Origins of isotopomeric polymorphism.
Origins of isotopomeric polymorphism.
复制标题
同位素异构体多态性的起源。
DOI：
10.1080/10256010600840093
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发表时间：
2006
影响因子：
1.3
通讯作者：
Harbison,GerardS
中科院分区：
文献类型：
--
作者：
Zhou,Jun;Kye,Young-Sik;Kolesnikov,AlexanderI;Harbison,GerardS
4-甲基吡啶与五氯苯酚（4MPPCP）形成的配合物在三斜空间群中结晶。如果由氘化五氯酚合成相同的络合物，则结晶为完全不同的单斜晶型。利用固体核磁共振合成的样品与全范围的氘化水平，结晶从溶液或熔体，在存在或不存在种子，我们已经证实，同位素确实有不同的热力学稳定的晶体结构。这种同位素多晶现象的根源显然在于多晶之间氢键的差异。三斜型的氢键相对较短。高场固体核磁共振显示，氢键中氢的1h化学位移和2h电四极耦合都强烈依赖于温度，表明氢键纵向振动处于低洼激发态。通过对4MPPCP同位素体的非弹性中子散射，我们确定了氢键中氢在29.7、145和205 meV（240、1168和1651 cm−1）下的三种正交振动模式。纵向模式的能量最低，它表示一个稍微不对称的低势垒双阱势。这种势的内在特征是质子化和氘化形式之间的零点能（zpe）差别很小。相比之下，单斜晶型具有相对正常的氢键，其中质子和氘核的zpe相差约500 cm−1。可以设想三斜质子化形式的能量低于单斜质子化形式，但三斜氘化形式的能量高于单斜氘化形式的情况。这显然说明了两种形式在同位素取代后相对稳定性的差异。
The complex formed between 4-methylpyridine and pentachlorophenol (4MPPCP) crystallises in a triclinic space group. If the same complex is synthesized from deuterated pentachlorophenol, it crystallizes in an entirely different monoclinic polymorph. Using solid-state NMR of samples synthesized with a full range of deuteration levels, crystallized from solution or the melt, and in the presence or absence of seeds, we have confirmed that the isotopomers indeed have different thermodynamically stable crystal structures. The roots of this phenomenon of isotopomeric polymorphism apparently lie in the differences in hydrogen bonding between the polymorphs. The triclinic form has a relatively short hydrogen bond. High-field solid-state NMR shows both the1H chemical shift and the2H electric quadrupole coupling of the hydrogen involved in the bond to be strongly temperature-dependent, indicating a low-lying excited state of the hydrogen bond longitudinal vibration. Inelastic neutron scattering of isotopomers of 4MPPCP has allowed us to identify the three orthogonal vibrational modes of the hydrogen in the hydrogen bond, at 29.7, 145, and 205 meV (240, 1168, and 1651 cm−1). The longitudinal mode is the lowest in energy, and it indicates a slightly asymmetric low-barrier double-well potential. Intrinsic to such potentials is a very small difference in zero-point energies (ZPEs) between the protonated and deuterated forms. As a contrast, the monoclinic form has a comparatively normal hydrogen bond, in which the proton and deuteron ZPEs should be different by approximately 500 cm−1. A scenario can be envisaged where the triclinic protonated form is lower in energy than the monoclinic protonated form, but the triclinic deuterated form is higher in energy than the monoclinic deuterated form. This evidently accounts for the difference in relative stabilities of the two forms upon isotope substitution.