Neopentane and solid acids: direct hydron exchange before cracking

The hydrogen/deuterium exchange reaction of 2,2-dimethylpropane (neopentane) over D(2)O-exchanged zeolites (MOR, FAU, BEA, MFI) using a batch recirculation reactor was studied by means of gas chromatography coupled with mass spectrometer. In the temperature range 473-573 K, H/D exchange proceeds without side reaction such as cracking at short contact times. Indeed the C-H bond has appeared favorably involved in the activation of neopentane compared to the less accessible C-C bond. The transition state allowing hydron exchange is most likely a carbonium species (pentacoordinated carbon) as in the case of the H/D exchange between methane and solid acid. The activation energies of the H/D exchange between neopentane and zeolites are the same for all zeolites indicating a common carbonium ion type transition state. On the basis of previous results in the case of the exchange between methane and liquid superacids, the deuterium exchange rates in neopentane were tentatively related to the acidity of the solids. However the order of activity MOR > MFI > BEA > FAU seems to be related to the size of the pores, which may suggest the involvement of a confinement effect in the zeolites cavities. Moreover we found that H/D exchange takes also place between neopentane and deuterated sulfated zirconia (SZ) emphasizing its strong acidity.     

Gas Phase H/D Exchange of Sodiated Amino Acids: Why Do We See Zwitterions?

The gas-phase interaction of sodiated amino acids and sodiated amino acid methyl esters with various deuterium donors is investigated by combining results of H/D exchange reactions with those from density functional theory and molecular dynamics calculations. Discrepancy between experimentally and theoretically obtained structures for sodium cationized amino acids is explained by deuterium donor caused perturbation of the most stable amino acid conformation. Detailed study of H/D exchange mechanism on sodiated amino acids shows that the H/D exchange reaction is preceded by a multistep quasi-isoenergetic transition (perturbation) from a charge solvated to zwitterionic structure in the amino acid. Although the computation refers to the system AlaNa(+) and D(2)O, these mechanisms apply to all amino acids, except those where a functional side-chain group takes part in the perturbation process. The suggested perturbation mechanism applies also for other deuterium donors such as CD(3)OD or even ND(3) and indicates that a single water molecule suffices to convert the sodiated amino acid from charge solvated to zwitterionic form.     

F~2+2HCl→2HF+Cl~2反应机理的密度泛函理论研究

用密度泛函理论(DFT)B3LYP方法,在6-311G^*^*基组下,计算研究了反应F~2+2HCl→2HF+Cl~2的机理。求得各可能反应途径的系列过渡态,并通过振动分析和内禀反应坐标(IRC)分析加以证实。比较反应能垒(理论计算活化能)发现,标题反应若以分子与分子作用机理进行,则需克服的最大能垒为150.63kJ.mol^-^1;若以F~2分子先裂解为F原子再反应的机理进行,则需越过能垒154.82kJ.mol^-^1,求得反应F+HCl→HF+Cl的线形和三角形两种过渡态,以三角形较稳定;求得反应HCl+Cl→H+Cl~2的两种过渡态,以线形较稳定。     

三氟化氯和环氧丙烷反应的理论研究

应用密度泛函理论对三氟化氯和环氧丙烷反应产生C3H5O和C1F2自由基的机理进行了研究。在B3PW91/6-31+G(d,p)水平优化了12个不同反应通道上各驻点(反应物、中间体、过渡态和产物) 的几何构型,并计算了它们的振动频率和零点振动能。采用CCSD(T)/6-31+ G(d,p) // B3PW91/6-31+G(d,p)单点能计算方法求得各物种的能量,并作了零点能校正。计算结果表明,三氟化氯和环氧丙烷反应可经过不同的反应路径引发C3H5O自由基和C1F2自由基,其中,三氟化氯呈对称的F原子与环氧丙烷的C(1)上与CH3在同一侧的上的H原子结合的活化能最低,仅为16.81 kJ/mol。     

The nature of superacid electrophilic species in HF/SbF(5): a density functional theory study

