Intrinsic Reaction Coordinate Analysis Research Articles

Ab Initio and DFT Studies of the Thermal Rearrangement of Trimethylsilylsilylene

A thermal reaction scheme for the rearrangement of trimethylsilylsilylene (Me3Si−S̈i−H) proposed in a previous study [Organometallics 1986, 5, 698] was studied by the MP2 and DFT methods. The reaction pathways were searched by intrinsic reaction coordinate analysis. We report structures and energies of various silicon species. Activation energies for the C−H insertions by the divalent silicon centers in Me2HSiCH2−S̈i−H and MeHSiCH2−S̈i−Me are predicted to be high, 121 and 110 kJ/mol, respectively, in excellent agreement with an experimental value of 121 kJ/mol. However, the energy barrier for the C−H insertion by the divalent silicon center in Me3S−S̈i−H, predicted to be relatively small, 53 kJ/mol, is lower than that for the 1,2-Me shift (78 kJ/mol) in Me3Si−S̈i−H.

Mechanisms for the Deamination Reaction of Cytosine with H2O/OH−and 2H2O/OH−: A Computational Study

Mechanisms for the deamination reaction of cytosine with H 2O/OH (-) and 2H 2O/OH (-) to produce uracil were investigated using ab initio calculations. Optimized geometries of reactants, transition states, intermediates, and products were determined at MP2 and B3LYP using the 6-31G(d) basis set and at B3LYP/6-31+G(d) levels of theory. Single point energies were also determined at MP2/G3MP2Large and G3MP2 levels of theory. Thermodynamic properties (Delta E, Delta H, and Delta G), activation energies, enthalpies, and free energies of activation were calculated for each reaction pathway investigated. Intrinsic reaction coordinate (IRC) analysis was performed to characterize the transition states on the potential energy surface. Seven pathways for the deamination reaction were found. All pathways produce an initial tetrahedral intermediate followed by several conformational changes. The final intermediate for all pathways dissociates to product via a 1-3 proton shift. The activation energy for the rate-determining step, the formation of the tetrahedral intermediate for pathway D, the only pathway that can lead to uracil, is 115.3 kJ mol (-1) at the G3MP2 level of theory, in excellent agreement with the experimental value (117 +/- 4 kJ mol (-1)).

Intrinsic Reaction Coordinate Analysis of the Activation of CH4by Molybdenum Atoms: A Density Functional Theory Study of the Crossing Seams of the Potential Energy Surfaces

Density functional theory (DFT) calculations were performed to investigate the quintet, triplet, and singlet potential energy surfaces associated with the C-H activation of methane by laser-ablated...

Theoretical study of the cracking mechanisms of linear α-olefins catalyzed by zeolites

The cracking reactions of linear C 4–C 10 α-olefins over zeolites have been studied by using density functional theory at the B3LYP/6-31G(d,p) level. The obtained results reveal that the β-scission processes of C 4–C 10 olefins have the same reaction mechanism. Every pathway only involves a transition state corresponding to the rupture of the C–C bond, while the intrinsic reaction coordinate analysis indicates that the protonated intermediate species is formed during the reaction process. Furthermore, it is found that this intermediate species is not usually highly stable alkoxy group but adsorbed short-lifetime carbocation. This phenomenon can well explain why the carbocations are seldom observed inside the zeolite's cavities. The calculated real activation energy for this pathway is lower than the experimental value for corresponding alkane cracking contrary to the previously reported pathway via an alkoxide intermediate. Therefore, the reaction pathway via a carbocation intermediate species is energetically much more favorable. In addition, the study also shows that the real activation energies of olefin cracking are nearly independent of the olefin chain length, which is in agreement with the existing experimental results of alkane cracking.

Local Modifications of Single-Wall Carbon Nanotubes Induced by Bond Formation with Encapsulated Fullerenes

Defected fullerenes in nanopeapods form bonds with the encapsulating single-walled carbon nanotubes when irradiated by an electron beam leading to changes in the guest (fullerene) and the host (nanotube). Intrinsic reaction coordinate (IRC) analysis based on B3LYP hybrid density functional theory shows that a C1-C59 defect with a single protruding C atom is initially formed from the C60(Ih) cage. The high activation energy for this step (8.37 eV (193.0 kcal/mol)), being assumed to be accessible during irradiation, is lower than that of the Stone-Wales rearrangement on the sp2 network. The binding of the defected fullerene to the nanotube is preferential, orthogonal bonds relative to the tube axis being slightly preferred. Because of the covalent bonds formed between the guest and host, the carbon network on the nanotube is locally perturbed in the vicinity of the binding site. As a result of the new bonds, bisnorcaradiene-like as well as quinonoid-like patterns appear near the binding site. These results are interpreted using orbital interaction and Clar diagram arguments. The changes in the bonding pattern on the nanotube should be significant in further functionalization of carbon nanotubes.

