Subsequent Hydrogen Transfer Research Articles

Enhancing reductive conversion of levulinic acid and levulinates to γ-valerolactone: Role of oxygen vacancy in MnOx catalysts

Oxygen vacancies (Ov) in metal oxides play a crucial role in modifying the electronic and acidic properties of catalysts, thereby influencing their catalytic activity. This study explores the impact of Ov in MnOx catalysts on their acidic and catalytic properties for the Meerwein–Ponndorf–Verley reduction of levulinic acid (LA) and levulinate to γ-valerolactone (GVL). Various characterization techniques demonstrate that surface Ov significantly modulate the acidic properties of MnOx catalysts, positively correlating with Lewis/Brønsted acid ratio and GVL yield. In situ DRIFTS and DFT calculations further unveil the reaction mechanism, revealing that Ov facilitate the activation and dehydrogenation of isopropanol and subsequent hydrogen transfer and hydrogenation of LA, leading to enhanced GVL production. These insights underscore the pivotal role of Ov in MnOx catalysts for the efficient conversion of LA to GVL, highlighting their importance in improving catalytic performance.

Porous Composite‐Mediated Bimetallic Cluster POMs/Zr‐MOF for Catalytic Transfer Hydrogenation of Biomass‐Derived Aldehydes and Ketones

AbstractSynergistic catalytic effects and metal node modulation are pivotal to the performance of multifunctional catalysts.We present a versatile strategy for constructing bimetallic M1M2 cluster catalysts (polyoxometalates (POMs)/Zr‐based metal‐organic frameworks (Zr‐MOF); M1 = Zr; M2 = W, V, Mo) by a microwave‐assisted in‐situ precoordination‐postmodification approach, which consisted of the grafting of POM single clusters onto Zr–O clusters of hierarchical porous composites (defective Zr‐MOF (HUiO‐66x)) and the modulation of metal nodes in Zr‐MOF. Hierarchical porosity and double active centers in the POMs/Zr‐MOF result in greater accessibility to catalytic sites and adjustable acid–base properties, thus providing opportunities for the catalytic transfer hydrogenation of biomass‐derived aldehydes/ketones. The synergistic acid–base couple sites (W6+–O2−–Zr4+) not only bridge hydrogen donors into the reaction system but also promote the dissociation of α‐C(sp3)‐H from isopropanol and subsequent hydrogen transfer to the activated carbonyl in furfural. Enhanced substrate adsorption and decreased activation barrier of the rate‐determining step are keys for the direct hydrogen transfer from carbonyl compounds to near‐quantitative corresponding alcohols via an eight‐membered‐ring transition state to achieving unprecedented low‐temperature catalytic performance over bimetallic Zr–W clusters. The proposed general method of bimetallic cluster catalysts reveals the roles of accessible metal modulation and synergistic catalytic centers toward specific catalysis.

Beyond the triple bond: unlocking dinitrogen activation with tailored superbase phosphines.

Activating atmospheric dinitrogen (N2), a molecule with a remarkably strong triple bond, remains a major challenge in chemistry. This theoretical study explores the potential of superbase phosphines, specifically those decorated with imidazolin-2-imine ((ImN)3P) and imidazolin-2-methylidene ((ImCH)3P) to facilitate N2 activation and subsequent hydrazine (H2NNH2) formation. Using density functional theory (DFT) at the M06L/6-311++G(d,p) level, we investigated the interactions between these phosphines and N2. Mono-phosphine-N2 complexes exhibit weak, noncovalent interactions (-0.6 to -7.1 kcal mol-1). Notably, two superbasic phosphines also form high-energy hypervalent complexes with N2, albeit at significantly higher energies. The superbasic nature and potential for the hypervalency of these phosphines lead to substantial N2 activation in bis-phosphine-N2 complexes, where N2 is "sandwiched" between two phosphine moieties through hypervalent P-N bonds. Among the phosphines studied, only (ImN)3P forms an exothermic sandwich complex with N2, stabilized by hydrogen bonding between the ImN substituents and the central N2 molecule. A two-step, exothermic hydrogen transfer pathway from (ImN)3P to N2 results in the formation of a bis-phosphine-diimine (HNNH) sandwich complex. Subsequent hydrogen transfer leads to the formation of a bis-phosphine-hydrazine (H2NNH2) complex, a process that, although endothermic, exhibits surmountable activation barriers. The relatively low energy requirements for this overall transformation suggest its potential feasibility under the optimized conditions. This theoretical exploration highlights the promise of superbase phosphines as a strategy for metal-free N2 activation, opening doors for the development of more efficient and sustainable nitrogen fixation and utilization methods.