A density functional theory study at the B3LYP/6-31++G** + RECP(Sb) level of the HF/SbF(5) superacid system was carried out. The geometries of possible electrophilic species, such as H(2)F(+).Sb(2)F(11)(-) and H(3)F(2)(+).Sb(2)F(11)(-), were calculated and correspond with available experimental results. Calculations of different equilibrium reactions involving HF and SbF(5) allowed the relative concentration of the most energetically favorable species present in 1:1 HF/SbF(5) solutions to be estimated. These species are H(+).Sb(2)F(11)(-), H(2)F(+).Sb(2)F(11)(-), H(3)F(2)(+).Sb(2)F(11)(-), and H(4)F(3)(+).Sb(2)F(11)(-), which correspond to 36.9, 16.8, 36.9, and 9.4%, respectively. Calculations of the acid strength of the electrophilic species were also performed and indicated that, for the same anion, the acid strength increases with the solvation degree. The entropic term also plays a significant role in proton-transfer reactions in superacid systems.     

钒硼双原子阳离子活化甲烷研究

Methane activation by transition metal species has been extensively investigated over the past few decades. It is observed that ground-state monocations of bare 3d transition metals are inert toward CH₄ at room temperature because of unfavorable thermodynamics. In contrast, many mono-ligated 3d transition metal cations, such as MO⁺ (M = Mn, Fe, Co, Cu, Zn), MH⁺ (M = Fe, Co), and NiX⁺ (X = H, CH₃, F), as well as several bis-ligated 3d transition metal cations including OCrO⁺, Ni(H)(OH)⁺, and Fe(O)(OH)⁺ activate the C―H bond of methane under thermal collision conditions because of the pronounced ligand effects. In most of the above-mentioned examples, the 3d metal atoms are observed to cooperate with the attached ligands to activate the C―H bond. Compared to the extensive studies on active species comprising of middle and late 3d transition metals, the knowledge about the reactivity of early 3d transition metal species toward methane and the related C―H activation mechanisms are still very limited. Only two early 3d transition metal species HMO⁺ (M = Ti and V) are discovered so far to activate the C―H bond of methane via participation of their metal atoms. In this study, by performing mass spectrometric experiments and density functional theory calculations, we have identified that the diatomic vanadium boride cation (VB⁺) can activate methane to produce a dihydrogen molecule and carbon-boron species under thermal collision conditions. The strong electrostatic interaction makes the reaction preferentially proceed the V side. To generate experimentally observed product ions, a two-state reactivity scenario involving spin conversion from high-spin sextet to low-spin quartet is necessary at the entrance of the reaction. This result is consistent with the reported reactions of 3d transition metal species with CH₄, in which the C―H bond cleavage generally occurs in the low-spin states, even if the ground states of the related active species are in the high-spin states. For VB⁺ + CH₄, the insertion of the synergetic V―B unit (rather than a single V or B atom) into the H₃C―H bond causes the initial C―H bond activation driven by the strong bond strengths of V―CH₃ and B―H. The mechanisms of methane activation by VB⁺ discussed in this study may provide useful guidance to the future studies on methane activation by early transition metal systems.     

Methane activation and oxidation in sulfuric acid

The H/D exchange observed when methane is contacted with D(2)SO(4) at 270-330 degrees C shows that the alkane behaves as a sigma base and undergoes rapid and reversible protonation at this temperature. DFT studies of the hydrogen exchange between a monomer and a dimer of sulfuric acid and methane show that the transition states involved in the exchange are bifunctional, that is one hydrogen atom is transferred from a hydroxy group in sulfuric acid to methane, while one hydrogen atom is abstracted from methane by a non-hydroxy oxygen atom in sulfuric acid. All the transition states include a CH(5) moiety, which shows similarities to the methanium ion CH(5) (+). The calculated potential activation energy of the hydrogen exchange for the monomer is 174 kJ mol(-1), which is close to the experimental value (176 kJ mol(-1)). Solvation of the monomer and the transition state of the monomer with an extra sulfuric acid molecule, decrease the potential activation energy by 6 kJ mol(-1). The acid-base process is in competition, however, with an oxidative process involving methane and sulfuric acid which leads to CO(2), SO(2), and water, and thus to a decrease of acidity and loss of reactivity of the medium.     

