Abstract:Various disparate experimental results are explained by the hypothesis that reactions of anionic nucleophiles with allylic halides are generally S(N)2. The S(N)2' reactions that do occur proceed generally with anti stereochemistry. Reactions with ion pair nucleophiles occur preferentially as S(N)2' reactions with syn stereochemistry. This hypothesis is consistent with a variety of computations at the HF, B3LYP, mPW1PW91 and MP2 levels with the 6-31+G(d) basis set of reactions of Li and Na fluoride and chloride… Show more
“…As shown in Figure , the F‐doped PMIA nanofibers and SiO 2 −F co‐doped PMIA nanofibers separator can be almost infiltrated by the applied electrolyte, but the contact angle of the pure PMIA nanofibers membrane to the liquid electrolyte is about 43°. The reasons of the enhanced absorbing properties to liquid electrolyte can be ascribed to the remarkable complexation between the fluorinated polymer and lithium salts in liquid electrolyte due to Lewis acid‐base binding effect, and the fine fiber diameter and high specific surface area of the F doped membranes proved by the above SEM images analysis. Furthermore, the crystallinity of PMIA membrane decreases.…”
In this study, a novel double‐layer composite separator based on F‐doped poly‐m‐phenyleneisophthalamide (PMIA) and silicon dioxide (SiO2)−F co‐doped PMIA membrane is designed and prepared by electrospinning technology. The combination of F‐doped PMIA membrane and SiO2−F co‐doped PMIA membrane endows the functional separator with exceedingly high electrolyte uptake and retention, greatly prominent thermostability, and prevented shrinkage. Moreover, its strong physical inhibition and chemisorption can also confine polysulfides. Thus, the lithium‐sulfur (Li−S) batteries using the prepared membrane presented a high initial discharge capacity of 1274.8 mAh g−1 and maintained a reversible discharge capacity of 814.8 mAh g−1 with high Coulombic efficiency of 98.42 % after 600 cycles at 0.5 C. Meanwhile, the cell exhibited small interfacial resistance and excellent rate capacity. The excellent properties were attributed to high ionic conductivity offered by the large pore volume of unique 3D nanofiber network, excellent liquid electrolyte uptake of SiO2−F co‐doped PMIA membrane, and less “shuttle effect” suppressed by both the physical inhibition of polysulfides by the formed gel polymer electrolyte based on F‐doped PMIA membrane and chemical confinement of polysulfides by the addition of F and SiO2. The results indicated that the composite separator through the inexpensive, unsophisticated, and efficient preparation method can provide a good choice for high‐performance Li−S batteries.
“…As shown in Figure , the F‐doped PMIA nanofibers and SiO 2 −F co‐doped PMIA nanofibers separator can be almost infiltrated by the applied electrolyte, but the contact angle of the pure PMIA nanofibers membrane to the liquid electrolyte is about 43°. The reasons of the enhanced absorbing properties to liquid electrolyte can be ascribed to the remarkable complexation between the fluorinated polymer and lithium salts in liquid electrolyte due to Lewis acid‐base binding effect, and the fine fiber diameter and high specific surface area of the F doped membranes proved by the above SEM images analysis. Furthermore, the crystallinity of PMIA membrane decreases.…”
In this study, a novel double‐layer composite separator based on F‐doped poly‐m‐phenyleneisophthalamide (PMIA) and silicon dioxide (SiO2)−F co‐doped PMIA membrane is designed and prepared by electrospinning technology. The combination of F‐doped PMIA membrane and SiO2−F co‐doped PMIA membrane endows the functional separator with exceedingly high electrolyte uptake and retention, greatly prominent thermostability, and prevented shrinkage. Moreover, its strong physical inhibition and chemisorption can also confine polysulfides. Thus, the lithium‐sulfur (Li−S) batteries using the prepared membrane presented a high initial discharge capacity of 1274.8 mAh g−1 and maintained a reversible discharge capacity of 814.8 mAh g−1 with high Coulombic efficiency of 98.42 % after 600 cycles at 0.5 C. Meanwhile, the cell exhibited small interfacial resistance and excellent rate capacity. The excellent properties were attributed to high ionic conductivity offered by the large pore volume of unique 3D nanofiber network, excellent liquid electrolyte uptake of SiO2−F co‐doped PMIA membrane, and less “shuttle effect” suppressed by both the physical inhibition of polysulfides by the formed gel polymer electrolyte based on F‐doped PMIA membrane and chemical confinement of polysulfides by the addition of F and SiO2. The results indicated that the composite separator through the inexpensive, unsophisticated, and efficient preparation method can provide a good choice for high‐performance Li−S batteries.
“…Mechanistic studies of S N 2 reactions that take into account the effects of ion pairing are comparatively few ( Harder et al., 1995 ; Streitwieser et al. 1997 , 2008 ; Chen et al., 2009 ; Streitwieser and Jayasree, 2007 ; Zheng et al., 2010 ; Westaway, 2011 ; Li et al., 2015 ; Laloo et al., 2016 ). Scheme 2 Analysis of Nucleophilic Reactions Using the Concept of Transition-State Expansion (A) The transition state in an S N 2 reaction when the counter is ignored.…”
Summary
Ionic reactions are the most common reactions used in chemical synthesis. In relatively low dielectric constant solvents (e.g., dichloromethane, toluene), ions usually exist as ion pairs. Despite the importance of counterions, a quantitative description of how the paired 'counterion' affects the reaction kinetic is still elusive. We introduce a general and quantitative model, namely transition-state expansion (TSE), that describes how the size of a counterion affects the transition-state structure and the kinetics of an ionic reaction. This model could rationalize the counterion effects in nucleophilic substitutions and gold-catalyzed enyne cycloisomerizations.
“…The peak at 1121 cm −1 was aligned to the vibration of bond C−O−C, and shifted to 1132 cm −1 when mixed with LiF obviously. It is because the DME has the high donor number constant of 24 as a Lewis alkali, which LiF could interact with through Li atom . The interaction between LiF and DME increased the bond force constant, resulting in the peak shifting.…”
Sulfur, as a cathode material in lithiumÀsulfur batteries, has a very high theoretical specific capacity of 1675 mAh g À1 , but there is still a large challenge, because of polysulfides' (PSs) that cause a severe shuttle effect. To suppress this effect, a simple way of modifying separator by introducing lithium fluoride (LiF) as a coating layer was developed. Owing to the interaction between LiF and dimethoxymethane (DME, an electrolyte solvent), a dense and viscous sol layer was formed that suppresses PSs passing from the cathode to the anode. The presence of this layer was confirmed by using Fourier transform infrared spectroscopy, thermogravimetric/differential thermogravimetric, and scanning electron microscopy. The linear sweep voltammetry test had shown that the LiF-coated separator had a wide electrochemical window above 5 V vs. Li/Li + , and the cell assembled with the LiF-coated separator exhibited an excellent cycling performance with a capacity retention rate of 69.3%. Even without LiNO 3 as an electrolyte additive, a high coulombic efficiency of 93% after 200 cycles at 0.2 C was achieved.
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