Theoretical Investigation On The Electronic States Of Tri-atomic Halogenated Carbenes

Tri-atomic halogenated carbenes are important active intermediates of variousreactions in environmental, atmosphere, combustion chemistry and organic synthesis.As the numerous interactions between the electronic states, such as Renner Tellereffect, spin orbit coupling, avoided crossing, or nonadiabatic interaction leading topredissociation, the spectroscopy and photodissociation dynamics of halocarbenesare rather complicated, especially the high excited state,which stimulate scientists tocontinuously perform experimental and theoretical studies on these importantreactive intermediates and are viewed as model systems to understand the structure,spectrum, and dynamics of radicals. Experimental and theoretical studies of theelectronic states of tri-atomic halogenated carbenes are of great importance.However, such information is limited in the literature, especially for the carbenescontaining fluorine or high-Z atoms. In this dissertation, we perform a high-level abinitio investigation of electronic states of several tri-atomic halogenated carbenes.The main results are presented in the following.In the first part of the dissertation, high level calculations using internallycontracted multireference configuration interaction including Davidson correction(icMRCI+Q) method have been carried out for the ground singlet states, the firstexcited states and the lowest triplet states of a series of fluorine-substituted carbenesFCX (X=H, F, Cl, Br, and I). Equilibrium geometries and vibrational frequencies ofthe three electronic states, adiabatic transition energy of the first excited singlet state, as well as the ground singlet-lowest triplet energy gap (S-T gap) of each of FCXcarbenes have been obtained. Effects of the basis set of icMRCI+Q calculation onthe geometries and energies have been investigated. In addition, various corrections,including the scalar relativistic effect, spin-orbit coupling and Core-Valencecorrelation, have been studied in calculating the transition energies and the S-T gaps,especially for heavy-atom carbenes. The present results have been compared withprevious calculations using a variety of methods. Our icMRCI+Q results are in verygood agreement with the high-resolution laser-based spectroscopic results whereavailable. Some structure and spectroscopic constants of the fluorine-substitutedcarbenes which are void in the literature have been provided with consistent highlevel calculations. The calculation of our study and previous calculations usingvarious methods are well consistent and in relative good agreement with availableexperimental results.The basis set effect on the calculations of FCBr is examined;the results indicate that large basis set may be necessary for calculations of carbenescontaining high-Z elements.Both the geometries and the energies of FCX carbenesincrease monotonically with increasing electronegativity of the substituted atoms.The effect of basis sets as well as the SOC effect, the SR-DK effect, and the CVcorrelation on the calculation of FCBr were investigated. The results indicate thatwhile the basis sets and the SOC effect have a very minor correction on thecalculated energies, the effects of scalar relativistic effect and CV correlation arequite significant for heavy FCX.In the second part of the dissertation, the first singlet state potential energysurface (PES) of FCBr(I) along C Br(I) bond distance was carefully examined atCASPT2/icMRCI+Q+DK3level, by optimizing C F bond and F C Br(I) angle atevery C Br bond length in contrast to fix them at the equilibrium values. On thebasis of the PES, a reliable barrier height of the A1A″state was obtained fromCASPT2and MRCI+Q calculations with different basis sets,considering the scalarrelativistic effect, spin orbit coupling, and core valence correlation. the barrier ofthe state was calculated at CASPT2and icMRCI+Q+DK3level. Our calculationgive a reliable dissociation barrier of the A state. There is no studies in the literature for electronic states with energy higher than the A state. Then we investigate the PESof the12electronic states with the vertical transition energy up to7ev of the FCBrand FCI at CASPT2/cc-pV5Z and icMRCI+Q+DK3/ANO-RCC level, respectively.The curves along the C-Br (I) bond length, C-F bond length and the F-C-Br(I)angle for each state of the carbene were obtained. The results of the FCBr give areasonable explanation of previous experimental results, and an assignment of theelectronic states that may involve in the193-nm dissociation process. Theinformation the excited electronic states are of great value for further experimentalinvestigation of halogenated carbenes.In the third part of the dissertation, the equilibrium geometries, excitationenergies, force constants and for several electronic states, X1A1,3B1,1B1,3A2,lA2,3B2and1B2, of CF2and CCl2have been calculated at the icMRCI level. On the basisof the PES,we will have the future work about the PES of the radicals and thedynamic in the UV region.