J. Phys. Chem. B 2009, 113, 9035–9040
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Striking Structural Transformation from Cyclic Oligosaccharide to Aromatic Series by Means of the Effect of Lithium Carbonate Based on Gas Chromatography Coupled to Time-of-Flight Mass Spectrometry Le Xin Song* and Lei Bai Department of Chemistry, UniVersity of Science and Technology of China, Hefei, 230026 Anhui, China ReceiVed: March 28, 2009; ReVised Manuscript ReceiVed: May 17, 2009
An interesting paradigm in which the conformation of glucopyranose of β-CD in the presence of Li2CO3 is transformed into both an aromatic structure, such as the heterocyclic compound C6H6O+ with a prominent relative abundance (RA) and the tropylium ions C3H3+, C5H5+, and C7H7+ at comparatively low temperatures, and a linear structure such as C18H30O+ and C4H10O3+ is reported in the present work. The efficient transformation (higher RA, lower temperature) from a sugar structure to an aromatic structure is ascribed to the contribution of the molecule-ion interaction between β-CD and Li2CO3. Such a transformation is significant because it provides new insight into the link between supramolecular chemistry and other fields such as organic synthesis, biomineralization, environmental protection, and energy utilization. Introduction
Experimental Section
β-Cyclodextrin (β-CD) is a cyclic oligosaccharide, formed by seven R-1,4-D-glucopyranosyl units.1-3 Because it possesses a hydrophobic pocket, β-CD has a well-known ability to trap a wide variety of guests to form supramolecular complexes, which gives it great potential for application to industry.4-8 The thermal behaviors of the inclusion complexes of β-CD with many guests, such as organic compounds, polymers, and coordination compounds, have already been reported by other workers.9-15 Our previous results showed that, on the one hand, the thermal stabilities of organic guests (G) were changed by the presence of a G-β-CD binding structure; on the other hand, the decomposition behavior of complexed β-CD also experiences changes caused by the binding interaction with G.16 In our recent studies, we found that β-CD had a significant effect on the thermal decomposition of guests, such as organic amines17 and sodium arsenite.18 However, there have been very few studies to date concerning the effect of guests, especially inorganic salts, on the thermal decomposition behaviors of β-CD in solid state. Therefore, an inorganic salt lithium carbonate (Li2CO3) with a wide application in medicine, material, and electrochemistry is selected as an objective inorganic guest. A solid adduct, Li2CO3sβ-CD, was prepared and analyzed by gas chromatography coupled to time-of-flight mass spectrometry (GC-TOF-MS). This present result is both surprising and inspiring in that it demonstrated the structural transformation from a cyclic oligosaccharide of β-CD to various aromatic series such as 2-methylenepyran with a higher relative abundance (RA) and several tropylium ions occurring at lower temperatures by introduction of Li2CO3. Undoubtedly, the finding has a profound implication for the formation and evolution of organic compounds in nature. Further, β-CD is only one of numerous carbohydrates such as simple sugars, starch, and cellulose. Hence, observations in the current work provide impetus for future research.
Materials. β-CD was purchased from Shanghai Chemical Reagent Company and recrystallized twice from deionized water. Li2CO3 was purchased from Shanghai Guanghua Technology Company and used as received without further purification. Calcium carbonate (CaCO3) was purchased from Sinopharm Chemical Reagent Co., Ltd., and used without further purification. All other chemicals were of general-purpose reagent grade unless otherwise stated. Preparation of Solid Sample. One millimoles of β-CD (1.135 g) and 1 mmol of Li2CO3 (0.074 g) were resolved in 120 mL of deionized water and stirred at 333.2 K for 6 h in a round-bottom flask. After the solvent was drawn off below 323.2 K, the residue was dried under vacuum at 393.2 K. Then the product (Li2CO3sβ-CD) was obtained as a white powder. The stoichiometry of the solid adduct of β-CD and Li2CO3 was determined to be 1:1 based on elemental analyses of the solid sample (Anal. Calcd for Li2C43H70O38 · 3H2O: Li+, 1.10%; C, 40.86%; H, 6.02%. Found: Li+, 1.07%; C, 40.92%; H, 6.13%). Also, 1 mmol of β-CD (1.135 g) and 1 mmol of CaCO3 (0.100 g) were resolved in 120 mL of deionized water and stirred at 333.2 K for 6 h in a round-bottom flask. However, because of the considerable low solubility of CaCO3 (Ksp, 2.8 × 10-9, 298.15 K), the undissolved CaCO3 was filtered out. After the solvent was drawn off below 323.2 K, the moist powder was dried under vacuum at 393.2 K. The stoichiometry of the solid adduct of β-CD and CaCO3 was determined to be 18:1 based on elemental analyses of the solid sample (Anal. Calcd for (CaCO3)(C42H70O35)18 · 25H2O: Ca2+, 0.19%; C, 43.30%; H, 6.24%. Found: Ca2+, 0.19%; C, 43.50%; H, 6.41%). Methods. The mass percentage of Li+ or Ca2+ in the samples was determined by using an Atomscan Advantage (Thermo Jarrell Ash Corporation, Franklin, MA) instrument and the mass percentage of C and H was determined by an elemental analyzer (Vario EL III, Germany). GC-TOF-MS measurements with a programmed temperature operation were carried out with a Micromass GCT-MS spectrometer using the standard direct insertion probe for a solid sample, with an increasing temperature. The detailed description of the appointed heating program
* To whom correspondence should be addressed. E-mail: solexin@ ustc.edu.cn.
