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求助翻译啊 ，关于核磁共振分析的 ，急用！！
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The 13C NMR spectra indicated that glucose was in the forms of a-pyranose (2) and b-pyranose (1; Scheme 1) in [BMIM]Cl at room temperature in the absence of GeCl4 (Figure 9 a). However, when 1 equivalent of GeCl4 was added into the mixture, a new peak at d=76.4 ppm appeared (Figure 9b). The ability of heavier main-group elements to accommodate more than eight electrons in their valence shells, thus forming so-called hypervalent or hypercoordinate compounds, is well established.[ 20] The cations of Group 14 elements can be stabilized thermodynamically by introduction of auxiliary NR2 or OR groups, the lone electron pairs of which could, in principle, be capable of bonding to the empty p orbital of the Group 14 element.[ 21] Therefore, we speculated that GeIV interacted with the two O atoms of 1,2-dihydroxy group of a-glucose to form a stable intermediate 3 (Scheme 1). When the 13C NMR spectrum was recorded at 508C, the relative intensity of the peak at d= 76.4 ppm decreased, and two new peaks at d=152.1 and 103.6 ppm appeared. The intensity of the signal at d= 152.1 ppm increased when the reaction was held at higher temperatures (Figure 9d ande) and also increased over time (data not shown). Therefore, this signal was assigned to the 13C-labeled carbon of HMF at the 2-position. Assignment of the signal at d=103.6 ppm was more tricky. Because this signal was found only at 508C, it should be associated to a relatively stable intermediate with a short lifetime along the reaction pathway to HMF. In other words, at higher temperature, the turnover of this intermediate was greatly accelerated. Therefore, we suppose it was from fructose,[22] which indicated that the intermediate fructose was formed through glucose isomerization. Moreover, the signal at d=76.4 ppm was missing at 808C, indicating that the conversion of 3 was fast. As proposed by zhao et al., fructose was formed from glucose through an enediol intermediate catalyzed by GeCl4. [10] Fructose then lost one water molecular to form the fructofuranosyl oxocarbenium ion.[18] Due to the basicity and nucleophilicity of the halide ion (X), the oxocarbenium ion lost HX to form the enolic species through the nucleophile or the base pathway, followed by loss of two water molecules to form HMF. Our results also suggested that increasing the reaction temperature would enhance the interaction of glucose and GeCl4, and thus promote a ring-opening step of the intermediate 3 to form the straight-chain intermediate 4 through hydrogen transfer, which could be enolated to form 5, followed by the release of GeCl4 to give fructose (Scheme 1). At 80 8C, three new signals at d=56.1, 110.9 and 124.3 ppm appeared, which were attributed to C6, C4 and C3, respectively, of HMF. A new peak at d=80.1 ppm was also observed at 80 8C and 100 8C, which was attributed to a byproduct formed during the reaction. |
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