Nonadiabatic Molecular Dynamics Simulation On Photodissociation Reaction Of Deuterated And Normal Acetone

The?-CC dissociation reaction of excited acetone is a typical Norrish I reaction.The study of its kinetics is of great significance for understanding the nature of the photochemical and photophysical processes of carbonyl compounds.Compared with the S₁state reaction that has been deeply analyzed,the previous research on the mechanism and kinetic mechanism of the resonance dissociation of the S₂state is still controversial.For example:In which electronic state does the dissociation process take place,the nature of the excited state intermediate and product energy layout etc.In addition,there have been no reports of full-dimensional non-adiabatic kinetic calculations of this reaction process.In this paper,a set of newly developed Zhu-Nakamura non-adiabatic transition probability algorithm is used,which is combined with trajectory surface jumping dynamics?abbreviated as ZN surface jumping dynamics?to compile the calculation program,and applied to deuterated and normal acetone the full-dimensional non-adiabatic molecular dynamics calculation of the dissociation process after n?3s excitation.In the analytical algorithm,the internal conversion probability only needs to be calculated by fitting the adiabatic potential energy surface and the corresponding energy gradient to the effective coupling parameters and the effective collision energy.For the intersystem crossing probability,the orbital coupling matrix element must be used to replace the original adiabatic coupling term.The algorithm realizes efficient synchronization processing of internal state transitions and intersystem transitions along with dynamic trajectory evolution,and can be used for ab initio full-dimensional non-adiabatic molecular dynamics simulation calculations for larger molecular systems.Using the above methods,this paper firstly carried out a kinetic simulation study on the photodissociation of S₂deuterated acetone.Based on the simulated 385 trajectories,we confirmed that the S₂deuterated acetone crossed the dissociation energy barrier to achieve first-order dissociation after a certain period of vibrational relaxation?S2D mechanism?,which solved the previous dissociation of S₂?S₁?S1D mechanism?and T₁?T1D mechanism?state disputes.In most trajectories,the deuterated acetone in the S₂state will directly cross the energy barrier and rapidly dissociate into deuterated acetyl and methyl radicals,but in a small part of the trajectory?6.6%?,the molecular CC bond length will be it undergoes stretching oscillations around 2.0?,and then through coupling with the stretching vibration mode to obtain enough energy to further complete the dissociation process.The lifetimes of the deuterated acetone S₂state obtained by exponential fitting layout attenuation curve and arithmetic average are 17.2 and 10.4 ps,which are in good agreement with the experimental data.In addition,the decay of the acetyl radical of the D₂state of the dissociation product to the D₁state can be carried out through the two CI regions,and due to the resonance relaxation of the deuterated acetyl group between the double potential wells,the lifetime of the D₂state will be distributed in the interval of 0.1-20.0 ps,explaining the various rapid decay processes observed in the experiment.In addition,the velocity distribution of the first-order deuterated methyl radicals given by the trajectory simulation and the deuterated acetone ionic strength layout curve can reproduce the experimental data,further confirming the correctness of the S2D mechanism.In order to explore the effect of isotope effects,this paper also uses the same non-adiabatic surface jump kinetic method to conduct a preliminary study on the dissociation process of normal acetone starting from the S₂state.Through analysis of 320trajectories,it is found that the S2D mechanism is still the main reaction channel,but due to the stronger methyl CH stretching and umbrella vibration in the initial state molecule,the speed of the acetone molecule over the S₂dissociation energy barrier is greatly accelerated.The average life span is only 5.3 ps.In addition,the lifetime of the acetyl group in the D₂state and its internal conversion to the D₁state also give preliminary data and can reproduce the results of experimental studies.