MINDO-SCF-MO-berechnung der cis-trans-isomerisierung von N-methyl- und N-phenylazomethin

MINDO/2-SCF-MO calculations for the ground state properties of N-methyl- and N-phenyl-azomethin have been carried out. The calculated rotation barrier for the methyl group in N-methyl-azomethin was 0·8 kcal/mol, the eclipsed conformation being most stable. The calculated rotation barrier about the CN bond in the protonated methylazomethin was 27·9 kcal/mol. MINDO/1-SCF-MO treatment for the N-inversion barrier of the unprotonated species yielded 13·00 kcal/mol. Similar MINDO/2 calculations for N-phenylazomethin yielded 4·0 kcal/mol for the rotation barrier of the phenyl ring around the CN= bond, the perpendicular conformation of the ring to the CNC plane being most stable. For the corresponding N protonated derivative the value 27·3 kcal/mol was calculated for the rotation barrier around the CN bond. MINDO/1 treatment yielded an inversion barrier of 14·0 kcal/mol for N-phenylazomethin.     

NMR.-Study of nitrogen inversion and conformation of 1,5-dihydro-isoalloxazines ("reduced flavin"). Studies in the flavin series, XIX

The pyramidal inversion of the N(5)-centre of several reduced flavins was measured by NMR. The inversion barrier was found to be ～10 kcal/mol in acetone solutions and to be independent of the size of the N(5) substituent. An increase of the inversion barrier of ～5 kcal/mol was observed in the case where the N(5) substituent could only be in axial position, and an increase of ～3.5 kcal/mol was observed for an acyl-like N(5) substituent. In aqueous solution the inversion barrier increases by ～3 kcal/mol. The stereochemistry of reduced flavin and its potential relevance in flavin-dependent biological dehydrogenations is discussed.     

Is the protein surrounding the active site critical for hydrogen peroxide reduction by selenoprotein glutathione peroxidase? An ONIOM study

In this ONIOM(QM:MM) study, we evaluate the role of the protein surroundings in the mechanism of H2O2 reduction catalyzed by the glutathione peroxidase enzyme, using the whole monomer (3113 atoms in 196 amino acid residues) as a model. A new optimization scheme that allows the full optimization of transition states for large systems has been utilized. It was found that in the presence of the surrounding protein the optimized active site structure bears a closer resemblance to the one in the X-ray structure than that without the surrounding protein. H2O2 reduction occurs through a two-step mechanism. In the first step, the selenolate anion (E-Se(-)) formation occurs with a barrier of 16.4 kcal/mol and is endothermic by 12.0 kcal/mol. The Gln83 residue plays the key role of the proton abstractor, which is in line with the experimental suggestion. In the second step, the O-O bond is cleaved, and selenenic acid (R-Se-OH) and a water molecule are formed. The calculated barrier for this process is 6.0 kcal/mol, and it is exothermic by 80.9 kcal/mol. The overall barrier of 18.0 kcal/mol for H2O2 reduction is in reasonable agreement with the experimentally measured barrier of 14.9 kcal/mol. The protein surroundings has been calculated to exert a net effect of only 0.70 kcal/mol (in comparison to the "active site only" model including solvent effects) on the overall barrier, which is most likely due to the active site being located at the enzyme surface.     

