Department of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario L8S 4M1, Canada.
J Phys Chem A. 2011 Aug 25;115(33):9207-16. doi: 10.1021/jp205375f. Epub 2011 Aug 4.
Cyclohexane is an extremely flexible molecule that oscillates, at room temperature, between two clearly distinct and extreme conformations that cannot be distinguished at room temperature; so much so that the NMR spectrum is a single line that includes all 12 protons be they axial or equatorial. This raises the interesting question as to what happens when there are equal substituents at the 1 and 4 carbon atoms of the ring. Therefore substitution in the 1,4-positions in the cyclohexane ring has been the subject of considerable interest because some form of interconversion between extreme conformations could lead to the existence of a rather unusual behavior. To study this problem, the interconversion in (di- or tetra-1,4)-substituted six-membered cyclohexane-type rings, trans-1,4-dibromo-1,4-dicyanocyclohexane, was found to be a particularly suitable candidate. Although X-ray diffraction studies on the crystalline solid found the molecule to be centrosymmetric, it still shows a significant dipole moment μ in solution, as determined with a procedure that leads to the vapor phase values of μ. Furthermore, the low magnetic field proton NMR spectrum at ambient temperature appears as a single line, a situation that changes with increasing field intensity and different solvents. Both these effects are attributed to dynamics, because small distortions can easily disrupt the exact cancellation of the individual dipoles (which are quite strong) associated with each end of the molecule. The molecule can exist in two forms, with both the bromines in an axial geometry or both in an equatorial position. Interconversion between these forms is observed, as in the parent cyclohexane. The single NMR line observed at low magnetic fields is due to fast exchange and requires that the two forms have roughly equal populations. Spectra obtained at low temperature confirm this, and variable-temperature studies allow measurement of the rates, leading to an enthalpy of activation of 62 kJ mol(-1). More details of the interconversion are provided by some new calculation methods. Even for a relatively small molecule like this, calculation of a full potential energy surface is prohibitive. However, methods are now available to follow the molecule along the reaction coordinate in quite an efficient way. The results of these calculations lead to an extremely detailed picture of chair-chair interconversion in a di- and tetrasubstituted six-membered ring of the cyclohexane family.
环己烷是一种非常灵活的分子，在室温下，它在两种明显不同且极端的构象之间振荡，室温下无法区分；以至于 NMR 谱是一条单一的线，包括所有 12 个质子，无论是轴向还是赤道。这就提出了一个有趣的问题，即在环上的 1 号和 4 号碳原子上有相等的取代基时会发生什么。因此，环己烷环的 1,4-位取代一直是相当关注的话题，因为极端构象之间的某种相互转换可能导致出现一种相当特殊的行为。为了研究这个问题，发现（二或四-1,4）-取代的六元环己烷型环，反式-1,4-二溴-1,4-二氰基环己烷的相互转换是一个特别合适的候选物。尽管对晶体固体的 X 射线衍射研究发现该分子是中心对称的，但在溶液中仍显示出显著的偶极矩 μ，这是通过一种导致气相 μ 值的程序确定的。此外，在环境温度下，低磁场质子 NMR 谱在增加磁场强度和不同溶剂时，呈现出一条单一的线，这种情况会发生变化。这两种效应都归因于动力学，因为微小的扭曲很容易破坏与分子两端相关的各个偶极子（非常强）的精确抵消。该分子可以以两种形式存在，两个溴原子均处于轴向几何形状或均处于赤道位置。在这些形式之间观察到互变，就像在母体环己烷中一样。在低磁场下观察到的单 NMR 线是由于快速交换引起的，并且需要这两种形式具有大致相等的浓度。在低温下获得的光谱证实了这一点，并且变温研究允许测量速率，从而导致活化焓为 62 kJ mol(-1)。互变的更多细节由一些新的计算方法提供。即使对于像这样相对较小的分子，也无法计算完整的势能表面。然而，现在有一种方法可以相当有效地沿着反应坐标跟踪分子。这些计算的结果导致了环己烷家族中双取代和四取代六元环椅式互变的极其详细的图像。
