A computational study into the reactivity of epichlorohydrin and epibromohydrin under acidic conditions in the gas phase and aqueous solution

Ab initio molecular orbital calculations have been carried out upon epichlorohydrin and epibromohydrin at the Hartree–Fock (HF) and Møller–Plesset (MP2) levels of theory to explore the reactivity of these species with respect to nucleophilic attack by water under acidic conditions in the gas phase and aqueous solution. These results suggest that nucleophilic attack occurs preferentially at the epoxy carbon atoms in both the gas phase and aqueous solution. These results are in contrast to those found for nucleophilic attack under basic conditions, where attack at the halocarbon atom is competitive with that at the epoxy carbon atoms. Copyright © 2007 John Wiley & Sons, Ltd.     

Computational study of the mechanism of thermal decomposition of xanthates in the gas phase (the Chugaev reaction)

A theoretical study on the mechanism of the thermal decomposition of a series of xanthates, O‐alkyl S‐methyl and S‐alkyl O‐methyl dithiocarbonates, has been carried out, and the alkyl groups being ethyl, isopropyl, and tert‐butyl. Kinetically, these xanthates can be classified in two groups: those where the oxygen atom is involved in the bonding changes of the transition state (properly the Chugaev reaction), and those where it is not, O‐alkyl S‐methyl and S‐alkyl O‐methyl dithiocarbonates, respectively. We have studied not only the thermal elimination reactions but also the other possible reactions such as the thione‐to‐thiol rearrangement and the nucleophilic substitution to give ethers or thioethers. Two possible mechanisms for the thermal elimination reactions, in one and in two steps, respectively, have been studied. Calculations were made at the MP2/6‐31G(d) level of theory, and the progress of the reactions has been followed by means of the Wiberg bond indices. Copyright © 2008 John Wiley & Sons, Ltd.     

A computational study of stereospecifity in the thermal elimination reaction of menthyl benzoate in the gas phase

This paper reports a theoretical study, at the B3LYP/6–31 + G(d,p) and M05‐2X/6–31G + (d,p) levels, on the thermal decomposition of menthyl benzoate (2‐isopropyl‐5‐methylcyclohexyl benzoate). It undergoes a unimolecular first‐order elimination to give 3‐menthene (1‐isopropyl‐4‐methylcyclohexene), 2‐menthene (3‐isopropyl‐6‐methylcyclohexene), and benzoic acid. We studied two possible mechanisms trying to explain the formation of 2‐ and 3‐menthene, via six‐membered or four‐membered cyclic transition states. Rate constants were calculated at two temperatures, 587.1 and 598.6 K, and they agree well with the experimentally determined values. We verify that 3‐menthene is the product mainly formed at both temperatures. The progress of the reactions has been followed by means of the Wiberg bond indices. Intrinsic reaction coordinate (IRC) calculations have been carried out to verify that the localized transition state structures connect with the reactants and products and also to verify that the parent compound, menthyl benzoate, is taking the cis‐configuration needed in the reaction. Copyright © 2009 John Wiley & Sons, Ltd.     

双自由基CH燃烧反应机理的研究

应用量子理论从头算和密度泛函理论(DFT)对双自由基CH(X2Π)与O2(X3∑g-)的反应机理进行了研究.在B3PW91/6-311G**水平上优化了反应通道上各驻点(反应物、中间体、过渡态和产物)的几何构型,并计算了零点能和过渡态的虚频率.并由B3PW91/6-311G**给出了各物种的总能量.计算表明,反应物中自由基CH与O2反应主要在二重态势能面上进行,CH中的C原子可以插在O2分子中两个氧原子中间形成中间体1(2HCO2),中间体1(2HCO2)可以经过不同的反应通道形成不同的产物P1(1CO2 2H)和P2(1CO 2OH),各反应通道的反应热的计算与实验值吻合较好.     

The mechanism of alkaline hydrolysis of amides: a comparative computational and experimental study of the hydrolysis of N‐methylacetamide,N‐methylbenzamide,and acetanilide

Theoretical computations and experimental kinetic measurements were applied in studying the mechanistic pathways for the alkaline hydrolysis of three secondary amides: N‐methylbenzamide, N‐methylacetamide, and acetanilide. Electronic structure methods at the HF/6‐31+G(d,p) and B3LYP/6‐31+G(d,p) levels of theory are employed. The energies of the stationary points along the reaction coordinate were further refined via single point computations at the MP2/6‐31+G(d,p) and MP2/6‐311++G(2d,2p) levels of theory. The role of water in the reaction mechanisms is examined. The theoretical results show that in the cases of N‐methylbenzamide and N‐methylacetamide the process is catalyzed by an ancillary water molecule. The influence of water is further assessed by predicting its role as bulk solvent. The alkaline hydrolysis process in aqueous solution is characterized by two distinct free energy barriers: the formation of a tetrahedral adduct and its breaking to products. The results show that the rate‐determining stage of the process is associated with the second transition state. The entropy terms evaluated from theoretical computations referring to gas‐phase processes are significantly overestimated. The activation barriers for the alkaline hydrolysis of N‐methylbenzamide and acetanilide were experimentally determined. Quite satisfactory agreement between experimental values and computed activation enthalpies was obtained. Copyright © 2008 John Wiley & Sons, Ltd.     

