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Title: Substituent effects on dynamics at conical intersections: Allene and methyl allenes

Abstract

We report a joint experimental and theoretical study on the ultrafast excited state dynamics of allene and a series of its methylated analogues (1,2-butadiene, 1,1-dimethylallene, and tetramethylallene) in order to elucidate the conical intersection mediated dynamics that give rise to ultrafast relaxation to the ground electronic state. We use femtosecond time-resolved photoelectron spectroscopy (TRPES) to probe the coupled electronic-vibrational dynamics following UV excitation at 200 nm (6.2 eV). Ab initio multiple spawning (AIMS) simulations are employed to determine the mechanistic details of two competing dynamical pathways to the ground electronic state. In all molecules, these pathways are found to involve as follows: (i) twisting about the central allenic C–C–C axis followed by pyramidalization at one of the terminal carbon atoms and (ii) bending of allene moiety. Importantly, the AIMS trajectory data were used for ab initio simulations of the TRPES, permitting direct comparison with experiment. For each molecule, the decay of the TRPES signal is characterized by short (30 fs, 52 fs, 23 fs) and long (1.8 ps, 3.5 ps, [306 fs, 18 ps]) time constants for 1,2-butadiene, 1,1-dimethylallene, and tetramethylallene, respectively. However, AIMS simulations show that these time constants are only loosely related to the evolution of electronic charactermore » and actually more closely correlate to large amplitude motions on the electronic excited state, modulating the instantaneous vertical ionization potentials. Furthermore, the fully substituted tetramethylallene is observed to undergo qualitatively different dynamics, as displacements involving the relatively massive methyl groups impede direct access to the conical intersections which give rise to the ultrafast relaxation dynamics observed in the other species. These results show that the branching between the “twisting” and “bending” pathways can be modified via the selective methylation of the terminal carbon atoms of allene. The interplay between inertial and potential effects is a key to understanding these dynamical branching pathways. The good agreement between the simulated and measured TRPES confers additional confidence to the dynamical picture presented here.« less

Authors:
 [1];  [2]; ;  [1]
  1. Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5 (Canada)
  2. State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071 (China)
Publication Date:
OSTI Identifier:
22493616
Resource Type:
Journal Article
Journal Name:
Journal of Chemical Physics
Additional Journal Information:
Journal Volume: 144; Journal Issue: 1; Other Information: (c) 2016 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0021-9606
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ALLENE; ATOMS; BRANCHING RATIO; BUTADIENE; CARBON; COMPARATIVE EVALUATIONS; EV RANGE; EXCITATION; EXCITED STATES; METHYLATION; MOLECULES; PHOTOELECTRON SPECTROSCOPY; POTENTIALS; PROBES; RELAXATION; TIME RESOLUTION

Citation Formats

Neville, Simon P., Wang, Yanmei, Boguslavskiy, Andrey E., Stolow, Albert, National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Department of Physics, University of Ottawa, 150 Louis Pasteur, Ottawa ON K1N 6N5, Schuurman, Michael S., E-mail: michael.schuurman@nrc-cnrc.gc.ca, and National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6. Substituent effects on dynamics at conical intersections: Allene and methyl allenes. United States: N. p., 2016. Web. doi:10.1063/1.4938561.
Neville, Simon P., Wang, Yanmei, Boguslavskiy, Andrey E., Stolow, Albert, National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Department of Physics, University of Ottawa, 150 Louis Pasteur, Ottawa ON K1N 6N5, Schuurman, Michael S., E-mail: michael.schuurman@nrc-cnrc.gc.ca, & National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6. Substituent effects on dynamics at conical intersections: Allene and methyl allenes. United States. https://doi.org/10.1063/1.4938561
Neville, Simon P., Wang, Yanmei, Boguslavskiy, Andrey E., Stolow, Albert, National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Department of Physics, University of Ottawa, 150 Louis Pasteur, Ottawa ON K1N 6N5, Schuurman, Michael S., E-mail: michael.schuurman@nrc-cnrc.gc.ca, and National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6. 2016. "Substituent effects on dynamics at conical intersections: Allene and methyl allenes". United States. https://doi.org/10.1063/1.4938561.
@article{osti_22493616,
title = {Substituent effects on dynamics at conical intersections: Allene and methyl allenes},
author = {Neville, Simon P. and Wang, Yanmei and Boguslavskiy, Andrey E. and Stolow, Albert and National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6 and Department of Physics, University of Ottawa, 150 Louis Pasteur, Ottawa ON K1N 6N5 and Schuurman, Michael S., E-mail: michael.schuurman@nrc-cnrc.gc.ca and National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6},
abstractNote = {We report a joint experimental and theoretical study on the ultrafast excited state dynamics of allene and a series of its methylated analogues (1,2-butadiene, 1,1-dimethylallene, and tetramethylallene) in order to elucidate the conical intersection mediated dynamics that give rise to ultrafast relaxation to the ground electronic state. We use femtosecond time-resolved photoelectron spectroscopy (TRPES) to probe the coupled electronic-vibrational dynamics following UV excitation at 200 nm (6.2 eV). Ab initio multiple spawning (AIMS) simulations are employed to determine the mechanistic details of two competing dynamical pathways to the ground electronic state. In all molecules, these pathways are found to involve as follows: (i) twisting about the central allenic C–C–C axis followed by pyramidalization at one of the terminal carbon atoms and (ii) bending of allene moiety. Importantly, the AIMS trajectory data were used for ab initio simulations of the TRPES, permitting direct comparison with experiment. For each molecule, the decay of the TRPES signal is characterized by short (30 fs, 52 fs, 23 fs) and long (1.8 ps, 3.5 ps, [306 fs, 18 ps]) time constants for 1,2-butadiene, 1,1-dimethylallene, and tetramethylallene, respectively. However, AIMS simulations show that these time constants are only loosely related to the evolution of electronic character and actually more closely correlate to large amplitude motions on the electronic excited state, modulating the instantaneous vertical ionization potentials. Furthermore, the fully substituted tetramethylallene is observed to undergo qualitatively different dynamics, as displacements involving the relatively massive methyl groups impede direct access to the conical intersections which give rise to the ultrafast relaxation dynamics observed in the other species. These results show that the branching between the “twisting” and “bending” pathways can be modified via the selective methylation of the terminal carbon atoms of allene. The interplay between inertial and potential effects is a key to understanding these dynamical branching pathways. The good agreement between the simulated and measured TRPES confers additional confidence to the dynamical picture presented here.},
doi = {10.1063/1.4938561},
url = {https://www.osti.gov/biblio/22493616}, journal = {Journal of Chemical Physics},
issn = {0021-9606},
number = 1,
volume = 144,
place = {United States},
year = {Thu Jan 07 00:00:00 EST 2016},
month = {Thu Jan 07 00:00:00 EST 2016}
}