A Quasi-Diabatic Representation of the 1,21A States of Methylamine

We report a full 15-dimensional two-state quasi-diabatic potential energy matrix (PEM) for the 1,21A states of methylamine (CH3NH2) suitable for the description of its two distinct photodissociation channels, CH3NH2(11A) + hν → CH3NH2(21A) → CH3 + NH2 or CH3NH + H. The PEM is fit to ab initio electronic structure data (energies, energy gradients, and derivative couplings) obtained from multireference configuration interaction single and double excitation wave functions at 7732 geometries, using a diabatic representation based on symmetry-adapted polynomials. The root-mean-square error is 78.34 cm–1 for the energies and 2.98% for the gradients. The computed T0(21A) = 42 461 cm–1 (41 669 cm–1), D0(CH3NH+H) = 33 639 cm–1 (34 250 cm–1), and D0(CH3+NH2) = 28 543 cm–1 (29 300 cm–1) are in good agreement with the experimental values given in the parentheses. The absorption spectrum of CH3NH2 is computed using this diabatic PEM and a normal mode based kinetic energy operator. Agreement with experimental data is quite satisfactory. Our accurate representation of the PEM over a wide range of nuclear configuration space offers the possibility for quality nonadiabatic dynamics simulations involving the CH3 + NH2 and CH3NH + H dissociation channels. Surface hopping trajectories are used to illustrate the types of nuclear motion supported by the PEM.本研究报道了甲胺（CH₃NH₂）的1¹A、2¹A电子态的全15维两态准 diabatic 势能矩阵（quasi-diabatic potential energy matrix, PEM），该矩阵可用于描述其两种不同的光解通道：CH₃NH₂(1¹A) + hν → CH₃NH₂(2¹A) → CH₃ + NH₂ 或 CH₃NH + H。本势能矩阵基于对称适配多项式构建 diabatic 表象，拟合了7732个核几何构型下由多参考单双激发组态相互作用波函数得到的从头算电子结构数据（能量、能量梯度与导数耦合项）。其中能量的均方根误差为78.34 cm⁻¹，梯度的均方根误差为2.98%。计算得到的T0(2¹A)=42461 cm⁻¹（实验值41669 cm⁻¹）、D0(CH₃NH+H)=33639 cm⁻¹（实验值34250 cm⁻¹）以及D0(CH₃+NH₂)=28543 cm⁻¹（实验值29300 cm⁻¹）均与括号内给出的实验值吻合良好。基于该 diabatic PEM 与基于简正模的动能算符，我们计算了甲胺的吸收光谱，其与实验数据的吻合度极佳。我们在宽泛的核构型空间范围内对该势能矩阵的精准表征，为涉及CH₃+NH₂与CH₃NH+H解离通道的高精度非绝热动力学模拟提供了可能。本研究采用面跳跃轨迹来展示该势能矩阵所支持的核运动类型。