用自然原子轨道电荷估测取代吡啶的pK_a值

使用密度泛函理论(DFT)和B3LYP/3-21G基组,优化了24种取代吡啶类分子结构,发现吡啶环上氮原子的自然原子轨道电荷(NBO)值与其实验pKa值之间存在良好的线性关系(R=-0.623 23),比其原子核静电势电荷(ESP)值拟合的好。计算了12种未知pKa值的多取代吡啶化合物的NBO参数,代入拟合出的线性参数方程,发现与流行软件ACD Lab 6.0预测得到的多取代吡啶的pKa值非常接近,最大偏差ΔpKa小于±0.07,新方法可以估测到pKa值小数点后3位数。     

Kinetic Study of the Radical Scavenging of Capsaicin in Homogeneous Solutions and Aqueous Triton X‐100 Micellar Suspensions

A kinetic study of capsaicin (CAP) toward radicals has been performed using stopped‐flow spectrophotometry in detail. The second‐order rate constants (k₂) for the reaction of CAP toward 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) and galvinoxyl have been measured in methanol, ethanol, 2‐propanol/water (5:1, v/v), and aqueous micellar suspensions containing 5% Triton X‐100 (pH 4.0 to 10.0), respectively. The decay rates of DPPH and galvinoxyl for the reaction with CAP increased linearly in a concentration‐dependent manner in homogeneous solutions and aqueous micellar suspensions. However, the k₂ for CAP obtained in an aqueous micellar suspension showed notable pH dependence; that is, the reactivity of CAP increased with an increasing pH value from 4 to 10. In addition, a good correlation between the k₂ value and the molar fraction of CAP (phenolate anion (CAP‐O^?)/undeprotonated form (CAP‐OH)) was observed. These properties are associated with the pKa of CAP. Furthermore, it was found that the CAP‐O^? reacts with galvinoxyl about 6 times as fast as the CAP‐OH. These results indicate that sequential proton loss electron transfer from the phenolic hydrogen of CAP may be responsible for the scavenging of radicals in an aqueous micellar suspensions.