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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Solid-State ~(93)Nb, ~(19)F, and ~(113)Cd Nuclear Magnetic Resonance Study of Niobium Oxyfluorides: Characterization of Local Distortions and Oxygen/Fluorine Ordering
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Solid-State ~(93)Nb, ~(19)F, and ~(113)Cd Nuclear Magnetic Resonance Study of Niobium Oxyfluorides: Characterization of Local Distortions and Oxygen/Fluorine Ordering

机译：固态氟化铌的〜（93）Nb，〜（19）F和〜（113）Cd核磁共振：表征局部畸变和氧/氟有序

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摘要

The local structures and oxygen/fluoride ordering of Cdpy_4NbOF_5 (py = C_5H_5N) and [pyH]_2[Cdpy_4(NbOF_5)_2] have been investigated with ~(19)F, ~(113)Cd, and ~(93)Nb solid-state NMR spectroscopy. The ~(93)Nb magic-angle spinning (MAS) NMR spectra acquired at an ultrahigh magnetic field strength (19.6 T) and at ultrahigh spinning speeds (～43 kHz) of both compounds are dominated not only by the quadrupolar interaction but also by the chemical shielding (CS) interaction, consistent with highly asymmetric environments for niobium. A methodology is presented to extract the nuclear quadrupole coupling constant C_Q, the asymmetry parameter η, and the isotropic chemical shift δ_(iso) in the presence of a large chemical shift anisotropy (CSA). The CS tensor was then obtained from the simulations of the ~(93)Nb (I = 9/2) static NMR spectrum. The separations between the centerband of the central transition and sidebands of the ±3/2 <-> ± 5/2 satellite transition in the ~(93)Nb MAS NMR spectra are sensitive to the value of C_Q(~(93)Nb), while the line shapes are mainly determined by η and the relative orientation between the electric field gradient (EFG) and CS tensors. Thus, simulations of the MAS NMR spectra provided additional constraints on the fitting of all these parameters. The ~(93)Nb MAS NMR spectra acquired at lower field and lower spinning speeds, where the spinning sidebands are not separated from the centerband, were also reproduced in the simulations. The following parameters were determined for the two compounds at room temperature: C_Q = 36.0(5)/33.2(5) MHz, η = 0.50(5)/0.40(5), δ_(iso)(~(93)Nb) = -1310(10)/-1320(10) ppm, Ω = 1000(20)/1100(20) ppm, κ = 1.00(5)/0.8(1), α = 0(20)°/60(20)°, β = 0(2)°/5(2)°, γ = 0(20)°/0(20)° for Cdpy_4NbOF_5/[pyH]_2[Cdpy_4(NbOF_5)_2], respectively. A very distorted multiplet in the solid-state ~(19)F MAS NMR spectra was observed, which arises from J-coupling [~1J(~(93)Nb, ~(19)F) = 362(2) and 350(5) Hz, for Cdpy_4NbOF_5 and [pyH]_2[Cdpy_4(NbOF_5)_2], respectively] and residual dipolarcoupling between the ~(19)F and ~(93)Nb nuclei. Only one ~(113)Cd NMR resonance is observed for Cdpy_4-NbOF_5, which, together with the ~(19)F NMR, suggests that the O-substitution is not random. On the basis of the ~(113)Cd NMR, the NbOF_5 octahedra are most likely oriented in opposite directions in different chains so as to allow the acentric NbOF_5 octahedra to occupy a center of inversion in the crystal structure of this compound determined by X-ray diffraction (the "interchain" cancellation mechanism proposed by Heier et al., in J. Solid State Chem. 1997, 133, 576-579).
机译：使用〜（19）F，〜（113）Cd和〜（93）Nb固体研究了Cdpy_4NbOF_5（py = C_5H_5N）和[pyH] _2 [Cdpy_4（NbOF_5）_2]的局部结构和氧/氟有序性核磁共振谱。两种化合物在超高磁场强度（19.6 T）和超高旋转速度（〜43 kHz）下获得的〜（93）Nb魔角纺丝（MAS）NMR谱不仅由四极相互作用主导，而且由化学屏蔽（CS）相互作用，与铌的高度不对称环境一致。提出了一种在存在较大化学位移各向异性（CSA）的情况下提取核四极偶合常数C_Q，不对称参数η和各向同性化学位移δ_（iso）的方法。然后，通过〜（93）Nb（I = 9/2）静态NMR光谱的模拟获得CS张量。 〜（93）Nb MAS NMR光谱中中心跃迁的中心带与±3/2 <->±5/2卫星跃迁的边带之间的间隔对C_Q（〜（93）Nb）的值敏感，而线形主要由η和电场梯度（EFG）与CS张量之间的相对方向确定。因此，MAS NMR光谱的模拟对所有这些参数的拟合提供了额外的限制。在较低的磁场和较低的纺丝速度下获得的〜（93）Nb MAS NMR谱图也再现了模拟中的纺丝边带未与中心带分离的情况。在室温下确定了这两种化合物的以下参数：C_Q = 36.0（5）/33.2（5）MHz，η= 0.50（5）/0.40（5），δ_（iso）（〜（93）Nb）= -1310（10）/-1320（10）ppm，Ω= 1000（20）/ 1100（20）ppm，κ= 1.00（5）/0.8（1），α= 0（20）°/ 60（20）对于Cdpy_4NbOF_5 / [pyH] _2 [Cdpy_4（NbOF_5）_2]，分别为°，β= 0（2）°/ 5（2）°，γ= 0（20）°/ 0（20）°。固态〜（19）F MAS NMR光谱中观察到非常扭曲的多重峰，这是由于J耦合[〜1J（〜（93）Nb，〜（19）F）= 362（2）和350（ 5）Hz，分别用于Cdpy_4NbOF_5和[pyH] _2 [Cdpy_4（NbOF_5）_2]]和〜（19）F和〜（93）Nb原子核之间的残留偶极耦合。对于Cdpy_4-NbOF_5，仅观察到一个〜（113）Cd NMR共振，与〜（19）F NMR一起表明O-取代不是随机的。根据〜（113）Cd NMR，NbOF_5八面体最有可能在不同链中指向相反的方向，从而使无心NbOF_5八面体占据该化合物由X-确定的晶体结构的反转中心。射线衍射（Heier等人在J.Solid State Chem.1997，133，576-579中​​提出的“链间”消除机理）。

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