固体超强酸研究的最新进展

简要介绍了近年来固体超强酸的发展。目前研究较多的固体超强酸有负载硫酸超强酸、Ｃｓ２．５盐、ＡｌＣｌ３／ＰＳ－ＤＶＢ磺酸树脂体系和全氟磺酸树脂（Ｎａｆｉｏｎ－Ｈ－Ｈ ＰＥＩＥＰ）四种。文章主要阐述了这几种固体超强酸的制备方法及它们的性质比较。     

Hydrogen/deuterium exchange of myoglobin ions in a linear quadrupole ion trap

The hydrogen/deuterium (H/D) exchange of gas-phase ions of holo- and apo-myoglobin has been studied by confining the ions in a linear quadrupole ion trap with D(2)O or CD(3)OD at a pressure of several mTorr. Apo-myoglobin ions were formed by collision-induced dissociation of holo-myoglobin ions between the orifice and skimmer of the ion sampling system. The exchange takes place on a time scale of seconds. Earlier cross section measurements have shown that holo-myoglobin ions can have more compact structures than apo-myoglobin. Despite this, both holo-myoglobin and apo-myoglobin in charge states +8 to +14 are found to exchange nearly the same number of hydrogens (ca. 103) in 4 s. It is possible the ions fold or unfold to new conformations on the much longer time scale of the exchange experiment compared with the cross section measurements.     

Collisionally-induced dissociation of substituted pyrimidine antiviral agents: Mechanisms of ion formation using gas phase hydrogen/deuterium exchange and electrospray ionization tandem mass spectrometry

ESI and CID mass spectra were obtained for four pyrimidine nucleoside antiviral agents and the corresponding compounds in which the labile hydrogens were replaced by deuterium using gas-phase exchange. The number of labile hydrogens, x, was determined from a comparison of ESI spectra obtained with N(2) and with ND(3) as the nebulizer gas. CID mass spectra were obtained for [M + H](+) and [M - H](-) ions and the exchanged analogs, [M(D(x)) + D](+) and [M(D(x)) - D](-), produced by ESI using a SCIEX API-III(plus) mass spectrometer. Protonated pyrimidine antiviral agents dissociate through rearrangement decompositions of base-protonated [M + H](+) ions by cleavage of the glycosidic bonds to give the protonated bases with a sugar moiety as the neutral fragment. Cleavage of the glycosidic bonds with charge retention on the sugar moiety eliminates the base moiety as a neutral molecule and produces characteristic sugar ions. CID of protonated pyrimidine bases, [B + H](+), occurs through three major pathways: (1) elimination of NH(3) (ND(3)), (2) loss of H(2)O (D(2)O), and (3) elimination of HNCO (DNCO). Protonated trifluoromethyl uracil, however, dissociates primarily through elimination of HF followed by the loss of HNCO. CID mass spectra of [M - H](-) ions of all four antiviral agents show NCO(-) as the principal decomposition product. A small amount of deprotonated base is also observed, but no sugar ions. Elimination of HNCO, HN(3), HF, CO, and formation of iodide ion are minor dissociation pathways from [M - H](-) ions.     

氟代苯阳离子的理论研究

用B3LYP方法及6-311G(d,p)和6-311 G(d,p)基组,对十二种氟代苯阳离子做了理论研究,优化了它们的电子基态的结构,计算了对应分子的垂直电离势(VIP)和绝热电离势(AIP)．依据Jahn．Teller理论,计算确定了1,3,5-C6H3F^ 3和C6F^ 6离子分别具有C2v(2↑B)和D2h(2↑B2g)结构(对应分子分别为D3h和D6h结构)．其余十个离子的构型的对称点群与对应分子相同,但构型参数值有明显差别．自然布居分析计算表明这些离子的正电荷主要分布在与F原子相连的C原子和各H原子上．B3LYP／6-311 G(d,p)级别上计算的各氟代苯分子的VIP和AIP值和实验值符合得很好．     

Role of van der Waals resonances in the vibrational relaxation of HF by collisions with H atoms

Vibrational relaxation of HF(v) in collisions with H atoms can occur by three pathways: inelastic scattering with and without H atom exchange, and, for v>or=3, the HF+H-->F+H2 reaction. Fully quantum, reactive scattering calculations on the Stark-Werner FH2 potential energy surface reveal narrow peaks in the energy dependence of the integral cross sections for each of these processes. By means of an adiabatic-bender analysis, we show that each of these peaks corresponds to the position of quasibound HF-H vibrational states trapped in the weak van der Waals well. The width of these resonances indicates that the lifetime of the quasibound states is up to 30 periods of the HF-H van der Waals vibration.     

