Benzoxazine oligomers: Evidence for a helical structure from solid-state NMR spectroscopy and DFT-based dynamics and chemical shift calculations
Benzoxazine oligomers: Evidence for a helical structure from solid-state NMR spectroscopy and DFT-based dynamics and chemical shift calculations
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DOI：
10.1021/ja029059r
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发表时间：
2003-05-14
影响因子：
15
通讯作者：
Ishida, H
中科院分区：
文献类型：
--
作者：
Goward, GR;Sebastiani, D;Ishida, H
结合分子模拟，密度泛函理论计算，和先进的固态核磁共振实验是用来阐明一系列的苯并恶嗪低聚物的超分子结构。分子内氢键的特点和确定为三聚体和四聚体单元的环状和螺旋构象的驱动力。在快速MASH NMR谱中，形成氢键的质子的共振可以被分配，并用于通过基于DFT的几何优化和H-1化学位移计算来验证和细化结构。同样支持这些结构的还有homomethyl H-1-H-1双量子NMR谱，它确定了每种材料中的局部质子-质子邻近性。此外，定量N-15-H-1的距离测量得到的偶极自旋边带图案的分析确认优化的几何形状的四聚体。这些结果清楚地支持预测的苯并恶嗪聚合物的螺旋几何形状。这种几何形状，其中N. H... O和O H... O氢键在螺旋内部受到保护，可以解释聚苯并恶嗪材料的许多示例性化学性质。先进的实验固态NMR光谱与计算几何优化，总能量和NMR光谱计算的结合是结构分析的有力工具。它的结果比单独的测量或计算提供了更大的置信度，特别是因为许多无序系统的微观结构由于缺乏长程有序而无法通过常规方法阐明。
A combination of molecular modeling, DFT calculations, and advanced solid-state NMR experiments is used to elucidate the supramolecular structure of a series of benzoxazine oligomers. Intramolecular hydrogen bonds are characterized and identified as the driving forces for ring-shape and helical conformations of trimeric and tetrameric units. In fast MASH NMR spectra, the resonances of the protons forming the hydrogen bonds can be assigned and used for validating and refining the structure by means of DFT-based geometry optimizations and H-1 chemical-shift calculations. Also supporting these proposed structures are homonuclear H-1-H-1 double-quantum NMR spectra, which identify the local proton-proton proximities in each material. Additionally, quantitative N-15-H-1 distance measurements obtained by analysis of dipolar spinning sideband patterns confirm the optimized geometry of the tetramer. These results clearly support the predicted helical geometry of the benzoxazine polymer. This geometry, in which the N...H...O and O...H...O hydrogen bonds are protected on the inside of the helix, can account for many of the exemplary chemical properties of the polybenzoxazine materials. The combination of advanced experimental solid-state NMR spectroscopy with computational geometry optimizations, total energy, and NMR spectra calculations is a powerful tool for structural analysis. Its results provide significantly more confidence than the individual measurements or calculations alone, in particular, because the microscopic structure of many disordered systems cannot be elucidated by means of conventional methods due to lack of long-range order.