Microhydration Effects on the Intermediates of the SN2 Reaction of Iodide Anion with Methyl Iodide
Microhydration Effects on the Intermediates of the SN2 Reaction of Iodide Anion with Methyl Iodide
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DOI：
10.1002/anie.201207697
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发表时间：
2013-01-01
影响因子：
16.6
通讯作者：
Inokuchi, Yoshiya
中科院分区：
文献类型：
--
作者：
Doi, Keisuke;Togano, Eijiro;Inokuchi, Yoshiya
卤化物阴离子与甲基卤化物X~(2+)+CH~3Y！XCH3+Y是近几十年来研究较多的双分子亲核取代(SN2)反应。[1]图1显示了对称X+CH3X！XCH3+X沿着反应坐标进行SN2反应。在气相中，PES有两个极小值，对应于稳定的X原子(CH3X)络合物。[2]PES被溶剂化作用严重扭曲。由于负电荷在过渡态的(X···CH3···X)基团上离域，过渡态的溶剂化稳定能比X+CH3X反应物或X-(CH3X)络合物的稳定能小.在溶液中，反应物和产物之间存在很大的势垒。据报道，这些反应在质子性溶剂中的反应速率常数比在非质子性溶剂中小几个数量级；这一趋势是由于卤化物阴离子周围质子化分子的溶剂化壳层的形成。[1，3]Morokuma曾报道过一项关于Cl2+CH3Cl2！ClCH3+Cl与少量H2O分子发生Sn2反应。H2O分子与氯离子(CH3Cl)反应体系的结合产生亚稳态异构体，影响反应机理。[4]Johnson和他的同事利用光解离光谱研究了卤化物阴离子络合物在气相中的结构和反应。[5]在这里，我们报道了I-(CH3I)(H2O)n(n=1-3)簇合物的红外光解离(IRPD)光谱结果。红外光谱的分析，以及理论计算的帮助，提供了关于这些配合物的稳定结构的有价值的信息。从这些稳定的结构中，我们得到了PES沿I+CH3I的形变信息！ICH3+i与Sn2反应配位是由微水化作用引起的。在水、甲醇、乙醇和丙酮中测定了该反应的速率常数；发现在水中的速率常数比在丙酮中的速率常数小四个数量级。[3，6]I-阴离子与H2O之间的结合能为43kJ·mol·L~(-1)，这一值与I~-阴离子与CH3I的结合能(35kJ·mol·L~(-1))相当。图2显示了在C-H和O-H伸缩区域测得的I-(CH_3I)(H_2O)1-3团簇的IRPD和计算的IR光谱。在3200-3700 cm/1区域的强带是由H2O组分的O-H伸缩振动引起的。我们用一个大的基组(I原子的MP2/Aug-cc-pVDZ-PP，C、H和O原子的ogcc-pVDZ)进行了非谐分析，以获得图2中的振动频率；本研究中使用的电子结构计算和基组的水平是根据重现气相中I+(H2O)的红外光谱的能力仔细确定的(见支持信息)。如图2中虚线所示，计算的红外光谱很好地再现了观察到的红外光谱。测得的红外光谱可归因于每个簇的两个异构体。图3显示了I-(CH3I)-(H2O)1-3络合物的结构，通过比较它们的计算非谐频率与IRPD光谱确定的。对于n=1的离子，3369和3696 cm-1谱带被指定为异构体1A的氢键和自由O-H伸缩振动，这是n=1最稳定的形式。
Reactions of halide anions with methyl halides XĀ+ CH3Y! XCH3+ YĀ are bimolecular nucleophilic substitution (SN2) reactions that have been well investigated in the last few decades.[1] Figure 1 shows the typical potential energy surfaces (PES) proposed for symmetric XĀ+ CH3X! XCH3+ XĀ SN2 reactions along the reaction coordinate. In the gas phase, the PES has two minima, which correspond to the stable XĀ (CH3X) complexes.[2] The PES is substantially distorted by solvation. As the negative charge is delocalized over the (X··· CH3··· X) Ā moiety at the transition state, the stabilization energy gained by solvation is smaller for the transition state than that for the XĀ+ CH3X reactants or the XĀ (CH3X) complexes. In solution, a large potential barrier exists between the reactants and products. The rate constants of these reactions in protic solvents were reported to be a few orders of magnitude smaller than those in aprotic solvents; this trend was explained by the formation of solvation shells of protic molecules around the halide anions.[1, 3] Morokuma has previously reported a theoretical study on the PES of the ClĀ+ CH3Cl! ClCH3+ ClĀ SN2 reaction with a few H2O molecules. The attachment of H2O molecules to the ClĀ (CH3Cl) reactive system produces metastable isomers, which affect the reaction mechanism.[4] Johnson and coworkers have extensively investigated the structures and reactions of halide anion complexes in the gas phase using photodissociation spectroscopy.[5] Herein, we report the results of IR photodissociation (IRPD) spectroscopy on IĀ (CH3I)(H2O) n (n= 1–3) clusters. An analysis of the IRPD spectra, along with the aid of theoretical calculations, provides valuable information about the stable structures of these complexes. From these stable structures we obtain information about the deformation of the PES along the IĀ+ CH3I! ICH3+ IĀ SN2 reaction coordinate caused by microhydration. The rate constant of this reaction has been measured in water, methanol, ethanol, and acetone; the rate constant in water was found to be four orders of magnitude smaller than that in acetone.[3, 6] The binding energy between IĀ anion and H2O is 43 kJ molĀ1, a value that is comparable to that of the IĀ anion with CH3I (35 kJ molĀ1).[7] Other important previous studies related to this subject are discussed in the Supporting Information. Figure 2 shows the measured IRPD and calculated IR spectra of the IĀ (CH3I)(H2O) 1–3 clusters in the CĀH and OĀH stretching regions. The strong bands in the 3200–3700 cmĀ1 region are due to the OĀH stretching vibrations of the H2O constituents. We have performed anharmonic analysis with a large basis set (MP2/aug-cc-pVDZ-PP for I atoms, and augcc-pVDZ for C, H, and O atoms) to obtain the vibrational frequencies in Figure2; the level of electronic structure calculations and basis sets used in this study are carefully determined from the ability to reproduce the IR spectrum of IĀ (H2O) in the gas phase (see the Supporting Information). The observed IR spectra are well reproduced by the calculated ones, as indicated by the dotted lines in Figure 2. The measured IRPD spectra can be attributed to two isomers for each cluster. Figure 3 shows the structure of the IĀ (CH3I)-(H2O) 1–3 complexes, as determined by a comparison of their computed anharmonic frequencies with the IRPD spectra. For the n= 1 ion, the 3369 and 3696 cmĀ1 bands are assigned to the hydrogen-bonded and free OĀH stretching vibrations of isomer 1A, which is the most stable form for n= 1. The