Superhalogen and Superacid

A superhalogen F@C₂₀(CN)₂₀ and a corresponding Brønsted superacid were designed and investigated on DFT and DLPNO-CCSD(T) levels of theory. Calculated compounds have outstanding electron affinity and deprotonation energy, respectively. We consider superacid H[F@C₂₀(CN)₂₀] to be able to protonate molecular nitrogen. The stability of these structures is discussed, while some of the previous predictions concerning neutral Brønsted superacids of record strength are doubted. © 2019 Wiley Periodicals, Inc.     

Hydrogen bonding and catalysis of solvolysis of 4-methoxybenzyl fluoride

Values of k(o) = 8.0 x 10(-3) s(-1) and k(H) = 2.5 x 10(-2) M(-1) s(-1), respectively, were determined for the spontaneous and the acid-catalyzed cleavage of 4-methoxybenzyl fluoride (1-F) to form the 4-methoxybenzyl carbocation (1+). Values of k(F) = 1.8 x 10(7) M(-1) s(-1) and k(HF) = 7.2 x 10(4) M(-1) s(-1) were determined for addition of F- and HF to 1+ for reaction in the microscopic reverse direction. Evidence is presented that the reversible addition of HF to 1+ to give 1-F + H+ proceeds by a concerted reaction mechanism. The relatively small 250-fold difference between the reactivities of fluoride ion and neutral HF toward 1+ is attributed to the tendency of the strong aqueous solvation of F- to decrease its nucleophilic reactivity and to the advantage for the concerted compared with the usual stepwise pathway for addition of HF. There is no significant stabilization of the transition state for cleavage of 1-F from general acid catalysis by 0.80 M cyanoacetate buffer at pH 1.7. The estimated 3 kcal/mol larger Marcus intrinsic barrier for heterolytic cleavage of 1-F than for cleavage of 1-Cl is attributed to a lag in the development at the transition state of the ca. 30 kcal/mol greater stabilizing solvation of the product ion F- compared with Cl-. The decrease in the electronegativity of X along the series X = F, OH, Cl is accompanied by a ca. 10(10)-fold increase in the carbon basicity compared with the proton basicity of X-.     

Energetics and structural elucidation of mechanisms for gas phase H/D exchange of protonated peptides

Hydrogen/deuterium exchange reactions involving protonated triglycine and deuterated ammonia (ND(3)) have been examined in the gas phase using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Ab initio and density functional theory (DFT) calculations have been carried out to model the exchanges and to obtain energetics and vibrational frequencies for molecules involved in the proposed exchange mechanisms. Structural optimization and frequency calculations have been performed at the B3LYP level of theory with the 6-311+G(d,p) basis set. Transition states have been calculated at the same level of theory and basis set as above using the QST2 and QST3 methods. Single-point energy calculations have been performed at the MP2/6-311+G(d,p) level. Six labile sites of protonated triglycine were found to undergo H/D exchange. Of these six labile hydrogens, two are amide, three are ammonium, and one is carboxyl. Detailed mechanisms for each of these transfers are proposed. Qualitative onium ion and tautomer mechanisms for the exchanges of ammonium and amide hydrogens, respectively, using semiempirical calculations were suggested in previous studies by Beauchamp et al. As shown by the current ab initio and DFT calculations completed during this study, the mechanisms proposed in that study are notionally correct; however, the tautomer mechanisms are shown here to be the result of the fact that a second stable isomer of protonated triglycine exists in which the amide1 carbonyl oxygen is protonated. The exchange of the carboxyl hydrogen is found to proceed via a transition state resembling an ammonium ion interacting with a carboxylate moiety via two hydrogen bonds. The current work thus provides significant mechanistic and structural detail for a considerably more in-depth understanding of the processes involved in gas phase H/D exchange of peptides.     