Ring opening of boriranes vis‐à‐vis aziridines: An ab initio and DFT probe on the mechanisms

AbstractBorirane undergoes ring opening reaction with NOCl and HNF2 yielding the corresponding alkenes. Ab initio and density functional investigations of this reaction with cis‐ and trans‐2,3‐dimethylboriranes reveal that these reactions take place in a single step through the formation of a prereactive complex and a transition state giving the alkene with the same stereochemistry. Calculations clearly show that the concerted cleavage of CB bonds leads to retention of stereochemistry. Further, it shows that HNF2 cleaves boriranes more efficiently than does NOCl. Intrinsic reaction coordinate analyses and bond order analysis describe the nature of the transition state very well and fix the reaction mechanism. Solvent effect calculations through PCM model, with acetonitrile and CCl4 as solvents, do not alter the gas phase results significantly. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Proton Exchanges between Phenols and Ammonia or Amines: A Computational Study

Density functional theory calculations at the B3LYP/6-31+G(d,p) level of theory have been performed to explore proton exchanges between phenols and ammonia or amines, which can be used to account for previous NMR experiments. For the parent phenol-NH(3) system, a transition state with a symmetric phenolate-NH(4)(+)-like structure, which lies about 35 kcal mol(-1) in energy above the hydrogen-bonded complex, has been successfully located. An intrinsic reaction coordinate (IRC) analysis indicates that the proton exchange is a concerted process, which can be roughly divided into four continuous subprocesses. A series of para-substituted phenol-NH(3) systems have been considered to investigate the substituent effect. Whereas introduction of an electron-withdrawing group on the phenol appreciably reduces the barrier, an opposite effect is observed for an electron-donating group. Moreover, it has been disclosed that there exists a good linear correlation between the activation barriers and the interaction energies between the phenols and NH(3), indicating the important role of proton transfer (or hydrogen bonding) in determining the proton exchange. Also considered are the proton exchanges between phenol and amines and those for some sterically hindered systems. The results show that the phenol tends to exchange hydrogen with the amines, preferably the secondary amines, and that the steric effect is favorable for the proton exchange, which imply that, as the IRC analysis suggested, besides the proton transfer, the flip of the ammonium-like moiety may play a significant role in the course of proton exchange. For all of these systems, we investigated the solvent effects and found that the barrier heights of proton exchange decrease remarkably as compared to those in a vacuum due to the ion pair feature of the transition state. Finally, we explored the phenol radical cation-NH(3) system; the barrierless proton transfer and remarkably low barrier (5.2 kcal mol(-1)) of proton exchange provide further evidence for the importance of proton transfer in the proton exchange.

Computational Study of the Deamination Reaction of Cytosine with H2O and OH-

The mechanism for the deamination reaction of cytosine with H(2)O and OH(-) to produce uracil was investigated using ab initio calculations. Optimized geometries of reactants, transition states, intermediates, and products were determined at RHF/6-31G(d), MP2/6-31G(d), and B3LYP/6-31G(d) levels and for anions at the B3LYP/6-31+G(d) level. Single-point energies were also determined at B3LYP/6-31+G(d), MP2/GTMP2Large, and G3MP2 levels of theory. Thermodynamic properties (DeltaE, DeltaH, and DeltaG), activation energies, enthalpies, and free energies of activation were calculated for each reaction pathway that was investigated. Intrinsic reaction coordinate analysis was performed to characterize the transition states on the potential energy surface. Two pathways for deamination with H(2)O were found, a five-step mechanism (pathway A) and a two-step mechanism (pathway B). The activation energy for the rate-determining steps, the formation of the tetrahedral intermediate for pathway A and the formation of the uracil tautomer for pathway B, are 221.3 and 260.3 kJ/mol, respectively, at the G3MP2 level of theory. The deamination reaction by either pathway is therefore unlikely because of the high barriers that are involved. Two pathways for deamination with OH(-) were also found, and both of them are five-step mechanisms. Pathways C and D produce an initial tetrahedral intermediate by adding H(2)O to deprotonated cytosine which then undergoes three conformational changes. The final intermediate dissociates to product via a 1-3 proton shift. Deamination with OH(-), through pathway C, resulted in the lowest activation energy, 148.0 kJ/mol, at the G3MP2 level of theory.

