Low-barrier Process Research Articles

Low-temperature hydrogenation of butanal proceeding via keto-enol tautomerization

Hydrogenation of carbonyl-containing compounds to alcohols is a crucial step in various technological processes, including reversible green hydrogen storage. Activation of the highly stable C=O bond, however, is highly challenging and typically proceeds under harsh reaction conditions. Recent theoretical predictions on low-barrier hydrogenation pathway of carbonyl compounds suggest that this step proceeding over a high activation barrier can be overcome in an alternative low barrier process, which comprises two steps: keto-enol tautomerization followed by hydrogenation of the C=C bond in the newly formed enol. The major challenge of this predicted pathway is the low stability of enol species, which would readily convert to ketone if not intentionally stabilized.In this study, we performed atomistic-level investigation on keto-enol tautomerization and low-temperature hydrogenation of butanal over well-defined Pd(111) model catalyst by combination of surface sensitive infrared reflection absorption spectroscopy (IRAS) and molecular beam techniques. Specifically we show that different types of enol species can be formed on the H-containing Pd(111) surface, whose appearance and abundance strongly dependent on the surface temperature. Importantly, the evolution of the partly hydrogenated product of butanal – C-butoxy species attached to Pd via C atom – was detected already at cryogenic temperatures, which points to a possibility of low temperature hydrogenation. A very clear correlation between the abundance of the enol species and the C-butoxy hydrogenated product was detected: the growing concentration of enol species was found to result in the formation of the C-butoxy product. In contrast, strong decrease in the abundance of enol adsorbates lead to full vanishing of the hydrogenation product C-butoxy. With this, we experimentally show a strong link between the population of the enol species and hydrogenation of butanal at cryogenic temperatures. Finally, we discuss the chemical nature of different enol species, which appear to be stabilized on the surface in different ways via hydrogen bonding, and propose the reaction mechanisms of low-barrier butanal hydrogenation that are consistent with all experimental observations.Obtained atomistic-level insights in low-barrier hydrogenation of carbonyl compounds provide important information on the elementary reaction steps of this potentially highly promising class of reactions for green hydrogen storage. Related phenomena can be employed for controlling keto-enol tautomerization and enol stabilization via mutual lateral interactions on functionalized catalytic surfaces for the broad range of aldehydes and ketones to enable their low-barrier hydrogenation.

Four-Electron Reduction of O2 Using Distibines in the Presence of ortho-Quinones.

This study, which aims to identify atypical platforms for the reduction of dioxygen, describes the reaction of O2 with two distibines, namely, 4,5-bis(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethylxanthene and 4,5-bis(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethyldihydroacridine, in the presence of an ortho-quinone such as phenanthraquinone. The reaction proceeds by oxidation of the two antimony atoms to the + V state in concert with reductive cleavage of the O2 molecule. As confirmed by 18O labeling experiments, the two resulting oxo units combine with the ortho-quinone to form an α,α,β,β-tetraolate ligand that bridges the two antimony(V) centers. This process, which has been studied both experimentally and computationally, involves the formation of asymmetric, mixed-valent derivatives featuring a stibine as well as a catecholatostiborane formed by oxidative addition of the quinone to only one of the antimony centers. Under aerobic conditions, the catecholatostiborane moiety reacts with O2 to form a semiquinone/peroxoantimony intermediate, as supported by NMR spectroscopy in the case of the dimethyldihydroacridine derivative. These intermediates swiftly evolve into the symmetrical bis(antimony(V)) α,α,β,β-tetraolate complexes via low barrier processes. Finally, the controlled protonolysis and reduction of the bis(antimony(V)) α,α,β,β-tetraolate complex based on the 9,9-dimethylxanthene platform have been investigated and shown to regenerate the starting distibine and the ortho-quinone. More importantly, these last reactions also produce two equivalents of water as the product of O2 reduction.

