Theoretical study on the proton tunneling in the excited-state hydrogen bond

使用相关性一致基集（aug-cc-pVTZ）和多参考组态相互作用（MRCI+D）及限制性耦合团簇（CCSD（T））方法，计算了基态、激发态HCl_2体系的势能面随Cl-Cl距离和质子离Cl原子中心距离的变化（2^1SIGMA和PI），以及根式（^2SIGMA和^2PI）。同时也阐明了激发态HCl_2阴离子和基态阴离子垂直电子跃迁产生的自由基的势能曲线随质子沿（固定）Cl-Cl距离沿着运动的函数关系。采用全价CASSCF方法计算了HCl_2阴离子和自由基的自然轨道，得到了质子沿着两个Cl原子运动的反对称振动模式下的双阱势。势能垒在7（^2SIGMA）到18（^2PI）kcal/mol之间。这些理论研究清楚地预言，如果垂直电子跃迁产生的物种具有足够的内能有效地激发反对称振动模式，那么在解离成碎片（激发态Cl阴离子和HCl分子或Cl原子和HCl分子）之前的短时间内，质子可能通过反对称振动模式发生转移。

Using the correlation consistent basis set (aug-cc-pVTZ) and the multi-reference configuration interaction (MRCI+D) and restricted coupled-cluster (CCSD (T) ) methods, the potential energy surface as a function of two coordinates, the Cl-Cl distance and the distance of the proton from the center of the two Cl atoms, was investigated in the ground-state HCl_2, excited-state HCl_2 (2^1SIGMA and PI), and the radicals (^2SIGMA and ^2PI). The potential energy curve as a function of the proton movement along the (fixed) Cl-Cl distance was also clarified in the excited HCl_2 anion and the radicals, which are produced by the vertical electronic transitions from the ground-state anions. The natural orbitals obtained in the proceeding full-valence CASSCF method was used in the MRCI+D calculations.All in the excited-state HCl_2 anions and the radicals in the present study, the double-well potentials were obtained in the anti-symmetric vibrational mode which corresponds to the proton movement along the two Cl atoms. The potential energy barriers range from 7 (^2SIGMA) to 18 (^2PI)kcal/mol. These theoretical studies clearly predict that the proton transfer may occur through the anti-symmetric vibrational mode within the short-period before before the dissociation to the fragments (the excited-state Cl anion and the HCl molecule or the Cl atom and the HCl molecule), if the species produced by the vertical electronic transitions have enough internal energies which can effectively excite the anti-symmetric vibrational mode.