Ab initio and DFT investigations on the stereochemistry of ring opening of episulfides

Cleavage of cis and trans-2,3-dimethylepisulfides by triethyl phosphite have been experimentally accomplished by Neureiter and Bordwell and they have reported that the reaction proceeded with complete retention of stereochemistry. This reaction has been modeled here at ab initio and DFT level to look at the mechanism of the reaction and to understand how the stereochemistry of the reaction is controlled. Calculations reveal that the reaction takes place in a single step passing through a concerted synchronous transition state. In this step, cleavage of two C–S bonds takes place with the simultaneous formation of a new P–S bond and this results in the formation of the alkene with the same stereochemistry, in total agreement with the experiment. Intrinsic reaction coordinate analysis explains the synchronous nature of the transition state very well and smoothly connects the reactants and products. For comparison, single point energy calculations have been performed at higher levels and that also confirms the same trend. High exothermicity of the reaction indicates the release of the ring strain during the reaction in the episulfide.

Ab initio study of the decomposition of formamidine

The decomposition of formamidine yielding hydrogen cyanide and ammonia has been investigated by ab initio calculations. Optimized geometries for reactants, transition states, and products were determined at the HF/6-31G(d) and MP2/6-31G(d) levels of theory. Energies were also determined at the G1, G2, G2MP2, G3, G3B3, G3MP2, and G3MP2B3 levels of theory. The role of water in the decomposition reaction of formamidine was examined. Intrinsic reaction coordinate (IRC) analysis was carried out for all transition states. Activation energies, enthalpies and free energies of activation were also calculated for each reaction pathway. G3 level of theory predicts the gas-phase decomposition of formamidine to have a high activation energy of 259.1 kJ mol–1. Adding one water molecule catalyses the reaction by forming a cyclic hydrogen-bonded transition state, reducing the barrier to 169.4 kJ mol–1 at the G3 level. Addition of a second water, which acts as a "solvent" molecule, further reduces the barrier to 151.1 kJ mol–1 at the G3 level. These values are still high and explain why rather extreme conditions are necessary to achieve this reaction experimentally. Thermodynamic properties (ΔE, ΔH, and ΔG) for each reaction pathway studied were also calculated. The G3 heats of reaction (ΔE) of the gas-phase decomposition of formamidine, its complex with one water molecule, and its complex with two water molecules are 0.9, 2.2, and –5.1 kJ mol –1, respectively. The G3 heat of reaction for the gas-phase decomposition to yield separated products is 22.3 kJ mol–1. Free energies of reaction and of activation in aqueous solution were calculated with PCM using the KLAMT cavity model. At MP2 the formamidine reaction is found to be exergonic in aqueous solution and to favour formation of the separated products (NH3 + HCN). The solvent model predicts a significant lowering of the free energy of activation (16–18 kJ mol–1) for the unimolecular reaction and 21–42 kJ mol–1 for the water-mediated reaction in aqueous solution relative to the gas phase. Key words: decomposition reaction, formamidine, Hartree–Fock, post Hartree–Fock, Gaussian-n theories, IRC, solvation models, PCM, KLAMT.

Photophysical properties of substituted intramolecularly hydrogen bonded compounds: A combined experimental and theoretical study

Photophysical properties of prototype excited state intramolecular proton transfer (ESIPT) system 4-methyl-2,6-diformyl phenol (MFOH) and its derivatives were studied by steady state and time-resolved fluorescence spectroscopy as well as by ab-initio quantum chemical calculation. It has been found that nonradiative decay process is the most important deactivation channel in all the cases and the hydrogen bonded enol conformer is stable in the ground state whereas, the proton transferred keto form is energetically favoured in the S 1(ππ *) state. However, the net gain in stabilization in the process of ESIPT is almost unaffected by the substitution. The reversal of stability in the excited state was explained on the basis of the nature of frontier molecular orbital in all the cases. Intrinsic reaction coordinate analysis showed that drastic change in nonbonded interoxygen distance R(O–O) in the proton transfer pathway causes the switch over from the enol to keto configuration. A close comparison of several properties like molecular geometry, hydrogen bond strength and atomic charge in different derivatives of MFOH were found to be consistent and in good agreement with the experimental results obtained from time-resolved fluorescence experiments.

