全氟烷基磺酸酯C—O键断裂的同面SN2反应

用密度泛函理论研究了CF₃SO₃CF₂CF₃＋F^－的碳氧键断裂反应的机理. 首先, 用DFT方法优化了反应物、中间体、过渡态、产物的平衡构型, 分析了碳氧键断裂反应的势能面变化. 发现在S_N2反应机理中, 除了S—O断裂S_N2反应外, 引起C—O键断裂的同面进攻也是一个可能的反应途径. 理论计算表明, 最终反应的产物是受热力学控制的, S—O键的断裂绝对地优于C—O的断裂. 因此, C—O断裂的同面机理虽然是可能的, 但却难以被实验观察到. 本文还讨论了端基 —F₃在同面S_N2反应中的邻位效应, 以及基组对这个效应的影响.     

全氟和多氟烷基磺酸的研究 X:在亲核取代反应中全氟烷基3-氧杂全氟烷基磺酸酯的唯一的硫-氧键断裂

Perfluoroalkyl 3-oxaperfluoralkanesulfonates XCF2OCF2CF2SO3CF2OCF2X (1) (X=CF2I (1a), CF2Cl (1b), HCF2 (1c), Cl2CF (1d)) reacted readily with various mucleophiles leading to S--O seission exclusively, thus 1 -- XCF2OCF2CF2SO2Y+XCF2OCF2COZ In the presence of a catalytic amount of halide (F-, Cl-, Br-, I-) and thiecyanate in diglyme 1 decomposed to give the corresponding sulfonly fluoride 2 (X=F) and acyl fluoride 3(Z=F). At room temperature 1 did not react with excess ethanol, but under refluxing for 12.5h, 1 was converted to 2 (Y=F) and 3 (Z=OEt). More powerful nucleophile ethoxide ion reacted readily with 1 at-60 - -50`C yielding Et2O and 3 (Z=OEt) but no 2 (Z=F). When the reaction was carried out at 80`C the yields of the products varied with the order of mixing of the reactants i.e. when 1 was added to excess ethoxide in ethanol, products are 3(Z=OEt), Et2O and 2(Y=F), but with ethoxide adding to 1 the yield of 2 (Y=F) was increased and that of ether decreased whereas the yield of 3 (Z=OEt) remained constant. Carboxylates (CF3CO2-, CH3CO2-) also caused S--O cleavage of 1 to give acetyl fluoride, 2 (y=F) and 3 (Z=F) as a result of decomposition of the intermediary mixed anhydride by the fluoride ion. In the same manner R2NH, C6H5NH2 reacted with 1 giving the products of S--O cleavage. In contrast to the nucleophilic reactions of α, α-di-H-perfluoroalkyl perfluoroalkanesulfonates (mainly C--O cleavage) it has been found that all nucleophies attack the sulfur atom of 1 exclusively. A possible interpretation is that the SN2 attack at sp3 carbon atom in highly fluorinated system is made impossible by the shielding effect of the two fluorine atoms in the alcoholic moiety and leaving the attack on the sulfur as the only alternative.     

SN1‐SN2 and SN2‐SN3 mechanistic changes revealed by transition states of the hydrolyses of benzyl chlorides and benzenesulfonyl chlorides

Hydrolysis reactions of benzyl chlorides and benzenesulfonyl chlorides were theoretically investigated with the density functional theory method, where the water molecules are explicitly considered. For the hydrolysis of benzyl chlorides (para‐Z? C₆H₄? CH₂? Cl), the number of water molecules (n) slightly influences the transition‐state (TS) structure. However, the para‐substituent (Z) of the phenyl group significantly changes the reaction process from the stepwise (S_N1) to the concerted (S_N2) pathway when it changes from the typical electron‐donating group (EDG) to the typical electron‐withdrawing one (EWG). The EDG stabilizes the carbocation (MeO? C₆H₄? CH₂⁺), which in turn makes the S_N1 mechanism more favorable and vice versa. For the hydrolysis of benzenesulfonyl chlorides (para‐Z? C₆H₄? SO₂? Cl), both the Z group and n influence the TS structure. For the combination of the large n value (n > 9) and EDG, the S_N2 mechanism was preferred. Conversely, for the combination of the small n value and EWG, the S_N3 one was more favorable. © 2014 Wiley Periodicals, Inc.     

