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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Dynamics and Kinetic Isotope Effect for the Double Proton Transfer in formamidine Monohydrated Complex Using Direct Semiempirical Dynamics Calculation
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Dynamics and Kinetic Isotope Effect for the Double Proton Transfer in formamidine Monohydrated Complex Using Direct Semiempirical Dynamics Calculation

机译：直接半经验动力学计算的甲am一水合物中双质子转移的动力学和动力学同位素效应

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摘要

The dynamics of the double proton transfer in formamidine monohydrated complex has been studied by the direct semiempirical dynamics approach with variational transition-state theory using multidimensional semiclassical tunneling approximations. High-level ab initio quantum mechanical calculations were performed to estimate the energetics of the double proton transfer. Dimerization energies and the barrier height have been calculated at the G2~* level of theory, which yields -7.50 and 16.65 kcal mol~(-1), respectively. A quantum mechanical potential energy surface has been constructed using the AM1 Hamiltonian with specific reaction parameters (AM1-SRP) which are obtained by adjusting the standard AM1 parameters to reproduce the energetics by high-level ab initio quantum mechanical calculation. The minimum energy path has been calculated on this potential energy surface, and other characteristics of the surface were calculated as needed. The two protons are transferred synchronously, so the transition state possesses C_s symmetry. The reaction path curvature near the transition state is small, but that far from the transition state is large. Therefore the microcanonical optimized multidimensional tunneling approximation was used to calculate the tunneling coefficient. The tunneling amplitude initiated by reaction coordinate motion as well as that initiated by the vibrational mode normal to the reaction coordinate is important over the entire reaction coordinate. The distance that the proton hops during tunneling is about 0.62 A at 300 K. This is a very long distance compared with the normal single proton transfer in solution. Before tunneling occurs, hydrogenic motion contributes minimally to the reaction path, which consists primarily of the heavy atoms moving to being the formamidine and water molecules closer. This heavy-atom motion assists the tunneling process. The kinetic isotope effect (KIE) was also calculated. The quasi-classical contribution to the KIE is quite large due to the synchronous motion of the two protons. The tunneling contribution to the KIE determines the characteristics of the overall KIE in terms of temperature.
机译：通过直接半经验动力学方法和变过渡态理论，使用多维半经典隧穿近似，研究了甲am一水合物中双质子转移的动力学。进行了高级从头算起的量子力学计算，以估计双质子转移的能量。二聚化能和势垒高度是在理论的G2〜*水平下计算得到的，分别得到-7.50和16.65 kcal mol〜（-1）。使用具有特定反应参数（AM1-SRP）的AM1哈密顿量构造了量子力学势能面，该特定反应参数是通过调整标准AM1参数以通过高级从头算起量子力学计算来重现能量而获得的。已经在该势能表面上计算了最小能量路径，并根据需要计算了该表面的其他特性。两个质子同步转移，因此过渡态具有C_s对称性。过渡状态附近的反应路径曲率小，而远离过渡状态的反应路径曲率大。因此，使用微规范优化的多维隧穿近似来计算隧穿系数。在整个反作用坐标上，由反作用坐标运动引发的隧穿振幅以及由垂直于反作用坐标的振动模态引发的隧穿振幅都很重要。在300 K时，质子在隧穿过程中跳跃的距离约为0.62A。与溶液中正常的单个质子转移相比，这是一个非常长的距离。在隧穿发生之前，氢运动对反应路径的贡献很小，反应路径主要由重原子移动为甲am和更紧密的水分子组成。这种重原子运动有助于隧穿过程。还计算了动力学同位素效应（KIE）。由于两个质子的同步运动，对KIE的准经典贡献很大。隧道对KIE的贡献决定了整个KIE的温度特性。

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