High Reaction Barrier Research Articles

Ab Initio Calculations of Thermal Decomposition Mechanism of LiPF6-Based Electrolytes for Lithium-Ion Batteries

Ab initio molecular orbital calculations were done to examine the thermal decomposition mechanism of LiPF6-based electrolyte such as LiPF6 dissolved into carbonate ester. We focused our study on LiPF6 decomposition (LiPF6 + H2O → LiF + POF3 + 2HF) and ethylene carbonate decomposition (EC + POF3 → CO2 + CH2FCH2OPF2O). Intrinsic reaction coordinate calculations showed that the latter reaction has a significantly high reaction barrier more than 3 eV. However, we found that it may be reduced by 0.97 eV by considering PF6− ion which acts as a kind of catalyst in the reaction. Although POF3 triggers EC ring opening reaction unless both Li+ and PF6− ions are considered in the model, it remains a spectator in the C-O bond breaking reaction of diethyl carbonate (DEC) by PF6−.

Theoretical Investigations on Decomposition of HCOOH Catalyzed by Pd7 Cluster

Density functional theory based calculations have been performed to investigate decomposition of HCOOH on a Pd(7) cluster in vacuum and solution. The adsorption of HCOOH on Pd(7) cluster occurs on a layer-by-layer quasi-planar conformation of Pd(7) with 4 atoms on top and 3 atoms below. Possible reaction pathways for the decomposition of HCOOH adsorbed on Pd(7) cluster in vacuum and solution are located and compared in terms of the reaction enengies and barriers. Formic acid prefers to decompose through dehydrogenation rather than dehydrate under the significant effect of solvent. The toxic species, CO generated on Pt surface, could not possibly appear in the catalytic decomposition of formic acid on Pd(7) cluster due to high reaction barrier, thus no poisoning of catalyst would occur on Pd surface. The Pd(7) cluster model rationalizes experimental observation, and the predictions are in good agreement with the ones based on the surface model.

Combined studies of DFT atomistic modelling and in situ FTIR spectroscopy on surface oxidants and CO oxidation at Ru electrodes

We report the combined studies of density functional theory (DFT) calculations and electrochemical in situ FTIR spectroscopy on surface oxidants and mechanisms of CO oxidation at the Ru(0001) electrodes. It is shown that CO can co-adsorb with both O and OH species at lower potential region where a low coverage of the (2×2)-O/OH adlayer formed; the oxidation of CO adsorbates takes place at higher potentials where a high coverage of the (1×1)-O/OH adlayer formed. Surface O species are not the active oxidants under all coverages studied, due to the high reaction barriers between CO and O (>1eV). However, surface OH species with higher coverage are identified as the active oxidants, and CO oxidation takes place via a two-steps’ mechanism of CO+3OH→COOH+2OH→CO2+H2O+OH, in which three nearby OH species are involved in the CO2 formation: CO reacts with OH, forming COOH; COOH then transfers the H to a nearby OH to form H2O and CO2, at the same time, another H in the H2O transfers to a nearby OH to form a weak adsorbed H2O and a new OH. The reaction barrier of these processes is reduced significantly to around 0.50eV. These new results not only provide an insight into surface active oxidants on Ru, which is directly relevant to fuel cell catalysis, but also reveals the extra complexity of catalytic reactions taking place at solid/liquid electrochemical interface in comparison to the relatively simpler ones at solid/gas phase.

Hydrogen bonding and reactivity of water to azines in their S1 (n,π*) electronic excited states in the gas phase and in solution

A unified picture is presented of water interacting with pyridine, pyridazine, pyrimidine, and pyrazine on the S(1) manifold in both gas-phase dimers and in aqueous solution. As (n,π*) excitation to the S(1) state removes electrons from the ground-state hydrogen bond, this analysis provides fundamental understanding of excited-state hydrogen bonding. Traditional interpretations view the excitation as simply breaking hydrogen bonds to form dissociated molecular products, but reactive processes such as photohydrolysis and excited-state proton coupled electron transfer (PCET) are also possible. Here we review studies performed using equations-of-motion coupled-cluster theory (EOM-CCSD), multireference perturbation theory (CASPT2), time-dependent density-functional theory (TD-DFT), and excited-state Monte Carlo liquid simulations, adding new results from symmetry-adapted-cluster configuration interaction (SAC-CI) and TD-DFT calculations. Invariably, gas-phase molecular dimers are identified as stable local minima on the S(1) surface with energies less than those for dissociated molecular products. Lower-energy biradical PCET minima are also identified that could lead to ground-state recombination and hence molecular dissociation, dissociation into radicals or ions, or hydration reactions leading to ring cleavage. For pyridine.water, the calculated barriers to PCET are low, suggesting that this mechanism is responsible for fluorescence quenching of pyridine.water at low energies rather than accepted higher-energy Dewar-benzene based "channel three" process. Owing to (n,π*) excitation localization, much higher reaction barriers are predicted for the diazines, facilitating fluorescence in aqueous solution and predicting that the as yet unobserved fluorescence from pyridazine.water and pyrimidine.water should be observable. Liquid simulations based on the assumption that the solvent equilibrates on the fluorescence timescale quantitatively reproduce the observed spectral properties, with the degree of (n,π*) delocalization providing a critical controlling factor.

