Theoretical study of neighboring carbonyl group participation in the elimination kinetics of chloroketones in the gas phase

The gas‐phase elimination of kinetics 4‐chlorobutan‐2‐one, 5‐chloropentan‐2‐one, and 4‐chloro‐1‐phenylbutan‐1‐one has been studied using electronic structure methods: B3LYP/6‐31G(d,p), B3LYP/6‐31++G(d,p), MPW91PW91/6‐31G(d,p), MPW91PW91/6‐31++G(d,p), PBEPBE/6‐31G(d,p), PBEPBE /6‐31++G(d,p), and MP2/6‐31++G(d,p). The above‐mentioned substrates produce hydrogen chloride and the corresponding unsaturated ketone. Calculation results of 4‐chlorobutan‐2‐one suggest a non‐synchronous four‐membered cyclic transition state (TS) type of mechanism. However, in the case of 5‐chloropentan‐2‐one and 4‐chloro‐1‐phenylbutan‐1‐one, the carbonyl group assists anchimerically through a polar five‐membered cyclic TS mechanism. The polarization of the C? Cl bond, in the sense of C^δ+…Cl^δ?, is a rate‐determining step in these elimination reactions. The significant increase in rates in the elimination of 5‐chloropentan‐2‐one and 4‐chloro‐1‐phenylbutan‐1‐one is attributed to neighboring group participation due to the oxygen of the carbonyl group assisting the C? Cl bond polarization in the TS. Copyright © 2010 John Wiley & Sons, Ltd.     

Theoretical study on the mechanism of the gas-phase elimination kinetics of alkyl chloroformates

The theoretical calculations on the mechanism of the homogeneous and unimolecular gas-phase elimination kinetics of alkyl chloroformates– ethyl chloroformate (ECF), isopropyl chloroformate (ICF), and sec-butyl chloroformate (SCF) – have been carried out by using CBS-QB3 level of theory and density functional theory (DFT) functionals CAM-B3LYP, M06, MPW1PW91, and PBE1PBE with the basis sets 6-311++G(d,p) and 6-311++G(2d,2p). The chlorofomate compounds with alkyl ester C_β–H bond undergo thermal decomposition producing the corresponding olefin, HCl and CO₂. These homogeneous eliminations are proposed to undergo two different types of mechanisms: a concerted process, or via the formation of an unstable intermediate chloroformic acid (ClCOOH), which rapidly decomposes to HCl and CO₂ gas. Since both elimination mechanisms may occur through a six-membered cyclic transition state structure, it is difficult to elucidate experimentally which is the most reasonable reaction mechanism. Theoretical calculations show that the stepwise mechanism with the formation of the unstable intermediate chloroformic acid from ECF, ICF, and SCF is favoured over one-step elimination. Reasonable agreements were found between theoretical and experimental values at the CAM-B3LYP/6-311++G(d,p) level.     

Homogeneous catalysis on the gas-phase dehydration reaction of tertiary alcohols by hydrogen bromide. Density functional theory calculation

The gas-phase thermal dehydration mechanism of tert-butanol, 2-methyl-2-butanol, 2-methyl-2-pentanol and 2,3-dimethyl-2-butanol by homogeneous catalysis of hydrogen bromide was examined by density functional theory calculations with the hybrid functionals: M062X, CAMB3LYP and WB97XD. Reasonable agreements were found between theoretical and experimental enthalpy values at the WB97XD/6-311++G(d,p) level. The dehydration mechanism of tert-butanol with and without catalysis was evaluated in order to examine the catalyst effect on the mechanism. The elimination reaction without catalysis involves a four-membered transition state (TS), while the reaction with catalysis involves a six-membered TS. The mechanism without catalysis has enthalpy activation over 150 kJ mol^–1 greater than the catalysed reaction. In all these reactions, the elongation of the C–O bond is significant in the TS. The un-catalysed reaction is controlled by breaking of C–O bond, and it was found to be more synchronous (Sy ≈ 0.91) than the hydrogen bromide catalysed reactions (Sy ≈ 0.75–0.78); the latter reactions are dominated by the three reaction coordinates associated with water formation. No significant effect on the enthalpies of activation was observed when the size of the alkyl chain was increased.     

