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

利用相关一致基集（aug-cc-pVTZ）、多参考组态相互作用（MRCI+D）和限制耦合簇（CCSD (T)）方法，研究了基态HCl_2、激发态HCl_2 （2^1SIGMA和PI）和自由基（^2SIGMA和^2PI）中Cl-Cl距离和质子到两个Cl原子中心距离的函数势能面。在激发的HCl_2阴离子和由基态阴离子垂直电子跃迁产生的自由基中，质子沿（固定）Cl-Cl距离运动的势能曲线也得到了澄清。在之前的全价CASSCF方法中得到的自然轨道被用于MRCI+D计算。在本研究的激发态HCl_2阴离子和自由基中，双阱势均以反对称振动模式得到，对应于质子沿两个Cl原子的运动。势垒范围从7 （^2SIGMA）到18 (^2PI)kcal/mol。这些理论研究清楚地预测，如果垂直电子跃迁产生的物质具有足够的内能，能够有效地激发反对称振动模式，质子在解离到碎片（激发态Cl阴离子与HCl分子或Cl原子与HCl分子）之前的短时间内，就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.