Activation Strain Analyses of Counterion and Solvent Effects on the Ion-Pair SN2 Reaction of and CH3Cl

We have computationally studied the bimolecular nucleophilic substitution (S_N2) reactions of M_nNH₂⁽ⁿ⁻¹⁾ + CH₃Cl (M⁺ = Li⁺, Na⁺, K⁺, and MgCl⁺; n = 0, 1) in the gas phase and in tetrahydrofuran solution at OLYP/6-31++G(d,p) using polarizable continuum model implicit solvation. We wish to explore and understand the effect of the metal counterion M⁺ and of solvation on the reaction profile and the stereochemical preference, that is, backside (S_N2-b) versus frontside attack (S_N2-f). The results were compared to the corresponding ion-pair S_N2 reactions involving F⁻ and OH⁻ nucleophiles. Our analyses with an extended activation strain model of chemical reactivity uncover and explain various trends in S_N2 reactivity along the nucleophiles F⁻, OH⁻, and , including solvent and counterion effects. © 2019 Wiley Periodicals, Inc.     

Ab initio direct classical trajectory investigation on the SN2 reaction of F- with NH2F: nonstatistical central barrier recrossing dynamics

The bimolecular nucleophilic substitution (S(N)2) reaction of F(a)(-) with NH(2)F(b) has been investigated with the ab initio direct classical trajectory method. According to our trajectory calculations, a dynamic behavior of nonstatistical central barrier recrossing is revealed. Among the 64 trajectories calculated in this work, 45 trajectories follow the dynamic reaction pathways as assumed by statistical theory and other 19 trajectories with central barrier recrossings are nonstatistical. For the nonstatistical trajectories, the central barrier recrossings may originate from the inefficient kinetic energy transfer from the intramolecular modes of the NH(2)F(a) moiety in the dynamic F(b)(-)…H-NH-F(a) complex to the intermolecular modes of the dynamic F(b)(-)…H-NH-F(a) complex on the exit-channel potential energy surface. With respect to the dynamic behavior of the nonstatistical central barrier recrossing, the statistical theories such as the Rice-Ramsperger-Kassel-Marcus and transition state theories without further corrections cannot be used to model the reaction kinetics for this S(N)2 reaction.     

An investigation on the reaction mechanism of the F2+Cl2→2ClF using the B3LYP method

The reaction mechanism of F₂+Cl₂→2ClF has been investigated with the density functional theory at the B3LYP/6‐311G* level. Six transition states have been found for the three possible reaction paths and verified by the normal mode vibrational and IRC analyses. Ab initio MP2/6‐311G* geometry optimizations and CCSD(T)/6‐311G(2df)//MP2/6‐311G* single‐point energy calculations have been performed for comparison. It is found that when the F₂ (or Cl₂) molecule decomposes into atoms first and then the F (or Cl) atom reacts with the molecule Cl₂ (or F₂) nearly along the molecular axis, the energy barrier is very low. The calculated energy barrier of F attacking Cl₂ is zero and that of Cl attacking F₂ is only 15.57 kJ?mol^?1 at the B3LYP level. However, the calculated dissociation energies of F₂ and Cl₂ are as high as 145.40 and 192.48 kJ?mol^?1, respectively. When the reaction proceeds through a bimolecular reaction mechanism, two four‐center transition states are obtained and the lower energy barrier is 218.69 kJ?mol^?1. Therefore, the title reaction F₂+Cl₂→2ClF is most probably initiated from the atomization of the F₂ molecule and terminated by the reaction of F attacking Cl₂ nearly along the Cl? Cl bond. MP2 calculations lead to the same conclusion, but the geometry of TS and the energy barrier are somewhat different. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2002     