Characterization of hydrogen dissociation over aluminium-doped zinc oxide using an efficient massively parallel framework for QM/MM calculations

A task-farm parallelization framework has been implemented in the ChemShell computational chemistry environment to provide a facility for parallelizing common chemical calculations, including finite-difference Hessian evaluation, the nudged elastic band method for reaction path optimization, and population-based methods for global optimization. The optimization methods are provided by a parallel interface to the DL-FIND optimization library. As ChemShell can already exploit parallel external programs for energy and gradient evaluations, the new methods result in a two-level approach to parallelization that gives significantly improved performance for massively parallel calculations. For typical systems, speed-up factors of five to eight times have been observed compared with non-task-farmed calculations. The task-farming version of ChemShell has been used to study the heterolytic dissociation of a hydrogen molecule over a polar oxygen-terminated surface of aluminium-doped zinc oxide using an embedded cluster hybrid QM/MM approach. We calculate a 42 kcal mol −1 heat of reaction and a 30 kcal mol −1 activation energy, which is equivalent to a high backward reaction barrier of 72 kcal mol −1 per H 2 molecule, in close agreement with temperature programmed desorption experiments. The dissociation path includes a stable intermediate comprising a hydride ion in an oxygen vacancy and physisorbed atomic hydrogen.

Diffractive and reactive scattering of H2 from Ru(0001): experimental and theoretical study

We present a combined experimental and theoretical study of the diffraction of H(2) from Ru(0001) in the incident energy range 78-150 meV, and a theoretical study of dissociative chemisorption of H(2) in the same system. Pronounced out-of-plane diffraction was observed in the whole energy range studied. The energy dependence of the elastic diffraction intensities was measured along the two main symmetry directions for a fixed parallel translational energy. The data were compared with quantum dynamics calculations performed by using DFT-based, six-dimensional potential energy surfaces calculated with both the PW91 and RPBE functionals, as well as with a functional obtained from a weighted average of both (the MIX functional, which was earlier shown to perform quite well for H(2) + Cu(111)). Our results show that the PW91 functional describes the H(2) diffraction intensities more accurately than the RPBE and the MIX functionals, although the absolute values of these intensities are overestimated in the calculations. For the reaction probabilities a preference for one or the other functional cannot be given over the entire energy range probed by the sticking experiments. The PW91 functional yields too high reaction probabilities over the entire investigated energy range, but is better than RPBE at low collision energies (<0.1 eV). The RPBE functional gives too low reaction probabilities at low energy and somewhat too high reaction probabilities at high energy, but agrees better with experiment than PW91 for energies >0.1 eV. The results suggest that, in order to get a better description of both H(2) diffraction and dissociative chemisorption for this system, a specific reaction parameter functional for H(2) + Ru(0001) is needed that is a weighted average of functionals other than PW91 and RPBE. We speculate that differences between the H(2) + Ru(0001) system (early and low reaction barrier) and H(2) + Cu(111) (late and high reaction barrier) may well lead to fundamentally different specific reaction parameter functionals, and that including a reasonable accurate description of the van der Waals interaction might be important for H(2) + Ru(0001) which has barriers localised far away from the surface. Based on our results we advocate new, systematic combined theoretical and experimental studies of H(2) interacting with transition metals in early and late barrier systems, with the aim of determining whether specific reaction parameter functionals for these systems might differ in a systematic way.

