Theoretical studies on the mechanism of α-hydroxy-acid gas-phase elimination process and its substituent effect

The α-hydroxy-acid gas-phase elimination process has been studied theoretically by HF/3-21G.The calculated results can be summed up as follows:(1) The elimination process is a stepwise reaction.In the first step,a 3-membered ring intermediate is formed via a 5-membered ring transition state;while the product is formed in the second step via a 3-membered ring transition state.(2) The obtained results of the substituent effect show that the increase of electronic donation of the alkyl groups is favorable for the reaction.Other substituents which show the electron-withdrawing inductive effect (e.g.-Cl,-CN,-CF3) are unfavorable for this process.     

Quasi-classical Trajectory Study of the Intramolecular Isotope Effect in the Reaction O（3p）＋H2/HD

Theoretical studies of the dynamics of the reactions O（3p）＋H2/HD（ν=0, j=0）→OH＋H have been performed with quasi-classical trajectory method （QCT） on an ab initio potential surface for the lowest triplet electronic state of H2O（aA＂）. The QCT-calculated integral cross sections are in good agreement with the earlier time-dependent quantum mechanics results. The state-resolved rotational distributions reveal that the product OH rotational distributions for O＋HD have a preference for populating highly internally excited states compared with the O＋H2 reaction. Distributions of differential cross sections show that directions of scattering are strongly dependent on the choice of quantum state. The polarization dependent generalized differential cross-sections and the distributions were calculated and a pronounced isotopic effect is revealed. The calculated results indicate that the product polarization is very sensitive to the mass factor.     

Computational Study on OH Addition Reactions of Three Sulfonamides in Aqueous Solution

OH addition reactions of cationic,neutral and anionic forms of three sulfonamides(sulfamethoxazole,sulfadiazine and sulfapyridine)in aqueous solution were theoretically studied using density functional theory(DFT)method at the M06-2X/6-311+G(3df,2p)level.Transition state theory was applied to estimate the secondary rate constants for these elementary reactions.The obtained results indicate that OH addition reactions of sulfonamides can take place spontaneously at standard conditions.The anionic form of three sulfonamides has the highest addition activity,while the corresponding cationic form is the most inactive addition reagent.The benzene ring of neutral forms of three sulfonamides is always a more favorable site for OH radical addition than the oxazole,pyrimidine or pyridine ring.C(3)or(and)C(5)atoms of benzene ring are the most favorable positions for OH addition occurring in benzene ring.Although the water solvent has no remarkable effect on OH addition reactions of neutral sulfonamides,it exerts a significant adverse influence on OH addition reactions of ionic sulfonamides.     

Ab initio studies on the pyrolysis mechanism of 2-haloacetic acids in the gas phase

The dehydrohalogenation mechanism of 2-haloacetic acids (XCH2CO2H, X=F, Cl and Br) has been studied theoretically by HF/3-21G and AM1 methods. The results indicate that these reactions are most probably proceeded in terms of a polar five-membered cyclic transition state in the gas phase. Their microscopic processes are beleived to be a stepwise reaction and the rate-determining step is the first one. By comparing the energy barriers of different 2-haloacetic acids, it can be realized that 2-fluoroacetic acid is easier to react than 2-chloroacetic and 2-bromoacetic acids.     

Theoretical Study on the Adsorption and Decomposition of Methanol over the Pt-Mo（111）/C Surface

The density functional theory(DFT) and self-consistent periodic calculation were used to investigate the methanol adsorption on the Pt-Mo(111)/C surface.The adsorption energies,equilibrium geometries and vibration frequencies of CH3OH on nine types of sites on the Pt-Mo(111)/C surface were predicted and the favorite adsorption site for methanol is the top-Pt site.Both sites of valence and conduction bands of doped system have been broadened,which are favorable for electrons to transfer to the cavity.The possible decomposition pathway was investigated with transition state searching and the calculation results indicate that the O-H bond is first broken,and then the methanol decomposes into methoxy.The activation barrier of O-H bond breaking with Pt-Mo catalyst is only 104.8 kJ mol-1,showing that carbon supported Pt-Mo alloys have promoted the decomposition of methanol.Comparing with the adsorption energies of CH3OH on the Pt(111)/C surface and that of CO,the adsorption energies of CO are higher,and Pt(111)/C is liable to be oxidized and loses the activity,which suggests that the catalyst Pt-Mo(111)/C is in favor of decomposing methanol and has better anti-poisoning ability than Pt(111)/C.     

STUDIES ON ORGANOPHOSPHORUS COMPOUNDS——OXIDATIVE PHOSPHINYLATION——A NEW ROUTE TO DIALKYLPHOSPHINIC ACID DERIVATIVES

A new method is reported for the preparation of dialkylphosphinic acids and their derivatives based on the reaction of dialkylphosphinous acid with a nucleophile and base in the presence of carbon tetraehloride. The influence of the reaction conditions and the structure of nueleophiles on the yield of the oxidative phosphinylation is examined. It is proposed that the reaction operates by an ionic process rather than a radical mechanism.     

