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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Interconversion study in 1,4-substituted six-membered cyclohexane-type rings. structure and dynamics of trans -1,4-dibromo-1,4-dicyanocyclohexane
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Interconversion study in 1,4-substituted six-membered cyclohexane-type rings. structure and dynamics of trans -1,4-dibromo-1,4-dicyanocyclohexane

机译：在1,4-取代的六元环己烷型环中进行相互转化研究。 -1,4-二溴-1,4-二氰基环己烷的结构与动力学

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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.
机译：环己烷是一种极富柔韧性的分子，在室温下在两个明显不同且极端的构象之间振荡，这些构象在室温下无法区分;如此之多，以致NMR光谱是一条直线，其中包括所有12个质子，无论是轴向的还是赤道的。这就提出了一个有趣的问题，即当环的1和4个碳原子上有相同的取代基时，会发生什么。因此，在环己烷环的1，4-位上的取代一直是引起广泛关注的主题，因为极端构象之间的某种形式的相互转化可能导致相当不正常的行为的存在。为了研究这个问题，发现特别适合的候选化合物是（二或四-1,4）取代的六元环己烷型环，即反式1,4-二溴-1,4-二氰基环己烷。 。尽管对结晶固体进行了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着的偶极矩μ，这是通过导致μ的汽相值的过程确定的。此外，环境温度下的低磁场质子NMR谱线显示为一条线，这种情况随着场强的增加和溶剂的不同而变化。这两种效应都归因于动力学，因为小的畸变会轻易破坏与分子每个末端相关的单个偶极子（相当强）的精确抵消。该分子可以两种形式存在，两种溴都呈轴向几何形状，或者两种都处于赤道位置。观察到这些形式之间的相互转化，就像在母体环己烷中那样。在低磁场下观察到的一条NMR谱线是由于快速交换而引起的，因此要求这两种形式具有大致相等的总体。在低温下获得的光谱证实了这一点，并且可变温度研究允许测量速率，从而导致62 kJ mol-1的激活焓。一些新的计算方法提供了互转换的更多详细信息。即使对于像这样的相对较小的分子，也无法计算出完整的势能面。但是，现在有许多方法可以以非常有效的方式沿着反应坐标跟踪分子。这些计算的结果得出了环己烷家族的二元和四元取代的六元环中的椅子-椅子相互转换的极其详细的图片。

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