Ring Cleavage of Aziridines by Difluoroamine: Mechanistic Insights from ab Initio and DFT Study

cis-2,3-Dimethylaziridine reacts with difluoroamine to give the corresponding alkene and nitrogen with retention of configuration. We have carried out a DFT study of this reaction to clarify the reaction mechanism by considering a multistep reaction pathway with possible intermediacy of several three- and four-membered cyclic intermediates and transition states (TSs). The energetics of this reaction shows that the reaction takes place in four steps including a three-membered azamine intermediate. Both the energetics and the stereochemical outcome of this reaction rule out the formation of a four-membered diazetine intermediate during the reaction. Although the first N-N bond formation step is rate determining, the final step, asynchronous concerted cleavage of the azamine intermediate, explains the stereochemistry of this reaction. The asynchronous nature of the final step makes the reaction Woodward-Hoffmann allowed, as reported by Yamabe and Minato (J. Phys. Chem. A 2001, 105, 7281). Computations at HF and MP2 levels confirm the same trends in energetics. Single point energy computations at B3LYP, MP2, and QCISD levels with the 6-311++G(d,p) and cc-pVTZ basis sets show that the larger basis sets predict higher free energies of activation and less negative free energies of reaction. Intrinsic reaction coordinate (IRC) analyses reveal the asynchronous nature of the first and the last steps of the reaction. The deamination of trans-2,3-dimethylaziridine was shown to follow a course of reaction similar to that of the cis isomer.

Structures and isomerization of neutral and zwitterion serine–water clusters: Computational study

AbstractCalculations are presented for the structure and the isomerization reaction of various conformers of the bare serine, neutral serine–(H2O)n and serine zwitterion–(H2O)n (n = 1, 2) clusters. The effects of binding water molecules on the relative stability and the isomerization processes are examined. Hydrogen bonding between serine and the water molecule(s) may significantly affect the relative stability of conformers of the neutral serine–(H2O)n (n = 1, 2) clusters. The sidechain (OH group) in serine is found to have a profound effect on the structure and isomerization of serine–(H2O)n (n = 1, 2) clusters. Conformers with the hydrogen bonding between water and the hydroxyl group of serine are predicted. A detailed analysis is presented of the isomerization (proton transfer) pathways between the neutral serine–(H2O)2 and serine zwitterion–(H2O)2 clusters by carrying out the intrinsic reaction coordinate analysis. At least two water molecules need to bind to produce the stable serine zwitterion–water cluster in the gas phase. The isomerization for the serine–(H2O)2 cluster proceeds by the concerted double and triple proton transfer mechanism occurring via the binding water molecules, or via the hydroxyl group. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Structures and properties of [CN⋯CN −] coupling system in doublet state

The structures and the properties of CN −(X 1Σ +)⋯CN(X 2Σ +) coupling system are investigated in detail in present paper with density functional theory (DFT) method at UB3LYP level employing the aug-cc-pVDZ basis set. The vertical electron affinity of CN is determined at the same level. Geometrical optimizations indicate that there are eight possible stable isomers (EC1: 2A′, C s; EC2: 2A′, C s ; EC3: 2B u, C 2h ; EC4: 2A′, C s ; EC5: 2A′, C s ; EC6: 2A′, C s; EC7: 2B u, C 2h ; EC8: 2A, C 2 ), which correspond to the genuine minima on the global potential energy surfaces (PES) for the doublet spin states. On the basis of the optimized geometries, the stabilization energies of the complexes are determined with a correction using counterpoise corrections method for the basis set superposition error (BSSE) and the Zero-point vibrational energy (ZPVE), and the relative stability of the coupling complexes, CN⋯CN −, are compared. The stabilization energy order of these complexes is EC3>EC4>EC5>EC2>EC1>EC6>EC7>EC8. The stabilization energy of the most stable complex, EC3, is 62.08 kcal/mol. The most interesting isomer is the EC8, which energy is higher than the sum of total energies of the two separated monomers. The electronic states of these complexes are all determined. Then the vibrational frequency analysis and the assignment of the vibrational frequency modes have been performed. The natural bond orbital analyses of these isomers have also been carried out in succession. The transition state searches and the intrinsic reaction coordinate (IRC) analysis have indicated that there are five transition states, in which TS1 correlates with EC1 and EC2, and TS3 correlates with EC4 and EC5, respectively. The forward and reverse activation energies are 5.37, 5.46 kcal/mol for TS1 and 8.26, 6.00 kcal/mol for TS3, respectively. No direct correlation is found for EC6 with the other complexes. The self isomerization of EC7 is more difficult to take place than the self isomerization processes of the most stable complex, EC3. TS5 is the transition state of EC8 in the dissociation process.

