Department of Chemistry, University of Rome La Sapienza, I-00185 Rome, Italy.
J Phys Chem A. 2012 Aug 9;116(31):8209-17. doi: 10.1021/jp303722w. Epub 2012 Jul 26.
The transmission of substituent effects through a benzene framework has been studied by a novel approach, based on the structural variation of the Ph group in p-Ph-C(6)H(4)-X molecules. The molecular structures of many 4-substituted biphenyls were determined from MO calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. The twist angle between the phenyl probe (ring B) and the benzene framework carrying the substituent (ring A) was set at 90° to prevent π-electron transfer from one ring to the other and at 0° to maximize it. The structural variation of the probe is best represented by a linear combination of the internal ring angles, termed S(F)(BIPH(o)) and S(F)(BIPH(c)) for the orthogonal and coplanar conformations of the molecules, respectively. Regression analysis of these parameters using appropriate explanatory variables reveals a composite field effect, a substantial proportion of which is originated by resonance-induced π-charges on the carbon atoms of ring A. Field-induced polarization of the π-system of ring A also contributes to the structural variation of the probe. Thus, the S(F)(BIPH(o)) parameter is very well reproduced by a linear combination of the π-charges on the ortho, meta, and para carbons of ring A, an uncommon example of a quantitative relationship between molecular geometry and electron density distribution. Comparison of S(F)(BIPH(o)) with the gas-phase acidities of para-substituted benzoic acids shows that, while the deprotonating carboxylic probe is more sensitive to π-electron withdrawal than donation, the phenyl probe is equally sensitive to both. While the ability of the orthogonal biphenyl system to exchange π-electrons with the para substituent is the same as that of the benzene ring in Ph-X molecules, an increase by about 18% occurs when the conformation is changed from orthogonal to coplanar. The structural variation of the probe becomes more complicated, however. This is due to π-electron transfer from one ring to the other, which is shown to introduce quadratic terms in the regressions.
通过一种新方法研究了取代基效应通过苯环的传递，该方法基于 Ph 基团在 p-Ph-C(6)H(4)-X 分子中的结构变化。通过 HF/6-31G* 和 B3LYP/6-311++G** 理论水平的 MO 计算确定了许多 4-取代联苯的分子结构。将苯探针（环 B）与携带取代基的苯环（环 A）之间的扭转角设置为 90°以防止π电子从一个环转移到另一个环，设置为 0°以最大化它。探针的结构变化最好通过内部环角的线性组合来表示，对于分子的正交和共面构象，分别称为 S(F)(BIPH(o))和 S(F)(BIPH(c))。使用适当的解释变量对这些参数进行回归分析揭示了复合场效应，其中很大一部分是由环 A 碳原子上的共振诱导的π电荷引起的。环 A 的π 体系的场诱导极化也有助于探针的结构变化。因此，S(F)(BIPH(o))参数可以很好地由环 A 的邻位、间位和对位碳上的π电荷的线性组合来重现，这是分子几何形状和电子密度分布之间定量关系的一个罕见示例。将 S(F)(BIPH(o))与对位取代苯甲酸的气相酸度进行比较表明，虽然去质子化的羧酸探针比供电子探针更敏感于π电子的抽取，但苯探针对两者同样敏感。虽然正交联苯系统与 Ph-X 分子中的苯环具有相同的能力来与对位取代基交换π电子，但当构象从正交变为共面时，增加约 18%。然而，探针的结构变化变得更加复杂。这是由于π电子从一个环转移到另一个环，这表明在回归中引入了二次项。