Energetics and mechanism of the decomposition of trifluoromethanol

The thermal instability of alpha-fluoroalcohols is generally attributed to a unimolecular 1,2-elimination of HF, but the barrier to intramolecular HF elimination from CF3OH is predicted to be 45.1 +/- 2 kcal/mol. The thermochemical parameters of trifluoromethanol were calculated using coupled-cluster theory (CCSD(T)) extrapolated to the complete basis set limit. High barriers of 42.9, 43.1, and 45.0 kcal/mol were predicted for the unimolecular decompositions of CH2FOH, CHF2OH, and CF3OH, respectively. These barriers are lowered substantially if cyclic H-bonded dimers of CF3OH with complexation energies of approximately 5 kcal/mol are involved. A six-membered ring dimer has an energy barrier of 28.7 kcal/mol and an eight-membered dimer has an energy barrier of 32.9 kcal/mol. Complexes of CF3OH with HF lead to strong H-bonded dimers, trimers and tetramers with complexation energies of approximately 6, 11, and 16 kcal/mol, respectively. The dimer, CH3OH:HF, and the trimers, CF3OH:2HF and (CH3OH)2:HF, have decomposition energy barriers of 26.7, 20.3, and 22.8 kcal/mol, respectively. The tetramer (CH3OH:HF)2 gives rise to elimination of two HF molecules with a barrier of 32.5 kcal/mol. Either CF3OH or HF can act as catalysts for HF-elimination via an H-transfer relay. Because HF is one of the decomposition products, the decomposition reactions become autocatalytic. If the energies due to complexation for the CF3OH-HF adducts are not dissipated, the effective barriers to HF elimination are lowered from approximately 20 to approximately 9 kcal/mol, which reconciles the computational results with the experimentally observed stabilities.     

Reactivity of alkyl versus silyl peroxides. The consequences of 1, 2-silicon bridging on the epoxidation of alkenes with silyl hydroperoxides and bis(trialkylsilyl)peroxides

The bond dissociation energies for a series of silyl peroxides have been calculated at the G2 and CBS-Q levels of theory. A comparison is made with the O-O BDE of the corresponding dialkyl peroxides, and the effect of the O-O bond strength on the activation barrier for oxygen atom transfer is discussed. The O-O bond dissociation enthalpies (DeltaH(298)) for bis (trimethylsilyl) peroxide (1) and trimethylsilyl hydroperoxide (2) are 54.8 and 53.1 kcal/mol, respectively at the G2 (MP2) and CBS-Q levels of theory. The O-O bond dissociation energies computed at G2 and G2(MP2) levels for bis(tert-butyl) peroxide and tert-butyl hydroperoxide are 45.2 and 48.3 kcal/mol, respectively. The barrier height for 1,2-methyl migration from silicon to oxygen in trimethylsilyl hydroperoxide is 47.9 kcal/mol (MP4//MP2/6-31G). The activation energy for the oxidation of trimethylamine to its N-oxide by bis(trimethylsilyl) peroxide is 28.2 kcal/mol (B3LYP/6-311+G(3df,2p)// B3LYP/6-31G(d)). 1,2-Silicon bridging in the transition state for oxygen atom transfer to a nucleophilic amine results in a significant reduction in the barrier height. The barrier for the epoxidation of E-2-butene with bis(dimethyl(trifluoromethyl))silyl peroxide is 25.8 kcal/mol; a reduction of 7.5 kcal/mol relative to epoxidation with 1. The activation energy calculated for the epoxidation of E-2-butene with F(3)SiOOSiF(3) is reduced to only 2.2 kcal/mol reflecting the inductive effect of the electronegative fluorine atoms.     

Mechanism of the hydration of carbon dioxide: direct participation of H2O versus microsolvation

