Veluthaparambath Ragima V P, Krishna G Vibha, Pancharatna Pattath D, Saha Binoy K
Department of Chemistry, Pondicherry University, Puducherry 605 014, India.
Acta Crystallogr C Struct Chem. 2022 Dec 1;78(Pt 12):693-701. doi: 10.1107/S205322962201018X. Epub 2022 Nov 8.
The single crystal of 4-iodobenzonitrile (CHIN) is brittle, whereas those of 4-bromobenzonitrile (CHBrN) and one of the two forms of 4-chlorobenzonitrile (CHClN) are compliant in nature. The chloro crystal exhibits elastic bending, but in spite of having stronger halogen bonds, the bromo crystal exhibits plastic bending. Crystal structures have been analyzed to understand the different bending properties of these three crystals. In all three cases, the molecules form C-X...N[triple-bond]C (X = halogen) halogen-bonded chains in their respective crystal structures. Statistical analyses and DFT calculations on the C-X...N[triple-bond]C halogen bonds reveal that the optimum geometry of all three halogen bonds is linear and the C-I...N[triple-bond]C bond is strongest among the three. However, when the geometry deviates from linearity, the energy loss is very high in the case of the C-I...N[triple-bond]C bond compared to the other two systems. This explains why 4-iodobenzonitrile is brittle, whereas the other two are flexible. The interactions in 4-bromobenzonitrile are more isotropic than those in 4-chlorobenzonitrile. The iodo and chloro compounds crystallize in centrosymmetric space groups, whereas the crystal of the bromo compound lacks inversion symmetry. In spite of this difference in their space groups, the chloro and bromo crystals have very similar crystal packing. In the case of the bromo crystal, the halogen-bonded chains are parallel to the bending axis (long axis) of the crystal. However, these chains are significantly tilted in the case of the chloro crystal. The isotropic/anisotropic interactions, presence/absence of an inversion centre and the different alignment of the halogen-bonded chains with respect to the bending axis could explain the different bending properties of the chloro and bromo crystals.
4-碘苯腈（CHIN）的单晶很脆，而4-溴苯腈（CHBrN）的单晶以及4-氯苯腈两种晶型之一的单晶本质上是柔顺的。氯代晶体表现出弹性弯曲，尽管溴代晶体具有更强的卤素键，但它表现出塑性弯曲。已对晶体结构进行分析，以了解这三种晶体不同的弯曲特性。在所有三种情况下，分子在各自的晶体结构中形成C-X...N≡C（X = 卤素）卤素键链。对C-X...N≡C卤素键的统计分析和密度泛函理论（DFT）计算表明，所有三种卤素键的最佳几何构型都是线性的，并且C-I...N≡C键在三者中最强。然而，当几何构型偏离线性时，与其他两个体系相比，C-I...N≡C键的能量损失非常高。这就解释了为什么4-碘苯腈是脆的，而其他两种是柔性的。4-溴苯腈中的相互作用比4-氯苯腈中的更具各向同性。碘代和氯代化合物结晶于中心对称空间群，而溴代化合物的晶体缺乏反演对称性。尽管它们的空间群存在这种差异，但氯代和溴代晶体具有非常相似的晶体堆积。在溴代晶体的情况下，卤素键链平行于晶体的弯曲轴（长轴）。然而，在氯代晶体中，这些链明显倾斜。各向同性/各向异性相互作用、有无反演中心以及卤素键链相对于弯曲轴的不同排列可以解释氯代和溴代晶体不同的弯曲特性。
