17O NQR and 13C NMR study of hydrogen‐bonded organic ferroelectric croconic acid

17O NQR and 13C NMR study of hydrogen‐bonded organic ferroelectric croconic acid
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氢键有机铁电克罗康酸的 17O NQR 和 13C NMR 研究

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发表时间：
2011
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通讯作者：
R. Blinc
R. Blinc
中科院分区：
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文献类型：
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作者：
J. Seliger;J. Plavec;P. Šket;V. Zagar;R. Blinc

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在室温下研究了多晶巴豆酸H_2C_5O_5的~1H~(17)O核四极双共振谱和~(13)C CP/MAS核磁共振谱。最近研究表明，巴豆酸在所有有机铁电体中具有最高的可切换自发极化，并在450 K左右保持极化到分解点。~(13)C核磁共振和~(17)O核四极共振(NQR)谱表明，对于给定的分子，存在五个晶体学上不等价的碳和氧位置，因此，两个OH…O债券是不等价的。根据17O四极共振谱的偶极结构确定了两个氢键的OH距离为0.099 ± 0.001 nm。在COH和CO…处的大17O四极耦合常数H氧位置和较短的OH距离表明OH…O氢键是强烈不对称的。17OH…的关联O和OH…17O四极耦合常数与O…的关系在几种有机酸中观察到O距离。巴豆酸的数据明显偏离了这一关联，这可能是由于影响氢键中电子分布的强长程铁电有序的结果。
The 1H17O nuclear quadrupole double resonance spectrum and the 13C CP/MAS NMR spectrum of polycrystalline croconic acid, H2C5O5, have been studied at room temperature. Croconic acid has been recently shown to have the highest switchable spontaneous polarization of all organic ferroelectrics and stays polarized up to the decomposition point at around 450 K. Both the 13C NMR and the 17O nuclear quadrupole resonance (NQR) spectra show that there are five crystallorgaphically non‐equivalent carbon and oxygen positions for a given molecule and that therefore the two OH…O bonds are non‐equivalent. From the dipolar structure of the 17O quadrupole resonance spectra the OH distance is determined as being 0.099 ± 0.001 nm in both hydrogen bonds. The large 17O quadrupole coupling constant at the COH as well as at the CO…H oxygen position and the short OH distance demonstrate that the OH…O hydrogen bonds are strongly asymmetric. A correlation of the 17OH…O and OH…17O quadrupole coupling constants versus the O…O distance has been observed in several organic acids. The data for croconic acid significantly deviate from this correlation, what may be the result of the strong long range ferroelectric ordering which influences the electron distribution in the hydrogen bonds.