Structure analysis of [P,N,O] species in singlet and triplet states at density functional theory and wavefunction-correlated levels

The geometries and the harmonic vibrational frequencies for several PNO isomers in the singlet and triplet states have been predicted at density functional theory levels with a 6-311+G* basis set. Results have indicated that there are three linear structures, one bent and one cyclic structure in the singlet state, and three bent structures and one cyclic structure exist in triplet state. The ground state corresponds to a linear 1Σ PNO structure. The detailed bonding character is discussed, and the state–state energy separations of various stable states relative to the ground state are calculated. The state–state correlation is also predicted by searching the transition states and the intrinsic reaction coordinate analysis.

Effects of Microsolvation on the Structures and Reactions of Neutral and Zwitterion Alanine: Computational Study

Calculations are presented for the structures and the reactions of various conformers of the bare alanine, neutral alanine−(H2O)n, and alanine zwitterion−(H2O)n (n = 1 and 2) clusters. The effects of the binding water molecules on the relative thermodynamic stability and the isomerization reaction of alanine are examined. Hydrogen bonding between alanine and the water molecule(s) may significantly affect the thermodynamic stability of conformers of the neutral alanine−(H2O)n (n = 1 and 2). clusters. Detailed analysis is presented on the isomerization (proton transfer) pathways between the neutral alanine−(H2O)2 and the alanine zwitterion−(H2O)2 clusters including the structures of the transition states by carrying out the intrinsic reaction coordinate analysis. We find that at least two water molecules need to bind to produce the stable alanine zwitterion−water cluster in the gas phase. The isomerization reaction for the alanine−(H2O)2 cluster proceeds by the concerted double proton-transfer mechanism via the binding water molecules. The effects of continuum water are also examined by employing the supramolecule/continuum type model to study the properties of aianine in aqueous solution. The water continuum is predicted to considerably alter the thermodynamic and kinetic stability of the neutral and zwitterioniic alanine−(H2O)n (n = 0−2) clusters.

Ab Initio Study of the Deamination of Formamidine

The mechanism for the deamination of formamidine with OH-, H2O, and H3O+ was investigated using computational techniques. Optimized geometries for reactants, intermediates, products, and transition states were determined at the RHF/6-31G(d) and MP2/6-31G(d) levels of theory. Single-point energies were determined at G1 and G2 and using density functional theory (B3LYP) at both optimized geometries and the 6-311G(d,p), 6-311+G(d,p), 6-311++G(d,p) (with OH- only), 6-311G(2df,p), and 6-311+G(3df,2p) basis sets. Frequencies were calculated for all structures studied at both geometries. Intrinsic reaction coordinate analysis was carried out for all transition states. Activation energies were calculated for each pathway investigated. Deamination with H3O+ involves the formation of the same tetrahedral intermediate from the addition of water to either possible protonated formamidine cation followed by a 1,3-proton shift and dissociation to products. Deamination is an unlikely event because of the high activation-energy barrier to the formation of the intermediate (242.4 kJ mol-1 for pathway A and 243.5 kJ mol-1 for pathway B using G2 data). Two pathways for deamination with H2O were studied. Pathway A produced a tetrahedral intermediate by the addition of water followed by a 1,3-proton shift and dissociation to products, and pathway B involved the formation of the tautomer of the formamide product by reaction with water followed by a 1,3-proton shift to yield formamide. Deamination by either pathway is unlikely because of the high activation-energy barriers involved (212.7 kJ mol-1 for the formation of the tetrahedral intermediate of pathway A and 249.2 kJ mol-1 for the formation of the formamide tautomer of pathway B using G2 data). Deamination with OH- yields a tetrahedral intermediate with a much lower activation-energy barrier than for the reaction with H3O+ and H2O (58.2 kJ mol-1 using G2 data), but the deprotonation of formamidine by hydroxide appears to be the most likely process because of the resonance-stabilized deprotonated formamidine anion/water complex formed (ΔE = −163.2 kJ mol-1).