Anomalous temperature dependence of self-interstitial diffusivity in metallic lithium and sodium

Despite the abundance and importance of alkali metals for various applications, the behavior of point and dimensional defects in them has remained terra incognita. At the same time, the growing interest to high-capacity Li-metal batteries and the study of Li electrodeposition mechanisms dictates the need to investigate the defect dynamics. Here, we report the results of MD simulations of atomic motions in bulk Li and Na, including vacancy and interstitial (particularly <111>-crowdion) defect dynamics. Our main finding is an anomalous temperature dependence of the apparent diffusion coefficient for self-interstitial atoms (SIA), namely, the diffusivity decrease with the temperature rise. To uncover the underlying mechanism, we analyzed the interplay between 1D motions in terms of mean squared displacement (MSD) and 3D motions due to SIA rotations or switching. We found that the coefficient for 1D crowdion migration along the 〈111〉 axes is virtually temperature independent in case of both Li and Na. This implies linear temperature dependence of the drag coefficient γ, in contrast to other BCC metals. High rotation frequency reaching several THz at 300 K seems to drive the 1D diffusion into a semi-ballistic mode, therefore also affecting the temperature dependence of the apparent 3D diffusion coefficient. We believe that our results contribute to the atomistic understanding of low-barrier processes in a crystal lattice.

Insight into the Role of Active Site Protonation States and Water Molecules in the Catalytic Inhibition of DPP4 by Vildagliptin.

Vildagliptin (VIL) is an antidiabetic drug that inhibits dipeptidyl peptidase-4 (DPP4) through a covalent mechanism. The molecular bases for this inhibitory process have been addressed experimentally and computationally. Nevertheless, relevant issues remain unknown such as the roles of active site protonation states and conserved water molecules nearby the catalytic center. In this work, molecular dynamics simulations were applied to examine the structures of 12 noncovalent VIL-DPP4 complexes encompassing all possible protonation states of three noncatalytic residues (His126, Asp663, Asp709) that were inconclusively predicted by different computational tools. A catalytically competent complex structure was only achieved in the system with His126 in its ε-form and nonconventional neutral states for Asp663/Asp709. This complex suggested the involvement of one water molecule in the catalytic process of His740/Ser630 activation, which was confirmed by QM/MM simulations. Our findings support the suitability of a novel water-mediated mechanism in which His740/Ser630 activation occurs concertedly with the nucleophilic attack on VIL and the imidate protonation by Tyr547. Then, the restoration of His740/ Tyr547 protonation states occurs via a two-water hydrogen bonding network in a low-barrier process, thus describing the final step of the catalytic cycle for the first time. Additionally, two hydrolytic mechanisms were proposed based on the hydrogen bonding networks formed by water molecules and the catalytic residues along the inhibitory mechanism. These findings are valuable to unveil the molecular features of the covalent inhibition of DPP4 by VIL and support the future development of novel derivatives with improved structural or mechanistic profiles.

Self-Uptake Mechanism of Polymyxin-Based Lipopeptide against Gram-Negative Bacterial Membrane: Role of the First Adsorbed Lipopeptide.

Research in the continuously increasing threat of polymyxin-resistant multidrug-resistant Pseudomonas aeruginosa, which causes severe infection in immunocompromised patients, has resulted in the development of several polymyxin-derived cyclic lipopeptides containing l-α-γ- diamino butyric acid-like FADDI-019 (F19). In this work, F19's insertion into a minimal model of the asymmetric outer membrane of the bacterium, which contained only penta-acylated lipid A (LipA) and lacked keto-d-octulosonic acid and O-antigens, in the top leaflet and phospholipids in the bottom leaflet, was studied. F19 exhibited all of the hallmarks of the self-uptake mechanism into the asymmetric bilayer. While a single monomer of the lipopeptide did not get partitioned into the inside of the bilayer, it competitively displaced Ca2+ from the membrane surface, observed as a decrease in Ca2+ coordination number with phosphate groups (1.89 vs 1.718), resulting in membrane destabilization. This resulted in an increment of the average defect size and the probability of interplay between lipid tails and hydrophobic residues of another F19. When more than one monomer was present in the system, the first monomer remained docked on the surface, while other monomers intercalated into the bilayer interior with their hydrophobic moieties "sleeved" by lipid acyl chains. The free energy barrier for partial insertion of the lipopeptide into a bilayer in the presence of surface-docked second F19 was recorded at ∼1.3 kcal/mol using two-dimensional (2D) well-tempered metadynamics, making it a low barrier process at 300 K. This study is an attempt to demonstrate the self-uptake mechanism of F19 during intercalation process into the bilayer interior, which may help in the design of better alternates for polymyxins to work against polymyxin resistance.

