Özcan Nergiz, Vähäkangas Jarkko, Lantto Perttu, Vaara Juha
Department of Chemistry, University of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1), 00014 Helsinki (Finland).
Chemphyschem. 2014 Jun 23;15(9):1799-808. doi: 10.1002/cphc.201301184. Epub 2014 May 7.
Nuclear magnetic resonance (NMR) spectroscopy is an important molecular characterisation method that may aid the synthesis and production of graphenes, especially the molecular-scale graphene nanoislands that have gathered significant attention due to their potential electronic and optical applications. Herein, carbon-13 NMR chemical shifts were calculated using density functional theory methods for finite, increasing-size fragments of graphene, hydrogenated graphene (graphane) and fluorinated graphene (fluorographene). Both concentric hexagon-shaped (zigzag boundary) and crenellated (armchair) fragments were investigated to gain information on the effect of different types of flake boundaries. Convergence trends of the (13)C chemical shift with respect to increasing fragment size and the boundary effects were found and rationalised in terms of low-lying electronically excited states. The results predict characteristic behaviour in the (13)C NMR spectra. Particular attention was paid to the features of the signals arising from the central carbon atoms of the fragments, for graphene and crenellated graphene on the one hand and graphane and fluorographene on the other hand, to aid the interpretation of the overall spectral characteristics. In graphene, the central nuclei become more shielded as the system size increases whereas the opposite behaviour is observed for graphane and fluorographene. The (13)C signals from some of the perimeter nuclei of the crenellated fragments obtain smaller and larger chemical shift values than central nuclei for graphene and graphane/fluorographene, respectively. The diameter of the graphenic quantum dots with zigzag boundary correlates well with the predicted carbon-13 chemical shift range, thus enabling estimation of the size of the system by NMR spectroscopy. The results provide data of predictive quality for future NMR analysis of the graphene nanoflake materials.
核磁共振（NMR）光谱学是一种重要的分子表征方法，它可能有助于石墨烯的合成与生产，特别是分子尺度的石墨烯纳米岛，由于其潜在的电子和光学应用而备受关注。在此，使用密度泛函理论方法计算了石墨烯、氢化石墨烯（石墨烷）和氟化石墨烯（氟石墨烯）有限且尺寸不断增加的片段的碳-13 NMR化学位移。研究了同心六边形（锯齿形边界）和锯齿状（扶手椅形）片段，以获取不同类型薄片边界效应的信息。发现了（13）C化学位移随片段尺寸增加的收敛趋势以及边界效应，并根据低电子激发态进行了合理化解释。结果预测了（13）C NMR光谱中的特征行为。特别关注了片段中心碳原子产生的信号特征，一方面是石墨烯和锯齿状石墨烯，另一方面是石墨烷和氟石墨烯，以帮助解释整体光谱特征。在石墨烯中，随着系统尺寸的增加，中心原子核的屏蔽作用增强，而在石墨烷和氟石墨烯中则观察到相反的行为。锯齿状片段一些周边原子核的（13）C信号，对于石墨烯而言，其化学位移值比中心原子核小，而对于石墨烷/氟石墨烯而言，则比中心原子核大。具有锯齿形边界的石墨烯量子点的直径与预测的碳-13化学位移范围具有良好的相关性，因此能够通过NMR光谱学估计系统的大小。这些结果为未来石墨烯纳米薄片材料的NMR分析提供了具有预测性的质量数据。