Matrix Metalloproteinase 2 (MMP2) Inhibition: DFT and QM/MM Studies of the Deprotonation-Initialized Ring-Opening Reaction of the Sulfoxide Analogue of SB-3CT

(4-Phenoxyphenylsulfonyl)methylthiirane (SB-3CT) is the selective inhibitor of matrix metalloproteinase 2 (MMP2). The inhibition mechanism of MMP2 by SB-3CT involves C-H deprotonation with concomitant opening of the three-membered heterocycle. In this study, the energetics of the deprotonation-induced ring-opening of (4-phenoxyphenylsulfinyl)methylthiirane, the sulfoxide analogue of SB-3CT, are examined computationally using DFT and QM/MM calculations. A model system, 2-(methylsulfinylmethyl)thiirane, is used to study the stereoelectronic and conformational effects of reaction barriers in methanol. For the model system in methanol solution (using the polarizable continuum model), the reaction barriers range from 17 to 23 kcal/mol with significant stereoelectronic effects. However, the lowest barriers of the (R,R) and (S,R) diastereomers are similar. Two diastereomers of the sulfoxide analogue of SB-3CT are studied in the active site of MMP2 by QM/MM methods with an accurate partial charge fitting procedure. The ring-opening reactions of these two diastereomers have similar reaction energetics. Both are exothermic from the reactant to the ring-opening product (thiolate). The protonation of the thiolate by a water molecule is endothermic in both cases. However, the deprotonation/ring-opening barriers in the MMP2 active site using QM/MM methods for the (R,R) and (S,R) inhibitions are quite different (23.3 and 28.5 kcal/mol, respectively). The TSs identified in QM/MM calculations were confirmed by vibrational frequency analysis and following the reaction path. The (R,R) diastereomer has a hydrogen bond between the sulfoxide oxygen and the backbone NH of Leu191, while the (S,R) has a hydrogen bond between the sulfoxide oxygen and a water molecule. The dissimilar strengths of these hydrogen bonds as well as minor differences in the TS structures contribute to the difference between the barriers. Compared to SB-3CT, both diastereomers of the sulfoxide analogue have higher reaction barriers and have less exothermic reaction energies. This agrees well with the experiments, where SB-3CT is a more effective inhibitor of MMP2 than its sulfoxide analogue.

Double Group Transfer Reactions: Role of Activation Strain and Aromaticity in Reaction Barriers

Double group transfer (DGT) reactions, such as the bimolecular automerization of ethane plus ethene, are known to have high reaction barriers despite the fact that their cyclic transition states have a pronounced in-plane aromatic character, as indicated by NMR spectroscopic parameters. To arrive at a way of understanding this somewhat paradoxical and incompletely understood phenomenon of high-energy aromatic transition states, we have explored six archetypal DGT reactions using density functional theory (DFT) at the OLYP/TZ2P level. The main trends in reactivity are rationalized using the activation strain model of chemical reactivity. In this model, the shape of the reaction profile DeltaE(zeta) and the height of the overall reaction barrier DeltaE( not equal)=DeltaE(zeta=zeta(TS)) is interpreted in terms of the strain energy DeltaE(strain)(zeta) associated with deforming the reactants along the reaction coordinate zeta plus the interaction energy DeltaE(int)(zeta) between these deformed reactants: DeltaE(zeta)=DeltaE(strain)(zeta)+DeltaE(int)(zeta). We also use an alternative fragmentation and a valence bond model for analyzing the character of the transition states.

A note on the vibrational efficacy in molecule-surface reactions

The effectiveness of vibrational energy in promoting dissociation of molecules colliding with surfaces can be measured through the so-called vibrational efficacy. It is thought by many to be a pure "energetic" measure and therefore believed to be limited from below by zero (in the case that there is no increase in dissociation probability upon vibrational excitation) and from above by one (in the case that all of the vibrational excitation energy is used to promote reaction). However, the quantity vibrational efficacy is clearly linked to the detailed dynamics of the system, and straightforward considerations lead to the conclusion that it is not limited either from below or above. Here we discuss these considerations together with a quasiclassical dynamics study of a molecule-surface system, N(2)/Ru(0001), for which a vibrational efficacy bigger than one has been found both experimentally and theoretically. We show that an analysis of the vibrational efficacy only in terms of energy transfer from vibration to translation can be too simple to describe the behavior of systems for which the potential energy surfaces present (high) reaction barriers, potential corrugation and anisotropy, and curved reaction paths.

Why Do Cysteine Dioxygenase Enzymes Contain a 3-His Ligand Motif Rather than a 2His/1Asp Motif Like Most Nonheme Dioxygenases?

