Zhang Peng, Maeda Satoshi, Morokuma Keiji, Braams Bastiaan J
Department of Chemistry and Cherry L. Emerson Center for Scientific Computation, Emory University, Atlanta, Georgia 30322, USA.
J Chem Phys. 2009 Mar 21;130(11):114304. doi: 10.1063/1.3085952.
Accurate ab initio calculations using the multireference configuration interaction method have been performed to characterize the potential energy surfaces (PESs) of low-lying excited states (S(1) and T(1)) of formaldehyde (H(2)CO) and hydroxymethylene (HCOH) with emphasis on their isomerization, dissociation, and the possible role of the T(1) state in the nonadiabatic photodissociation of H(2)CO. Two regions on the T(1) PES are found to contribute to the nonadiabatic transition to the ground (S(0)) state. Three minima on the seam of crossing (MSXs), 80-85 kcal/mol (above the S(0) global minimum), are located in the HCOH region; they, however, are blocked by a high-energy isomerization transition state at approximately 107 kcal/mol. The other MSX discovered in the H(2)CO region is reachable with energy </=91 kcal/mol and strong spin-orbit interaction; this may be a more important pathway for the T(1) to S(0) transition. A full-dimensional PES is generated for the T(1) state, fitted by a weighted least-squares method employing a many-body expansion in which each term is a function of the internuclear distances and is invariant under permutations of like atoms. The single global function covers the formaldehyde and the HCOH regions as well as dissociation pathways. The high quality of the fitted PES is demonstrated by the small root-mean-square fitting error of 119 cm(-1) and the close agreement between the critical points from ab initio calculations and from the fitted PES.
利用多参考组态相互作用方法进行了精确的从头算计算，以表征甲醛（H₂CO）和羟亚甲基（HCOH）低激发态（S(1)和T(1)）的势能面（PESs），重点研究它们的异构化、解离以及T(1)态在H₂CO非绝热光解离中可能发挥的作用。发现T(1)势能面上的两个区域有助于向基态（S(0)）的非绝热跃迁。在HCOH区域位于交叉缝（MSXs）上有三个极小值，比S(0)全局极小值高80 - 85 kcal/mol；然而，它们被一个约107 kcal/mol的高能异构化过渡态所阻挡。在H₂CO区域发现的另一个MSX在能量≤91 kcal/mol且有强自旋 - 轨道相互作用时可达；这可能是T(1)到S(0)跃迁更重要的途径。为T(1)态生成了一个全维势能面，通过加权最小二乘法拟合，采用多体展开，其中每一项都是核间距的函数，并且在同类原子置换下不变。单个全局函数涵盖了甲醛和HCOH区域以及解离途径。拟合势能面的高质量通过119 cm⁻¹的小均方根拟合误差以及从头算计算和拟合势能面的临界点之间的紧密一致性得到证明。
