Solute-solvent complex switching dynamics of chloroform between acetone and dimethylsulfoxide-two-dimensional IR chemical exchange spectroscopy.
Solute-solvent complex switching dynamics of chloroform between acetone and dimethylsulfoxide-two-dimensional IR chemical exchange spectroscopy.
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DOI：
10.1021/jp806035w
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发表时间：
2008-11-06
影响因子：
3.3
通讯作者：
Fayer, Michael D.
中科院分区：
文献类型：
--
作者：
Kwak, Kyungwon;Rosenfeld, Daniel E.;Chung, Jean K.;Fayer, Michael D.
C-H与各种氢键受体之间形成的氢键在蛋白质和有机晶体的结构以及C-H键断裂反应的机制中发挥着重要作用。氯仿是一种 C-H 氢键供体，可以与丙酮和二甲基亚砜 (DMSO) 形成弱氢键配合物。当氯仿溶解在丙酮和DMSO的混合溶剂中时，两种类型的氢键配合物都存在。氯仿-丙酮和氯仿-DMSO这两种复合物处于平衡状态，并且它们通过氯仿交换氢键受体而快速相互转化。使用超快二维红外 (2D IR) 振动回波化学交换光谱来探测这种快速氢键受体取代反应。实验中使用氘代氯仿，测量了C-D拉伸模式的二维红外光谱。通过观察二维红外光谱中非对角线峰随时间的增长来跟踪氯仿氢键伙伴的化学交换。测得的丙酮分子取代氯仿-DMSO复合物中的DMSO分子的取代率为1/30 ps，DMSO分子取代氯仿-丙酮复合物中的丙酮分子的取代率为1/45 ps。混合溶剂中存在游离氯仿，它在取代反应中充当反应中间体，类似于SN1型反应。根据丙酮和 DMSO 的测量速率和平衡浓度，发现氯仿-DMSO 和氯仿-丙酮复合物的解离速率分别为 1/24 ps 和 1/5.5 ps。测量的完全取代反应速率与复合物解离速率之间的差异对应于扩散限制速率。估计的扩散限制率与扩散反应的 Smoluchowski 处理结果非常吻合。
Hydrogen bonds formed between C-H and various hydrogen bond acceptors play important roles in the structure of proteins and organic crystals, and the mechanisms of C-H bond cleavage reactions. Chloroform, a C-H hydrogen bond donor, can form weak hydrogen bonded complexes with acetone and with dimethylsulfoxide (DMSO). When chloroform is dissolved in a mixed solvent consisting of acetone and DMSO, both types of hydrogen bonded complexes exist. The two complexes, chloroform-acetone and chloroform-DMSO, are in equilibrium, and they rapidly interconvert by chloroform exchanging hydrogen bond acceptors. This fast hydrogen bond acceptor substitution reaction is probed using ultrafast two dimensional infrared (2D IR) vibrational echo chemical exchange spectroscopy. Deuterated chloroform is used in the experiments, and the 2D IR spectrum of the C-D stretching mode is measured. The chemical exchange of the chloroform hydrogen bonding partners is tracked by observing the time-dependent growth of off-diagonal peaks in the 2D IR spectra. The measured substitution rate is 1/30 ps for an acetone molecule to replace a DMSO molecule in a chloroform-DMSO complex and 1/45ps for a DMSO molecule to replace an acetone molecule in a chloroform-acetone complex. Free chloroform exists in the mixed solvent, and it acts as a reactive intermediate in the substitution reaction, analogous to a SN1 type reaction. From the measured rates and the equilibrium concentrations of acetone and DMSO, the dissociation rates for the chloroform-DMSO and chloroform-acetone complexes are found to be 1/24 ps and 1/5.5 ps respectively. The difference between the measured rate for the complete substitution reaction and the rate for complex dissociation corresponds to the diffusion limited rate. The estimated diffusion limited rate agrees well with the result from a Smoluchowski treatment of diffusive reactions.
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