Theoretical investigation of high-pressure phase transitions in Mg1-xSrxO

The stability of Mg_1-xSr_xO solid solution has been analyzed using charge transfer interaction potential (CTIP) model as well as density functional theory-based ab initio approach with Perdew–Burke–Ernzerhof (PBE) type parameterized generalized gradient approximation. The present CTIP model consists of long-range part as modified coulomb interactions and charge transfer forces whereas short-range part includes the van der Waals as well as Hafemeister Flygare type overlap repulsive interactions effective up to next nearest neighbor ions. The present study finds that under the influence of pressure host binary oxides as well as their solid solutions undergo B1→B2 structural phase transition in the pressure range of 54–495 GPa. The variation of ground state properties and transition pressures have also been analyzed as a function of Sr composition. The observed results for the end point members are in agreement to their experimental counterparts and the deviations have been discussed in terms of interactions taken into consideration in two approaches.     

Pressure-induced phase transition and electronic properties of AlN nanowires: an ab initio study

The structural stability of AlN nanowires have been analyzed in wurtzite (B4), zincblende (B3), rocksalt (B1) and CsCl (B2) type phases using density functional theory based ab initio approach. The total energy calculations have been performed in a self-consistent manner using local density approximation as exchange correlation functional. The analysis finds the B4 type phase as most stable amongst the other phases taken into consideration and observes the structural phase transition from B4?→?B3, B4?→?B1, B4?→?B2, B3?→?B1 and B3?→?B2 at 42.7, 76.54, 142, 30.4 and 108.9?GPa respectively. Lattice parameter, bulk modulus and pressure derivatives of AlN nanowires have also been calculated for all the stable phases. The electronic band structure analysis of AlN nanowires shows a semiconducting nature in its B4, B3 and B1 type phases, whereas the B2 type phase is found to be metallic.     

Experimental and theoretical study of the mechanism for the kinetic of elimination of methyl carbazate in the gas phase

The elimination kinetic of methyl carbazate in the gas phase was determined in a static system over the temperature range of 340–390 °C and pressure range of 47–118 Torr. The reaction is homogeneous, unimolecular, and obeys a first order rate law. The decomposition products are methyl amine, nitrous acid, and CO gas. The variation of the rate coefficients with temperatures is given by the Arrhenius expression: log k₁ (s^?1) = (11.56 ± 0.34) ? (180.7 ± 4.1) kJ mol^?1(2.303 RT)^?1. The estimated kinetics and thermodynamics parameters are in good agreement to the experimental values using B3LYP/6‐31G (d,p), and MP2/6‐31G (d,p) levels of theory. These calculations imply a molecular mechanism involving a concerted non‐synchronous quasi three‐membered ring cyclic transition state to give an unstable intermediate, 1,2‐oxaziridin‐3‐one. Bond order analysis and natural charges implies that polarization of O (alkyl)? C (alkyl) bond of the ester is rate determining in this reaction. Copyright © 2008 John Wiley & Sons, Ltd.     

A refined estimate of the bond length of methane

The atmospheric reaction of H₂S with Cl was investigated using high level ab initio calculations and Canonical Variational Transition State Theory (CVTST). The adduct formation step is the dynamical bottleneck, and the rate constant was calculated to be 1.2 × 10^?9 cm³ molecule^?1 s^?1, which is around ten times greater than the upper experimental value. Additional ab initio classical trajectory calculations show that the adduct formed in the initial collision can easily dissociate, recrossing the variational transition state. The stabilization of this species depends on the vibrational excitation of H₂S molecule, which requires an almost collinear SH-Cl collision. These dynamical effects provide an explanation for the substantial error in the rate constant obtained using CVTST.     

First-principles study of phase transition and elastic properties of ScSb and YSb compounds

The phase transition of ScSb and YSb from the NaCl-type (B1) structure to the CsCl-type (B2) structure is investigated by the ab initio plane-wave pseudopotential density functional theory method. It is found that the pressures for transition from the B1 structure to the B2 structure obtained from the equal enthalpies are 38.3 and 32.1 GPa for ScSb and YSb, respectively. From the variations of elastic constants with pressure, we find that the B1 phase of ScSb and YSb compounds are unstable when applied pressures are larger than 46.3 and 64.2 GPa, respectively. Moreover, the detailed volume changes during phase transition are analyzed.     