HArF和HNO二聚体相互作用氢键的理论研究

用二阶微扰理论结合6-311+G**、6-311++G**和6-311++G(2d,2p)基组对氢键相互作用二聚体HNO···HArF进行研究．在MP2/6-311+G**、MP2/6-311++G**和MP2/6-311++G(2d,2p)水平上，利用标准方法和均衡校正方法对二聚体进行了几何优化、振动频率和相互作用能的计算．对于相互作用能采用G2MP2方法计算．计算结果表明存在两种稳定的二聚体HNO···HArF结构，在这两种结构中，Dimer I(H···F)比Dimer II(H···O)更加稳定．通过振动频率的计算表明，在Dimer I(H···F)中存在N-H···F蓝移氢键，在DimerII(H···O)中存在Ar-H···O红移氢键，并对蓝移氢键加以确认．利用电子密度拓扑学分析和自然键轨道分析对于氢键红移和蓝移进行了合理解释．     

Theoretical study on the mechanism of thieno[3,2‐b]benzofuran bromination: the importance of Lewis and non‐Lewis type NBOs interactions along the reaction path

A density functional theory study has been performed to estimate the electrophilic thieno[3,2‐b]benzofuran bromination reaction. Optimized structures for all stationary points were examined by employing the B3LYP and BMK at the 6‐31++G(d,p), 6‐311G(d,p), and 6‐311++G(d,p) levels of theory. The solvent polarity has a significant effect on a reduction of activation energies barriers. The reaction involves the formation of a triangle complex, migration of a proton through the bromine moiety followed by ionization of the bromine bond, and activation to the σ‐complex. Finally, the σ‐complex transforms into the reaction products. The natural bond orbital (NBO) population analysis was performed along the reaction minimal energy path defined as a function of the intrinsic reaction coordinate (IRC). The evolution of interaction energies between filled and empty NBOs along IRC has been estimated. The importance of these interactions for the disruption of Br?Br and C?H bonds and creation of C?Br and H?Br bonds have been emphasized. The changes in NBOs hybridization, covalency effects, electrostatic potential density maps, and occupancy of natural bonds have been investigated along IRC. The results obtained explain well the essence of bonding transformations and electron density changes during the reaction. Copyright © 2015 John Wiley & Sons, Ltd.     

FT-IR,FT-Raman spectra and quantum mechanical study of piperidine-3-carboxylic acid and its tautomers,isomers

The mid-IR, far-IR, and Raman spectra of piperidine-3-carboxylic acid were measured and interpreted with support of the MP2 and B3LYP/6-311++G(d, p) calculated harmonic vibrational spectra. 10 stable piperidine-3-carboxylic acid tautomers/isomers were found after B3LYP, calculations. The experimental absorption bands of carboxylate (COO^?) group show that the free piperidine-3-carboxylic acid molecule exists in zwitterionic form and the most stable tautomer (NAT-1) can be stabilized by an intramolecular N-H...O hydrogen bond. All vibrational frequencies of NAT-1 assigned in detail with the help of total energy distribution (TED). The experimental vibrational wave numbers were compared with the calculated data.     

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.     

The influence of CH … π interaction on coupling constants across N … H–F hydrogen bond in a substituted T-shaped configuration: a theoretical study

For studying the influence of CH?…?π interaction on coupling constants across N?…?H–F hydrogen bond in a substituted T-shaped configuration, X-benzene⊥(FH?…?pyrazine?…?HF) complexes are chosen as a working model. NMR calculations are performed at B3LYP/6-311++G(d,p) and PBE0/6-311++G(d,p) levels. Here, correlations between energetic, geometrical and topological parameters and coupling constants are investigated. The results indicate that direct correlations exist between strength of N?…?H hydrogen bond, electron-donating power of substituents and |^2hJ_N?F|. Also, |^2hJ_N?F| increases as cooperative and synergistic energies become more negative. These behaviours are reversed for ^1hJ_N?H. Due to contradictory behaviours of FC and PSO terms, an irregular trend is observed for ¹J_H?F.     

