Xu Minzhong, Bacić Zlatko
Department of Chemistry, New York University, New York, New York 10003, USA.
J Phys Chem A. 2007 Aug 9;111(31):7653-63. doi: 10.1021/jp072218e. Epub 2007 May 27.
We report accurate quantum three-dimensional calculations of highly excited intermolecular vibrational states of the van der Waals (vdW) complexes tetracene.He and pentacene.He in the S1 excited electronic state. The aromatic molecules were taken to be rigid and the intermolecular potential energy surfaces (IPESs) were modeled as a sum of atom-atom Lennard-Jones pair potentials. The IPESs are corrugated in the direction of the long (x) axis of the aromatic molecules, due to the presence of the symmetrically equivalent global double minimum for tetracene.He, and a triple minimum (central global minimum and two equivalent local minima) for pentacene.He, on each side of the aromatic plane. Both IPESs have two additional minor equivalent local minima further away from the center of the molecule. The vdW vibrational states analyzed in this work cover about 80% of the well depths of the IPESs. The mode coupling is generally weak for those states whose out-of-plane (z) mode is unexcited. However, the z-mode fundamental is strongly coupled to the short-axis (y) in-plane mode, so that the pure z-mode excitation could not be identified. The He atom exhibits large in-plane spatial delocalizaton already in the ground vdW vibrational state, which increases rapidly upon the excitation of the in-plane x and y modes, with little hindrance by the corrugation of the aromatic microsurfaces. For the vdW vibrational energies considered, the He atom spatial delocalization reaches Deltax and Deltay values of approximately 5 and 4 A, respectively, and is limited only by the finite size of the aromatic substrates. Side-crossing delocalization of the wave functions on both sides of the molecular plane is found at excitation energies >30 cm(-1), giving rise to the energy splittings of the pairs of states symmetric/antisymmetric with respect to the aromatic plane; the splittings show strong vdW vibrational mode specificity.
我们报告了对处于S1激发电子态的范德华（vdW）配合物并四苯·氦和并五苯·氦的高激发分子间振动态进行的精确量子三维计算。芳香族分子被视为刚性分子，分子间势能面（IPESs）被建模为原子 - 原子 Lennard - Jones 对势的总和。由于并四苯·氦存在对称等效的全局双极小值，以及并五苯·氦在芳香族平面两侧存在三重极小值（中央全局极小值和两个等效局部极小值），IPESs在芳香族分子的长（x）轴方向上呈波纹状。两个IPESs在离分子中心更远的位置还有另外两个等效的小局部极小值。本工作中分析的vdW振动态覆盖了IPESs阱深的约80%。对于那些面外（z）模式未被激发的态，模式耦合通常较弱。然而，z模式基频与短轴（y）面内模式强烈耦合，因此无法识别纯z模式激发。氦原子在基态vdW振动态时就已经表现出较大的面内空间离域，在面内x和y模式激发时迅速增加，几乎不受芳香族微表面波纹的阻碍。对于所考虑的vdW振动能量，氦原子的空间离域在x和y方向上分别达到约5 Å和4 Å的Δx和Δy值，并且仅受芳香族底物有限尺寸的限制。在激发能量>30 cm⁻¹时，发现分子平面两侧的波函数存在侧向交叉离域，导致相对于芳香族平面的对称/反对称态对出现能量分裂；这些分裂表现出强烈的vdW振动模式特异性。
