Structure and vibrational dynamics of the benzene dimer
Structure and vibrational dynamics of the benzene dimer
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DOI：
10.1063/1.479338
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发表时间：
1999-07-08
影响因子：
4.4
通讯作者：
Hobza, P
中科院分区：
文献类型：
--
作者：
Spirko, V;Engvist, O;Hobza, P
逐点评估耦合簇单双三[CCSD（T）]稳定化能量被用来参数化的非经验模型（NEMO）经验分子间的潜在的苯二聚体在基态的电子状态。的潜力是用于理论解释的二聚体结构和其分子间运动的动力学。结果表明，T型结构对应于一个能量最小值，平行位移结构是分隔对称等效T型结构的一级跃迁结构。由于相对较高的跃迁势垒（类似于170 cm（-1）），相互转换隧穿在可用转动光谱所涵盖的能量区域中并不重要，因此被忽略（因此，用于解释可用实验光谱的分子对称群与具有两个可行的内部旋转和非等效单体的T形结构有关）。二聚体经历了一个几乎自由的内部旋转沿着连接的苯质量中心的T形平衡几何形状的轴和受阻的内部旋转（障碍是类似于46厘米（-1））沿着的轴是垂直于"几乎自由"的内部旋转轴。在转动光谱中观察到的隧穿分裂很可能是由于这种受阻的转动。假设后者的旋转是一个独立的运动，并使用纯粹的振动隧穿分裂（通过外推到旋转量子数的零值获得）的分析表明，受阻旋转势垒的真实值几乎是其从头算值的两倍。类似地，苯单体的质量中心距离的从头算（平衡）和实验（基态）值之间的差值Δ R = 0.25埃，这有力地证明了我们的理论势能比真正的势能要浅得多。在3 - 10 cm（-1）区域观察到的拉曼谱带似乎涉及与近自由旋转和"能量最小路径"弯曲运动有关的态。较弱的拉曼特征的非对称分配是不可行的，部分原因是理论势的准确性的限制，部分原因是缺乏对二聚体的极化率张量和光谱拍摄温度的了解。(C)1999年美国物理学会。[S0021 - 9606（99）00526 - 7]。
Point-wise evaluated coupled-cluster single double triple [CCSD(T)] stabilization energies are used to parameterize the nonempirical model (NEMO) empirical intermolecular potential of the benzene dimer in the ground electronic state. The potential is used for theoretical interpretation of the dimer structure and the dynamics of its intermolecular motions. Only one energy minimum, corresponding to the T-shaped structure, is found. A parallel displaced structure is the first-order transition structure separating the molecular symmetrically equivalent T-shaped structures. Due to a relatively high transition barrier (similar to 170 cm(-1)), the interconversion tunneling is unimportant in the energy region spanned by the available rotational spectra and is thus neglected (accordingly, the molecular symmetry group which is used for interpretation of the available experimental spectra is related to the T-shaped structure with two feasible internal rotations and nonequivalent monomers). The dimer undergoes a nearly free internal rotation along the axis connecting the benzene centers of mass in the T-shaped equilibrium geometry and a hindered internal rotation (the barrier being similar to 46 cm(-1)) along the axis that is perpendicular to the "nearly free" internal rotation axis. The tunneling splittings observed in the rotational spectrum are likely due to this hindered rotation. An analysis assuming the latter rotation as an independent motion and using purely vibrational tunneling splittings (obtained by extrapolating to zero values of the rotational quantum numbers) indicates that the genuine value of the hindered rotation barrier is nearly twice higher than its ab initio value. Similarly, the difference Delta R=0.25 Angstrom between the ab initio (equilibrium) and experimental (ground state) values for the distance of the mass centers of the benzene monomers is strong evidence that our theoretical potential is much shallower than the genuine one. The Raman bands observed at the 3-10 cm(-1) region seem to involve states associated with the nearly free rotation and the "energy minimum path" bending motion. Unambiguous assigning of the weaker Raman features is infeasible, partly due to limitations in the accuracy of the theoretical potential, and partly due to the lack of knowledge of the polarizability tensor of the dimer and temperature at which the spectra were taken. (C) 1999 American Institute of Physics. [S0021-9606(99)00526-7].