Computer Modeling of Extended PnX3n + 2 Chains (X = F, Cl)

Quantum-chemical calculations of extended nonmetal chains of general formula PnX3n + 2 (X = F, Cl) and the corresponding infinite chains -(PX3−PX3)∞- have been performed. It has been shown that all high-symmetry PnX3n + 2 structures (n = 2–9) correspond to minima of the PES. Rotation around the P-P bond for fluoro derivatives is a low-barrier process; in the case, of chloro derivatives, it is accompanied by dissociation of the molecule. Calculations of the phonon spectrum for the infinite chain have demonstrated that both -(PX3−PX3)∞- structures exhibit dynamic stability. According to calculations of the electronic band structure, such chains are wide-gap semiconductors.

Fluorinated Vitrimer Elastomers with a Dual Temperature Response.

Vitrimers are an emerging new class of permanently cross-linked polymeric materials that show a liquid behavior upon heating wherein the macroscopic deformation is controlled by the rate of internal chemical bond exchange reactions. Thus, quite uniquely among polymeric materials, flow rates and material viscosities can be enhanced or controlled by the addition of catalysts and additives. We now report a catalyst-free vitrimer system, prepared from mixing two simple components, wherein two competing bond exchange mechanisms coexist, each showing a strikingly different temperature dependence, related to the large difference in activation energy for the different exchange pathways (60 vs 130-170 kJ/mol). The low barrier process is predominant at lower temperatures, but is outcompeted by the high barrier process that becomes dominant at higher temperatures because of its much more pronounced temperature dependence. The result is an interesting and highly unusual dual viscosity profile for this new class of vitrimer materials: a very gradual decrease in viscosity at lower temperatures, intercepted by a much sharper drop in viscosity at higher temperatures. The highly counterintuitive effect where a higher barrier pathway is dominant over a much lower barrier process can be rationalized by the exchange mechanisms that involve different reactive species, but lead to the overall same exchange. We observed this unusual but highly promising behavior first for fluorinated vitrimer elastomers, aimed at high performance materials, but the effect was also shown to hold in related nonfluorinated elastomers. A new way to control and design the rheological behavior of vitrimers toward finely tuned and precisely controlled processing applications has thus been provided.

An energetic and kinetic investigation of the role of different atomic grain boundaries in healing radiation damage in nickel

In this paper, we investigate the role that the atomic structure of grain boundaries (GBs) in Ni has in the interaction with point defects produced during the collision cascades. Using molecular dynamics and statics, we study the influence of different GB structures on the production and evolution of defects, through calculation of defect formation energy, vacancy diffusion barrier, interstitial-vacancy annihilation barrier, and the corresponding energetic and kinetic influence ranges near pristine and damaged GBs. We observe that, as previously reported, GBs preferentially absorb interstitials over vacancies, which result in accumulation of vacancies in the bulk grain and interstitials to highly localize at the GBs. Also, we find that, in several layers near the GBs, the formation energy of defects is substantially reduced, indicating that the GBs serve as a sink for the radiation-produced defects. Further investigation also reveals that the sink strength of the GBs is significantly stronger for interstitials than vacancies. Also in terms of kinetic behavior, we find that the energy barrier of vacancy diffusion reduces as the defect migrates to the close vicinity of the GBs, implying that in the presence of thermally assisted events, vacancies diffuse toward the GBs via low-barrier processes, which result in healing of the damaged boundaries at high temperatures. Moreover, we observe that in the neighboring region of the defect-loaded GBs, the annihilation of a close vacancy–interstitial pair is spontaneous. Finally, via continuous ion bombardment simulation, we study the stability of the GBs in severe irradiation condition. We find that the GBs lose their stability as the number of trapped interstitials increases at the GB plane. Besides, we conclude that the GBs have high efficiency in defect removal on picosecond time scale and act as efficient defect sinks, despite the instability and saturation observed in repeated ion bombardments.

