Spin-Orbit Ab Initio Calculation Of Photodissociation Of Methyl Iodide

Methyl iodide is the most simple alkyl iodide.The photodissociation of alkyl iodide will produce excited state atom I ( P1 /2) which is an active medium for the iodine laser. Magnetic dipole transition between I * ( 2 P1 /2) and I ( 2 P3 /2) of Iodine atom will produce laser with wavelength of 1.315μm .The wavelength of iodine laser after frequency multiplying is at the atmospheric window and it has favorable transmission characteristic in atmosphere.In addition, the fiber transmission performance of the iodine laser is preferable, hence, it's proper for the iodine laser to transmit in silica fiber with efficiency and for long distance. Due to the above merits, the study of the iodide laser has been attracting more attention. The potential energy curves of electronic states of CH₃I which are derived from Ab Initio calculations can provide theoretical support for molecular photodissociation dynamics and photochemical reactions. In this paper, we study the two questions through the procedure of GAMESS.1.Effective potential energy curves of CH₃I dissociating along C-I bond and quantum yields of I* ( 2 P1 /2)The effective potential energy curves of CH₃I correlated to the dissociation limits of I( 2 P3 /2) and I* ( 2 P1 /2) atom are calculated using the second-order multiconfiguration quasidegenerate perturbation theory (MCQDPT) and the second-order spin-orbit multiconfiguration quasidegenerate perturbation theory (SO-MCQDPT). In the SO-MCQDPT calculations, there are three low-lying singlet states and triplet states, so six spin-free states in all are coupled.The effective potential energy curves of CH₃I indicate that all the excited states are repulsive except for the 3 Q0 state.Based on the potential energy curves calculated with SO-MCQDPT, we fit the relative absorption spectra. The photodissociation processes of CH₃I molecule are analyzed and the quantum yields of I* ( 2 P1 /2) atom are estimated. The calculated data agree well with the experimental results at 240nm and 275nm, but are slightly higher than experimental results in the middle of the band 248nm-266nm. This is because the calculated theoretical results don't consider that the radiationless transition between 3 Q 01Q→at crossing point of the two states and the shallow potential well of 3 Q0 state at 0.35nm will reduce quantum yields. The present calculations can be used to interpret the experimental results.2. The affect of relaxation effect of the alkyl radical in dissociation of CH₃IThe one-dimensional rigid potential energy curves and optimized effective potential energy curves of low-lying states are calculated separately with second-order spin-orbit multiconfiguration quasidegenerate perturbation theory (SO-MCQDPT). The relaxation effect of the alkyl radical is indicated apparently from one-dimensional effective potential energy curves. The umbrella angle of H-C-H has an increase of 2.2o at the crossing point of 3 Q0state and 1Q state compared with the equilibrium structure. The 3Q 0and 1Q 1 crossing is found in both two set of curves. The crossing position of the effective curves is at RC - I= 0.2370±0.0001nm, corresponding crossing position of the rigid curves is at RC - I= 0.2389 - 0.2390nm.3. Ab initio study of CF₃I and C 2 H 2 F3IEmploying the multiconfiguration self-consistent field method and the multiconfiguration quasi-degenerate perturbation method, the adiabatic potential energy curves and vertical excitation energies of alkyl iodides CF₃I and C 2 H 2 F3I are calculated for the low-lying states,respectively. It is found that the low-lying excited states of the two molecules are repulsive, and the calculated dissociation energies for their ground states are 2.473eV and 2.835eV respectively, of which the former one agrees well with the experimental results.