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首页> 外文期刊>Applied Magnetic Resonance >The stabilization of unusual conformers in guest-host systems: Solid-state NMR investigations of 2-methylhexadecane in urea and thiourea
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The stabilization of unusual conformers in guest-host systems: Solid-state NMR investigations of 2-methylhexadecane in urea and thiourea

机译：来宾-宿主系统中异常构象异构体的稳定化：尿素和硫脲中2-甲基十六烷的固态NMR研究

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摘要

Solid-state nuclear magnetic resonance (NMR) spectroscopy is employed for the first time on urea and thiourea inclusion compounds (UICs and TICs) containing branched alkyl chains. In the present work, H-2 and C-13 NMR as well as X-ray diffraction studies of two selectively deuterated 2-methylhexadecanes in UIC and TIC are presented. An analysis of the derived T-1 data reveals significant differences between UICs and TICs, which can be attributed to differences in the motional features of the guest species. It is found that four different motional contributions have to be considered, namely, chain rotation, chain wobbling, trans-gauche isomerization and methyl group rotation. 2-Methylhexadecane in UIC exists in an almost all-trans conformation (gauche amount not more than 5%) and undergoes fast chain rotation (6-site jump process, activation energy E-A = 16.7 kJ/mol). The analysis of the H-2 NMR spectra of 2-methylhexadecane-1,1',2-d(7), in urea proves that the branched chain end exists in an eclipsed conformation. The T-1 data of 2-methylhexadecane-3-d(2) in thiourea can be reproduced by an overall rotation (E-A = 9.8 kJ/mol) and a trans-gauche isomerization with torsional jumps around the C-3-C-4 bond (E-A = 11.0 kJ/mol, gauche population = 15%). As for the corresponding UIC, the H-2 NMR spectra of 2-methylhexadecane-1,1',2-d(7) in TIC can be only explained by the existence of an eclipsed conformation at the branched chain end.
机译：固态核磁共振（NMR）光谱首次用于含有支链烷基链的尿素和硫脲包合物（UIC和TIC）。在本工作中，介绍了UIC和TIC中两个选择性氘代的2-甲基十六烷的H-2和C-13 NMR以及X射线衍射研究。对衍生的T-1数据的分析显示，UIC和TIC之间存在显着差异，这可以归因于客体物种运动特征的差异。发现必须考虑四个不同的运动贡献，即，链旋转，链摇摆，反式变位异构化和甲基旋转。 UIC中的2-甲基十六烷几乎以全反式构型存在（胶团含量不超过5％），并经历了快速链旋转（6-位跳跃过程，活化能E-A = 16.7 kJ / mol）。对尿素中的2-甲基十六烷-1,1'，2-d（7）的H-2 NMR光谱分析表明，支链末端以偏光构象存在。硫脲中2-甲基十六烷-3-d（2）的T-1数据可通过整体旋转（EA = 9.8 kJ / mol）和反式变位的g-cheche异构化得到，该扭转围绕C-3-C- 4键（EA = 11.0 kJ / mol，gauche人口= 15％）。对于相应的UIC，TIC中2-甲基十六烷-1,1'，2-d（7）的H-2 NMR光谱只能通过在支链末端存在蚀变构象来解释。

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