Structure And Photodissociation Mechanism Of Electronic States For Halogenated Carbenes And Nitrenes

Halogenated carbenes and nitrenes are important intermediates and play an important role in the combustion,interstellar space,atmospheric chemistry,organic synthesis.Research on the structures,spectroscopic properties,and kinetics of the excited states for halogenated carbenes and nitrenes is helpful to elucidate the micromechanism of photophysical and photochemical processes,which is of important value in application and theoretical investigations.The complexity of the structure for electronic states and interaction between the electronic states leads to complicated photodissociation dynamics of the excited states for halogenated carbenes and nitrenes.Especially for molecules containing high Z atoms,various correction effects including scalar relativistic effect,core-valence correlation effect,and spin-orbit coupling effect would have significant influence on the electronic states of halogenated carbenes and nitrenes.Relevant research is still lack.In this paper,high-level ab initio calculations are performed on the structure,spectroscopic constants and potential energy curves of the excited states for halogenated carbenes(CHCl,CHI)and halogenated nitrines(NX(X=Cl,Br,I),NOBr).The interaction between the electronic states and photodissociation mechanisms are discussed.The main contents and results are as follows:(1)We present herein a high-level ab initio study on the structure and photodissociation mechanisms of electronic excited states of the CHCl using internally contracted multireference configuration interaction method(icMRCI).A total of 13 electronic states with energy up to 7 eV have been investigated.The vertical transition energies,the oscillator strengths,electron configurations and transitions of electronic states of CHCl have been calculated.The potential energy curves(PECs)of the electronic states have been studied along the H-C-Cl angle,the C-H bond length and the C-Cl bond length,respectively.Particularly,the excited states involved in the 193-nm photodissociation as well as the corresponding dissociation mechanism have been discussed based on our calculation results.The results indicate that the CH(~2?)+Cl channel is the main products at 193-nm,which may be produced through the spin-orbit coupling of 4~1A'-3~3A''and the avoided crossing between 4~1A'-3~1A'.Our theoretical results support the experimental observations in the literature and shed more light on the photodissociation dynamics of CHCl at 193-nm.(2)The spin-orbit coupling effect(SOC)and dissociation mechanism of the electronic state of CHI molecule were investigated by using internally contracted multireference configuration interaction which employs wave functions that explicitly depend on the electron–electron distance(icMRCI-F12).The effect of the SOC on the geometric structure and harmonic vibrational frequencies of the ground state(X~1A'),the first excited singlet state(A~1A'')and the lowest triplet state(a~3A'')was calculated for the first time.The results indicate the X~1A'state and a~3A''states have strong spin-orbit coupling in the bending vibrational mode,which results in the change of the bond angle and bending vibration frequency of the spin-orbit states.The PECs of 20 spin-free states as well as 50 spin-coupled states generated from the spin-free states via the SOC were studied.The SOC matrix elements and the non-adiabatic coupling matrix elements of the electronic states have been calculated as well as the interactions and the photodissociation dynamics in the ultraviolet region of the electronic states of the CHI have been analyzed.It is indicated that the“CH+I”channel is the main products.Upon excitation from the ground X~1A'state to the2~1A'state near 331nm,the molecule could directly dissociate to CH(X~2?)+I(~2P_u);while both channels of CH(X~2?)+I(~2P_u)and CH(a~4?~-)+I(~2P_u)are attributed to the predissociation of the 4~1A'state at 248 nm,via the nonadiabatic coupling with the singlet states and the spin-orbit coupling with the triplet states.The study would add our understanding on the interactions and dynamics of the electronic excited states of halocarbenes.Our calculations provide a theoretical reference for further experimental studies.(3)Accurate spectroscopic constants of X~3?~-,a~1??b~1?~+states have been investigated for a series of halogenated nitrenes NX(X=Cl,Br,and I)using MRCI method.The effect of the different basis(aug-cc-pVXZ(X=T,Q,5)and complete basis set(CBS)limit)and various corrections(including the scalar relativistic effect,core-valence correlation,spin-orbit coupling effect and Davidson correction)on spectroscopic constants has been calculated.The results indicate that large basis set may be necessary for calculations of nitrenes and various corrections have substantial contribution to accurate prediction of the spectroscopic constants of the electronic states,especially for high-Z-halogen-atom-contained NXs.Based on our calculation,the accurate spectroscopy information is available for NX.(4)The structure and dissociation mechanism of the electronic state of NOBr were investigated at the icMRCI-F12 level.The vertical transition energies,the oscillator strengths,electron configurations and transitions of electronic states of a total of 17 electronic states with energy up to 7 eV of NOBr have been calculated.The potential energy curves(PECs)of the electronic states have been studied along the Br-N-O angle,the N-Br bond length and the N-O bond length,respectively.The interaction between the excited states as well as the corresponding dissociation dynamics in the ultraviolet-visible region has been discussed based on our calculation results.The results show that,upon excitation from the ground X~1A'state to the 1~1A''?2~1A'and 3~1A'states,the molecule could directly dissociate to NO(X~2?)+Br(~2P_u);while upon excitation from the ground X~1A'state to the 4~1A'state,the molecule could indirectly dissociate to NO(X~2?)+Br(~2P_u).After the vibrational wave packet of 4~1A'state through the tunneling,it could interact with 3~1A'state via the non-adiabatic coupling.Our theoretical research contributes to better understand the characteristics of excited states and the photochemical processes in the UV-visible region for NOBr.