Hydrogen Bonding Constrains Free Radical Reaction Dynamics at Serine and Threonine Residues in Peptides

Free radical-initiated peptide sequencing (FRIPS) mass spectrometry derives advantage from the introduction of highly selective low-energy dissociation pathways in target peptides. An acetyl radical, formed at the peptide N-terminus via collisional activation and subsequent dissociation of a covalently attached radical precursor, abstracts a hydrogen atom from diverse sites on the peptide, yielding sequence information through backbone cleavage as well as side-chain loss. Unique free-radical-initiated dissociation pathways observed at serine and threonine residues lead to cleavage of the neighboring N-terminal Cα-C or N-Cα bond rather than the typical Cα-C bond cleavage observed with other amino acids. These reactions were investigated by FRIPS of model peptides of the form AARAAAXAA, where X is the amino acid of interest. In combination with density functional theory (DFT) calculations, the experiments indicate the strong influence of hydrogen bonding at serine or threonine on the observed free radical chemistry. Hydrogen bonding of the side-chain hydroxyl group with a backbone carbonyl oxygen aligns the singly occupied π orbital on the β-carbon and the N-Cα bond, leading to low-barrier β-cleavage of the N-Cα bond. Interaction with the N-terminal carbonyl favors a hydrogen-atom transfer process to yield stable c and z(•) ions, whereas C-terminal interaction leads to effective cleavage of the Cα-C bond through rapid loss of isocyanic acid. Dissociation of the Cα-C bond may also occur via water loss followed by β-cleavage from a nitrogen-centered radical. These competitive dissociation pathways from a single residue illustrate the sensitivity of gas-phase free radical chemistry to subtle factors such as hydrogen bonding that affect the potential energy surface for these low-barrier processes.

Computational studies of the mechanistic aspects of olefin metathesis reactions involving metal oxo-alkylidene complexes

The reactions of Cl 2(O)MCH 2(M = Cr, Mo, W, Ru, Re) with ethylene, models of potential chain-carrying catalysts and olefins respectively in the metal-catalyzed olefin reaction, have been studied using hybrid density functional theory at the B3LYP/LACVP * and MO6/LACVP * levels of theory. It was found that formation of the metallacyclobutane, a key intermediate in the olefin metathesis reaction according to the Herrison–Chauvin mechanism, is a low-barrier process for each of the complexes studied, the highest barrier occurring in Ru (13.78 kcal/mol by B3LYP calculations and 4.74 kcal/mol by MO6 calculations) and the lowest barrier occurring in W (0.38 kcal/mol by B3LYP and 0.28 kcal/mol by MO6). However, for M = Cr the [3+2] addition of the olefinic bond of ethylene across the Cr O and Cr C bonds of the complex has a lower activation barrier than the metallacyclobutane formation step. Since the potential chain-terminating side reaction is more feasible than the metallacyclobutane formation step, Cl 2(O)CrCH 2 may not promote olefin metathesis. Also, for M = Ru the activation barrier of the [3+2] addition of ethylene across the Ru O and Ru Cl bonds is 3.61 kcal/mol lower than the barrier of the metallacyclobutane formation step, at the B3LYP level of theory, suggesting that Cl 2(O)RuCH 2 may not promote olefin metathesis. However, the MO6 results indicate that the barrier for metallacyclobutane formation is 3.55 kcal/mol lower than that for the [3+2] addition across the Ru O and Ru Cl bonds, suggesting that Cl 2(O)RuCH 2 may catalyze metathesis. At either level of theory, the difference in activation barriers between the productive [2+2] route and the [3+2] route is not high enough to preclude either pathway from manifesting. Thus, metallacyclobutane formation may occur in the Ru complex, albeit very unselectively. However, the metallacyclobutane intermediate formed is very stable (31.45 kcal/mol and 42.57 kcal/mol by B3LYP and MO6 respectively) and may not easily undergo cycloreversion to form the final product, i.e. a dead end in the catalytic cycle. For M = Mo, W, and Re, the metallacyclobutane formation pathway is more favorable than the potential side reactions, the differences between the productive [2+2] pathway and the lowest-barrier side reactions being 19.30, 25.87, 3.54 kcal/mol respectively at the B3LYP level and 13.82, 18.81 and 4.06 kcal/mol respectively at the MO6 level. Thus, the selectivity of the metallacyclobutane formation, which is marginal in Re, is in the order: W > Mo > Re. However, the metallacyclobutane formed in Re may be too stable to allow cycloreversion to form the final product. Thus metathesis is predicted to occur in Mo and W but not in Cr, Ru or Re.