Total Synthesis of Immunosuppressive Mycestericin E and G Enabled by a Highly Stereoselective Nitroso-Ene Cyclization.

This report describes a streamlined synthesis of immunosuppressive mycestericin E and G through a highly stereoselective nitroso-ene cyclization in 11-12 steps using readily available materials. The stereochemical outcome in the formation of a Nα-quaternary stereogenic center is rationalized by a trajectory based on the polar diradical intermediate and subsequent hydrogen transfer. Julia olefination offers a facile chain elongation method that presents a viable strategy for structural derivatization in future medicinal applications.

A nucleotide-copper(II) complex possessing a monooxygenase-like catalytic function.

The de novo design of artificial biocatalysts with enzyme-like active sites and catalytic functions has long been an attractive yet challenging goal. In this study, we present a nucleotide-Cu2+ complex, synthesized through a one-pot approach, capable of catalyzing ortho-hydroxylation reactions resembling those of minimalist monooxygenases. Both experimental and theoretical findings demonstrate that the catalyst, in which Cu2+ coordinates with both the nucleobase and phosphate moieties, forms a ternary-complex intermediate with H2O2 and tyramine substrates through multiple weak interactions. The subsequent electron transfer and hydrogen (or proton) transfer steps lead to the ortho-hydroxylation of tyramine, where the single copper center exhibits a similar function to natural dicopper sites. Moreover, Cu2+ bound to nucleotides or oligonucleotides exhibits thermophilic catalytic properties within the temperature range of 25 °C to 75 °C, while native enzymes are fully deactivated above 35 °C. This study may provide insights for the future design of oxidase-mimetic catalysts and serve as a guide for the design of primitive metallocentre-dependent enzymes.

Rational Design of Main Group Metal-Embedded Nitrogen-Doped Carbon Materials as Frustrated Lewis Pair Catalysts for CO2 Hydrogenation to Formic Acid.

Developing efficient and inexpensive main group catalysts for CO2 conversion and utilization has attracted increasing attention, as the conversion process would be both economical and environmentally benign. Here, based on the main group element Al, we designed several heterogeneous frustrated Lewis acid/base pair (FLP) catalysts and performed extensive first-principles calculations for the hydrogenation of CO2. These catalysts, including Al@N-Gr-1, Al@N-Gr-2, and Al@C2N, are composed of a single Al atom and two-dimensional (2D) N-doped carbon-based materials to form frustrated Al/C or Al/N Lewis acid/base pairs, which are all predicted to have high reactivity to absorb and activate hydrogen (H2). Compared with Al@N-Gr-1, both Al@N-Gr-2 and Al@C2N, especially Al@N-Gr-2, containing Al/N Lewis pairs exhibit better catalytic activity for CO2 hydrogenation with lower activation energies. CO2 hydrogenation on the three catalysts prefers to go through a three-step mechanism, i.e., the heterolytic dissociation of H2, followed by the transfer of the hydride near Al to CO2, and finally the activation of a second H2 molecule. Other IIIA group element (B and Ga)-embedded N-Gr-2 materials (B@N-Gr-2 and Ga@N-Gr-2) were also explored and compared. Both Al@N-Gr-2 and Ga@N-Gr-2 show higher catalytic activity for CO2 hydrogenation to HCOOH than B@N-Gr-2. However, the CO2 hydrogenation path on Ga@N-Gr-2 tends to follow a two-step mechanism, including H2 dissociation and subsequent hydrogen transfer. The present study provides a potential solution for CO2 hydrogenation by designing novel and effective FLP catalysts based on main group elements.

The catalytic mechanism of intercalated chlorine anions as active basic sites in MgAl-layered double hydroxide for carbonyl sulfide hydrolysis.

