Gill Gurvinder, Pawar Diwakar M, Noe Eric A
Department of Chemistry, Jackson State University, Jackson, Mississippi 39217-0510, USA.
J Org Chem. 2005 Dec 23;70(26):10726-31. doi: 10.1021/jo051654z.
[reaction: see text] Low-temperature 13C NMR spectra of cis-1,4-di-tert-butylcyclohexane (1) showed signals for the twist-boat (1a) and chair (1b) conformations. 13C NMR signals were assigned to specific carbons based on the different populations, different symmetries (time-averaged C(2v) for 1a 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 13C peaks were observed. Free-energy barriers of 6.83 and 6.35 kcal/mol were found for interconversion of 1a (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 1a-c. Calculations were repeated with ab 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.
[反应：见正文] 顺式-1,4-二叔丁基环己烷（1）的低温13C NMR光谱显示了扭船式（1a）和椅式（1b）构象的信号。基于不同的丰度、不同的对称性（1a的时间平均C(2v)和1b的时间平均C(s)）以及计算得到的化学位移（GIAO，HF/6-311+G*），将13C NMR信号归属到特定的碳原子上。除了椅式和扭船式构象之间缓慢的环反转和相互转化外，还发现了叔丁基的缓慢旋转。观察到了大多数预期的13C峰。在-148.1℃下，1a（主要）和1b（次要）相互转化的自由能垒分别为6.83和6.35 kcal/mol。使用Allinger的MM3和MM4程序搜索构象空间，并获得了几种低能量构象1a - c的自由能。用从头算方法重复计算，直至HF/6-311+G*水平。得到了分子对称性、相对自由能、相对焓和熵、频率以及NMR化学位移。通过从头算、MM3和MM4计算生成并优化了一个船式构象（1d；C(2v)对称性）作为过渡态。
