)Zr((13)CH(3))-OSi[triple bond] species. Two complementary active site kinetic assays for benzene hydrogenation show that, unlike typical heterogeneous and supported organometallic catalysts, 97 +/- 2% of all Cp'ZrMe(3) (3)/AlS sites are catalytically significant, demonstrating that the species identified by (13)C CPMAS NMR is indeed the active species. 3/AlS mediates benzene hydrogenation with a turnover frequency of 360 h(-1) at 25 degrees C/1.0 atm H(2). Active site assays were also conducted for ethylene polymerization and reveal that 87 +/- 3% of 3/AlS sites are catalytically active, again demonstrating that nearly all zirconium sites are catalytically significant. Relative rates of ethylene homopolymerization mediated by the catalysts prepared via Cp(2)Zr(CH(3))(2) (1), Cp'(2)Zr(CH(3))(2) (2), Cp'Zr(CH(3))(3) (3), Zr(CH(2)TMS)(4) (4), and Zr(CH(2)Ph)(4) (5) (Cp = eta(5)-C(5)H(5)) chemisorption on AlS are 5/AlS > or = 4/AlS > or = 3/AlS > 2/AlS > or = 1/AlS for ethylene homopolymerization at 150 psi C(2)H(4), 60 degrees C. Under identical conditions, the polymerization rate for 3/DA is approximately 1/10th that for 3/AlS.     

Addition-elimination in the reaction of alpha-hydroxyalkyl radicals with 3,5-pyridinedicarboxylic acid and nicotinic acid: example of inner sphere organic electron transfer

Reactions of alpha-hydroxyalkyl radicals with 3,5-pyridinedicarboxylic acid (3,5-PDCA) and nicotinic acid (NA) were studied at appropriate pHs in aqueous solutions by pulse radiolysis technique. At pH 1, CH(3)C*HOH and *CH(2)OH radicals were found to react with 3,5-PDCA by rate constants of 2.2 x 10(9) and 5.1 x 10(8) dm(3) mol(-1) s(-1), respectively, giving radical adduct species. The adduct species formed in the reaction of CH(3)C*HOH radicals with 3,5-PDCA underwent unimolecular decay (k = 9.8 x 10(4) s(-1)) giving pyridinyl radicals. Reaction of (CH(3))(2)C*OH, CH(3)C*HOH, and *CH(2)OH radicals with NA at pH 3.3 gave the adduct species which subsequently decayed to the pyridinyl radicals. At pH 1, wherein NA is present in the protonated form, (CH(3))(2)C*OH radicals directly transfer electrons to NA, whereas CH(3)C*HOH and *CH(2)OH radicals react with higher rate constants compared with those at pH 3.3, initially giving the adduct species which subsequently undergo elimination reaction giving pyridinyl radicals. Reactions of alpha-hydroxyalkyl radicals with 3,5-pyridinedicarboxylic acid and nicotinic acid are found to proceed by an addition-elimination pathway that provides one of the few examples of organic inner sphere electron-transfer reactions. Rate constant for the addition reaction as well as rate of elimination varies with the reduction potential of alpha-hydroxyalkyl radicals.     

Investigations by (13)C NMR spectroscopy of ethene-initiated catalytic CO hydrogenation

13C NMR spectroscopy shows that the n-alkene and n-alkane products from the catalytic hydrogenation of CO in the presence of (13)C(2)H(4) probes over Ru/150 degrees C, Co/180 degrees C, Fe/220 degrees C, or Rh/190 degrees C (1 atm, CO:H(2) 1:1, "mild conditions") contain terminal (13)CH(3)(13)CH(2)- units. This is consistent with their formation by a regiospecific polymerization of C(1) species derived from CO and initiated by (13)C(2)H(4). Although the activities toward individual products differed somewhat, similar distributions and similar product labeling patterns were obtained over all the four catalysts. 1-Butene and the higher 1-n-alkenes from all the catalysts were largely (13)CH(3)(13)CH(2)(CH(2))(n)()CH=CH(2) (n = 0-3), propene formed over Ru or Co was (13)CH(3)(13)CH=CH(2), while both (13)CH(3)(13)CH=CH(2) and (13)CH(2)=(13)CHCH(3) were formed over Fe or Rh. Comparison of the conclusions from these probe experiments with those from isotope transient experiments by other workers indicates that the ethene initiator does not significantly modify the course of the CO hydrogenation. The reaction products are largely kinetically determined, and the primary products are mainly linear 1-n-alkenes, while the n-alkanes and 2-n-alkenes largely arise via secondary processes. Since the distribution of products and the labeling in them is so similar, it is concluded that one basic primary mechanism applies over all the four metals. Several different reaction paths involving a polymerization of surface methylene, [CH(2(ad))], have been proposed. Although the predictions based on several of these mechanisms agree with many of the results, the alkenyl + [CH(2(ad))] mechanism, initiated by a surface vinyl [CH(2)=CH((ad))], most easily accommodates the experimental evidence. An alternative path involving sequential addition of surface methylidyne and hydride either to a growing alkylidene chain (alkylidene + [CH(ad) + H(ad)]) or to an alkyl chain (alkyl + [CH((ad)) + H(ad)]) has recently been proposed by van Santen and Ciobica. The [CH(2(ad))] mechanism offers an easier explanation for the formation of the various alkenes, the distribution of products, and of the initiation, while the [CH(ad) + H(ad)] mechanism can explain any n-alkanes formed as primary products and not derived from alkenes. At higher reaction temperatures over Ru and Co, considerable (13)C(1) incorporation (from natural abundance in the CO and from cleavage of the (13)C(2)H(4) probe) was found in all the hydrocarbons. Thus, at higher temperatures (13)C(1(ad)) in addition to (13)C(2(ad)) species participate in both chain growth and initiation. In summary, adsorbed CO is transformed very easily into surface C(1(ad)), probably [CH(2(ad))] in equilibrium with [CH((ad))+H(ad)], which act as the propagating species.     