Thermochemical parameters of carbonic acid and the stationary points on the neutral hydration pathways of carbon dioxide, CO 2 + nH 2O --> H 2CO 3 + ( n - 1)H 2O, with n = 1, 2, 3, and 4, were calculated using geometries optimized at the MP2/aug-cc-pVTZ level. Coupled-cluster theory (CCSD(T)) energies were extrapolated to the complete basis set limit in most cases and then used to evaluate heats of formation. A high energy barrier of approximately 50 kcal/mol was predicted for the addition of one water molecule to CO 2 ( n = 1). This barrier is lowered in cyclic H-bonded systems of CO 2 with water dimer and water trimer in which preassociation complexes are formed with binding energies of approximately 7 and 15 kcal/mol, respectively. For n = 2, a trimeric six-member cyclic transition state has an energy barrier of approximately 33 (gas phase) and a free energy barrier of approximately 31 (in a continuum solvent model of water at 298 K) kcal/mol, relative to the precomplex. For n = 3, two reactive pathways are possible with the first having all three water molecules involved in hydrogen transfer via an eight-member cycle, and in the second, the third water molecule is not directly involved in the hydrogen transfer but solvates the n = 2 transition state. In the gas phase, the two transition states have comparable energies of approximately 15 kcal/mol relative to separated reactants. The first path is favored over in aqueous solution by approximately 5 kcal/mol in free energy due to the formation of a structure resembling a (HCO 3 (-)/H 3OH 2O (+)) ion pair. Bulk solvation reduces the free energy barrier of the first path by approximately 10 kcal/mol for a free energy barrier of approximately 22 kcal/mol for the (CO 2 + 3H 2O) aq reaction. For n = 4, the transition state, in which a three-water chain takes part in the hydrogen transfer while the fourth water microsolvates the cluster, is energetically more favored than transition states incorporating two or four active water molecules. An energy barrier of approximately 20 (gas phase) and a free energy barrier of approximately 19 (in water) kcal/mol were derived for the CO 2 + 4H 2O reaction, and again formation of an ion pair is important. The calculated results confirm the crucial role of direct participation of three water molecules ( n = 3) in the eight-member cyclic TS for the CO 2 hydration reaction. Carbonic acid and its water complexes are consistently higher in energy (by approximately 6-7 kcal/mol) than the corresponding CO 2 complexes and can undergo more facile water-assisted dehydration processes.     

A semiempirical study of the optimized ground and excited state potential energy surfaces of retinal and its protonated Schiff base.

The electronic ground and first excited states of retinal and its Schiff base are optimized for the first time using the semiempirical AM1 Hamiltonian. The barrier for rotation about the C(11)-C(12) double bond is characterized by variation of both the twist angle delta(C(10)-C(11)-C(12)-C(13)) and the bond length d(C(11)-C(12)). The potential energy surface is obtained by varying these two parameters. The calculated ground state rotational barrier is equal to 15.6 kcal/mol for retinal and 20.5 kcal/mol for its Schiff base. The all-trans conformation is more stable by 3.7 kcal/mol than the 11-cis geometry. For the first excited state, S(1,) the 90 degrees twisted geometry represents a saddle point for retinal with the rotational barrier of 14.6 kcal/mol. In contrast, this conformation is an energy minimum for the Schiff base. It can be easily reached at room temperature from the planar minima since it is separated from them by a barrier of only 0.6 kcal/mol. The 90 degrees minimum conformation is more stable than the all-trans by 8.6 kcal/mol. We are thus able to present a reaction path on the S(1) surface of the retinal Schiff base with an almost barrier-less geometrical relaxation into a twisted minimum geometry, as observed experimentally. The character of the ground and first excited singlet states underscores the need for the inclusion of double excitations in the calculations.     

A multilayered representation,quantum mechanical and molecular mechanics study of the CH3F + OH− reaction in water

The bimolecular nucleophilic substitution (S_N2) reaction of CH₃F + OH^? in aqueous solution was investigated using a combined quantum mechanical and molecular mechanics approach. Reactant complex, transition state, and product complex along the reaction pathway were analyzed in water. The potentials of mean force were calculated using a multilayered representation with the DFT and CCSD(T) level of theory for the reactive region. The obtained free energy activation barrier for this reaction at the CCSD(T)/MM representation is 18.3 kcal/mol which agrees well with the experimental value at ～21.6 kcal/mol. Both the solvation effect and solute polarization effect play key roles on raising the activation barrier height in aqueous solution, with the former raising the barrier height by 3.1 kcal/mol, the latter 1.5 kcal/mol. © 2013 Wiley Periodicals, Inc.     