10.1021/jp902805b CCC: $40.75 2009 American Chemical Society Published on Web 06/09/2009
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Song and Bai
Figure 1. TIC curves of free β-CD (A) and Li2CO3sβ-CD (B).
for three samples free β-CD, Li2CO3sβ-CD, and CaCO3sβCD was the same as that of a recent report.17 Results and Discussion Total Ion Current Analysis of the Solid Adduct Li2CO3sβCD. GC-TOF-MS can be applied to explore the decomposition paths of samples in detail based on the composition and RA of decomposed fragments under different temperatures.15c,16-18 The total ion current (TIC) plots of free β-CD and Li2CO3sβ-CD are shown in Figure 1. Clearly, there are remarkable differences between the profiles of the two curves in the figure. First, the shape of curve B of Li2CO3sβ-CD is completely different from that of curve A of free β-CD, and from those of the inclusion complexes of β-CD with ethylenediamine and diethylenetriamine, though they were measured under the same conditions.17 Curve B has four obvious peaks at 20.72 (broad), 24.87 (strong), 31.70 (moderate), and 35.70 min (weak), whereas curve A has only double peaks at 25.63 and 27.23 min. This finding has a significant implication for the diversification and complexity of the thermal decomposition behavior of β-CD in the presence of Li2CO3. The occurrence of more peaks in a wide temperature range shows that Li2CO3sβ-CD has more frequent release events in comparison with free β-CD.17 Second, the strongest peak of free β-CD appears at 25.63 min, accompanied by a strong shoulder peak at 27.23 min. However, Li2CO3sβ-CD exhibits a large broad peak at 20.72 min and a strong sharp peak at 24.87 min. If they are regarded as the shift of the double peak in free β-CD, both of them occur sooner than in free β-CD. Besides, two new peaks appear at 31.70 and 35.70 min in Li2CO3sβ-CD. Third, there is a very weak peak at around 10.08 min in curve B, corresponding to a relatively low temperature of 463.2 K. Apparently, it does not appear in curve A. To examine whether the presence of the peak is due to the release of crystal water molecules in Li2CO3sβ-CD, the MS spectra of Li2CO3sβ-CD at 10.08 and 10.13 min are shown in Figure 2. Mass Spectrum Analysis of Li2CO3sβ-CD at 10.08 and 10.13 min. Figure 2A presents three major molecular ion peaks, corresponding to CO2+ (m/z at 43.990) with a RA of 100%, H2O+ (m/z at 18.011, RA, 17.91%), and C6H6O+ (m/z at 94.042, RA, 7.61%). The formation of the fragment CO2+ means that the rupture of several R-1,4-D-glycosidic linkages in β-CD is also successfully accomplished, at the moment, largely ahead of schedule. The phenomenon also is found in the mass spectrum of free β-CD.17 However, to our surprise, the fragments of H2O+ and C6H6O+ are not found in the mass spectrum of free β-CD at the corresponding temperature.17 Structurally, the fragment C6H6O+ should be one of the aromatic compounds with three unsaturated degrees. Eighteen probable structures corresponding to the fragment are listed in Table 1. According to the number and kind of atoms that it contains, and structural similarity to the
Figure 2. Mass spectra of Li2CO3sβ-CD at 10.08 min (A) and 10.13 min (B).
TABLE 1: Probable Structures Corresponding to C6H6O+
frame structure of β-CD, we show that it is most likely 2-methylenepyran ion. A proposed formation model of this fragment at the moment is illustrated in Figure 3. Li+ ions have a very small radius and a strong polarization potential for some atoms or anions. As shown in Figure 3, the interactions (HO · · · Li+ · · · O and HO · · · Li+ · · · OH) between a Li+ ion and two oxygen atoms can weaken some C-O bonds in β-CD, causing the formation of
From Cyclic Oligosaccharide to Aromatic Series
J. Phys. Chem. B, Vol. 113, No. 26, 2009 9037
Figure 3. Proposed formation model of the C6H6O+ ion at 10.08 min.
Figure 4. MSID curves of key fragments: m/z at 43.990 (A) in free β-CD, m/z at 43.990 in Li2CO3sβ-CD (B), and m/z at 94.042 in Li2CO3sβ-CD (C).
