碘甲烷在外电场下的光谱和解离特性

碘甲烷是一种有毒的甲基化试剂和土壤消毒剂，应用十分广泛，研究其基本的物理性质和使其降解的有效措施很有必要。使用密度泛函理论（density functional theory，DFT），在B3LYP/LANL2DZ水平上研究在外电场（0~0.04a.u.）作用下碘甲烷分子的解离特性以及多种物理性质。计算结果表明，在C-I键连线Z方向上，外电场从0逐渐增加到0.04a.u.时，分子体系能量逐渐减小，偶极矩单调增大.HOMO-LUMO能隙E_G却呈现先增大后减小的变化趋势，C-I和C-H键键长逐渐增大，更加易于裂解。在外电场逐渐增强时，解离特性表现为：CH₃I分子的C-I键方向扫描得到的势能曲线的束缚状态逐渐消失，势垒逐渐变小最后消失。计算发现，强度为0.04a.u.的外电场足以使CH₃I分子发生C-I键断裂而降解。该结果为保护环境和对碘甲烷进行电场降解提供理论依据。

Dissociation and physical properties of methyl iodide in external electric field

As a kind of toxic methylation reagent and disinfectant, methyl iodide (CH₃I) is widely used. It is important to study the basic physical properties of methyl iodide and to use effective measures to degrade it. The ground states of methyl iodide in different electric fields from 0 to 0.04a.u.(atomic unit) are optimized using the B3LYP calculation with the LANL2DZ basis set. Optimized parameters, total energies, bond lengths, dipole moments, the highest occupied molecular orbital energies, the lowest unoccupied molecular orbital energies, energy gaps, infrared spectra, and dissociation potential energy surface (PES) are obtained. The obtained results show that when the external electric field gradually increases from 0 to 0.04a.u. along the molecular axis Z (the C-I bond direction), the total energy decreases while the dipole moment increases. The C-I and C-H bond lengths increase gradually. The energy gap first increases and then decreases with the external electric field. Further studies show that when the external electric field increases from 0 to 0.04 a.u., the dissociation PES along the C-I bond becomes unbound with the potential energy barrier disappearing. The external electric field of 0.04 a.u. is sufficient to induce the degradation of methyl iodide with the C-I bond breaking. This work provides an important support for the degradation of methyl iodide in the external electric field.