A computational study of methanol-to-hydrocarbon conversion — Towards the design of a low-barrier process

Computational quantum chemistry has been employed to examine the production of ethylene with methanol-to-hydrocarbon (MTH) processes via a carbon pool mechanism. We find that the M05-2X functional performs well for the types of reactions that are involved. The methylation reactions of the aromatic cocatalyst are the most energy-demanding steps in the process. For the subsequent production of C2H4, we have identified a low-energy pathway that involves multiple methyl shifts, followed by concerted deprotonation and C2H4 elimination. The substitutions of the Al and Si atoms in the participating Si–OH–Al moiety of zeolite catalysts with Ga and Ge do not lead to lower barriers for the methylation reactions, nor does the use of a more electron-rich aromatic cocatalyst. However, we find that the use of two cocatalysts, a nucleophile and an aromatic carbon pool, can provide an overall low-energy pathway for the MTH process.

Complex behavior in a simple system: Low-temperature Ag/Ag(100) growth revisited

The experimentally observed nonmonotonic temperature dependence of the surface roughness in Ag/Ag(100) growth over the temperature range $T=55--180\text{ }\text{K}$ is examined. In general, we find that the surface roughness depends sensitively on a competition between a variety of low-barrier processes including downward funneling of depositing atoms, island relaxation via edge zipping and edge diffusion, atom attraction, and concerted interlayer diffusion at kinks. The short-range attraction of depositing atoms to microprotrusions also plays a crucial role in determining the surface roughness, especially at low temperature. By taking these processes into account in our simulations, good agreement with experiment is obtained over the entire temperature range.

Density functional theory studies of the mechanistic aspects of olefin metathesis reactions

The potential energy surfaces of the reactions of Cl(4)MCH(2) (M = Cr, Mo, W, Ru, Re) with ethylene, models of potential chain-carrying catalysts and olefins respectively in the transition metal-catalyzed olefin metathesis reaction, have been explored using density functional theory at the B3LYP/LACVP* level of theory. In Cl(4)MCH(2) (M = Cr, Ru), the carbenoid complexes Cl(3)MCH(2)-Cl have been found to be more stable than the corresponding carbene Cl(4)M[double bond, length as m-dash]CH(2) complexes whereas in Cl(4)MCH(2) (M = Mo, W, Re) the carbene complexes are more stable than the carbenoid complexes. The carbenoid complexes have been found not to favor the formation of metallacyclobutanes, a key step in the olefin metathesis reaction according to the Herrison-Chauvin mechanism, indicating that the active species for the metathesis reaction is a carbene complex and not a carbenoid complex. Therefore, even though the formation of metallacyclobutanes through formal [2+2] cycloaddition has been found to be a low-barrier process with each of the metal carbene complexes investigated, metathesis is likely to occur only in Cl(4)MCH(2)(M = Mo, W, Re) but not in Cl(4)MCH(2)(M = Cr, Ru) since the reaction surface in the latter complexes is likely to be populated by the carbenoid complex rather than the carbene complex. In Cl(4)W=CH(2) the kinetic and thermodynamic preference of the productive [2+2] pathway leading to the metallacyclobutane over the [3+2] pathway is unambiguous whereas in Cl(4)MoCH(2) the [3+2] pathway is likely to be competitive with the [2+2] pathway. The metallacyclobutane formation in W and Re has been found to have lower barriers than in Mo, suggesting that the W and Re complexes may have a greater metathesis activity than the Mo complex.

