Browsing by Author "Zheng, Xiaolei"

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Chiral conversion and periodical decay in bridged-azobenzene photoisomerization: an ab initio on-the-fly nonadiabatic dynamics simulation
(2016-01-01)
Gao, Wanqing
;
Yu, Le
;
Zheng, Xiaolei
;
Lei, Yibo
;
Zhu, Chaoyuan
With the implementation of global nonadiabatic switching probability algorithm based on the Zhu-Nakamura theory, we performed on-the-fly trajectory surface hopping simulations at the CASSCF level to analyze the cis <-> trans photoisomerization mechanism of bridged-azobenzene upon S-1 excitation. The cis-to-trans isomerization process was initiated by the pedal-like twist of -N=N- moiety and observed hopping to S-0 mainly via the cis-form conical intersection region. While, the trans-to-cis process is induced by the nonsymmetrical torsion motion toward the central eight-membered ring and encountered surface hopping through another conical region lying in the middle of the isomerization pathway. In cis-to-trans photoisomerization, we also observed a particular chiral conversion pathway with surface hopping occurs at CI-trans conical region. The trajectories followed this pathway possess relative longer excited state lifetimes as the similar to 40 fs initial stage oscillation in the cis Franck-Condon region. Moreover, the periodical population decay pattern of S-1 state in trans-to-cis process was observed with an interval of similar to 16 fs.
Photo-induced 1,3-cyclohexadiene ring opening reaction: Ab initio on-the-fly nonadiabatic molecular dynamics simulation
(2016-02-15)
Lei, Yibo
;
Wu, Huiyu
;
Zheng, Xiaolei
;
Zhai, Gaohong
;
Zhu, Chaoyuan
By a newly developed algorithm to compute global nonadiabatic switching probability only using electronic adiabatic potential energy surfaces and its gradients, we could able to perform on-the-fly trajectory surface hopping molecular dynamics at the SA-CASSCF(14, 8)/6-31G* quantum level to investigate the photo-induced 1,3-cyclohexadiene ring opening reaction from 1,3-cyclohexadiene (CHD) to 1,3,5-hexatriene (HT). We found four conical intersection zones within three singlet low-lying electronic states, in which 1B/2A and 1A/2A are well known to be the most important conical intersection zones to govern dynamics of ring opening reaction. The present simulation gained insight into nonadiabatic dynamics in which 516 sampling trajectories out of 600 can switch back and forth between two involved potential energy surfaces not only at 1B/2A, but also at 1A/2A. This insight is the key to influence quantum yields and time scales of the ring opening reaction. We estimated time constants as 46 fs, 82 fs, and 120 fs, respectively for crossing 1B/2A and 1A/2A, and excited-state lifetime in comparison with corresponding experimental values 56 fs, 80 fs and 130 fs. We estimated quantum yield 0.47/0.53 to HT/CHD in comparison to experiment values 0.30/0.70 in the gas-phase and 0.41/0.59 in the condense-phase. Moreover, the present simulation within 600 sampling trajectories indicates that there is no five membered ring forming in agreement with experiment results and with prediction from the previous wave-packet simulation as a rare case. (C) 2015 Elsevier B.V. All rights reserved.
Photoisomerization mechanisms from trans, trans-1,4-diphenyl-1,3-butadiene: CASSCF on-the-fly trajectory surface hopping dynamic simulations
(2016-04-07)
Zheng, Xiaolei
;
Zhai, Gaohong
;
Gao, Wanqing
;
Lei, Yibo
;
Yu, Le
We have employed the SA2-CAS(4,4)/6-31G ab initio method together with an on-the-fly global-switching trajectory surface hopping algorithm to simulate photoisomerization reaction dynamics from reactant trans, trans-1,4-diphenyl-1,3-butadiene (DPB) to products cis,trans-DPB and cis,cis-DPB. This topic has been extensively studied experimentally and the present theoretical study is the first to simulate DPB photoisomerization reaction dynamics as far as we know. With total 600 sampling trajectories, 300 actively contribute to isomerization reaction via two conical intersections between the electronic ground and the first excited states. Simulated quantum yields of photoisomerization to cis, trans-DPB and cis, cis-DPB are 0.09 and 0.045, which are in good agreement with the experimental values of 0.07-0.25 and 0.02, respectively. The lifetime of the first excited state is estimated as 702 fs. The present simulation has shown two reactive photoisomerization mechanisms, namely one bond twist (OBT) and bicycle pedal (BP), and two non-reactive photoisomerization mechanisms, namely single bond torsion (SBT) and reverse torsion (RT) with respect to the central backbone CC bonds. We believe that the present theoretical work can benefit the experiments on photoisomerization of DPB derivates.