In order to make clear the role of intercalated anions in layered double hydroxides (LDHs) for catalytic hydrolysis of carbonyl sulfide (COS), the adsorption and reaction characteristics of COS over the simple Mg2Al-Cl-LDH model catalyst were studied by both theoretical and experimental methods. Density functional theory (DFT) calculations by CASTEP found that the chloride ions in LDH function as the key Brønsted base sites to activate the adsorbed H2O with enlarged bond length and angle, facilitate the dissociative adsorption of intermediates including mono-thiocarbonic acid (MTA) and hydrogen thiocarbonic acid (HTA), and participate in the formation of transient states and subsequent hydrogen transfer process with decreased energy barriers during COS hydrolysis. COS hydrolysis will preferentially go through the dissociated intermediates of mono-thiocarbonates (MT) and hydrogen thiocarbonates (HT) with dramatically decreased energy barriers, and the rate-determining step of COS hydrolysis over Mg2Al-Cl-LDH will be the nucleophilic addition of C=O in COS by H2O (Ea = 1.10 eV). The experimental results further revealed that the apparent activation energy (0.89 eV) of COS hydrolysis over Mg2Al-Cl-LDH is close to theoretical value (1.10 eV), and the accumulated intermediates of MT, HT, or carbonate were also observed by FT-IR around 1363 cm-1 on the used Mg2Al-Cl-LDH, which are well in accordance with the theoretical prediction. The demonstrated participation of intercalated chlorine anions in the evolution of intermediates and transient states as Brønsted base sites during COS hydrolysis will give new insight into the basic sites in LDH materials.

Reactions of a Silylyne Complex with Aldehydes: Formation of W−Si−O−C Four‐Membered Metallacycles and Their Metathesis‐Like Fragmentation

A tungsten silylyne complex having a W≡Si triple bond reacted with two molecules of aldehydes at room temperature to give W-Si-O-C four-membered metallacycles by [2+2] cycloaddition and subsequent formyl hydrogen transfer from one aldehyde molecule to another. Upon heating to 70 °C, the four-membered metallacycles underwent metathesis-like fragmentation cleanly to afford carbyne complexes and "silanoic esters," in a manner similar to that of metallacyclobutadiene, an intermediate of alkyne metathesis reactions, and dimerization of the latter products gave 1,3-cyclodisiloxanes. The "silanoic ester" was also trapped by pivalaldehyde to give a [2+2] cycloaddition product in high yield.

Hydrogen transfer between dimethyl ether and the methoxy radical: Understanding and kinetic modeling with anharmonic torsions

Hybrid density functionals M06-2X and BMK with the MG3S basis set, as well as double-hybrid density functional B2PLYP with the TZVP basis set, were employed to study the reaction fundamental and kinetics of hydrogen-abstraction from dimethyl ether (DME) by the methoxy radical. The density functional energetics was validated against the calculations of the CBS-QB3, G4, and G4MP2 composite methods. Thermal rate constants were evaluated using the multi-structural canonical variational transition-state theory (MS-CVT) with multidimensional tunneling effect over the temperature range 200–2800K. It is found that the CH3O+DME reaction proceeds via a pre-reaction CH3O–DME adducts with subsequent intramolecular hydrogen transfer to form post-reaction CH3OH–CH3OCH2 complex and dissociation to produce the separate CH3OH and CH3OCH2. Torsions in transition state are found to be significantly coupled to generate five conformations whose contributions considerably influence the rate constant predictions and are included in the total partition functions via the multi-structural treatment. Variational effects on computed rate constants are observed to be negligibly small and tunneling effect quickly becomes insignificant with increasing temperature. Finally, the four-parameter Arrhenius expression 8.50×1011(T/300)0.58 exp[−3660.61(T−22.27)/(T2+22.272)] cm3mol−1s−1 was used to describe the temperature dependences of the M06-2X/MG3S rate constants evaluated by MS-CVT including small-curvature tunneling correction over the entire temperature range.

Theoretical insights into the cycloaddition reaction mechanism between ketenimine and methyleneimine: An alternative approach to the formation of pyrazole and imidazole

The cycloaddition reaction mechanism between interstellar molecules, ketenimine and methyleneimine, has been systematically investigated employing the second-order Møller-Plesset perturbation theory (MP2) method in order to better understand the reactivity of nitrogenous cumulene ketenimine with the C =N double bond compound methyleneimine. Geometry optimizations and vibrational analyses have been performed for the stationary points on the potential energy surfaces of the system. Calculations show that five-membered cyclic carbene intermediates could be produced through pericyclic reaction processes between ketenimine and methyleneimine. Through the subsequent hydrogen transfer processes, carbene intermediates can be isomerized to the pyrazole and imidazole compounds, respectively. The present study is helpful to understand the formation of prebiotic species in interstellar space. The cycloaddition reaction mechanism between interstellar molecules, ketenimine and methyleneimine, has been systematically investigated theoretically. The products of this reaction are pyrazole and imidazole compounds, respectively.

ChemInform Abstract: An Efficient Transformation of Primary Halides into Nitriles Through Palladium‐Catalyzed Hydrogen Transfer Reaction.