Combined Experimental and Theoretical Approach To Understand the Reactivity of a Mononuclear Cu(II)−Hydroperoxo Complex in Oxygenation Reactions

A copper(II) complex bearing a pentadentate ligand, [Cu(II)(N4Py)(CF(3)SO(3))(2)] (1) (N4Py = N,N-bis(2-pyridylmethyl)bis(2-pyridyl)methylamine), was synthesized and characterized with various spectroscopic techniques and X-ray crystallography. A mononuclear Cu(II)-hydroperoxo complex, [Cu(II)(N4Py)(OOH)](+) (2), was then generated in the reaction of 1 and H(2)O(2) in the presence of base, and the reactivity of the intermediate was investigated in the oxidation of various substrates at -40 degrees C. In the reactivity studies, 2 showed a low oxidizing power such that 2 reacted only with triethylphosphine but not with other substrates such as thioanisole, benzyl alcohol, 1,4-cyclohexadiene, cyclohexene, and cyclohexane. In theoretical work, we have conducted density functional theory (DFT) calculations on the epoxidation of ethylene by 2 and a [Cu(III)(N4Py)(O)](+) intermediate (3) at the B3LYP level. The activation barrier is calculated to be 39.7 and 26.3 kcal/mol for distal and proximal oxygen attacks by 2, respectively. This result indicates that the direct ethylene epoxidation by 2 is not a plausible pathway, as we have observed in the experimental work. In contrast, the ethylene epoxidation by 3 is a downhill and low-barrier process. We also found that 2 cannot be a precursor to 3, since the homolytic cleavage of the O-O bond of 2 is very endothermic (i.e., 42 kcal/mol). On the basis of the experimental and theoretical results, we conclude that a mononuclear Cu(II)-hydroperoxo species bearing a pentadentate N5 ligand is a sluggish oxidant in oxygenation reactions.

Activation mechanism of a signaling protein at atomic resolution from advanced computations.

Advanced computational techniques including transition path sampling and free energy calculations are combined synergistically to reveal the activation mechanism at unprecedented resolution for a small signaling protein, chemotaxis protein Y. In the conventional "Y-T coupling" model for response regulators, phosphorylation induces the displacement of the conserved Thr87 residue through hydrogen-bond formation, which in turn makes it sterically possible for Tyr106 to isomerize from a solvent exposed configuration to a buried rotameric state. More than 160 unbiased activation trajectories show, however, that the rotation of Tyr106 does not rely on the displacement of Thr87 per se. Free energy calculations reveal that the Tyr106 rotation is a low-barrier process in the absence of the Thr87-phosphate hydrogen bond, although the rotation is stabilized by the formation of this interaction. The simulations also find that structural change in the beta4-alpha4 loop does not gate the Tyr106 rotation as suggested previously; rather, the rotation of Tyr106 stabilizes the activated configuration of this loop. The computational strategy used and mechanistic insights obtained have an impact on the study of signaling proteins and allosteric systems in general.

Catalytic Mechanism of Dopamine β-Monooxygenase Mediated by Cu(III)−Oxo

Mechanisms of dopamine hydroxylation by the Cu(II)-superoxo species and the Cu(III)-oxo species of dopamine beta-monooxygenase (DBM) are discussed using QM/MM calculations for a whole-enzyme model of 4700 atoms. A calculated activation barrier for the hydrogen-atom abstraction by the Cu(II)-superoxo species is 23.1 kcal/mol, while that of the Cu(III)-oxo, which can be viewed as Cu(II)-O*, is 5.4 kcal/mol. Energies of the optimized radical intermediate in the superoxo- and oxo-mediated pathways are 18.4 and -14.2 kcal/mol, relative to the corresponding reactant complexes, respectively. These results demonstrate that the Cu(III)-oxo species can better mediate dopamine hydroxylation in the protein environment of DBM. The side chains of three amino acid residues (His415, His417, and Met490) coordinate to the Cu(B) atom, one of the copper sites in the catalytic core that plays a role for the catalytic function. The hydrogen-bonding network between dopamine and the three amino acid residues (Glu268, Glu369, and Tyr494) plays an essential role in substrate binding and the stereospecific hydroxylation of dopamine to norepinephrine. The dopamine hydroxylation by the Cu(III)-oxo species is a downhill and lower-barrier process toward the product direction with the aid of the protein environment of DBM. This enzyme is likely to use the high reactivity of the Cu(III)-oxo species to activate the benzylic C-H bond of dopamine; the enzymatic reaction can be explained by the so-called oxygen rebound mechanism.