Density functional theory calculations on the oxygen activation process in cysteine dioxygenase (CDO) and three active site mutants whereby one histidine group is replaced by a carboxylic acid group are reported. The calculations predict an oxygen activation mechanism that starts from an Fe(III)-O-O(*) complex that has close lying singlet, triplet, and quintet spin states. A subsequent spin state crossing to the quintet spin state surfaces leads to formation of a ring-structure whereby an O-S bond is formed. This weakens the central O-O bond, which is subsequently broken to give sulfoxide and an iron-oxo complex. The second oxygen atom is transferred to the substrate after a rotation of the sulfoxide group. A series of calculations were performed on cysteine dioxygenase mutants with a 2His/1Asp motif rather than a 3His motif. These calculations focused on the differences in catalytic and electronic properties of nonheme iron systems with a 3His ligand system versus a 2His/1Asp motif, such as taurine/alpha-ketoglutarate dioxygenase (TauD), and predict why CDO has a 3His ligand system while TauD and other dioxygenases share a 2His/1Asp motif. One mutant (H86D) had the ligand trans to the dioxygen group replaced by acetate, while in another set of calculations the ligand trans to the sulfur group of cysteinate was replaced by acetate (H88D). The calculations show that the ligands influence the spin state ordering of the dioxygen bound complexes considerably and in particular stabilize the quintet spin state more so that the oxygen activation step should encounter a lower energetic cost in the mutants as compared to WT. Despite this, the mutant structures require higher O-O bond breaking energies. Moreover, the mutants create more stable iron-oxo complexes than the WT, but the second oxygen atom transfer to the substrate is accomplished with much higher reaction barriers than the WT system. In particular, a ligand trans to the sulfur atom of cysteine that pushes electrons to the iron will weaken the Fe-S bond and lead to dissociation of this bond in an earlier step in the catalytic cycle than the WT structure. On the other hand, replacement of the ligand trans to the dioxygen moiety has minor effects on cysteinate binding but enhances the barriers for the second oxygen transfer process. These studies have given insight into why cysteine dioxygenase enzymes contain a 3His ligand motif rather than 2His/1Asp and show that the ligand system is essential for optimal dioxygenation activity of the substrate. In particular, CDO mutants with a 2His/1Asp motif may give sulfoxides as byproduct due to incomplete dioxygenation processes.

Rearrangement and Hydrogen Scrambling Pathways of the Toluene Radical Cation: A Computational Study

A computational study is undertaken to provide a unified picture for various rearrangement reactions and hydrogen scrambling pathways of the toluene radical cation (1). The geometries are optimized with the BHandHLYP density functional, and the energies are computed with the ab initio CCSD(T) method, in conjunction with the 6-311+G(d,p) basis set. In particular, four channels have been located, which may account for hydrogen scrambling, as they are found to have overall barriers lower than the observed threshold for hydrogen dissociation. These are a stepwise norcaradiene walk involved in the Hoffman mechanism, a rearrangement of 1 to the methylenecyclohexadiene radical cation (5) by successive [1,2]-H shifts via isotoluene radical cations, a series of [1,2]-H shifts in the cycloheptatriene radical cation (4), and a concerted norcaradiene walk. In addition, we have also investigated other pathways such as the suggested Dewar-Landman mechanism, which proceeds through 5, via two consecutive [1,2]-H shifts. This pathway is, however, found to be inactive as it involves too high reaction barriers. Moreover, a novel rearrangement pathway that connects 5 to the norcaradiene radical cation (3) has also been located in this work.

On the Mechanism and Stereochemistry of Chiral Lithium‐Carbenoid‐Promoted Cyclopropanation Reactions

An investigation into the mechanism and stereochemistry of chiral lithium-carbenoid-promoted cyclopropanation reactions by using density functional theory (DFT) methods is reported. Previous work suggested that this type of cyclopropanation reaction may proceed by competition between a methylene-transfer mechanism and a carbometalation mechanism. In this paper, it is demonstrated that the intramolecular cyclopropanation reactions promoted by chiral carbenoids 1 and 2 proceed by the methylene-transfer mechanism. The carbometalation mechanism was found to have a much higher reaction barrier and does not appear to compete with the methylene-transfer mechanism. The Lewis base group does not enhance the carbometalation pathway enough to compete with the methylene-transfer pathway. The present computational results are consistent with experimental observations for these cyclopropanation reactions. The factors governing the stereochemistry of the intramolecular cyclopropanation reaction by the methylene-transfer mechanism were examined to help elucidate the origin of the stereoselectivity observed in experiments. Both the directing group and the configuration at the C(1) centre were found to play a key role in the stereochemistry. Carbenoid 1 has a chiral C(1) centre of R configuration. The Lewis base group directs the cyclization of carbenoid 1 to form a single product. In contrast, the Lewis base group cannot direct the cyclization of carbenoid 2 to furnish a stereoselective product due to the S configuration of the chiral C(1) centre in carbenoid 2. This relationship of the stereochemistry to the chiral character of the carbenoid has implications for the design of new efficient carbenoid reagents for stereoselective cyclopropanation.