A computational study of the thermolysis of β‐hydroxy ketones in gas phase and in m‐xylene solution

Theoretical calculations at the M05‐2X/6‐31+G(d) level of theory have been carried out in order to explore the nature of the mechanism of the thermal decomposition reactions of the β‐hydroxy ketones, 4‐hydroxy‐2‐butanone, 4‐hydroxy‐2‐pentanone, and 4‐hydroxy‐2‐methyl‐2‐pentanone in gas phase and in m‐xylene solution. The mechanism proposed is a one‐step process proceeding through a six‐membered cyclic transition state. A reasonable agreement between experimental and calculated activation parameters and rate constants has been obtained, the tertiary : secondary : primary alcohol rate constant ratio being calculated, at T = 503.15 K, as 5.9:4.7:1.0 in m‐xylene solution and 44.1:5.0:1.0 in the gas phase, compared with the experimental values, 3.7:1.3:1.0 and 13.5:3.2:1.0, respectively. The progress of the thermal decomposition reactions of β‐hydroxy ketones has been followed by means of the Wiberg bond indices. The lengthening of the O₁–C₂ bond with the initial migration of the H₆ atom from O₅ to O₁ can be seen as the driving force for the studied reactions. Calculated synchronicity values indicate that the mechanisms correspond to concerted and highly synchronous processes. The transition states are “advanced”, nearer to the products than to the reactants. Copyright © 2012 John Wiley & Sons, Ltd.     

Relationship between experimental pKa values in aqueous solution and a gas phase bond length in bicyclo[2.2.2]octane and cubane carboxylic acids

Linear correlations were established between the calculated bond lengths and the pK_a or σ_I values for a series of 4‐substituted bicyclo[2.2.2]octane‐1‐carboxylic acid and 4‐cubane‐1‐carboxylic acid derivatives. The bond lengths have been calculated at a modest computational level, HF/6‐31G(d), both in the gas phase and with the continuum solvation model, polarisable continuum model (PCM). In general, the best correlations are obtained when the PCM model is taken into account, especially when neutral and charged molecules are considered together. The best models in each case show square correlation coefficients (R²) larger than 0.9 and indicate that they can be used as predictive tools. These results expand previous results that indicate the possibility of a relationship between gas phase bond length and pK_a values in aqueous solution and indicate that such relationships are more general than hitherto expected. Copyright © 2013 John Wiley & Sons, Ltd.     

A computational study on the experimentally observed sensitivity of Ga-doped ZnO nanocluster toward CO gas

Metal doped ZnO nanostructures have attracted extensive attention as chemical sensors for toxic gases. An experimental study has previously shown that Ga-doped ZnO nanostructures significantly show a higher electronic response than the undoped sample toward CO gas. Here, the electronic sensitivity of pristine and Ga-doped ZnO nanoclusters to CO gas is explored using density functional theory computations (at B3LYP, PBE, M06-2X, and ωB97XD levels). Our results reproduce and clarify the electrical behavior which has been observed experimentally from the ZnO nanoparticles after the exposure to CO gas. We showed that the calculated change of HOMO-LUMO gap may be a proper index for the change of electrical conductance which is measurable experimentally. It was found that, in contrast to the pristine ZnO nanocluster, the electronic properties of Ga-doped cluster are sharply sensitive to the presence of CO gas which is in good accordance with the results of the experimental study.     

Theoretical calculations for neighboring group participation in gas‐phase elimination kinetics of 2‐hydroxyphenethyl chloride and 2‐methoxyphenethyl chloride

Theoretical calculation of the kinetics and mechanisms of gas‐phase elimination of 2‐hydroxyphenethyl chloride and 2‐methoxyphenethyl chloride has been carried out at the MP2/6‐31G(d,p), B3LYP/6‐31G(d,p), B3LYP/6‐31 + G(d,p), B3PW91/6‐31G(d,p) and CCSD(T) levels of the theory. The two substrates undergo parallel elimination reactions. The first process of elimination appears to proceed through a three‐membered cyclic transition state by the anchimeric assistance of the aromatic ring to produce the corresponding styrene product and HCl. The second process of elimination occurs through a five‐membered cyclic transition state by participation of the oxygen of o‐OH or the o‐OCH₃ to yield in both cases benzohydrofuran. The B3PW91/6‐31G(d,p) method was found to be in good agreement with the experimental kinetic and thermodynamic parameters for both substrates in the two reaction channels. However, some differences in the performance of the different methods are observed. NBO analysis of the pyrolysis of both phenethyl chlorides implies a C? Cl bond polarization, in the sense of C^δ+…Cl^δ?, which is a rate‐determining step for both parallel reactions. Synchronicity parameters imply polar transition states of these elimination reactions. Copyright © 2009 John Wiley & Sons, Ltd.     