10]Annulene: bond shifting and conformational mechanisms for automerization

We report density-functional and coupled-cluster calculations on conformation change and degenerate bond shifting in [10]annulene isomers 1-5. At the CCSD(T)/cc-pVDZ//CCSD/6-31G level, conversion of the twist (1) to the heart (2) has a barrier of 10.1 kcal/mol, compared to Ea = 16.2 kcal/mol for degenerate "two-twist" bond shifting in 1. Pseudorotation in the all-cis boat isomer (3) proceeds with a negligible barrier. The naphthalene-like isomer 4 has a 3.9 kcal/mol barrier to degenerate bond shifting. The azulene-like isomer 5 is the only species for which the nature of the bond-equalized form (5-eq) depends on the method. At the CCSD(T)/cc-pVDZ//CCSD/6-31G level, 5-eq is 1.2 kcal/mol more stable than the bond-alternating form 5-alt. Conversion of 5-eq to 4 has a barrier of 12.6 kcal/mol. Despite being significantly nonplanar, both 5-eq and the transition state for bond shifting in 4 are highly aromatic based on magnetic susceptibility exaltations. On the basis of a detailed consideration of these mechanisms and barriers, we can now, with greater confidence, rule out 4 and 5 as candidates to explain the NMR spectra observed by Masamune. Our results support Masamune's original assignments for both isolated isomers.     

Ab initio study of the S(N)2 and E2 mechanisms in the reaction between the cyanide ion and ethyl chloride in dimethyl sulfoxide solution

[reaction: see text] Reliable theoretical calculations predict a free energy barrier for nitrile formation from the reaction between the cyanide ion and ethyl chloride in DMSO solvent of 24.1 kcal/mol, close to the experimental value of 22.6 kcal/mol. We have also predicted that the isonitrile formation is less favorable by 4.7 kcal/mol, while the elimination mechanism is less favorable by more than 10 kcal/mol. These results indicate that isonitrile formation and bimolecular elimination are not significant side reactions for primary alkyl chloride reactions.     

Reactions of 1,3-cyclohexadiene with singlet oxygen. A theoretical study.

A thorough study of the reaction of singlet oxygen with 1,3-cyclohexadiene has been made at the B3LYP/6-31G(d) and CASPT2(12e,10o) levels. The initial addition reaction follows a stepwise diradical pathway to form cyclohexadiene endoperoxide with an activation barrier of 6.5 kcal/mol (standard level = CASPT2(12e,10o)/6-31G(d); geometries and zero-point corrections at B3LYP/6-31G(d)), which is consistent with an experimental value of 5.5 kcal/mol. However, as the enthalpy of the transition structure for the second step is lower than the diradical intermediate, the reaction might also be viewed as a nonsynchronous concerted reaction. In fact, the concertedness of the reaction is temperature dependent since entropy differences create a free energy barrier for the second step of 1.8 kcal/mol at 298 K. There are two ene reactions; one is a concerted mechanism (DeltaH(double dagger) = 8.8 kcal/mol) to 1-hydroperoxy-2,5-cyclohexadiene (5), while the other, which forms 1-hydroperoxy-2,4-cyclohexadiene (18), passes through the same diradical intermediate (9) as found on the pathway to endoperoxide. The major pathway from the endoperoxide is O-O bond cleavage (22.0 kcal/mol barrier) to form a 1,4-diradical (25), which is 13.9 kcal/mol less stable than the endoperoxide. From the diradical, two low-energy pathways exist, one to epoxyketone (29) and the other to the diepoxide (27), where both products are known to be formed experimentally with a product ratio sensitive to the nature of substitutents. A significantly higher activation barrier leads to C-C bond cleavage and direct formation of maleic aldehyde plus ethylene.     

Force field parameters for sulfates and sulfamates based on ab initio calculations: Extensions of AMBER and CHARMm fields

