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）形成的范德瓦尔斯（van der Waals, vdW）配合物的结构与动力学特性展开研究。针对弱范德瓦尔斯相互作用扰动下的苯胺-氩（AnAr）配合物反转势能，本研究在计入范德瓦尔斯模式零点能以及配合物形成过程中苯胺简正模式频率偏移带来的细微修正后完成计算。通过对相互作用能进行大规模数值计算，并将解析多体展开式拟合至单点相互作用能数据集，本研究确定了AnAr配合物的分子间势能面（PES）。所得到的势能面存在两个较深的局域极小值，分别对应反式与顺式AnAr构象异构体。二者的能量差仅为15 cm⁻¹，却足以使反转波函数局域化并形成两类稳定构象。在能量较低的反式构象（能量较高的顺式构象）中，氩原子结合于苯胺环上与氨基氢相对（相邻）的位置。此外还存在多组能量更高的局域极小值，对应氩原子靠近苯胺环平面附近C-H键之间的区域。另有一处局域极小值位于两条N-H键之间且与氮原子相对的位置。然而这些高能极小值的势阱过于平缓，无法形成稳定构象。氨基（NH₂）对氩原子与苯环相互作用的扰动效应可通过范德瓦尔斯空穴加以描述，该空穴是激发态分子间振动态中出现异常强模间混合现象的核心成因。本研究对基态（S₀）以及第一激发电子态（S₁）下的上述振动态开展分析，解决了此前AnAr体系电子振动谱带指认中面临的难题。