Reaction mechanism of aqueous-phase conversion of γ-valerolactone(GVL) over a Ru/C catalyst

The present work explores the reaction pathways of γ-valerolactone(GVL) over a supported ruthenium catalyst. The conversion of GVL in aqueous phase over a 5% Ru/C catalyst was investigated in a batch reactor operating at 463 K under 500–1000 psi of H_2. The main reaction products obtained under these conditions were 2-butanol(2-BuOH), 1,4-pentanediol(1,4-PDO), 2-methyltetrahydrofuran(2-MTHF) and 2-pentanol(2-PeOH). A complete reaction network was developed, identifying the primary and/or secondary products. In this reaction network, production of 2-BuOH via decarbonylation of a ring-opened surface intermediate CH_3CH(O*)–(CH_2)_2–CO*is clearly the dominant pathway. From the evolution of products as a function of reaction time and theoretical(DFT) calculations, a mechanism for the formation of intermediates and products is proposed. The high sensitivity of 2-BuOH production to the presence of CO, compared to a much lower effect on the production of the other products indicates that the sites responsible for decarbonylation are particularly prone to CO adsorption and poisoning. Also, since the decarbonylation rate is not affected by the H2 pressure it is concluded that the direct decarbonylation path of the CH_3CH(O*)–(CH_2)_2–CO*intermediate does not required a previous dehydrogenation step, as is the case in decarbonylation of short alcohols.     

Mechanism for the addition of carbenoid CH2ClLi to formaldehyde

Ab initlo HF/6-31G* calculations have been performed for the addition mechanism of carbenoid CH2CILi with formaldehyde in tetrahydrofuran. An early complex of formaldehyde with CH2CILi is first formed with quite exothermic effect. Only a little activation energy of 14.6 kJ/mol is needed for the complex developing into the product through a transition state with coplanar bicyclic structure. In this process, the eletrophilic attack of carbonyl carbon of formaldehyde is more active than the nucleophilic attack of carbon of carbenoid. The exothermal effect of this addition process is up to 216.5 kJ/mol.     

How Effective Is the Single-point Energy Calculation in Investigating the Reaction Mechanisms？ New Decarboxylation Mechanism of Pyrrole-2-carboxylic Acid by Full Optimization with CPCM Solvation Model

The decarboxylation of pyrrole-2-carboxylic acid in acid solutions was elucidated by full optimization with the CPCM solvation model at the B3LYP/6-31 l＋＋G（d,p） level. Compared with the single-point energy calculation, CPCM full optimization is better to model solvent environments to gain reasonable reaction mechanisms. The π interactions play a significant role in the decarboxylation of pyrrole-2-carboxylic acid （R）. Firstly, the a hydrogen is protonated, but all of the carbonyl hydration pathways bear relatively higher energy barriers. The carbonyl group can rove over the pyrrole ring, but it does not lead to the speciation of pyrrole and protonated carbon dioxide for the latter is an energy-rich species. The decarboxylation mechanism proposed here is that, the protonated pyrrole-2-carboxylic acid （RH） decarboxylates via direct C-C bond cleavage with the aid of a water molecule to accommodate the proton on the carbonyl group.     

Studies on the Adsorption and Dissociation of Methane and Carbon Dioxide on Nickel Catalysts

The adsorption and dissociation of methane and carbon dioxide for reforming on nickel catalysts were extensively investigated by TPSR, TPD, XPS and pulse reaction methods. These studies showed that the decomposition of methane results in the formation of at least three kinds of surface carbon species on supported nickel catalysts. Carbidic Cα, carbonaceous Cβ and carbidic clusters C-γ surface carbon species formed by the decomposition of methane demonstrated different surface mobility, thermal stability and reactivity. Carbidic Cα is a very active and important intermediate in carbon dioxide reforming with methane, and the carbidic clusters Cγ species might be the precursor of surface carbon deposition. The partially dehydrogenated Cβ species can react with H2 or CO2 to form CH4 or CO. On the other hand, it was proven that CO2 can be weakly adsorbed on supported nickel catalysts, and only one kind of CO2 adsorption state is formed. The interaction mechanism between the species dissociated from CH4     

Studies in organometallic rearrangements IV—proton and carbon nuclear magnetic resonance spectra of group IVA substituted indenes and indenyl anions

The proton and carbon NMR spectra for a series of substituted indenes [C₉H₇R, R = H, CH₃, Si(CH₃)₃] are reported. The proton and carbon resonances for the 5-membered ring exhibit pronounced changes as a function of the substituents.     