Two-step one-pot transformation of primary halides into corresponding nitriles is successfully achieved. Nucleophilic substitution of primary halides with sodium azide and subsequent palladium-catalyzed hydrogen transfer proceeds smoothly in the presence of sterically bulky ligand dicyclohexyl(2′,4′,6′-triisopropylbiphenyl-2-yl)phosphine (XPhos) in acetone to produce nitriles in satisfactory to good yields.

Barium-mediated cross-dehydrocoupling of hydrosilanes with amines: a theoretical and experimental approach.

Alkaline-earth (most prominently barium) complexes of the type [Ae{N(SiMe3 )2 }2 ⋅(THF)x ] and [{N^N}Ae{N(SiMe3 )2 }⋅(THF)x ] are very active and productive precatalysts (TON=396, TOF up to 3600 h(-1) ; Ca<Sr<Ba) for NH/HSi cross-dehydrocoupling, with excellent chemoselectivity in the reaction of (di)amines with (di)hydrosilanes. Experimental and DFT investigations revealed that the reactions proceed by nucleophilic attack of a metal amide at the incoming silane and subsequent turnover-limiting hydrogen transfer to the metal center.

A DFT study on the mechanism of palladium-catalyzed divergent reactions of 1,6-enyne carbonates

The reaction mechanisms of palladium-catalyzed divergent reactions of 1,6-enyne carbonates have been investigated using DFT calculations at the B3LYP/6-31G(d,p) (LanL2DZ for Pd) level. Solvent effects on these reactions have been considered by the polarizable continuum model (PCM) for the solvent (DMF). The formation of vinylidenepyridines and vinylidenepyrrolidines were generated through 5-exo-dig cyclization or 6-endo-dig cyclization. Our calculation results suggested the following: (i) The first step of the whole cycle is the rate-determining step, which causes allenic palladium intermediate through two plausible pathways. This intermediate provides the corresponding products and releases the palladium catalyst by a subsequent hydrogen transfer and elimination process. (ii) For the catalyst CH 3OPdH, the reaction could occur through two possible pathways, but 5-exo-dig cyclization is favoured over 6-endo-dig cyclization. However, when the hydrogen atom is substituted with a phenyl group, the energy barriers for 5-exo-dig cyclization or 6-endo-dig cyclization become relatively high, 18.0–28.5 kcal/mol. The computational results provide good explanation for the experimental observations. The reaction mechanisms of palladium-catalyzed divergent reactions of 1,6-enyne carbonates have been investigated using DFT calculations. The formation of vinylidenepyridines and vinylidenepyrrolidines were generated through 5-exo-dig cyclization or 6-endo-dig cyclization. The computational results provide good explanation for the experimental observations of Liang group.

Unusual Mechanism for the Reaction of a Niobocene Hydride Complex with Activated Alkynes. Experimental and DFT Studies

The reactivity of the activated alkynes RC≡CR (R = CO2Me, CO2tBu) with the Nb–H bond of the 18e– niobocene hydride trimethyl phosphite complex [Nb(η5-C5H4SiMe3)2(H)(P(OMe)3)] (1) has been studied, and the new (E)-niobocene alkenyl trimethyl phosphite complexes [Nb(η5-C5H4SiMe3)2(C(CO2R)═C(CO2R)H)(P(OMe)3)] (R = Me (2a), tBu (2b)) have been obtained. DFT studies were carried out to identify the lowest energy mechanism and the possible intermediates in the process. An unusual nucleophilic d2 Nb(III) center was identified as a σ-donor entity responsible for the nucleophilic attack on a carbon atom of the activated (electrophilic) alkyne molecules. A subsequent hydrogen transfer to the other carbon atom produces the final (E)-niobocene alkenyl trimethyl phosphite complexes, the stereochemistries of which were elucidated by NMR spectroscopy.

Catalytic hydroconversion of extraction residue from Shengli lignite over Fe–S/ZSM-5