Ab initio self-consistent field (SCF) Hartree-Fock calculations of sulfates R? O? SO₃(?1) (R = Me, Et, i-Pr) and sulfamates R? NHSO₃(?1) (R = H, Me, Et, i-Pr) were performed at the 4-31G(*S*N) //3-21G(*S*N) basis set levels, where asterisks indicate d functions on sulfur and nitrogen atoms. These standard levels were determined by comparing calculation results with several basis sets up to MP2/6-31G*//6-31G*. Several conformations per compound were studied to obtain molecular geometries, rotational barriers, and potential derived point charges. In methyl sulfate, the rotational barrier around the C? O bond is 1.6 kcal/mol at the MP2 level and 1.4 kcal/mol at the standard level. Its ground state has one of three HCOS torsion angles trans and one of three COSO torsion angles trans. Rotation over 60° around the single O? S bond in the sulfate group costs 2.5 kcal/mol at the MP2 and 2.1 kcal/mol at the standard level. For ethyl sulfate, the calculated rotational barrier in going from the ground state, which has its CCOS torsion angle trans, to the syn-periplanar conformation (CCOS torsion angle cis) is 4.8 kcal/mol. However, a much lower barrier of 0.7 kcal/mol leads to a secondary gauchelike conformation about 0.4 kcal/mol above the ground state, with the CCOS torsion angle at 87.6°. Again, one of the COSO torsion angles is trans in the ground state, and the rotational barrier for a 60° rotation of the sulfate group amounts to 1.8 kcal/mol. For methyl sulfamate, the rotational barriers are 2.5 kcal/mol around the C? N bond and 3.3 kcal/mol around the N? S bond. This is noteworthy because sulfamate itself has a calculated rotational barrier around the N? S bond of only 1.7 kcal/mol. These and other data were used to parameterize the well-known empirical force fields AMBER and CHARMm. When the new fields were tested by means of vibrational frequency calculations at the 6-31G*//6-31G* level for methyl sulfate, sulfamate, and methyl sulfamate ground states, the frequencies compared favorably with the AMBER and CHARMm calculated frequencies. The transferability of the force parameters to β-D -glucose-6-sulfate and isopropyl sulfate appears to be better than to isopropyl sulfamate. © 1995 by John Wiley & Sons, Inc.     

Ab initio study of gas phase and water-assisted tautomerization of maleimide and formamide

Maleimide serves as an important starting material in the synthesis of drugs and enzyme inhibitors. In the present paper, knowing the importance of tautomerization in maleimide for its drug action, potential energy surface of maleimide is studied and its tautomerization has been discussed and compared with tautomerization of formamide. Gas phase tautomerization of maleimide requires large amount of energy (23·21 kcal/mol) in comparison to formamide (15·05 kcal/mol) at HF/6-31+G* level. Thus making the proton transfer reaction a difficult process in gas phase. Water molecule lowers the energy barrier of tautomerization thus facilitating the tautomerization of maleimide to 5-hydroxy-pyrrol-2-one. Water assisted tautomerization of maleimide requires 19·60 kcal/mol energy at HF/6-31+G* and 17·63 kcal/mol energy at B3LYP/6-31+G* level, a decrease of 3·61 and 5·96 kcal/mol over gas phase tautomerization. Whereas, tautomerization of formamide requires 14·16 and 12·84 kcal/mol energy, a decrease of 0·89 and 2·01 kcal/mol energy over gas phase tautomerization at HF/6-31+G* and B3LYP/6-31+G* level, respectively. Water-assisted tautomerization in maleimide and formamide showed that difference in energy barrier reduces to 2·83 kcal/mol from 10·41 kcal/mol (in gas phase) at B3LYP level, which resulted that maleimide readily undergoes tautomerization in water molecule.     

Computational study of the aminolysis of 2-benzoxazolinone

Three possible mechanisms (zwitterionic, neutral stepwise, and neutral concerted) of the ring-opening reaction of 2-benzoxazolinone (BO) upon aminolysis with methylamine were studied at the B3LYP/6-31G* level. In the gas phase, the neutral concerted mechanism is shown to be most favorable, which proceeds via a rate-determining barrier of 28-29 kcal/mol. The transition state, CTS, associated with this barrier is a four-centered one, where 1,2-addition of the N[bond]H of methylamine to the C[bond]O of BO ring occurs. The rate-determining barrier of the neutral stepwise pathway is found to be ca. 42 kcal/mol. The inclusion of solvent effects by a polarizable continuum model (PCM) does not change the conclusions based on the gas-phase study; the barrier at CTS is reduced to 20, 20, and 22 kcal/mol in water, ethanol, and acetonitrile, respectively.     