Ab initio study of charge transfer between lithium and para-disubstituted benzenes

The present work examines how substituents affect the interaction of lithium with the benzene ring and considers whether two configurations, “loose” and “tight,” can be found for the investigated set of complexes as in the lithium complexes with polyaromatic hydrocarbons. The two conformations were actually found for the electron-donating substituents, whereas for the electron-withdrawing substituents, CF₃ and NO₂, only tight conformation could be optimized. For complexes in the loose conformation [with the N(CH₃)₂, NH₂, OH, H, Cl, and CH₃ substituents], the transfer of electron density was directed from the region occupied by a disubstituted benzene to that occupied by Li. The amount of transferred density was very similar for all the complexes (0.21–0.25 a.u.). By contrast, in the tight complexes, the density transfer direction was opposite and the amount of the transferred density depended on the electronegativity of the substituents on benzene ring, being ?0.09, ?0.09, ?0.13, ?0.15, ?0.15, ?0.20, and ?0.30 a.u. for N(CH₃)₂, NH₂, OH, H, Cl, CF₃, and NO₂, respectively. Geometries of the loose and tight conformations differed notably. The six-membered ring was significantly more warped in the tight conformation than in the loose one: In the latter, the ring was nearly planar, whereas in the former, the ring had the boat structure.     

Theoretical study of thermal rearrangements of α‐silylalcohols: Effects of substituents attached to the silicon atom on the reactions

To investigate the effects of substituents attached to the silicon atom on the thermal rearrangement reactions of α‐silyl alcohols, the thermal rearrangement reactions of dimethylsilyl methanol (CH₃)₂SiHCH₂OH and vinylsilyl methanol CH₂?CHSiH₂CH₂OH were studied by ab initio calculations at the G3 level. Geometries of various stationary points were fully optimized at the MP2(full)/6‐31G(d) and MP2(full)/6‐311G(d,p) levels, and harmonic vibrational frequencies were calculated at the same levels. The reaction paths were investigated and confirmed by intrinsic reaction coordinate (IRC) calculations at the MP2(full)/6‐31G(d) level. The results show that two dyotropic reactions could occur when (CH₃)₂SiHCH₂OH or CH₂?CHSiH₂CH₂OH is heated. One is Brook rearrangement reaction (reaction A), and the dimethylsilyl or vinylsilyl groups migrates from carbon atom to oxygen atom coupled with a simultaneous migration of a hydrogen atom from oxygen atom to carbon atom passing through a double three‐membered ring transition state, forming dimethylmethoxylsilane (CH₃)₂SiHOCH₃ or methoxylvinylsilane CH₂?CHSiH₂OCH₃; the other is a hydroxyl group migration (reaction B) from carbon atom to silicon atom, coupled with a simultaneous migration of a hydrogen atom from silicon atom to carbon atom, via a double three‐membered ring transition state, forming trimethylsilanol (CH₃)₃SiOH or methylvinylsilanol CH₃SiH(OH)CH?CH₂. The G3 barriers of the reactions A and B were computed to be 312.8 and 241.4 kJ/mol for (CH₃)₂SiHCH₂OH, and 317.6 and 233.7 kJ/mol for CH₂?CHSiH₂CH₂OH, respectively. On the basis of the MP2(full)/6‐31G(d) optimized parameters, vibrational frequencies, and G3 energies, the reaction rate constants k(T) and equilibrium constants K(T) were calculated using canonical variational transition state theory (CVT) with centrifugal‐dominant small‐curvature tunneling (SCT) approximation over a temperature range of 400–1800 K. The influences of methyl and vinyl groups attached to the silicon atom on reactions are discussed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Researches on alkoxysilanes

Reaction of 1,2-, 1,3-, 1,4-, 1,5-, and 1,6-glycols, and of catechol, pentaerythritol, and glycerol with dialkyldichlorosilanes (in the presence of tertiary amines) dialkyldialkoxysilanes, or polydialkylsiloxanes gives 30 ethers of dialkylsilanediols containing 5, 6, 7, 8, 9, and 10-membered rings (16 being new). Among them are 8-membered ring ethers containing along with Si, O, and C, N or S, and 6-membered ring ethers containing NH₂, NO₂, and OH groups. 2-Sila -1,3-dioxolanes are obtained only when formation of the 1,3,6,8-tetraoxa-2, 7-disilacyclodecane ring is hindered or prevented by the presence of one or two substituents on each carbon atom with two substituents on the silicon. 2, 2-Dialkyl-1, 3-dioxa-2-silacyclanes, which contain 8 and 9-membered rings, can be obtained only by thermal depolymerization of originally formed polymeric ethers of the type [-O(CH₂)_nOSiR₂-]_m. Many of the monomeric cyclic ethers prepared dimerize or polymerize on keeping.For Part XVI see [1].     

