Vibrational Energy Redistribution In Nitromethane Using AR Initio Molecular Dynamics

Vibrational Energy Redistribution（VER）,which describes the energy transfer process from an excited vibration to the other vibrations or surroudings,is commonly involved in the energy transfer processes in molecules and plays an important role in the various physical/chemical phenomena.In recent years,advances in ultrafast spectroscopy especially femtosecond laser technology,as well as quantum chemical calculation and supercomputers,greatly promote the development of bond-specific chemistry and research in vibrational dynamics.For energetic materials（EMs）,VER does not only influences their basic properties like melting point and thermostability but also greatly affects the detonation process,hence investigation of VER in EMs has been of great interest for many years.The ultrafast time-resolved infrared spectroscopy is an important tool for measuring the vibrational dynamics,while resolution in frequency domain limits its distinguishing ability of vibrations both in excitation and detection processes.In addition,the ultrafast spectroscopy have shortcomings in technological hurdles and costs,hence it’s necessary to conduct theoretical research.Theoretical simulation of vibrational dynamics is mature in the research of materials with simple structures like semiconductors,while it is quite insufficient in the study of materials with comlex structures like energetic materials.There is a lack of computational methods and most of them are dependent on the classical molecular dynamics.Empirical force fields are difficult to obtain for the EMs which are relatively complex,and the empirical potentials are doubtful in accuracy for the neglection of quantum effects.This thesis takes nitromethane（CH₃NO₂,NM）which is the simplest nitro energetic material as the research object,and carries out deep research of VER in NM using ab initio molecular dynamics（AIMD）simulations.Firstly,after analyzing the theoretical methods of studying the phonon dynamics with AIMD in various materials and experimental results of VER in NM,we established a method consisting of normal mode decomposition（NMD）and spectral energy density（SED）analysis based on AIMD calculations for further research.After a comparison of the simulated（3D excitation-detection spectrum）with the experimental（3D IR-Raman spectrum）results under selective excitation of CH stretches in solid NM,the reliability of the method was confirmed.Moreover,SED spectra of the different vibrations indicate that the mode couplings are related to the vibrational symmetries.Secondly,mode couplings underlying the transfer processes were investigated in detail and three symmetry-dependent transfer pathways were pointed out.After the time-and frequency-domain analysis of the vibrational energy transfer in single NM molecule,the specific symmetry-dependent transfer pathways were concluded,which are direct couplings between modes with the same symmetry,overtone-assisted transfers from modes with various symmetries to those having A₁ symmetry and rotation-assited transfers between modes with B₁ and B₂ symmetry.The dependence of symmetry is related to momentum conservation.Discussion of the transfer efficiency in frequency spectrum suggests that the efficiencies of these transfer pathways are limited by energy conservation.Finally,VER in solid NM after selectively exciting all vibrations except CH torsion were studied systematically.Four 3D excitation-detection spectra were obtained,wherein the three spectra of low and middle frequency excitations are predictive for experiments.Besides the intuitive description,transfer pathways and efficiencies were also given quantitively.The symmetry-dependent transfer pathways,as well as the transfer among the adjacent vibrations were found to be dominant in the VER processes.The former helps the energy redistribute among different frequency regions and the latter greatly influences the relaxtion rates of parent mode.The three vibrations near 1000 cm^-1（M9（（?）（CH₃）,B₂）,M10（（?）（CH₃）,B₁）,M11（（?）（CN）,A₁））decays more slowly than other vibrations due to the lack of efficient transfer among themselves.Interestingly,despite its weak couplings and inefficient transfers with adjacent modes,CN stretch always accumulates vibrational energy slowly owing to its role as a bridge between methyl and nitro group.