Theoretical evidence for fast H-divacancy rotation on H/Pd(1 1 1)

By ab initio calculations, we show that when two H vacancies occupy neighboring fcc sites on a hydrogen-covered Pd(1 1 1) surface, a very low-barrier process exists, leading to a fast effective rotation of the divacancy. As a consequence, divacancies are likely to display a three lobed shape in typical STM images, as reported in a recent experimental work [T. Mitsui, M.K. Rose, E. Fomin, D.F. Ogletree, M. Salmeron, Nature 422 (2003) 705]. Our results allow for a better understanding of the experimental data, in showing the atomic-scale path responsible for the rotation and in providing a theoretical estimate of the activation energy of the process.

Outer sphere anion participation can modify the mechanism for conformer interconversion in Pd pincer complexes

Interconversion of the two chiral conformations of the square planar Pd(II) CCC pincer carbene complex, 1 (η3-C,C′,C″) (2,6-bis{[N-methyl-N′-methylene]imidazol-2-ylidene}phenyl)bromopalladium(II), and the CNC cation, 2, (η3-C,C′,N)(2,6-bis{[N-methyl-N′-methylene]imidazol-2-ylidene}pyridine)bromopalladium(II)(1+), is characterized by VT NMR spectroscopy. Combined DFT/experimental work indicates two alternative mechanisms. In the case of 1, having no counterion, and several derivatives of 2 with weakly nucleophilic counterions, the fluxional process goes in two steps via an unsymmetrical cationic 4-coordinate intermediate. In this case one carbene ring moves through the square plane before the other. In some cases for 2 with more nucleophilic counterions, such as [{CNC}PdI]I, a second lower-barrier process takes over that depends on the nature of the counterion. We propose that the outer sphere anion reversibly displaces the central N (pyridine) unit of the pincer in a rate limiting step to form a neutral dihalo intermediate that undergoes rapid conformer interconversion. This accounts for the counterion dependence and constitutes an unusual type of fluxionality that couples anion substitution at the metal with the conformational change of the ligand. A pyridine, even when present as the central element of a CNC pincer ligand, can therefore be labile even under mild conditions and reaction mechanisms involving decoordination of such group are therefore possible.

Probing high-barrier pathways of surface reactions by scanning tunneling microscopy.

The ability of scanning tunneling microscopy to probe the pathways of thermally activated high-barrier surface processes is frequently limited by competing low-barrier processes that can confuse measurement of the true initial and final configuration. We introduce an approach to circumvent this difficulty by driving the surface process with nanosecond laser heating. The method is applied to determine the pathway of recombinative desorption in the H/Si(001) system. The observed configuration of dangling bonds after laser heating reveals that the desorbed hydrogen molecules are not formed on single dimers, but rather from neighboring silicon dimers via an interdimer reaction pathway.

Effect of coordinated water on the mechanism of neutral hydrolysis of silicon dioxide in gas phase: a first principles study

Ab initio quantum chemical studies are reported at the B3LYP/6-311++G(d,p), MP2/6-311++G(d,p), and MP2/cc-pVTZ levels for the neutral hydrolysis of silicon dioxide in gaseous phase. These mechanistic studies predict that hydrolysis of silicon dioxide to orthosilicic acid is significantly controlled by the coordinated water molecule at the silicon. The catalytic activity of second water molecule coordinated to silicon lowers the reaction barrier by more than 50% than the first water molecule. The overall reaction is a low barrier process with barrier heights <2 kcal/mol at the MP2/cc-pVTZ level. This is in contrast to the hydration of carbon dioxide, where the second coordination at the carbon is less feasible. At all the three levels of calculations, the reaction follows similar trend.