New insight into the gas-phase bimolecular self-reaction of the HOO radical

The singlet and triplet potential energy surfaces (PESs) for the gas-phase bimolecular self-reaction of HOO*, a key reaction in atmospheric environments, have been investigated by means of quantum-mechanical electronic structure methods (CASSCF and CASPT2). All the reaction pathways on both PESs consist of a first step involving the barrierless formation of a prereactive doubly hydrogen-bonded complex, which is a diradical species lying about 8 kcal/mol below the energy of the reactants at 0 K. The lowest energy reaction pathway on both PESs is the degenerate double hydrogen exchange between the HOO* moieties of the prereactive complex via a double proton transfer mechanism involving an energy barrier of only 1.1 kcal/mol for the singlet and 3.3 kcal/mol for the triplet at 0 K. The single H-atom transfer between the two HOO* moieties of the prereactive complex (yielding HOOH + O2) through a pathway keeping a planar arrangement of the six atoms involves a conical intersection between either two singlet or two triplet states of A' and A" symmetries. Thus, the lowest energy reaction pathway occurs via a nonplanar cisoid transition structure with an energy barrier of 5.8 kcal/mol for the triplet and 17.5 kcal/mol for the singlet at 0 K. The simple addition between the terminal oxygen atoms of the two HOO* moieties of the prereactive complex, leading to the straight chain H2O4 intermediate on the singlet PES, involves an energy barrier of 7.3 kcal/mol at 0 K. Because the decomposition of such an intermediate into HOOH + O2 entails an energy barrier of 45.2 kcal/mol at 0 K, it is concluded that the single H-atom transfer on the triplet PES is the dominant pathway leading to HOOH + O2. Finally, the strong negative temperature dependence of the rate constant observed for this reaction is attributed to the reversible formation of the prereactive complex in the entrance channel rather than to a short-lived tetraoxide intermediate.     

Oxidative addition of the ethane C-C bond to Pd. An ab initio benchmark and DFT validation study

We have computed a state-of-the-art benchmark potential energy surface (PES) for the archetypal oxidative addition of the ethane C-C bond to the palladium atom and have used this to evaluate the performance of 24 popular density functionals, covering LDA, GGA, meta-GGA, and hybrid density functionals, for describing this reaction. The ab initio benchmark is obtained by exploring the PES using a hierarchical series of ab initio methods [HF, MP2, CCSD, CCSD(T)] in combination with a hierarchical series of five Gaussian-type basis sets, up to g polarization. Relativistic effects are taken into account either through a relativistic effective core potential for palladium or through a full four-component all-electron approach. Our best estimate of kinetic and thermodynamic parameters is -10.8 (-11.3) kcal/mol for the formation of the reactant complex, 19.4 (17.1) kcal/mol for the activation energy relative to the separate reactants, and -4.5 (-6.8) kcal/mol for the reaction energy (zero-point vibrational energy-corrected values in parentheses). Our work highlights the importance of sufficient higher angular momentum polarization functions for correctly describing metal-d-electron correlation. Best overall agreement with our ab initio benchmark is obtained by functionals from all three categories, GGA, meta-GGA, and hybrid DFT, with mean absolute errors of 1.5 to 2.5 kcal/mol and errors in activation energies ranging from -0.2 to -3.2 kcal/mol. Interestingly, the well-known BLYP functional compares very reasonably with a slight underestimation of the overall barrier by -0.9 kcal/mol. For comparison, with B3LYP we arrive at an overestimation of the overall barrier by 5.8 kcal/mol. On the other hand, B3LYP performs excellently for the central barrier (i.e., relative to the reactant complex) which it underestimates by only -0.1 kcal/mol.     

