Theoretical Calculation Of Photodissociation Of C₂F₅I

Perfluoroalkyl iodide, CnF2n+1I(CF3I ,C2F5I, etc.) acting as active medium has been applied extensively in the electric-discharge iodine laser, photo-dissociation iodine laser, and pulsed chemical oxygen-iodine laser in recent years. The main attention has been paid to the methyl iodide molecule, which has a key role in the study of photodissociation of polyatomic species. The perfluoroiodides have also received substantial attention because of their key role as a precursors of excited iodine atoms ( 2 P1 /2) in the active medium in photodissociation iodine lasers. Therefore it is necessary for us to explore the methyl iodide molecule, such as the structure, the potential energy surface and the dissociation. and in this work we studied on the C2F5I molecule. The content is summarized as following1. GeometryFirst, we have used 6-311G** basis set in all calculations. GAMESS suite of programme is used throughout. Relativistic effects are treated by the relative elimination of small components (RESC) approach. The geometry of the C2F5I is optimized for the ground in CS symmetry at the complete active space self-consistent field (CASSCF) level with the active space of six electrons in six orbitals. Two lone pair orbitals of the iodine and the carbon iodine bonding orbitalσand antibonding orbitalσ* are considered in the active space. The optimized geometry of C2F5I ,using the default tolerance in the gradient (10-4), is r(C-C)=0.1534(0.1523±0.0027 EXP.); r(C-F)=0.1316(0.1338±0.0004 EXP.); r(C-I)=0.2199(0.2142±0.0023 EXP.); angles a(C-C-F)=108.19(109.9±0.8 EXP.); a(C-C-I)=112.83(113.4±0.8 EXP.); a(I-C-F)=109.41(109.4±1.0 EXP.); (Distances are in nanometers and the angles are in degrees). By comparing with the values of experiment, we find that our calculated results are in good agreement with the experiment.2. Vertical excitation energiesThe vertical excitation energies were obtained with spin-orbit multiconfiguration quasidegenerate perturbation theory ( SO-MCQDPT) calculations, where six low-lying states ( 1 A', 1 A " , 1 A ' , 3 A ' , 3 A " , 3 A ') were coupled and full two-electron spin-orbit coupling terms were kept at the second order and only one-electron spin-orbit coupling terms were kept at the second order. The vertical excitation energies obtained with MCQDPT do not split, however, when considerating the spin-orbit coupling the exitated states 3 A ' , 3 A " , 3 A ' will split into nine states. Finally the original six states split into twelve states. Since the difficulty of experimental measurement, the experimental data of the vertical excitation energies are not complete hitherto. Our vertical excitation energy value for 3 3 A "( 3Q 0 ) is at 4.658 eV, and is in very good agreement with the experimental value.( 3Q 0 is Mulliken notation )3 Potential energy curves (PEC) along C-I bondWe calculate the spin-orbit electronic curves along the dissociation pathway of C-I bond using SO-MCQDPT( second order ). The geometry at each point is optimized for C2F5I in the ground state, that is, the relaxation of ethyl is included along the dissociation path of the C-I bond. From our calculation results, we can find out that all the excited state PECs are repulsive and have larger slope suggesting a higer velocity and productive rate on iodine. From the PECs, we can find that 3 3 A " , 1 1 A " , 2 1 A' cross in the range of 0.239nm and 0.242nm and the crossing of the states leads to two different dissociation channels. The two seprate dissociation channels can be clearly seen, the energy gap for the two channels is 7832 cm -1 at 0.6 nm and it is close the 2 P1 /2→2P3/2 SO splitting of 7603 cm -1 collected from NIST database. According to the recent experiment results, the excited-state atoms I ( 2 P1 /2) were found mainly from the excited state 3 3 A "( 3Q 0 ) and the ground-state atoms I( 2 P3 /2) were found to appear mainly from the primarily excited 3 3 A " state via curve-crossing and to a lesser extent from other states. The vertical excitation energy value for 3 3 A " is at 4.658 eV and the evolution time of the excited molecule to the point where the energy gap between the excited state 3 3 A " and the ground state 1 1A' potential energy surfaces drops to a value of about 12 440 cm -1 was found to be 52±13 fs. This time corresponds to about 0.08 nm extension of the C–I bond distance. Our calculation results are that the vertical excitation energy value is 4.658 eV and the energy gap of 12 440 cm -1 between 3 3 A " and 1 1A' correspons to about 0.09 nm , which are in good agreement with the experiment results.