Dynamically weighted multiconfiguration self-consistent field: multistate calculations for F+H2O-->HF+OH reaction paths

A novel method of dynamically adjusted weighting factors in state-averaged multiconfiguration self-consistent-field calculations (SA-MCSCF) is described that is applicable to systems of arbitrary dimensionality. The proposed dynamically weighted approach automatically weights the relevant electronic states in each region of the potential energy surface, smoothly adjusting between these regions with an energy dependent functional. This method is tested on the F(2P)+H2O-->HF+OH(2Pi) reaction, which otherwise proves challenging to describe with traditional SA-MCSCF methods due to (i) different asymptotic degeneracies of reactant (threefold) and product (twofold) channels, and (ii) presence of low-lying charge transfer configurations near the transition state region. The smoothly varying wave functions obtained by dynamically weighted multiconfigurational self-consistent field represent excellent reference states for high-level multireference configuration interaction calculations and offer an ideal starting point for construction of multiple state potential energy surfaces.     

Crystal and molecular structures and experimentally determined charge densities of fluorinated ethenes

Crystals of various fluorinated ethenes were grown by in situ crystallization from their melts on a diffractometer, allowing the structures of tetrafluoroethene (C2F4), trifluoroethene (C2HF)3, 1,1-difluoroethene (C2H2F2), (E)-1,2-difluoroethene (C2H2F2), and (Z)-1,2-difluoroethene (C2H2F2) to be determined by X-ray crystallography. Unexpectedly, the C=C bond lengths show only small variations arising from fluorine substitution. These findings are supported by ab initio calculations at a DFT level of theory and the results of topological analyses of the experimentally determined and theoretically calculated charge densities.     

N/F掺杂和N-F双掺杂锐钛矿相TiO2(101)表面电子结构的第一性原理计算

采用密度泛函理论(DFT)平面波赝势方法计算了N/F掺杂和N-F双掺杂锐钛矿相TiO2(101)表面的电子结构. 由于DFT方法存在对过渡金属氧化物带隙能的计算结果总是与实际值严重偏离的缺陷, 本文也采用DFT+U(Hubbard 系数)方法对模型的电子结构进行了计算. DFT的计算结果表明N掺杂后, N 2p轨道与O 2p和Ti 3d价带轨道的混合会导致TiO2带隙能的降低, 而F掺杂以及氧空位的引入对材料的电子结构没有明显的影响. DFT+U的计算却给出截然不同的结果, N掺杂并没有导致带隙能的降低, 而只是在带隙中引入一个孤立的杂质能级, 反而F掺杂以及氧空位的引入带来明显的带隙能降低. DFT+U的计算结果与一些实验测量结果能够较好地符合.     

N／F掺杂和N-F双掺杂锐钛矿相TiO2（101）表面电子结构的第一性原理计算

采用密度泛函理论(DFT)平面波赝势方法计算了N/F掺杂和N-F双掺杂锐钛矿相TiO2(101)表面的电子结构.由于DFT方法存在对过渡金属氧化物带隙能的计算结果总是与实际值严重偏离的缺陷,本文也采用DFT+U(Hubbard系数)方法对模型的电子结构进行了计算.DFT的计算结果表明N掺杂后,N2p轨道与O 2p和Ti 3d价带轨道的混合会导致TiO2带隙能的降低,而F掺杂以及氧空位的引入对材料的电子结构没有明显的影响.DFT+U的计算却给出截然不间的结果,N掺杂并没有导致带隙能的降低,而只是在带隙中引入一个孤立的杂质能级,反而F掺杂以及氧空位的引入带来明显的带隙能降低.DFT+U的计算结果与一些实验测量结果能够较好地符合.     

Statistical modelling of NH+/ND+ + H2/HD/D2 branching ratios

In this work we study hydrogen isotope fractionation along the proposed gas-phase ammonia formation pathway at temperatures relevant to interstellar modelling. Specifically we study the isotopically substituted variants of the NH(+) + H(2) reaction, for which the primary product is NH(2)(+) + H. Adiabatic capture theory calculations have been performed for the association reaction. A new potential-energy surface has been determined for the NH(2)(+) product. An extensive set of rovibrational energy levels has been calculated for the NH(2)(+) isotopologues described by this potential-energy surface. These rovibrational energy levels have been used to determine energy- and angular-momentum-resolved numbers of accessible product states from the NH(3)(+) isotopologue intermediates, which in turn have been averaged to give statistical branching fractions for all isotopically-substituted NH(+) + H(2) reactions. It is determined that in all cases where both hydrogen and deuterium are present, the NHD(+) product is preferred.