DFT study on abstraction reaction mechanism of oh radical with 2‐methoxyphenol

Reaction mechanism of 2‐methoxyphenol (2MP) (guaiacol) with OH radical has been performed using density functional theory methods BH&HLYP and MPW1K method with 6‐311++G(d,p) basis set. Single‐point energy calculations were done using CCSD(T)/6‐311++G(d,p). The theoretical results reveal that the hydrogen abstraction from methoxy group is found to be the dominant reaction channel with an energy barrier of 9.31 kcal/mol. Also, time‐dependent density functional theory calculations have been performed using BH&HLYP/6‐311++G(d,p) level of theory, and the results reveal that the reactions occur in ground state than the excited state. The results of reaction force profile indicate that structural rearrangements are most influential with high percentage than the relaxation process. The calculated theoretical rate constants (12.19 × 10^?11 cm³ molecule^?1 s^?1) are in good agreement with the experimental rate constant. The atmospheric lifetime of 2‐methoxyphenol with respect to OH radicals is 2.27 hours, which implies that OH radical plays an important role in the degradation of 2MP. The Wiberg bond index of the abstraction reaction reveals that the bond order is concerted, partially synchronic. The reactant‐like transition state satisfies Hammond postulate, which eventually results in an exothermic reaction, and the product‐like transition state reveals in endothermic nature.     

甲基乙烯醚和OH反应机理的理论研究

利用量子化学从头计算的方法对甲基乙烯醚的两个异构体之间的转化,羟基与顺式-甲基乙烯醚和反式-甲基乙烯醚的加成反应,以及羟基提取甲基上的氢原子的反应机理进行了研究.研究结果表明:顺式-甲基乙烯醚比反式-甲基乙烯醚更加稳定,在QCISD/6-31G(d,P)//BHandHLYP/6.311 G(d,P)理论水平下,OH加到顺式-甲基乙烯醚1号住的碳原子上需要跨越的能垒比其它反应通道需要跨越的能垒少7.5～34 KJ/mol,因此是主要的反应通道,而OH加在反式.甲基乙烯醚2号位的碳原子上所需要跨越的能垒比其它反应路径所需要跨越的能垒少8.3～26.7 kJ/mol,因此是主要的反应路径.利用经典过渡态理论计算了总的速率常数     

Theoretical investigation of the transition states leading to HCI elimination in 2-chloropropene

This paper describes ab initio electronic structure calculations on the planar transition states of 2-chloropropene leading to HCI elimination in the ground electronic state to form either propyne or allene as the cofragment. The calculations provide optimized geometries of the transition states for these two reaction channels, together with vibrational frequencies, barrier heights, and reaction endothermicities. The calculated barrier heights for the two distinct four-centre HCI elimination transition states, one leading to HCI and propyne and the other leading to HCI and allene, are 72.5kcalmol^?1 (77.8kcalmol^?1 without zero-point correction) and 73.2kcalmol^?1 (78.7kcalmol^?1) at the MP2/6-311G(d, p) level, 71.Okcalmol^?1 (76.3kcalmol^?1) and 70.5kcalmol^?1 (76.0kcalmol^?1) at the QCISD(T)/6-311 +G(d, p)//MP2/6-311G(d, p) level, and 66.9kcalmol^?1 (71.7kcalmol^?1) and 67.3kcalmol^?1 (72.1kcalmol¹) at the G3//B3LYP level of theory. Calculated harmonic vibrational frequencies at the B3LYP/6-31G(d) level along with transition state barrier heights from the G3//B3LYP level of theory are used to obtain RRKM reaction rate constants for each transition state, which determine the branching ratio between the two HCI elimination channels. Even at internal energies well above both HCI elimination barriers, the HCI elimination leading to propyne is strongly favoured. The smaller rate constant for the HCI elimination leading to allene can be attributed to the strong hindrance of the methyl rotor in the corresponding transition state.     

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.     