Theoretical prediction of the state–state correlation among doublet state SNO isomers

The geometries and the harmonic vibrational frequencies for several SNO doublet state isomers have been predicted at density functional theory levels with a 6-311+G* basis set. Results have indicated that there are nine doublet states (five bent, three nearly linear and one cyclic). The ground state corresponds to a bent 2 A ′ SNO structure. The state–state correlation and the isomerization mechanism are also predicted by searching the transition states and the intrinsic reaction coordinate analysis. Results have indicated that the ground state is the most favorable product. The nearly linear 2 A″ SNO state may be obtained from the bent 2 A ′ SNO state by electron transition.

Cl 2 molecular elimination reaction from 1,2-dichloroethane

The transition state energy for Cl 2 molecular elimination from 1,2-dichloroethane is computed at compostite CBS-Q (based on B3LYP/6-31++G(d) optimized structure) and G3MP2 (based on MP2/6-31++G(d) optimized structure) levels of theory. Forward rate constants are calculated as 4.25 E11 T 0.826· exp(−93.4 kcal mol −1/ RT) and 7.62 E10 T 0.892· exp(−98.5 kcal mol −1/ RT) s −1 , respectively for these two methods. Intrinsic reaction coordinate (IRC) analysis is performed to verify the transition state structures. While the overall, elimination of Cl 2 plus olefin formation has a relative low ΔH rxn; the reaction with these barriers is evaluated to be not important in thermal reactions of chlorocarbons.

Theoretical approaches of the isomerization mechanism of GaSO isomers in doublet states at density functional theory levels

The geometries and the harmonic vibrational frequencies for several GaSO doublet state isomers have been predicted at density functional theory levels with a 6-311+G* basis set. Results have indicated that there are five doublet states (two cyclic, two linear and one bent), but only four of them keep S–O bond linkage. The ground state corresponds to a cyclic 2 A ″ structure. The state–state energy separations of the low-lying excited states with a significant S–O linkage relative to the 2 A ″ ground state are 3.2 (linear GaOS( 2Σ )), 12.9 (linear SGaO( 2Σ )), 87.9 (cyclic 2 A ′) and 21.9 (bent 2 A ′) kcal/mol above, respectively. The GaSO species in the ground state ( 2 A ″) should be classified as the thiosuperoxide, while some other isomers exhibit different thiosuperoxide character. But they are not as ionic as the alkali metal superoxides and thiosuperoxides. The excited state without the S–O linkage possesses a linear structure and should be classified as the thio-oxide. The state–state correlation and the isomerization mechanism are also predicted by searching the transition states and the intrinsic reaction coordinate analysis. Results have indicated that the ground state 2 A ″ may directly correlate with two linear structures (GaOS( 2Σ ) and SGaO( 2Σ )) by experiencing a TS3 with a barrier of 4–6 kcal/mol and a TS5 with a barrier of ∼56 kcal/mol. Due to the unstability of the cyclic 2 A ′ state, its isomerization transition state (TS1) to the linear GaOS( 2Σ ) is not found. The isomerization between the linear GaOS and SGaO via a 2 A ′ pathway may be proceeded by experiencing a TS4 with a barrier of 45–48 kcal/mol. These height barriers should be attributed to the S–O bond rupture in proceeding from the linear GaOS( 2Σ ) or the cyclic 2 A ″ ground state to the linear SGaO( 2Σ ) structure. The bent 2 A ′ state may correlates with the cyclic 2 A ′ state (or linear GaOS( 2Σ ) state) and the linear SGaO( 2Σ ) state by undergoing TS2 and TS6 transition states, respectively, but these two isomerization pathways of the bent GaSO( 2 A ′) are energetically unfavorable due to the very high activation barriers being ∼85 kcal/mol (TS2) or 56–60 kcal/mol (TS6).