TY - JOUR

T1 - Direct dynamics simulations of the unimolecular dissociation of dioxetane

T2 - Probing the non-RRKM dynamics

AU - Malpathak, Shreyas

AU - Ma, Xinyou

AU - Hase, William L.

N1 - Funding Information:
The research reported here is based upon work supported by the National Science Foundation under Grant No. CHE-1416428 and the Robert A. Welch Foundation under Grant No. D-0005. The simulations were performed on the Chem-dynm computer cluster of the Hase Research Group and the Lonestar5 cluster of the Texas Advanced Computing Center (TACC) at the University of Texas, Austin. Shreyas Malpathak was supported by a S. N. Bose Fellowship from the S. N. Bose Scholars Program for Indian students. Discussions with Professor Anirban Hazra from the Indian Institute of Science Education and Research, Pune are gratefully acknowledged, as well as those with Professor Rui Sun at the University of Hawaii.
Publisher Copyright:
© 2018 Author(s).

PY - 2018/4/28

Y1 - 2018/4/28

N2 - In a previous UB3LYP/6-31G∗ direct dynamics simulation, non-Rice-Ramsperger-Kassel-Marcus (RRKM) unimolecular dynamics was found for vibrationally excited 1,2-dioxetane (DO); [R. Sun et al., J. Chem. Phys. 137, 044305 (2012)]. In the work reported here, these dynamics are studied in more detail using the same direct dynamics method. Vibrational modes of DO were divided into 4 groups, based on their characteristic motions, and each group excited with the same energy. To compare with the dynamics of these groups, an additional group of trajectories comprising a microcanonical ensemble was also simulated. The results of these simulations are consistent with the previous study. The dissociation probability, N(t)/N(0), for these excitation groups were all different. Groups A, B, and C, without initial excitation in the O-O stretch reaction coordinate, had a time lag to of 0.25-1.0 ps for the first dissociation to occur. Somewhat surprisingly, the C-H stretch Group A and out-of-plane motion Group C excitations had exponential dissociation probabilities after to, with a rate constant ∼2 times smaller than the anharmonic RRKM value. Groups B and D, with excitation of the H-C-H bend and wag, and ring bend and stretch modes, respectively, had bi-exponential dissociation probabilities. For Group D, with excitation localized in the reaction coordinate, the initial rate constant is ∼7 times larger than the anharmonic RRKM value, substantial apparent non-RRKM dynamics. N(t)/N(0) for the random excitation trajectories was non-exponential, indicating intrinsic non-RRKM dynamics. For the trajectory integration time of 13.5 ps, 9% of these trajectories did not dissociate in comparison to the RRKM prediction of 0.3%. Classical power spectra for these trajectories indicate they have regular intramolecular dynamics. The N(t)/N(0) for the excitation groups are well described by a two-state coupled phase space model. From the intercept of N(t)/N(0) with random excitation, the anharmonic correction to the RRKM rate constant is approximately a factor of 1.5.

AB - In a previous UB3LYP/6-31G∗ direct dynamics simulation, non-Rice-Ramsperger-Kassel-Marcus (RRKM) unimolecular dynamics was found for vibrationally excited 1,2-dioxetane (DO); [R. Sun et al., J. Chem. Phys. 137, 044305 (2012)]. In the work reported here, these dynamics are studied in more detail using the same direct dynamics method. Vibrational modes of DO were divided into 4 groups, based on their characteristic motions, and each group excited with the same energy. To compare with the dynamics of these groups, an additional group of trajectories comprising a microcanonical ensemble was also simulated. The results of these simulations are consistent with the previous study. The dissociation probability, N(t)/N(0), for these excitation groups were all different. Groups A, B, and C, without initial excitation in the O-O stretch reaction coordinate, had a time lag to of 0.25-1.0 ps for the first dissociation to occur. Somewhat surprisingly, the C-H stretch Group A and out-of-plane motion Group C excitations had exponential dissociation probabilities after to, with a rate constant ∼2 times smaller than the anharmonic RRKM value. Groups B and D, with excitation of the H-C-H bend and wag, and ring bend and stretch modes, respectively, had bi-exponential dissociation probabilities. For Group D, with excitation localized in the reaction coordinate, the initial rate constant is ∼7 times larger than the anharmonic RRKM value, substantial apparent non-RRKM dynamics. N(t)/N(0) for the random excitation trajectories was non-exponential, indicating intrinsic non-RRKM dynamics. For the trajectory integration time of 13.5 ps, 9% of these trajectories did not dissociate in comparison to the RRKM prediction of 0.3%. Classical power spectra for these trajectories indicate they have regular intramolecular dynamics. The N(t)/N(0) for the excitation groups are well described by a two-state coupled phase space model. From the intercept of N(t)/N(0) with random excitation, the anharmonic correction to the RRKM rate constant is approximately a factor of 1.5.

UR - http://www.scopus.com/inward/record.url?scp=85046446483&partnerID=8YFLogxK

U2 - 10.1063/1.5024908

DO - 10.1063/1.5024908

M3 - Article

C2 - 29716233

AN - SCOPUS:85046446483

VL - 148

JO - The Journal of Chemical Physics

JF - The Journal of Chemical Physics

SN - 0021-9606

IS - 16

M1 - 164309

ER -