Predominant 1,2-insertion of styrene in the Pd-catalyzed alternating copolymerization with carbon monoxide.

The regioselectivity of styrene insertion to an acyl-Pd bond was studied by NMR in (i) a stoichiomeric reaction and (ii) a copolymerization with CO. In the stoichiometric reaction of styrene with [(CH(3)CO)Pd(CH(3)CN)[(R,S)-BINAPHOS]].[B[3,5-(CF(3))(2)C(6)H(3)](4)], both 1,2- and 2,1-products were given. To mimic the real polymerization conditions, a polyketone-substituted complex [[CH(3)(CH(2)CHCH(3)CO)(n)]Pd[(R,S)-BINAPHOS]].[B(3,5-(CF(3))(2)C(6)H(3))(4)] (n approximately 14) was prepared. When this polymer-attached Pd species was treated with styrene, the 1,2-insertion product was the only detectable species. Thus, exclusive 1,2-insertion is demonstrated to be responsible for the styrene-CO copolymerization, in sharp contrast to the predominant 2,1-insertion with conventional nitrogen ligands. Chain-end analysis revealed that beta-hydride elimination took place from the 2,1-complex but not from the 1,2-complex. Thus, once 2,1-insertion occurs, rapid beta-hydride elimination proceeds to terminate the polymerization, as is common to the other phosphorus-ligand systems. The resulting Pd-H species re-initiates the copolymerization, as was proven by MALDI-TOF mass analysis of the product copolymers.     

Ab initio studies of alkyl radical reactions: Combination and disproportionation reactions of CH3 with C2H5, and the decomposition of chemically activated C3H8

This paper reports the first quantitative ab initio prediction of the disproportionation/combination ratio of alkyl+alkyl reactions using CH3+C2H5 as an example. The reaction has been investigated by the modified Gaussian-2 method with variational transition state or Rice-Ramsperger-Kassel-Marcus calculations for several channels producing (1) CH4+CH2CH2, (2) C3H8, (3) CH4CH3CH, (4) H2+CH3CHCH2, (5) H2+CH3CCH3, and (6) C2H6+CH2 by H-abstraction and association/decomposition mechanisms through singlet and triplet potential energy paths. Significantly, the disproportionation reaction (1) producing CH4+C2H4 was found to occur primarily by the lowest energy path via a loose hydrogen-bonding singlet molecular complex, H3CHC2H4, with a 3.5 kcal/mol binding energy and a small decomposition barrier (1.9 kcal/mol), instead of a direct H-abstraction process. Bimolecular reaction rate constants for the formation of the above products have been calculated in the temperature range 300-3000 K. At 1 atm, formation of C3H8 is dominant below 1200 K. Over 1200 K, the disproportionation reaction becomes competitive. The sum of products (3)-(6) accounts for less than 0.3% below 1500 K and it reaches around 1%-4% above 2000 K. The predicted rate constant for the disproportionation reaction with multiple reflections above the complex well, k1=5.04 x T(0.41) exp(429/T) at 200-600 K and k1=1.96 x 10(-20) T(2.45) exp(1470/T) cm3 molecule(-1) s(-1) at 600-3000 K, agrees closely with experimental values. Similarly, the predicted high-pressure rate constants for the combination reaction forming C3H8 and its reverse dissociation reaction in the temperature range 300-3000 K, k2(infinity)=2.41 x 10(-10) T(-0.34) exp(259/T) cm3 molecule(-1) s(-1) and k(-2)(infinity)=8.89 x 10(22) T(-1.67)exp(-46 037/T) s(-1), respectively, are also in good agreement with available experimental data.     

Surface reactions of 1-propanethiol on GaAs(100)

The adsorption and decomposition pathways of 1-propanethiol on a Ga-rich GaAs(100) surface have been investigated using the techniques of temperature programmed desorption, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). 1-Propanethiol adsorbs dissociatively on a clean GaAs(100) surface to form propanethiolate and hydrogen. Further reactions of these species to form new products compete with the recombinative desorption of molecular propanethiol. The C-S bond scission in the propanethiolate results in the formation of propyl species and elemental sulfur. The generation of propene via beta-hydride elimination then follows. In addition, propane and hydrogen form via reductive elimination processes. A recombinative high-temperature propanethiol desorption state is also observed. XPS and TOF-SIMS analyses confirm the presence of sulfur on the GaAs(100) surface following thermal decomposition. This paper discusses the mechanisms by which these products form on the GaAs(100) surface.