A highly dispersive Fe/ZSM-5 as the precursor of Fe–S/ZSM-5 was prepared by decomposing Fe(CO)5 onto ZSM-5 zeolite at 250°C. Shengli lignite (SL) was isolated to extractable portion (EP) and extraction residue (ER) by exhaustive extraction with isometric carbon disulfide/acetone mixed solvent (IMCDSAMS) under ultrasonic irradiation. The EP and ER were analyzed with a direct analysis in real time ionization source coupled to ion trap mass spectrometer. Both of them and SL were analyzed with a Fourier transform infrared spectrometer. The ER was subjected to non-catalytic hydroconversion (NCHC) and catalytic hydroconversion (CHC) over Fe–S/ZSM-5, which was formed by in situ reaction of Fe/ZSM-5 with sulfur added, in cyclohexane under pressurized hydrogen at 300°C. The reaction mixtures were sequentially extracted with petroleum ether, carbon disulfide, methanol, acetone, and IMCDSAMS to afford extracts 1–5 (E1–E5), which were analyzed with a gas chromatography/mass spectrometer (GC/MS). As model reactions, the CHCs of di(1-naphthyl)methane (DNM) and diphenylmethane (DPM) over Fe–S/ZSM-5 were also conducted in cyclohexane at 300°C. The results show that more than 50% of organic matter in the ER was converted to a soluble portion by the CHC, whereas the yield of soluble portion from the NCHC is only 10.52%. GC/MS-detectable species only appear in the E1, which mainly consists of alkanes, arenes, and arenols either from the NCHC or from the CHC. DNM was selectively hydrocracked to naphthalene and 1-methylnaphthalene over Fe–S/ZSM-5, while DPM hydrocracking did not proceed, indicating that Fe–S/ZSM-5 catalyzed the formation of radical hydrogen from molecular hydrogen and subsequent radical hydrogen transfer (RHT) to the ipso-position of naphthalene ring in DNM. Some CH2 and O in the ER could be connected to condensed aromatic rings (CARs). Analogously, RHT to the ipso-position of CARs in the ER resulted in the release of arenes and phenols from the ER.

DFT Studies on the Methane Elimination Reaction of a Trinuclear Rare-Earth Polymethyl Complex: σ-Bond Metathesis Assisted by Cooperation of Multimetal Sites

DFT studies have been performed for the process of methane elimination via an intramolecular C–H bond activation of the polymethyl trinuclear rare-earth-metal complex [(η5-C5Me4SiMe3)Tm(μ2-CH3)2]3. It has been found that intermetallic cooperation plays an important role in achieving such an intramolecular C–H bond activation process, and the cooperation of trimetallic centers makes the C–H bond activation kinetically easier in comparison with that of bimetallic centers. The methane elimination reaction occurs through rearrangement of Tm–CH3 connections and subsequent C–H bond activation. The C–H bond activation is the rate-determining step of the whole process, and the corresponding transition state is characterized by a σ-bond metathesis assisted by cooperation of multimetal sites. Such a C–H bond activation step was accomplished by a change in the coordination manner (from μ2 to μ3 fashion) of a methyl group and a subsequent hydrogen transfer, and the two events are asynchronous concerted processes. The computational results also suggest that the reactivity of the C–H bond of the metal-connected CH3 group follows the order μ3-C(H2)–H > μ2-C(H2)–H > μ1-C(H2)–H. This is unlike the case of a metal–CH3 bond, where the terminal methyl (monometal-connected μ1-CH3) is generally more reactive than multimetal-connected methyls. In addition, the effect of 4f electrons of such trinuclear organolanthanide compounds on the geometry and energy profile studied here is also discussed.

Mechanism of Silver(I)‐Catalyzed Enantioselective Synthesis of Axially Chiral Allenes Based on Propargylamines

AbstractThe silver(I)‐catalyzed synthesis picture of axially chiral allenes based on propargylamines has been outlined using density functional theory (DFT) method for the first time. Our calculations find that, the coordination of silver(I) into triple bond of propargylamines at anti‐position of nitrogen shows a stronger activation on the triple bond than that at cis‐position, which is favorable for the subsequent hydrogen transfer. The NBO charge analysis for the hydrogen transfer affirms the experimental speculation that this step is a hydride transfer process. The energy barrier of the anti‐periplanar elimination of vinyl‐silver is 26.9 kJ·mol−1 lower than that of the syn‐periplanar elimination, supporting that (−)‐allene is the main product of this reaction. In a word, the most possible route for this reaction is that the silver(I) coordinates into the triple bond of propargylamines at anti‐position of nitrogen, then the formed silver(I) complex undergoes a hydride transfer to give a vinyl‐silver, finally the vinyl‐silver goes through an anti‐periplanar elimination to give (−)‐allene. The hydride transfer with the energy barrier of 44.8 kJ·mol−1 is the rate‐limiting step in whole catalytic process. This work provides insight into why this reaction has a very high enantioselectivity.

