The effect of zero‐point energy in simulating organic reactions with post‐transition state bifurcation

Abstract

Ambimodal reactions involve a single transition state leading to multiple products. To assess the ratio of the bifurcation products, quasiclassical trajectories (QCTs) are initiated from transition state geometries that are randomly sampled using the zero‐point energy (ZPE) plus thermal energy of a molecule's vibrational modes. However, in the QCTs, the influence of ZPE percentage in the resulting bifurcation ratio and time gap between formation of bonds is not well understood. To benchmark the effect of varying the percentage of the molecules' inherent ZPE used in the simulation, three organic reactions with distinct mechanisms that all display post‐transition state bifurcation were selected. The three reactions studied include an intramolecular [6 + 4]/[4 + 2] cycloaddition, an asynchronous nitrene insertion, and an SN2/addition to a carbonyl on an α‐bromoketone. These reactions encompass various modes of bond formation and cleavage, offering a broad view of the influence of ZPE scaling in evaluating the product ratio resulted from post‐transition state bifurcation. Exploring means of tuning selectivity in simulation enables development of more accurate organic reaction modeling for synthetic prediction. This work will establish a basis for future QCT studies of organic reactions.

Dynamics study of the post-transition-state-bifurcation process of the (HCOOH)H+ → CO + H3O+/HCO+ + H2O dissociation: application of machine-learning techniques

“…Recently, Tremblay et al have also shown that the ZPE percentage affects the bifurcation ratio. 78 The HCO + + H 2 O fragments for RPMD are slightly less than those for classical MD, but both tendencies are similar. One possible reason of the slight difference at high temperature could be the procedure for solving the EOMs.…”

Machine learning‐assisted study of correlation between post‐transition‐state bifurcation and initial phase information at the ambimodal transition state

“…13 Previously, the reaction system was investigated using the QCT method but at the B3LYP/6-31G(d) level. 12 Here, we investigated this reaction at the GFN2-xTB level, and a schematic of the PES is shown in Figure 1B. The PES properties obtained from the GFN2-xTB calculations were compared with those obtained from the B3LYP and M06 DFT calculations in Figure S1b.…”

“…In this study, we investigated three PTSB systems, namely, the Diels-Alder (DA) cycloaddition of cyclopentadiene and nitroalkene, 11 intramolecular cycloaddition between a diene and triene, 12,13 and DA cycloaddition involving proton transfer, 14 and discussed the bifurcation dynamics properties with the initial vibrational features at the ambimodal TS. Previous theoretical studies demonstrated the PTSB properties of PESs in these reactions.…”

“…QCT calculations frequently exhibit the zero-point leakage phenomenon, in which the quantized zero-point vibrational energy of the reaction system is erroneously transferred to other nuclear degrees of freedom via internal energy redistribution, leading to a significant reduction in the vibrational energy below the zero-point level. [59][60][61] The artificial reduction of the initial zero-point energy percentage was proposed to avoid this zero-point energy leakage problem 12,[59][60][61] ; however, it should be emphasized that nuclear dynamics is still treated with classical mechanics and a general method for obtaining an appropriate reduction factor prior the dynamics calculation has not yet been reported.…”

Prediction of Kinetic Product Ratios: Investigation of a Dynamically Controlled Case

“…And is it sufficient merely to determine which branch of the bifurcation is initially selected in order to answer that question? By focusing on the symmetrical case, one can be certain that any answers highlight features of the dynamics that are not influenced by differences in gradient leading to the two products, as there would be in all unsymmetrical cases. − …”