A theoretical investigation into dimethylcarbene and its diamino and diphosphino analogs: effects of cyclization and unsaturation on the stability and multiplicity

High levels of ab initio and DFT calculations (B3LYP/6‐311++G**, B3LYP/AUG‐cc‐pVTZ, and CCSD(T)/6‐311++G** levels) coupled with isodesmic reactions are used to compare and contrast the multiplicities and relative stabilities of singlet (s) and triplet (t) acyclic carbenes, including: dimethylcarbene, diaminocarbene, and diphosphinocarbene along with their saturated and unsaturated cyclic ones. Cyclization is unfavorable for all acyclic carbenes while unsaturation of cyclic analogs appears favorable. The simultaneous cyclization and unsaturation of dimethylcarbene increases the singlet–triplet energy gap (ΔE_s–t), while for diphosphinocarbene the situation is reversed. For diaminocarbene the increase of ΔE_s–t is encountered only during cyclization. Copyright © 2009 John Wiley & Sons, Ltd.     

Ab initio study on the anisotropy of mechanical behavior and deformation mechanism for boron carbide

The mechanical properties and deformation mechanisms of boron carbide under a-axis and c-axis uniaxial compression are investigated by ab initio calculations based on the density functional theory.Strong anisotropy is observed.Under a-axis and c-axis compression,the maximum stresses are 89.0 GPa and 172.2 GPa respectively.Under a-axis compression,the destruction of icosahedra results in the unrecoverable deformation,while under c-axis compression,the main deformation mechanism is the formation of new bonds between the boron atoms in the three-atom chains and the equatorial boron atoms in the neighboring icosahedra.     

Power-law falloff in the kosterlitz-Thouless phase of a two-dimensional lattice Coulomb gas

We give a rigorous proof of power-law falloff in the Kosterlitz-Thouless phase of a two-dimensional Coulomb gas in the sense that there exists a critical inverse temperaturegb and a constant >0 such that for all> and all external charges R we have , whereG (x) is the two-point external charges correlation function,=dist(, Z), and for 0$$ " align="middle" border="0"> . In the case of a hard-core or standard Coulomb gas with activityz, we may choose=(z) such that(z)24 asz0.     

First principles study of the electronic structure and phonon dispersion of naphthalene under pressure

The structural, electronic and vibrational properties of crystalline naphthalene has been investigated within the framework of density functional theory including van der Waals interactions. The computed lattice parameters and cohesive energy have good agreement with experimental data. We study on the structural and electronic properties of the naphthalene under the hydrostatic pressure of 0–20 GPa. The isothermal equations of state calculated from the results show good agreement with experiment in the pressure intervals studied. The phonon dispersion curves have been computed at ambient and hydrostatic pressure of 10 and 20 GPa. We have also calculated the quasiparticle band structure of naphthalene with the G₀W₀ approximation.     

Theoretical calculations of the kinetics and mechanisms of the gas phase elimination of primary,secondary, and tertiary 2‐hydroxyalkylbenzenes

Theoretical study of the elimination kinetics of 2‐phenylethanol, 1‐phenyl‐2‐propanol, and 2‐methyl‐1‐phenyl‐2‐propanol in the gas‐phase has been carried out at the MP2/6‐31G(d,p), B3LYP/6‐31G(d,p), B3LYP/6‐31++G(d,p), MPW1PW91/6‐31G(d,p), MPW1PW91/6‐31++G(d,p), PBEPBE/6‐31G(d,p), and PBEPBE/6‐31++G(d,p) levels of theory. The three substrates undergo two parallel elimination reactions. The first elimination appears to proceed through a six‐membered cyclic transition state to give toluene and the corresponding aldehyde or ketone. The second parallel elimination takes place through a four‐membered cyclic transition state producing water and the corresponding unsaturated aromatic hydrocarbon. Results from MP2/6‐31G(d,p) and MPW1PW91/6‐31++G(d,p) methods were found to be in good agreement with the experimental kinetic and thermodynamic parameters in the formation of toluene and the corresponding carbonyl compound. However, the results for PBEPBE/6‐31G(d,p) were in better agreement with the experimental data for the second parallel reaction yielding water and the corresponding unsaturated aromatic hydrocarbon. The charge distribution differences in the TS related to the substitution by methyl groups in the substrates can account for the observed reaction rate coefficients. The synchronicity parameters imply semi‐polar transition states for these elimination reactions. Copyright © 2009 John Wiley & Sons, Ltd.