Performance of the correlation consistent composite approach for transition states: a comparison to G3B theory

The correlation consistent composite approach (ccCA) was applied to the prediction of reaction barrier heights (i.e., transition state energy relative to reactants and products) for a standard benchmark set of reactions comprised of both hydrogen transfer reactions and nonhydrogen transfer reactions (i.e., heavy-atom transfer, SN2, and unimolecular reactions). The ccCA method was compared against G3B for the same set of reactions. Error metrics indicate that ccCA achieves "chemical accuracy" with a mean unsigned error (MUE) of 0.89 kcal/mol with respect to the benchmark data for barrier heights; G3B has a mean unsigned error of 1.94 kcal/mol. Further, the greater accuracy of ccCA for predicted reaction barriers is compared to other benchmarked literature methods, including density functional (BB1K, MUE=1.16 kcal/mol) and wavefunction-based [QCISD(T), MUE=1.10 kcal/mol] methods.     

Theoretical studies of proton-transfer reactions of 2-hydroxypyridine--(H2O)n (n = 0-2) in the ground and excited states

The potential energy profiles for proton-transfer reactions of 2-hydroxypyridine and its complexes with water were determined by MP2, CASSCF and MR-CI calculations with the 6-31G** basis set. The tautomerization reaction between 2-hydroxypyridine (2HP) and 2-pyridone (2PY) does not take place at room temperature because of a barrier of approximately 35 kcal/mol for the ground-state pathway. The water-catalyzed enol-keto tautomerization reactions in the ground state proceed easily through the concerted proton transfer, especially for the two-water complex. The S1 tautomerization between the 2HP and 2PY monomers has a barrier of 18.4 kcal/mol, which is reduced to 5.6 kcal/mol for the one-water complex and 6.4 kcal/mol for the two-water complex. The results reported here predict that the photoinduced tautomerization reaction between the enol and keto forms involves a cyclic transition state having one or two water molecules as a bridge.     

Ab initio rate constants from hyperspherical quantum scattering: application to H+C2H6 and H+CH3OH

The dynamics and kinetics of the abstraction reactions of H atoms with ethane and methanol have been studied using a quantum mechanical procedure. Bonds being broken and formed are treated with explicit hyperspherical quantum dynamics. The ab initio potential energy surfaces for these reactions have been developed from a minimal number of grid points (average of 48 points) and are given by analytical functionals. All the degrees of freedom except the breaking and forming bonds are optimized using the second order perturbation theory method with a correlation consistent polarized valence triple zeta basis set. Single point energies are calculated on the optimized geometries with the coupled cluster theory and the same basis set. The reaction of H with C2H6 is endothermic by 1.5 kcal/mol and has a vibrationally adiabatic barrier of 12 kcal/mol. The reaction of H with CH3OH presents two reactive channels: the methoxy and the hydroxymethyl channels. The former is endothermic by 0.24 kcal/mol and has a vibrationally adiabatic barrier of 13.29 kcal/mol, the latter reaction is exothermic by 7.87 kcal/mol and has a vibrationally adiabatic barrier of 8.56 kcal/mol. We report state-to-state and state-selected cross sections together with state-to-state rate constants for the title reactions. Thermal rate constants for these reactions exhibit large quantum tunneling effects when compared to conventional transition state theory results. For H+CH3OH, it is found that the CH2OH product is the dominant channel, and that the CH3O channel contributes just 2% at 500 K. For both reactions, rate constants are in good agreement with some measurements.     

Theoretical study on the reaction pathways of HFCO + H2O

For the reaction of methanoyl fluoride with water, both optimized structures and vibrational wavenumbers of reaction intermediates, transition structures and product complexes were calculated and characterized with theory at the MP2/6-311++G(d,p) level. Including a catalytic path and concerted and stepwise hydrolysis paths, possible reaction mechanisms were also investigated. The catalytic reaction of HFCO yielding HF and CO has the smallest activation barrier, 29.6 kcal/mol, whereas for the concerted hydrolysis 33.0 kcal/mol is required to overcome the barrier to form transoid HCOOH + HF, which is less than for the stepwise counterpart, 42.0 kcal/mol.