Understanding the Dehydrogenation Mechanism of Tetrahydrocarbazole over Palladium Using a Combined Experimental and Density Functional Theory Approach

The mechanism of the dehydrogenation of tetrahydrocarbazole to carbazole over palladium has been examined for the first time. By use of a combination of deuterium exchange experiments and density functional theory calculations, a detailed reaction profile for the aromatization of tetrahydrocarbazole has been identified and validated by experiment. As with many dehydrogenation reactions, the initial hydrogen abstraction is found to have the highest reaction barrier. Tetrahydrocarbazole has four hydrogens which can, in principle, be cleaved initially; however, the theory and experiment show that the reaction is dominated by the cleavage of the carbon hydrogens at the carbon atoms in positions 1 and 4. The two pathways originating from these two C−H bond cleavage processes are found to have similar reaction energy profiles and both contribute to the overall reaction.

Quantum approaches for the insertion dynamics of the H++D2 and D++H2 reactive collisions

The H(+)+D(2) and D(+)+H(2) reactive collisions are studied using a recently proposed adiabatic potential energy surface of spectroscopic accuracy. The dynamics is studied using an exact wave packet method on the adiabatic surface at energies below the curve crossing occurring at approximately 1.5 eV above the threshold. It is found that the reaction is very well described by a statistical quantum method for a zero total angular momentum (J) as compared with the exact ones, while for higher J some discrepancies are found. For J >0 different centrifugal sudden approximations are proposed and compared with the exact and statistical quantum treatments. The usual centrifugal sudden approach fails by considering too high reaction barriers and too low reaction probabilities. A new statistically modified centrifugal sudden approach is considered which corrects these two failures to a rather good extent. It is also found that an adiabatic approximation for the helicities provides results in very good agreement with the statistical method, placing the reaction barrier properly. However, both statistical and adiabatic centrifugal treatments overestimate the reaction probabilities. The reaction cross sections thus obtained with the new approaches are in rather good agreement with the exact results. In spite of these deficiencies, the quantum statistical method is well adapted for describing the insertion dynamics, and it is then used to evaluate the differential cross sections.

Theoretical study of the Wanzlick equilibrium

A theoretical study of the tetraaminoethylene–diaminocarbene dissociation reaction shows that equilibrium is more readily attained for benzimidazol-2-ylidenes and doubly bridged imidazol-2-ylidenes. For imidazolin-2-ylidenes, the ‘Wanzlick equilibrium’ is less likely to occur due to the high reaction barrier. We demonstrate that the equilibrium reaction can be catalyzed by an electrophile.

Mechanism of Reppe's nickel-catalyzed ethyne tetramerization to cyclooctatetraene: a DFT study.

In this B3 LYP model study, homoleptic nickel(0) ethyne complexes have been predicted as the catalyst resting state for the title reaction. Ethyne ligand coupling of Ni(C(2)H(2))(3) yields monoethyne nickelacyclopentadiene in the rate-determining step. Ethyne coordination is followed by insertion of an ethyne ligand into the Ni--C sigma bond. A highly strained monoethyne trans-nickelacycloheptatriene is formed. This trans intermediate is unable to reductively eliminate benzene without prior isomerization to a cis-structure. Instead, it rapidly collapses to a nickelacyclononatetraene. Ethyne coordination induces reductive elimination to the cyclooctatetraene complex Ni(eta(2)-C(2)H(2))(eta(2)-C(8)H(8)), followed by facile ligand exchange. Other ethyne coupling pathways have been computed to be less favored. The cyclooctatetraene ligand binds significantly weaker to nickel(0) than ethyne, both for mononuclear, and for dinuclear species. For this reason, C--C bond formation steps at Ni(2)(micro-cot) fragments have been predicted to feature prohibitively high overall reaction barriers.

