The role of second-order saddle in the isomerization dynamics was investigated by considering the \(E-Z\) isomerization of guanidine. The potential energy profile for the reaction was mapped using the ab initio wavefunction method. The isomerization path involved a torsional motion about the imine (C-N) bond in a clockwise or an anticlockwise fashion resulting in two degenerate transition states corresponding to a barrier of 23.67 kcal/mol. An alternative energetically favorable path (\(\sim\)1 kcal/mol higher than the transition states) by an in-plane motion of the imine (N-H) bond via a second-order saddle point on the potential energy surface was also obtained. The dynamics of the isomerization was investigated by ab initio classical trajectory simulations. The trajectories reveal that isomerization happens via the transition states as well as the second-order saddle. The dynamics was found to be nonstatistical with trajectories exhibiting recrossing and the higher energy second-order saddle pathway preferred over the traditional transition state pathway. Wavelet based time-frequency analysis of internal coordinates indicate regular dynamics and existence of long-lived quasi-periodic trajectories in the phase space.

通过考虑胍的\（EZ \）异构化研究了二阶鞍在异构化动力学中的作用。使用从头算波函数方法绘制了反应的势能分布图。异构化路径涉及以顺时针或逆时针方式绕亚胺（CN）键的扭转运动，从而导致两个简并的过渡态，对应于23.67 kcal / mol的势垒。还获得了通过势能表面上的二阶鞍点通过亚胺（NH）键的面内运动获得的另一条能量有利的路径（比过渡态高\（\ sim \） 1 kcal / mol） 。从头开始研究了异构化的动力学经典轨迹模拟。轨迹表明异构化是通过过渡态以及二阶鞍形发生的。发现动力学是非统计的，轨迹表现出交叉，并且较高能量的二阶鞍形路径优于传统的过渡态路径。基于小波的内坐标时频分析表明，相空间中存在规律的动力学和长寿命的准周期轨迹。