A study has been made of the isomeric composition of secondary chloroheptanes formed upon γ-irradiation at 77 K and subsequent melting of heptane containing 1 mol% 2-chlorohexane and various concentrations of octane. It is observed that the relative importance of 2-chloroheptane increases as a result of the presence of octane in the crystallites. This increase is attributed to selective proton transfer from ground state heptane radical cations to penultimate CâH bonds in heptane molecules. The proton transfer is induced by (partial) dislocation of heptane molecules adjacent to octane solute molecules, which brings the penultimate heptane CâH bonds into close contact with planar chain-end CâH bonds in heptane radical cations. Proton transfer from extended all-trans octane radical cations (on which the positive hole temporarily resides) to heptane molecules may also contribute to the observed effect. The results have important implications with respect to the molecular packing in binary n-alkane crystals. In binary n-alkane crystals, in which the shorter component is predominant, molecules of the longer component cannot be fully accommodated in one molecular layer of the crystal, even with a chain length mismatch of only one methylene unit; instead (at least partial) dislocation of adjacent molecules of the shorter component takes place. Such dislocations do not extend indefinitely over the crystal, however, and crystal order is restored by squeezing, deformation and changes in conformation of the appropriate molecules, including molecules of the shorter component.
对含有 1 mol% 2-
氯己烷和不同浓度
辛烷的
庚烷在 77 K 下经过δ-辐照并随后熔化后形成的仲
氯庚烷的异构体组成进行了研究。据观察,由于晶体中存在
辛烷,
2-氯庚烷的相对重要性增加。这种增加归因于质子从基态
庚烷自由基阳离子向
庚烷分子中倒数第二个 CâH 键的选择性转移。质子转移是由邻近
辛烷溶质分子的
庚烷分子(部分)错位引起的,错位使倒数第二位
庚烷 CâH 键与
庚烷自由基阳离子中的平面链端 CâH 键紧密接触。从扩展的全反式
辛烷自由基阳离子(正空穴暂时位于其上)向
庚烷分子的质子转移也可能有助于观察到的效果。这些结果对于二元正
烷烃晶体中的分子堆积具有重要意义。在二元正
烷烃晶体中,短组分占主导地位,即使只有一个亚甲基单元的链长不匹配,长组分的分子也不能完全容纳在晶体的一个分子层中;相反,相邻的短组分分子会发生(至少部分)错位。然而,这种错位不会无限地延伸到整个晶体,晶体秩序会通过挤压、变形和适当分子(包括较短组分的分子)的构象变化而得到恢复。