# Second-order Saddle Dynamics in Isomerization Reaction

TL;DR: In this article, the role of second-order saddle in the isomerization dynamics of guanidine was investigated by considering the potential energy profile for the reaction using the ab initio wavefunction method.

Abstract: 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.

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TL;DR: In this paper , the role of second-order saddle points on the dynamics of the thermal denitrogenation of 1-pyrazoline using ab initio classical trajectory simulations at the CASSCF(4,4)/6-31+G* level of theory, for total energies of 130, 140, and 150 kcal mol-1 available to the system.

Abstract: The transition state, a first-order saddle point on the potential energy surface, plays a central role in understanding the mechanism, dynamics, and rate of chemical reactions. However, we recently identified energetically accessible second-order saddles (SOS) in certain reactions and showed that the SOS plays a crucial role in the dynamics of the reactions [Pradhan et al., Phys. Chem. Chem. Phys., 2019, 21, 12837; Rashmi et al., Regul. Chaotic Dyn., 2021, 26, 119]. In the present work, we investigated the role of second-order saddle points on the dynamics of the thermal denitrogenation of 1-pyrazoline using ab initio classical trajectory simulations at the CASSCF(4,4)/6-31+G* level of theory, for total energies of 130, 140, and 150 kcal mol-1 available to the system. In this unimolecular dissociation reaction, the SOS point is 4 kcal mol-1 higher in energy than the synchronous bond-breaking transition state and opens up an additional reaction pathway. We found that the fraction of molecules following the synchronous bond-breaking pathway decreased with an increase in the total available energy in the reaction, accompanied by an increase in the fraction following the asynchronous pathway. To further understand the competition between the transition state and the SOS pathways, we investigated the mechanism of halogen-substituted 1-pyrazolines where the SOS energies are comparable to that of the transition states.

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TL;DR: In this article, the authors analyze phase space dynamics of a three degree of freedom Hamiltonian that models multiple bond breaking and forming reactions and provide insights into the role of the transverse modes by studying the delay times between the formation of two bonds.

Abstract: We analyze the classical phase space dynamics of a three degree of freedom Hamiltonian that models multiple bond breaking and forming reactions. The model Hamiltonian, inspired from studies on double proton transfer reactions, allows for exploring the dynamical consequences of higher index saddles on multidimensional potential energy surfaces. Studies have shown that coupling of low frequency transverse modes to the reaction coordinate can significantly influence the reaction mechanism, concerted or sequential, as inferred from a reduced dimensional analysis. Using the notion of dynamically concerted and sequential pathways, we provide insights into the role of the transverse modes by studying the delay times between the formation of two bonds. The delay time distribution, used extensively in earlier studies, is placed on a firm dynamical footing by correlating it with the phase space manifolds, determined using the technique of Lagrangian descriptors. We establish the utility of Lagrangian descriptors in identifying the phase space manifolds responsible for the dynamically concerted and dynamically sequential pathways.

2 citations

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TL;DR: In this article , the authors analyze phase space dynamics of a three degree of freedom Hamiltonian that models multiple bond breaking and forming reactions and provide insights into the role of the transverse modes by studying the delay times between the formation of two bonds.

Abstract: We analyze the classical phase space dynamics of a three degree of freedom Hamiltonian that models multiple bond breaking and forming reactions. The model Hamiltonian, inspired from studies on double proton transfer reactions, allows for exploring the dynamical consequences of higher index saddles on multidimensional potential energy surfaces. Studies have shown that coupling of low frequency transverse modes to the reaction coordinate can significantly influence the reaction mechanism, concerted or sequential, as inferred from a reduced dimensional analysis. Using the notion of dynamically concerted and sequential pathways, we provide insights into the role of the transverse modes by studying the delay times between the formation of two bonds. The delay time distribution, used extensively in earlier studies, is placed on a firm dynamical footing by correlating it with the phase space manifolds, determined using the technique of Lagrangian descriptors. We establish the utility of Lagrangian descriptors in identifying the phase space manifolds responsible for the dynamically concerted and dynamically sequential pathways.