Ab initio study on the spin-forbidden reaction HNO( 1A′)+H 2→NH( 3Σ)+H 2O

The spin-forbidden reaction HNO( 1A′)+H 2→NH( 3Σ)+H 2O was investigated by ab initio and DFT calculations. We first investigated the HNO+H 2→NH( 1Δ)+H 2O singlet reaction and found three possible pathways with significantly high reaction barriers. The lowest barrier height was calculated to be 193.3 kJ/mol at the MP4/cc-pVTZ//MP2/cc-pVTZ level. A crossing between the singlet and triplet potential energy surfaces occurs at the product region after which the NH 2OH complex is formed. The energy at the crossing point was estimated to be lower than the barrier height of the formation of the NH 2OH complex. It is then concluded that the reaction of HNO+H 2→NH( 3Σ)+H 2O proceeds by the addition of H atom of the H 2 molecule to the N atom of HNO molecule and forms the NH 2OH complex through two transition states and an intermediate on the singlet PES, and finally crosses with the triplet surface.

Density Functional Theory Studies of Ruthenium-Catalyzed Bis-Diels−Alder Cycloaddition of 1,5-Cyclooctadiene with Alkynes

Density functional theory calculations at the B3LYP level have been performed to study the reaction mechanism of the Ru-catalyzed cycloaddition of 1,5-cyclooctadiene (COD) with alkynes. Our calculations point toward the proposed mechanism that the cycloaddition reaction occurs via an intermediate formed by the active species [CpRu(COD)]+ and an alkyne substrate molecule. The first C−C coupling step was found to be the rate-determining step. The active species [CpRu(COD)]+, which is crucial for the catalytic reaction, can be obtained through the ionization of Cl- from [CpRu(COD)Cl] under the polar solvent of MeOH. The extra π⊥ bond of alkynes in comparison to alkenes has been found to play a key role in stabilizing the relevant reaction intermediate as well as lowering the reaction barriers. In the olefin case, the absence of the extra π⊥ bond leads to the instability of the corresponding intermediate as well as the high reaction barriers. Reaction mechanisms that are deemed possible on the basis of known fundamental organometallic reactions, such as oxidative coupling, ligand isomerization, and Cp ring slippage, were found to have higher reaction barriers.

Beyond Schmittel and Myers-Saito cyclizations: rearrangements of 4-heteroatom-1,2-hexa-diene-5-ynes.

The thermal rearrangements of 4-heteroatom-1,2-hexadiene-5-ynes (2) were studied at the BLYP/6-311+G//BLYP/6-31G level of theory. Cyclization of 2 to heteroatom-containing cyclopentadienyl structures (6) competes with the Claisen-type rearrangement to acyclic, allenic structures. Cyclizations to cyclobutene (4)- and cyclohexadiene (8)-derived heterocycles are not feasible as a result of high reaction barriers and lower-lying alternative pathways. [structure: see text]

Defect sites at the (001) surface of mordenite: An ab initio study

The mechanisms and energetics of the formation of various defects upon dehydration of the surface of pure-siliceous and Al-monosubstituted mordenite are investigated using a periodic ab initio density functional theory technique. An energetically favorable defect at the pure-siliceous surface is a strained two-membered Si–O ring (2MR) formed via elimination of a water molecule from a pair of neighboring terminal silanol groups. Assuming the formation of two-membered rings, the dehydration-energy of the (001) surface of pure-silica mordenite is 133 kJ/mol. A relatively high reaction barrier of 179 kJ/mol coincides with the experimental observation that these defects are formed at high temperatures &amp;gt;700 K. Despite a short Si–Si distance of 2.35 Å across the 2MR which is comparable to the bond length between Si atoms in silicon in diamond structure, the electron-localization function reveals no bonding interaction between Si atoms on the 2MR. In the Al-substituted surfaces, the dehydration proceeds via proton transfer from the Brønsted-acid site (BA) to a neighboring terminal hydroxyl group. The low values of two subsequent energetic barriers of dehydration of 13 and 10 kJ/mol suggest that the surface BA sites are likely to be destroyed at even modest temperatures. The most stable defects formed in this mechanism are ones containing a threefold-coordinated Al atom and a defect with both an Al atom and a bridging OH group located on a two-membered ring. The heat of reaction of only 9 kJ/mol and the activation energy of the transformation between these two configurations of 26 kJ/mol suggest that both defects occur with similar probability.