CH_3I~(2+)的二体、三体解离过程的理论研究

采用密度泛函和耦合簇理论从过渡态的观点对CH_3I~(2+)离子的解离过程进行了计算和分析.优化得到了CH_3I,CH_3I+和CH_3I~(2+)的稳定结构及解离过渡态的几何构型并给出了相应能量,计算的第一、二电离能与实验结果符合.基于母体离子、过渡态和解离碎片的几何结构和能量,对CH_3I~(2+)的两体解离过程和三体解离过程进行了详细分析和讨论.给出了二体、三体解离的可能解离通道,并分析了实验上观测到的解离碎片离子的产生机制.计算结果表明,三重和单重绝热势能面上的一些解离过程表现出明显的差异.

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 I^q+(q≤3), CH_n⁺(n≤3), 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.