Nishikyushu University Junior College, Saga, 840-0806, Japan.
J Phys Chem B. 2013 Feb 14;117(6):1896-900. doi: 10.1021/jp309959m. Epub 2013 Feb 4.
Ultrasonic absorption coefficients have been measured in aqueous solution of imipramine {3-(10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl)-N,N-dimethylpropan-1-amine} in the frequency range of 0.8-220 MHz at 25 °C. The frequency dependences of the observed absorption was characterized by a Debye-type relaxational equation with two relaxation frequencies, although only one relaxation had been observed in aqueous solutions of the related molecule amitriptyline. Both of the relaxation frequencies in imipramine solutions were found to be independent of the solute concentration and the amplitudes of the relaxational absorptions increase linearly with increasing solute concentration. It was therefore concluded that these two relaxations are associated with unimolecular reactions, such as a structural change due to rotational motions of the bond in the specified group in the imipramine molecule. To analyze quantitatively the source of the relaxations, semiempirical molecular orbital methods have been applied to determine the standard enthalpy of formation of the imipramine molecule at various dihedral angles around the bonds in the alkylamine side chain. According to the results, only one rotational motion of carbon-carbon bond in the side chain was found to be appropriate and the three minima of the standard enthalpy of formation was obtained as a function of the rotational angle. At the three minimum positions, the values of the standard enthalpy of formation are almost the same. With the assumptions (a) that rotational motion is not accompanied by a volume change of the reaction and (b) that the standard free energy change is close to the difference in the values between the standard enthalpies of formation, the equilibrium constants for the rotational isomerization have been calculated to be near unity. Hence, the forward and backward rate constants of the isomerization reactions are nearly the same. If one assumes that there are two kinds of rotational motions in one bond of the molecule, one proceeds with a rate constant on the order of 10(8) s(-1), whereas the other with a rate constant on the order of 10(6) s(-1). The faster and slower processes are also distinguished by the height of the standard enthalpy of formation.
在 25°C 下，于 0.8-220MHz 的频率范围内，测量了水中丙咪嗪（3-（10,11-二氢-5H-二苯并[b,f]氮杂卓-5-基）-N,N-二甲基丙-1-胺）溶液的超声吸收系数。观察到的吸收的频率依赖性由德拜弛豫方程描述，其中有两个弛豫频率，尽管在相关分子阿米替林的水溶液中仅观察到一个弛豫。在丙咪嗪溶液中，两个弛豫频率均被发现与溶质浓度无关，并且弛豫吸收的幅度随溶质浓度的增加线性增加。因此，可以得出结论，这两种弛豫与单分子反应有关，例如由于指定基团中键的旋转运动导致的结构变化。为了定量分析弛豫的来源，应用半经验分子轨道方法来确定丙咪嗪分子在烷基胺侧链中键的不同二面角处的标准生成焓。根据结果，发现只有侧链中碳-碳键的一种旋转运动是合适的，并且作为旋转角度的函数获得了标准生成焓的三个最小值。在三个最小位置，标准生成焓的值几乎相同。在假设 (a) 旋转运动不伴随反应体积变化和 (b) 标准自由能变化接近于标准生成焓值之间的差值的情况下，计算了旋转异构化的平衡常数，接近 1。因此，异构化反应的正向和反向速率常数几乎相同。如果假设分子中一个键有两种旋转运动，一种以 10(8) s(-1)的速率常数进行，而另一种以 10(6) s(-1)的速率常数进行。较快和较慢的过程也可以通过标准生成焓的高度来区分。
