π-Hydrogen Bonding Probes the Reactivity of Aromatic Compounds: Nitration of Substituted Benzenes

The shifts of phenol O-H stretching vibration frequencies [Delta nu(OH)(exp)] upon pi-hydrogen bonding with aromatic compounds is proposed as a spectroscopic probe of the reactivity of aromatic substrates toward electrophiles. A single infrared spectrum reflecting the Delta nu(OH)(exp) shift for an aromatic species in a reference solvent (CCl4 in this study) provides a good estimate of reactivity. The methodology is applied in rationalizing reactivity trends for the BF3 catalyzed nitration by methylnitrate in nitromethane of 20 aromatic reactants, including benzene, 11 methylbenzenes, several monoalkyl benzenes, the four halobenzenes, and anisole. Literature kinetic data are employed in the analysis. Very good correlations between relative rates of nitration and Delta nu(OH)(exp) are obtained. The approach is best applied to reactions, where the initial interactions between the reactants controls the rates. A new theoretical quantity, the shifts (with respect to benzene) of the molecular electrostatic potential at 1.5 angstrom over the centroid of the aromatic ring [Delta nu(1.5)] is defined and shown to provide a good description of substituent effects on properties of the aromatic species. B3LYP density functional and MP2 ab initio methods combined with the 6-311++G(3df,2pd) basis set are employed in evaluating the Delta nu(1.5) values.

酚的O - H伸缩振动频率的位移[Δν(OH)(exp)]在与芳香族化合物形成π - 氢键时，被提议作为芳香族底物对亲电试剂反应性的一种光谱探针。反映一种芳香族物质在参考溶剂（本研究中为四氯化碳）中Δν(OH)(exp)位移的单一红外光谱能很好地估计反应性。该方法被用于解释20种芳香族反应物（包括苯、11种甲基苯、几种单烷基苯、四种卤代苯和苯甲醚）在硝基甲烷中由硝酸甲酯在三氟化硼催化下进行硝化反应的反应性趋势。分析中采用了文献中的动力学数据。得到了硝化相对速率与Δν(OH)(exp)之间非常好的相关性。该方法最适用于反应物之间的初始相互作用控制反应速率的反应。定义了一个新的理论量，即芳香环质心上方1.5埃处分子静电势相对于苯的位移[Δν(1.5)]，并表明它能很好地描述取代基对芳香族物质性质的影响。采用B3LYP密度泛函和MP2从头算方法结合6 - 311++G(3df,2pd)基组来计算Δν(1.5)值。