2 citations

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TL;DR: In this article , the second-order saddle (SOS) is used in the isomerization reaction of guanidine and the role of vibrational energy redistribution (IVR) on the reaction dynamics is investigated.

Abstract: Our recent work on the E - Z isomerization reaction of guanidine using ab initio chemical dynamics simulations [Rashmi et al, Regul. Chaotic Dyn. 2021, 26, 119] emphasized the role of second-order saddle (SOS) in the isomerization reaction; however we could not unequivocally establish the non-statistical nature of the dynamics followed in the reaction. In the present study, we performed thousands of on-the-fly trajectories using forces computed at the MNDO level to investigate the influence of second-order saddle in the E - Z isomerization reaction of guanidine and the role of intramolecular vibrational energy redistribution (IVR) on the reaction dynamics. The simulations reveal that while majority of the trajectories follow the traditional transition state pathways, 15% of the trajectories follow the SOS path. The dynamics was found to be highly non-statistical with the survival probabilities of the reactants showing large deviations from those obtained within the RRKM assumptions. In addition, a detailed analysis of the dynamics using time-dependent frequencies and the frequency ratio spaces reveal the existence of multiple resonance junctions that indicate the existence of regular dynamics and long-lived quasi-periodic trajectories in the phase space associated with non-RRKM behavior.

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5,052 citations

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TL;DR: In this paper, the probability of the activated state is calculated using ordinary statistical mechanics, and the probability multiplied by the rate of decomposition gives the specific rate of reaction, and necessary conditions for general statistical treatment to reduce to the usual kinetic treatment are given.

Abstract: The calculation of absolute reaction rates is formulated in terms of quantities which are available from the potential surfaces which can be constructed at the present time. The probability of the activated state is calculated using ordinary statistical mechanics. This probability multiplied by the rate of decomposition gives the specific rate of reaction. The occurrence of quantized vibrations in the activated complex, in degrees of freedom which are unquantized in the original molecules, leads to relative reaction rates for isotopes quite different from the rates predicted using simple kinetic theory. The necessary conditions for the general statistical treatment to reduce to the usual kinetic treatment are given.

4,718 citations

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TL;DR: An overview of NWChem is provided focusing primarily on the core theoretical modules provided by the code and their parallel performance, as well as Scalable parallel implementations and modular software design enable efficient utilization of current computational architectures.

Abstract: The latest release of NWChem delivers an open-source computational chemistry package with extensive capabilities for large scale simulations of chemical and biological systems. Utilizing a common computational framework, diverse theoretical descriptions can be used to provide the best solution for a given scientific problem. Scalable parallel implementations and modular software design enable efficient utilization of current computational architectures. This paper provides an overview of NWChem focusing primarily on the core theoretical modules provided by the code and their parallel performance.

4,666 citations

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01 Jan 1989

TL;DR: The transition from the macroscopic to the microscopic level is discussed in this article, where the transition state theory is applied to the transition from macroscopy to the microscopic level.

Abstract: 1. Basic Concepts of Kinetics. 2. Complex Reactions. 3. Kinetic Measurements. 4. Reactions in Solution. 5. Catalysis. 6. The Transition from the Macroscopic to the Microscopic Level. 7. Potential Energy Surfaces. 8. Dynamics of Biomolecular Collisions. 9. Experimental Chemical Dynamics. 10. Statistical Approach to Reaction Dynamics: Transition State Theory. 11. Unimolecular Reaction Dynamics. 12. Dynamics Beyond the Gas Phase. 13. Information-Theoretic Approach to State-to-State Dynamics. 14. Kinetics of Multicomponent Systems: Combustion Chemistry. 15. Kinetics of Multicomponent Systems: Atmospheric Kinetics. Appendix 1. Quantum Statistical Mechanics. Appendix 2. Classical Statistical Mechanics. Appendix 3. Databases in Chemical Kinetics.

1,187 citations