Theoretical study of substituent effects on the acidity of the methyl group: Structure of anions CH2X−

Geometries have been optimized using molecular-orbital calculations (a) with a 4-31G Gaussian basis set for carbanions CH₂X^? where X = H, CH₃, NH₂, OH, F, C?CH, CH?CH₂, CHO, COCH₃, CN, and NO₂; and (b) with an STO -3G basis set for methyl acetate and acetyl deprotonated methyl acetate. All the carbanions containing unsaturated substituents are planar, with a considerable shortening of the C? X bond. Carbanions containing saturated substituents are pyramidal with the out-of-plane angle α increasing with the electronegativity of the substituent. Double-zeta basis set calculations give proton affinities over the range 449 (for CH₃CH₂^?) to 355 kcal/mol (for CH₂NO₂^?), with all unsaturated anions having smaller affinities than saturated anions. The correlation of proton affinities with 1s binding energies, and with charges on both the carbon of the anion and on the acidic proton of the neutral molecule are examined.     

Mndo study of fragmentations in mass spectrometry. Part II. Substituent effects in the ioinization of [CH3?CO?R] compounds and their enol tautomers

A semiempirical MNDO method was used in calculations on monosubstituted neutral and cationic compounds of the [CH₃? CO? R] type with a wide range of substituents (R = H, F, Cl, CH₃, NH₂, OH, OCH₃, NHCH₃, CH?CH₂). The results obtained are interpreted with respect to the effect of the individual substituents on the geometry and electron distribution in the systems studied and allow the conclusion that the ratios of partial charges in the individual substructures in a radical cation correspond to those of the respective peak intensities in the mass spectrum.     

Relationships between the acidity of the Z and E isomers of N-Nitroso-N-alkyl-α-amino acids and their conformations

The acidity constants of both Z and E conformational isomers of five N-nitroso-N-alkyl-α-amino acids, ON? N(R₁)? CH(R₂)? COOH, are determined by the observation of selected pH titrated ¹H NMR signals. For two glycine derivatives (1, R₁?CH₃, R₂?H, ON? Sar; 2, R₁?C₂H₅, R₂?H, ON? EtGly) and two alanine derivatives (3, R₁?CH₃, R₂?CH₃, ON? MeAla; 4, R₁?C₂H₅, R₂?CH₃, ON? EtAla) the E isomers appear to be stronger acids than the Z while for the third alanine derivative (5, R₁?n-C₃H₇, R₂?CH₃, ON? PrAla) the opposite is observed. These results, also including anisotropy effects associated with the N? NO group, are discussed in terms of conformations. A 7-membered ring conformation with an ? NO…HOOC? intramolecular hydrogen bond is proposed to be statistically important in the Z isomers of 1, 2, 3 and, to a lesser extent, 4.     

Proximity and the heteroatom effects on the carbon-13 chemical shifts of methyl-substituted phenols,anilines and thiophenols

Upfield substituent-induced ¹³C chemical shifts for aryl carbons of polymethyl substituted benzenes, phenols, anilines and thiophenols were investigated as a function of the proximity between substituents X and CH₃ (X = CH₃, NH₂, OH and SH). The results indicate that the induced shifts of the substituted aryl carbons are, in general, independent of the polar substituent but depend on the number of adjacent substituted aryl carbons. A ?2.0 ppm upfield shift was found for a substituted aryl carbon adjacent to one substituted aryl carbon and a ?3.8 ppm upfield shift for a substituted aryl carbon bound by two substituted aryl carbons. It is suggested that the near additivity of the upfield shifts is the result of changes in the bond order between the aromatic ring carbons in the region of the substituted aryl carbons due to distortion of the ring. The ¹³C chemical shifts of the methyl substituents for methyl substituted phenols, anilines and thiophenols were determined, and it was found that the values could be predicted from the additivity parameters reported for the analogous methylbenzenes plus an additional pair-interaction term associated with the through-space electronic influence of the heteroatom.     

Synthesis and proposed conformations of l-prolyl-l-leucyl-β-alanine alkylamides

The synthesis of H-Pro-Leu-β-Ala-NH₂, H-Pro-Leu-β-Ala-NHCH₃ and H-Pro-Leu-β-Ala-N(CH₃)₂ is described. On the basis of IR and ¹H NMR spectral data, a 7-membered ring including the NH of β-alanine with the CO of proline should be assigned for the H-Pro-Leu-β-Ala-N(CH₃)₂. Consequently, the plausible conformations for H-Pro-Leu-β-Ala-NH₂ and H-Pro-Leu-β-Ala-NHCH₃ derive from the formation of an 11-membered ring, between the trans amide proton and the CO of Pro, or from the formation of an 8-membered ring, between this carboxamide proton and the CO of Leu, plus the aforementioned 7-membered ring.