Structure and Dynamics of the Aniline−Argon Complex as Derived from its Potential Energy Surface

The structure and dynamics of the van der Waals (vdW) complex of aniline (An) with argon (Ar) are studied
using ab initio methods. The inversion potential of the aniline−argon (AnAr) complex perturbed by the weak
vdW interaction is calculated taking into account subtle corrections from the zero-point energy of the vdW
modes and from the frequency shifts of the An normal modes modified by the complexation. The intermolecular
potential energy surface (PES) of the AnAr complex is determined by performing a large-scale computation
of the interaction energy and the fitting of the analytical many-body expansion to the set of single-point
interaction energies. The PES determined shows two deep local minima corresponding to the anti and syn
AnAr conformers. The difference in the energies of these two minima is only 15 cm-1, but it is sufficient to
localize the inversion wave functions and to form the two conformers. In the conformer anti (syn) of lower
(higher) energy, Ar is bound to the An ring opposite (adjacent) the amino-hydrogens. In the additional local
minima higher in energy, Ar approaches the aniline ring between the C−H bonds near its plane. An additional
local minimum is located opposite of nitrogen between the two N−H bonds. The high-energy minima are,
however, too flat to form stable conformers. The perturbation of the interaction of Ar with the phenyl ring
by the NH2 group is described by the vdW hole, which is responsible for unusually strong intermode mixing
in the excited intermolecular vibrational states. The analysis of these states calculated for the ground (S0) as
well as the first excited electronic state (S1) resolves difficulties faced earlier with the assignment of the
observed vibronic bands of AnAr.

本研究采用从头算（ab initio）方法，对苯胺（An）与氩（Ar）构成的范德瓦尔斯（van der Waals，vdW）复合物的结构与动力学特性开展系统性研究。本研究计算了受弱范德瓦尔斯相互作用扰动的苯胺-氩（AnAr）复合物的反转势能（inversion potential），计算中纳入了范德瓦尔斯模式的零点能（zero-point energy），以及络合作用改性后苯胺简正模式（normal modes）的频移带来的细微修正。AnAr复合物的分子间势能面（intermolecular potential energy surface, PES）通过两步流程确定：首先开展相互作用能的大规模量化计算，再将解析形式的多体展开（many-body expansion）拟合至单点相互作用能数据集。所得势能面呈现两个较深的局域极小值（local minima），分别对应反式（anti）与顺式（syn）两种AnAr构象异构体（conformers）。这两个局域极小值的能量差仅为15 cm⁻¹，但该差值足以使反转波函数（inversion wave functions）定域，并形成两种稳定构象。在能量更低的反式构象（以及能量更高的顺式构象）中，氩原子结合于苯胺环上与氨基氢相对（相邻）的位点。其余高能局域极小值则对应氩原子靠近苯胺环平面附近C-H键间隙的构型。另有一个局域极小值位于两个N-H键之间且与氮原子相对的位置。但上述高能局域极小值的势阱过浅，无法形成稳定构象。氨基（NH₂）对氩原子与苯环（phenyl ring）之间相互作用的扰动可通过范德瓦尔斯空穴（vdW hole）阐释，该空穴是激发态分子间振动模式间出现异常强耦合的关键成因。针对基态（ground state, S₀）与第一电子激发态（first excited electronic state, S₁）下计算得到的上述振动态开展分析，解决了此前AnAr振电子带（vibronic bands）归属工作中遇到的难题。