Computational study of two-body and three-body dissociation of CH3I2+

As one of the simplest alkyl halides, methyl iodide is extensively investigated in the research fields of the photodissociation and photoionization. In the experimental investigations of ionization and dissociation, many molecular fragments, such as Iq+(q3), CHn+(n3), H+, etc., are observed in the mass spectrum of CH3I. While the mechanisms for dissociation and ionization are not completely understood. As the doubly-ionized product, CH3I2+ exhibits different isomer structures and isomerization reactions. The dissociation channels of different isomers in combination with the corresponding transition states of CH3I2+ are helpful for better understanding the dissociation and ionization dynamics of CH3I in an intense laser field.In our present work, the dissociation channels of CH3I2+ are investigated by the density functional and couple cluster theory. The geometries and energies corresponding to the local isomers and the transition states of CH3I, CH3I+ and CH3I2+ are computed. The first and second ionization energies we measured are in good agreement with experimental values. Our computational results show that the ground state of the CH3I2+ is a triplet one with 3A2 symmetry. Totally 11 two-body and 15 three-body dissociation channels of the CH3I2+ on both the lowest singlet and the lowest triplet potential energy surfaces are computed and analyzed in detail. Our computations indicate that seven two-body dissociations channels, i.e., six singlet and one triplet ones, are exergonic, in which CH3I2+(1A')CH2++HI+(4A1) is the easiest process to achieve; four exergonic three-body dissociation channels with three on singlet potential energy surface and one on triplet potential energy surface are found. The possible mechanisms for producing the dissociative ionized fragments observed in experiments, CH3+, H+, and I+, are presented; furthermore, the dissociation channels generating other ions not observed in experiments, such as H3+ et al, are also given for further experimental study. The detailed information about dissociation channels and fragments is summarized for further experimental comparisons. In the computations, we find that the density functional theory and CCSD(T) methods give different energy orders for a few dissociation potential energy surfaces; and in this work, the analysis and discussion are performed based on the CCSD(T) results. Our computations indicate that the dissociation channels on singlet and triplet potential energy surface exhibit different behaviors.