C6H6O+ ion. Further, the rupture of some C-O bonds also results in the occurrence of water molecules. Herein, special emphasis should be put on the structural comparison between C6H6O+ and β-CD. The former is a heterocyclic compound with an aromatic structure, but the latter is a macrocyclic compound with an oligosaccharide structure. The observation of the structural transformation gives us a strong impression that a glucopyranose can be transformed into an aromatic compound expediently. It is worth noting that the occurrence of the fragment is not transient because it still appears at 10.13 min with a higher RA of 7.96% as shown in Figure 2B. To gain further insight into the formation of the fragment, namely, the relationship between RA and heating time, massselective ion detection (MSID) is performed. The MSID curves of several key fragments are plotted in Figure 4. MSID Curves of CO2+ and C6H6O+. The fragment of CO2+ ion results from the rupture of R-1,4-D-glycosidic linkages of β-CD.17 From the profile comparison of Figure 4A,B, the release behavior of the fragment CO2+ in free β-CD is similar to that of Li2CO3sβ-CD in a lower temperature range, each having two small peaks at 10.08 and 17.33 min. However, there exists a large difference between them in a higher temperature range. For example, free β-CD shows a strong double peak at 25.87 min and a moderately broad peak at 31.98 min, but Li2CO3sβCD shows three single peaks at 25.17 (strong), 31.69 (weak), and 35.76 min (moderate). The release data of CO2+ at high temperatures suggest that the presence of Li2CO3 extends the temperature range involved in the rupture of R-1,4-D-glycosidic linkages. As shown in Figure 4C, the MSID curve of C6H6O+ shows four peaks at 10.08 (very weak), 17.33 (the strongest), 25.35 (moderate), and 31.43 min (weak). The positions of the latter three peaks approximately accord with those of corresponding peaks in curves A and B, but the intensity ratio of the three peaks in each curve is completely different from one another. The formation of this aromatic compound mainly occurs at 17.33 min (553.2 K), by this β-CD does not start to decompose to a significant degree under the conditions of measurement used here. In addition, there are no signals concerning the fragment after 33.32 min.
Figure 5. Mass spectra of Li2CO3sβ-CD at 17.33 (A) and 25.41 min (B).
The findings reveal that most of the aromatic compound has been generated at a relatively lower temperature, instead of coming from the subsequent rapid decomposition of β-CD. This may provide an important clue that the structural transformation from a sugar ring into an aromatic ring can be carried out with the existence of Li2CO3 under moderate conditions because, in the case of free β-CD, C6H6O+ ion cannot be observed until 25.40 min and its RA values in all mass spectra are always very low ( 5%) of tropylium ions are detected, indicating that the thermal decomposition behavior of β-CD in the adducts has a close relationship with the nature and content of inorganic salts. Figure 12 illustrates the MSID curve of the key fragment at m/z 94.042 in the case of CaCO3sβ-CD. Clearly, the profile of the curve is highly similar to that of the TIC curve (see Figure 10) of CaCO3sβ-CD itself, but distinct from that of the MSID curve of the fragment in the Li2CO3sβ-CD system. Besides, the data in Table 3 indicate that, different from Li2CO3, the presence of CaCO3 not only cannot lead to the earlier appearance of the aromatic fragment at m/z 94.042 but also does not lead to an improvement in the RA values of the fragment at higher temperatures. Also, the three tropylium ionssC3H3+ (39.024), C5H5+ (65.039), and C7H7+ (91.054)sdo not appear in CaCO3sβ-CD at lower temperatures, just like in the case of free β-CD. Although they occur at higher temperatures, their RA values are different from those in the case of free β-CD or Li2CO3sβ-CD. These results show that different inorganic salts cause different effects on the thermal decomposition behavior of β-CD. The different effects may be involved
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in different molecule-ion interactions between β-CD and inorganic ions, which are related to the nature and content of inorganic salts. Conclusion The data presented in this paper suggest a striking and interesting paradigm in which the conformation of glucopyranose of β-CD in the presence of Li2CO3 is transformed into both an aromatic structure such as the heterocyclic compound C6H6O+ with a higher RA and the tropylium ions C3H3+, C5H5+, and C7H7+ occurring at lower temperatures, and a linear structure such as C18H30O+ and C4H10O3+. The efficient conformation transformation (higher RA, lower temperature) from a sugar structure to an aromatic structure is ascribed to the contribution of the molecule-ion interaction between β-CD and Li2CO3. However, many of those phenomena occurring in Li2CO3sβCD are changed or do not occur in CaCO3sβ-CD. This suggests that different inorganic salts cause different effects on the thermal decomposition behavior of β-CD. The different effects may be ascribed to differences in molecule-ion interactions between β-CD and inorganic ions, and the differences are related to the nature and content of inorganic salts. What is the effect of other inorganic salts on the thermal decomposition of β-CD and other carbohydrates? Undoubtedly, such a structural transformation is significant because it provides new insight into the link between supramolecular chemistry and other fields such as organic synthesis, biomineralization, environmental protection, and energy utilization. Acknowledgment. We acknowledge the funding support received for this research project from the Innovation Foundation of Graduate Students at the University of Science and Technology of China, Natural Science Foundation of Anhui Province (No. 090416228), and NSFC. References and Notes (1) Szejtl, J. Chem. ReV. 1998, 98, 1743–1753. (2) (a) Connors, K. A. Chem. ReV. 1997, 97, 1325–1357. (b) Song, L. X.; Bai, L.; Xu, X. M.; He, J.; Pan, S. Z. Coord. Chem. ReV. 2009, 253, 1276–1284. (3) (a) Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 803–822. (b) Harata, K. Chem. ReV. 1998, 98, 1803–1827.
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