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

用关联相容基组(AUG-CC-pVTZ)和多参考组态相互作用(MRCI+D)和限制耦合团簇(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)的势能面随两个坐标--氯-氯距离和质子到两个氯原子中心的距离的变化.在被激发的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，也阐明了质子沿(固定的)氯-氯距离运动的势能曲线。在MRCI+D计算中使用了全价CASSCF方法得到的自然轨道，在本研究的所有激发态HCl_2阴离子和自由基中，双势垒都是在反对称振动模式下得到的，对应于质子沿两个氯原子的运动。势垒范围从7(^2SIGMA)到18(^2pi)千卡/摩尔。这些理论研究清楚地预测，如果垂直电子跃迁产生的物种有足够的内能有效地激发反对称振动模，质子转移可能在解离到碎片(激发态氯离子和HCl分子或氯原子和HCl分子)之前的短时间内通过反对称振动模发生。

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.