Reactions of Aromatic N-Heterocycles with d0fn-Metal Alkyl Complexes Supported by Chelating Diamide Ligands

Aromatic heterocycles are a prominent feature within natural products and pharmaceuticals and considerable efforts are directed toward their synthesis and functionalization. These molecules also appear as unwanted impurities in carbon-based fuels, and processes that fragment them are of increasing interest. Early transition metal-carbon bonds show diverse reactivity toward aromatic heterocycles: researchers have reported both functionalization, relevant to synthetic efforts, and ring opening, relevant to their removal from fuels. In particular, chelating ferrocene-diamides possess unique electronic characteristics as ancillary ligands that enable a wide range of reactivity behaviors for the resulting metal complexes. In this Account, we describe our efforts to understand the reactivity of group 3 metal and uranium alkyl complexes supported by these organometallic ligands toward aromatic N-heterocycles. Two geometrically related ancillary ligands were investigated: 1,1'-ferrocenylene-diamides and pincer-type pyridine-diamides. A substrate-dependent behavior was observed. For example, all the benzyl metal complexes cleaved 1-methylimidazole. In the case of pyridines, differences in reactivity were identified: C-H activation and C-C coupling occurred with substituted pyridines, while alkyl transfer predominated with isoquinoline and chelating pyridines. The products of the C-C coupling or the alkyl-transfer reactions underwent subsequent hydrogen transfer: within the same ring for the substituted pyridines and between two different heterocycles for isoquinoline and chelating pyridines. The comparison between yttrium and lutetium benzyl complexes supported by ferrocene- or pyridine-diamide ligands indicated that similar reactions occurred for specific substrates (1-methylimidazole, 2-picoline, and isoquinoline). A broader range of reaction types and a larger substrate scope were identified, however, for the ferrocene than for the pyridine-type complexes. Based on the reactions discussed in this Account and on isolated examples drawn from the literature, we conclude that the ferrocene-diamides represent a versatile ligand framework. We propose that iron's ability to accommodate changes in the electronic density at the metal center more readily than classical supporting ligands leads to the privileged status of these organometallic ancillary ligands.

DFT and ONIOM(DFT:MM) Studies on Co−C Bond Cleavage and Hydrogen Transfer in B12-Dependent Methylmalonyl-CoA Mutase. Stepwise or Concerted Mechanism?

The considerable protein effect on the homolytic Co-C bond cleavage to form the 5'-deoxyadenosyl (Ado) radical and cob(II)alamin and the subsequent hydrogen transfer from the methylmalonyl-CoA substrate to the Ado radical in the methylmalonyl-CoA mutase (MMCM) have been extensively studied by DFT and ONIOM(DFT/MM) methods. Several quantum models have been used to systematically study the protein effect. The calculations have shown that the Co-C bond dissociation energy is very much reduced in the protein, compared to that in the gas phase. The large protein effect can be decomposed into the cage effect, the effect of coenzyme geometrical distortion, and the protein MM effect. The largest contributor is the MM effect, which mainly consists of the interaction of the QM part of the coenzyme with the MM part of the coenzyme and the surrounding residues. In particular, Glu370 plays an important role in the Co-C bond cleavage process. These effects tremendously enhance the stability of the Co-C bond cleavage state in the protein. The initial Co-C bond cleavage and the subsequent hydrogen transfer were found to occur in a stepwise manner in the protein, although the concerted pathway for the Co-C bond cleavage coupled with the hydrogen transfer is more favored in the gas phase. The assumed concerted transition state in the protein has more deformation of the coenzyme and the substrate and has less interaction with the protein than the stepwise route. Key factors and residues in promoting the enzymatic reaction rate have been discussed in detail.

Nanometer-thick SGOI structures produced by Ge+ ion implantation of SiO2 films and subsequent hydrogen transfer of Si layers

Nanometer-thick silicon-germanium-on-insulator (SGOI) structures have been produced by the implantation of Ge+ ions into thermally grown SiO2 layer and subsequent hydrogen transfer of silicon film on the Ge+ ion implanted substrate. The intermediate nanometer-thick Ge layer has been formed as a result of the germanium atom segregation at the Si/SiO2 bonding interface during annealing at temperatures 800–1100оС. From a thermodynamic analysis of Si/Ge/SiO2 system, it has been suggested that the growth of the epitaxial Ge layer is provided by the formation of a molten layer at the Si/SiO2 interface due to the Ge accumulation. The effect of germanium on the hole mobility in modulation-doped heterostructures grown over the 3–20nm thick SGOI layers was studied. An increase in the Hall hole mobility in SGOI-based structures by a factor of 3–5 was obtained in comparison with that in respective Ge-free SOI structures.