Theoretical study of the gas‐phase ion pairs SN2 reactions of LiX with CH3SY (X,Y = F,Cl, Br,I)

The gas‐phase ion pair S_N2 reactions at saturated sulfur LiX + CH₃SY → CH₃SX + LiY (X, Y = F, Cl, Br, I) are investigated using the CCSD(T) calculations. The calculated results show that the reactions LiX + CH₃SY are exothermic only when the nucleophile is a heavier lithium halide. Central barrier heights are found to depend primarily on the identity of nucleophile LiX, decreasing in the order LiF > LiCl > LiBr > LiI. Another interesting feature of the ion pair reactions at sulfur is the good correlation between the reaction barriers with geometrical looseness of Li? X and S? Y bonds in the transition state structures. The data for the reaction barriers show good agreement with the prediction of the Marcus equation and its modification. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Hybrid QM/MM molecular dynamics simulations for an ionic SN2 reaction in the supercritical water: OH- + CH3Cl --> CH3OH + Cl-

A hybrid real space quantum mechanical/molecular mechanical (RS-QM/MM) method has been applied to an ionic S(N)2 reaction (OH- + CH3Cl --> CH3OH + Cl-) in water solution to investigate dynamic solvation effects of the supercritical water (SCW) on the reaction. It has been demonstrated that the approaching process of OH- to methyl group is prevented by water molecules in the ambient water (AW), while the reaction takes place easily in the gas phase. Almost the same solvation effect on the dynamics of OH- is observed in the SCW, though the bulk density of water is substantially reduced compared with that of the AW. It has been shown that the solvation of the SCW around the OH anion is locally identical to that of the AW due to the strong ion-dipole interactions between OH- and water molecules. At the transition state, the QM/MM simulations have revealed that the excess electron is quite flexible, and the charge volume, as well as the fractional charges on atoms, vary seriously depending on the instantaneous solvent configurations. However, it has been found that the solvation energy in the SCW can be qualitatively related to the HOMO volume of the system by Born's equation.     

Theoretical investigation of ion pair SN2 reactions of alkali isothiocyanates with alkyl halides. Part 1. Reaction of lithium isothiocyanate and methyl fluoride with inversion mechanism

The gas‐phase ionic S_N2 reactions NCS^‐ + CH₃F and ion pair S_N2 reaction LiNCS + CH₃F with inversion mechanism were investigated at the level of MP2(full)/6‐311+G**//HF/6‐311+G**. Both of them involve the reactants complex, inversion transition state, and products complex. There are two possible reaction pathways in the ionic S_N2 reaction but four reaction pathways in the ion pair S_N2 reaction. Our results indicate that the introduction of lithium significantly lower the reaction barrier and make the ion pair displacement reaction more facile. For both ionic and ion pair reaction, methyl thiocyanate is predicted to be the major product, but the latter is more selective. More‐stable methyl isothiocyanate can be prepared by thermal rearrangement of methyl thiocyanate. The theoretical predictions are consistent with the known experimental results. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Br2+2HI=2HBr＋I2应机理的密度泛函理论

用密度泛函理论(DFT)B3LYP方法，取3-21G**基组研究了气相反应Br2＋2HI=2HBr＋I2的机理，求得一系列四中心和三中心的过渡态.双分子基元反应Br2＋HI→HBr＋IBr和IBr＋HI→I2＋HBr的活化能(81.02和121.08 kJ•mol－1)小于Br2、HI和IBr的解离能(249.21、320.16和232.42 kJ•mol－1)，故从理论上证明了标题反应将优先以分子与分子作用形式分两步完成.同时发现I原子与Br2分子反应生成较稳定的IBr2是一个无能垒过程，IBr2分解为IBr和Br原子的能垒为70.88 kJ•mol－1.