Reaction mechanism and rate constants of the CH+CH4 reaction: a theoretical study

Ab initio and density functional calculations have been performed to elucidate the mechanism of CH radical insertion into methane. The results show that the reaction can be viewed to occur via two stages. On the first stage, the CH radical approaches methane without large structural changes to acquire proper positioning for the subsequent stage, where H-migration occurs from CH₄ to CH, along with a C–C bond formation. Where the first stage ends and the second begins, a tight transition state was located using the B3LYP/6-311G(d,p) and MP4(SDQ)/6-311++G(d,p) methods. Using a rigid rotor – harmonic oscillator approach within transition state theory, we show that at the MP5/6-311++G(d,p)//MP4(SDQ)/6-311++G(d,p) level the calculated rate constants are in a reasonably good agreement with experiment in a broad temperature range of 145–581 K. Even at low temperatures, the insertion reaction bottleneck is found about the location of the tight transition state, rather than at long separations between the CH and CH₄ reactants. In addition, high level CCSD(T)-F12/CBS calculations of the remainder of the C₂H₅ potential energy surface predict the CH+CH₄ reaction to proceed via the initial insertion step to the ethyl radical which then can emit a hydrogen atom to form highly exothermic C₂H₄+H products.     

Theoretical determination of the 1H NMR spectrum of ethanol

GIAO calculations of the ¹H NMR chemical shifts for ethanol at the SCF and DFT levels of theory are presented. The importance of molecular geometry and basis set is discussed. Vibrational correction to the hydroxyl proton chemical shift is also considered in calculations for the monomer of ethanol. The final theoretical results for the monomer obtained at the optimized DFT/B3LYP/6-311G(d,p) geometry with the 6-311G++ (d,p) basis set for NMR are in very good agreement with gas phase experimental data. For the liquid phase ethanol the hydrogen bonding effects are taken into account by performing calculations on various clusters of ethanol. It is shown that inaccuracy due to molecular geometry and basis set in the monomer of ethanol is magnified significantly in calculations for its clusters. In this context the structure of liquid ethanol as predicted recently by quantum cluster equilibrium (QCE) theory is discussed.     

Theoretical study of mechanisms for NCO with CH3 reaction

A detailed computational study has been performed at the QCISD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) level for the NCO with CH₃ reaction by constructing singlet and triplet potential energy surfaces (PES). The results show that the title reaction is more favorable for the singlet PES than the triplet PES. On the singlet PES, the dominant channel is the barrierless addition of the O or N atom to the C atom of the methyl group to form CH₃NCO (IM1) and CH₃OCN (IM2). On the triplet PES, the favorable channel is the barrierless addition of the N atom to the C atom of the methyl group to form an intermediate CH₃NCO (³IM2), which then undergoes a N–C bond scission process to give out CH₃N + CO.     

Electrocyclic [1,5] hydrogen shift in the thermal elimination kinetics of phenyl acetate and p-tolyl acetate in the gas phase: a density functional theory study

The kinetics and mechanisms of thermal decomposition of phenyl acetate and p-tolyl acetate in the gas phase were studied by means of electronic structure calculations using density functional theory methods: B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), B3PW91/6-31G(d,p), B3PW91/6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31++G(d,p), PBE/6-31G(d,p) and PBE/6-31++G(d,p). Two possible mechanisms have been considered: mechanism A is a stepwise process involving electrocyclic [1,5] hydrogen shift to eliminate ketene through concerted six-membered cyclic transition-state structure, followed by tautomerisation of cyclohexadienone or by 4-methyl cyclohexadienone intermediate to give the corresponding phenol. Mechanism B is a one-step concerted [1,3] hydrogen shift through a four-membered cyclic transition-state geometry, to produce ketene and phenol or p-cresol. Theoretical calculations showed reasonable agreement with experimental activation parameters when using the Perdew, Burke and Ernserhof (PBE)functional, through the stepwise [1,5] hydrogen-shift mechanism. For mechanism B, large deviation for the entropy of activation was observed. No experimental data were available for p-tolyl acetate; however, theoretical calculations showed similar results to phenyl acetate, thus supporting the stepwise mechanism for both phenyl acetate and p-tolyl acetate.     

