Intersystem crossing (ISC) dynamics plays an important role in determining the product branching in the O((3)P) + C2H2 reaction despite the necessarily small spin-orbit coupling constant values. In this study we investigate the effect of collision energy on the extent of the contribution of a spin non-conserving route through ISC dynamics to the product distributions at the initial collision energies 8.2, 9.5, and 13.1 kcal/mol. A direct dynamics trajectory surface hopping method is employed with potential energy surfaces generated at the unrestricted B3LYP/6-31G(d,p) level of theory to perform nonadiabatic dynamics. To make our calculation simpler, nonadibatic transitions were only considered at the triplet-singlet intersections. At the crossing points, Landau-Zener transition probabilities were calculated using spin-orbit coupling constant values computed at the same geometry. The Landau-Zener model for the title reaction is validated against a more rigorous Tully's fewest switches method and found to be working reasonably well as expected because of weak spin-orbit coupling. We have compared our results with the recent crossed molecular beam experiments and observed a very good agreement with respect to the primary product branching ratios. Our calculation revealed that there is no noticeable effect of the initial collision energy on the overall product distributions that corroborates the recent experimental findings. Our calculation indicates, however, that the extent of intersystem crossing contributions varies significantly with collision energy, needed to be verified, experimentally.

The importance of intersystem crossing dynamics for the O((3)P)+C(2)H(2) reaction is demonstrated in this work. A direct dynamics trajectory surface hopping method has been employed to study the intersystem crossing effects. Our study reveals that there is a significant contribution from the spin nonconserving route to the chemical dynamics of the O((3)P)+C(2)H(2) reaction, despite small spin-orbit coupling constant values (<70 cm(-1)).

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We report multimode vibronic coupling of the energetically low-lying electronic states of phenol and pentafluorophenol in this article. First principles nuclear dynamics calculations are carried out to elucidate the optical absorption spectrum of both of the molecules. This is motivated by the recent experimental measurements [S. Karmakar et al., J. Chem. Phys. 142, 184303 (2015)] on these systems. Diabatic vibronic coupling models are developed with the aid of adiabatic electronic energies calculated ab initio by the equation of motion coupled cluster quantum chemistry method. A nuclear dynamics study on the constructed electronic states is carried out by both the time-independent and time-dependent quantum mechanical methods. It is found that the nature of low-energy πσ* transition changes, and in pentafluorophenol the energy of the first two πσ* states, is lowered by about half an eV (vertically, relative to those in phenol), and they become energetically close to the optically bright first excitedππ* (S) state. This results in strong vibronic coupling and multiple multi-state conical intersections among the ππ* and πσ* electronic states of pentafluorophenol. The impact of associated nonadiabatic effects on the vibronic structure and dynamics of the ππ* state is examined at length. The structured vibronic band of phenol becomes structureless in pentafluorophenol. The theoretical results are found to be in good accord with the experimental finding at both high energy resolution and low energy resolution.

Amyloid polymorphism has emerged as an important topic of research in recent years to identify the particular species responsible for several neurodegenerative disorders, whereas the concept is overlooked in the case of the simplest building block, that is, l-phenylalanine (l-Phe) self-assembly. Here, we report the first evidence of l-Phe polymorphism and the conversion of metastable helical fibrillar to thermodynamically stable rodlike crystalline morphologies with increasing time and temperature. Furthermore, only the fibrillar l-Phe polymorph shows a significant modulation of the model membrane. In addition, the l-Phe molecules prefer to arrange in a multilayered rodlike fashion than in a lateral arrangement, which reduces the membrane binding ability of the l-Phe polymorph due to the decrease in the partial charge of the N-terminal of l-Phe units. The present work exemplifies a different approach to understanding l-Phe self-assembly and provides an effective strategy for the therapy of phenylketonuria by scrutinizing the discrete membrane activity of different l-Phe polymorphs.