Conformational study of cis-1,4-di-tert-butylcyclohexane by dynamic NMR spectroscopy and computational methods.: Observation of chair and twist-boat conformations
Conformational study of cis-1,4-di-tert-butylcyclohexane by dynamic NMR spectroscopy and computational methods.: Observation of chair and twist-boat conformations
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DOI：
10.1021/jo051654z
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发表时间：
2005-12-23
影响因子：
3.6
通讯作者：
Noe, EA
中科院分区：
文献类型：
--
作者：
Gill, G;Pawar, DM;Noe, EA
顺式-1，4-二叔丁基环己烷（1）的低温~（13）C NMR谱显示了扭船（1a）和椅式（1b）构象的信号。基于不同的群体、不同的对称性（Ia的时间平均C-2 v和Ib的时间平均C-s）和计算的化学位移（GIAO，HF/6-311+G*），将C-13 NMR信号分配给特定的碳。除了缓慢的环反转和椅子和扭船构象的相互转换外，还发现了叔丁基的缓慢旋转，观察到了大多数预期的C-13峰。在-148.1 ℃下，发现la（主要）和1b（次要）相互转化的自由能垒为6.83和6.35 kcal/mol。用Allinger的MM 3和MM 4程序对构象空间进行搜索，得到了几种低能构象la-c的自由能。用从头算方法在HF/6-311+G* 水平上重复计算。得到了分子的对称性、相对自由能、相对熵、频率和NMR化学位移。通过从头算、MM 3和MM 4计算，生成并优化了船构象（1d; C-2 v对称性）作为过渡态。
Low-temperature C-13 NMR spectra of cis-1,4-di-tert-butyleyclohexane (1) showed signals for the twist-boat (1a) and chair (1b) conformations. C-13 NMR signals were assigned to specific carbons based on the different populations, different symmetries (time-averaged C-2v for la and time-averaged C-s for 1b), and calculated chemical shifts (GIAO, HF/6-311+G*). In addition to slow ring inversion and interconversion of the chair and twist-boat conformations, slow rotation of the tert-butyl groups was found. Most of the expected C-13 peaks were observed. Free-energy barriers of 6.83 and 6.35 kcal/mol were found for interconversion of la (major) and 1b (minor) at -148.1 degrees C. Conformational space was searched with Allinger's MM3 and MM4 programs, and free energies were obtained for several low-energy conformations la-c. Calculations were repeated with A initio methods up to the HF/6-311+G* level. Molecular symmetries, relative free energies, relative enthalpies and entropies, frequencies, and NMR chemical shifts were obtained. A boat conformation (1d; C-2v symmetry) was generated and optimized as a transition state by ab initio, MM3, and MM4 calculations.