Theoretical investigation of the photochemical reaction mechanism of cyclopropenone decarbonylation

The gas-phase decomposition mechanism of the photochemical and thermal reaction of cyclopropenone leading to carbon monoxide and acetylene has been investigated theoretically. We employed the B3LYP, MP2, and CASSCF methods with the 6-311?+?G** basis set to determine the pathways and the potential energy surface (PES) of this reaction. PES minima were characterized by the absence of any imaginary frequencies and compared with the transition states that contained single imaginary frequencies. The intrinsic reaction coordinate (IRC) method was used to find the minimum energy paths in which reactants and products were connected to the transition states. Activation barrier, thermodynamic, and IRC analyses were performed using the above three methods. Our computations indicated that the decomposition of cyclopropenone proceeds through a stepwise mechanism containing two transition states (TS1 and TS2) and an intermediate. The results show that TS1, the critical transition state, determines the rate of the cyclopropenone decomposition reaction. Therefore, we employed natural bond order (NBO) calculations to probe the structure of the intermediate. The calculations showed that the intermediate has resonance structures containing a carbene and a zwitterion. Our results are in good agreement with previous theoretical and experimental studies.     

Natural Bond Orbital(NBO),Natural Population Analysis and Mulliken Analysis of Atomic Charges of 4-Amino-3-Phenylbutanoic Acid

The Molecular Structure of 4-Amino-3-phenylbutanoic acid conformers have been studied in the gas phase. Natural Bond Orbital Analysis (NBO) and Mulliken analysis of atomic charges of 4-Amino-3-phenylbutanoic acid have been performed by DFT level of theory using B3LYP/6-311++G(d,p) basis set. The atomic charges, electronic exchange interaction and charge delocalization of the molecule have been performed by Natural Bond Orbital(NBO) analysis and Natural Population Analysis(NPA) have been constructed at B3LYP/6-311++G(d,p) level.     

(LiH)_n(n=1～5)团簇与NH_3反应的密度泛函理论研究

采用密度泛函理论中杂化密度泛函B3LYP/6-311G(d,p)方法,对(LiH)_n(n=1～5)团簇结构进行计算,得到最稳定构型,并计算分析其与NH_3的反应机理.对各反应的中间体和过渡态进行频率分析和内禀反应坐标(IRC)计算,以验证反应的正确性.用QCISD/6-311G(d,p)方法计算各驻点的单点能,得到能量信息.结果表明:各反应所释放H_2中的两个氢原子分别来源于NH_3和(LiH)_n(n=1～5)团簇.弱化N-H键的作用有利于反应能垒的降低,是反应脱氢的关键.LiH团簇尺寸变化对反应能垒没有太大影响.     

The mechanism of the homogeneous,unimolecular gas‐phase elimination kinetic of 1,1‐dimethoxycyclohexane: experimental and theoretical studies

The gas‐phase elimination of 1,1‐dimethoxycyclohexane yielded 1‐methoxy‐1‐cyclohexene and methanol. The kinetics were determined in a static system, with the vessels deactivated with allyl bromide, and in the presence of the free radical inhibitor cyclohexene. The working temperature was 310–360 °C and the pressure was 25–85 Torr. The reaction was found to be homogeneous, unimolecular, and follows a first‐order rate law. The temperature dependence of the rate coefficients is given by the following Arrhenius equation: log k(s^?1) = [(13.82 ± 0.07) – (193.9 ± 1.0)(kJ mol^?1)](2.303RT)^?1; r = 0.9995. Theoretical calculations were carried out using density functional theory (DFT) functionals B3LYP, MPW1PW91, and PBE with the basis set 6‐31G(d,p) and 6‐31G++(d,p). The calculated values for the energy of activation and enthalpy of activation are in reasonably good agreement with the experimental values using the PBE/6‐31G (d,p) level of theory. Both experimental results and theoretical calculations suggest a molecular mechanism involving a concerted polar four‐membered cyclic transition state. The transition state structure of methanol elimination from 1,1‐dimethoxycyclohexane is characterized by a significantly elongated C? O bond, while the C_β? H bond is stretched to a smaller extent, as compared to the reactant. The process can be described as moderately asynchronic with some charge separation in the TS. Copyright © 2010 John Wiley & Sons, Ltd.