THEORETICAL STUDY FOR ELECTRONIC STRUCTURES OF THE DIMER CATIONS OF AROMATIC COMPOUNDS

最近，我们在CASSCF和MRSDCI水平上用从头算MO计算表明，苯二聚体阳离子（C_6H_6）_2^+的全局最小值在扭曲的三明治状C_<2h>结构，比三明治状D_<6h>和t形C_<2v>结构分别低0.016和0.162 eV。在本研究中，我们对苯三聚体阳离子（C_6H_6）_3^+进行了类似的计算，发现三聚体阳离子的全局最小值是扭曲的C_<2v>三明治结构，比D_<6h>三明治结构低0.032 eV。解离能（D_e）相对于（C_6H_6）_2^++C_6H_6计算值为0.43 eV，而实验值（D_0）为0.34±0.02 eV。我们的计算表明，三聚体阳离子的大部分电荷集中在中心苯环上，其总电荷为+0.9。这一观察结果与实验学家的推测相反，实验学家认为电荷集中在二聚体核心或分散在所有三个苯环上。我们还研究了苯酚二聚体及其阳离子可能的几何结构。计算得到的苯酚二聚体结合能与实验值和其他理论值相当吻合。计算得到二聚体阳离子在MP2和CASSCF水平下的结合能约为28 kcal/mol；这是中性二聚体的三倍多。苯酚二聚体阳离子中PAP的OH拉伸频率（3616 ~ 3618 cm^<-1>）与观测值（3620±3 cm^<-1>）吻合良好。二聚体阳离子中的PDP在2434 ~ 2447 cm^<-1>之间，比裸苯酚小1210 ~ 1223 cm^<-1>。苯酚二聚体阳离子中PDP的OH拉伸频率大幅降低可归因于参与氢键的0-H键的伸长，在阳离子中形成比中性二聚体中更强的氢键。这些值的羟基拉伸频率的PDP在苯酚二聚体阳离子将为今后的参考。少

Recently, we showed using ab initio MO calculations at levels of CASSCF and MRSDCI that the global minimum of the benzene dimer cation, (C_6H_6)_2^+, are at distorted sandwich C_<2h> geometries, which is lower than sandwich D_<6h> and T-shaped C_<2v> structures by 0.016 and 0.162 eV, respectively. In this study, we performed similar calculations for the benzene trimer cation, (C_6H_6)_3^+, and found that that the global minimum of the trimer cation is the distorted C_<2v> sandwich structure, which is 0.032 eV lower than the D_<6h> sandwich structure. The dissociation energy (D_e). relative to (C_6H_6)_2^++C_6H_6 was calculated to be 0.43 eV, in comparison to the experimental value (D_0) of 0.34±0.02 eV.Our calculations revealed that most of the charge of the trimer cation is localized in the central benzene ring, whose gross charge is +0.9. This observation contrasts with speculations by experimentalists that the charge is localized in the dimer core or delocalized over all three benze … More ne rings.We investigated also possible geometrical structures for the phenol dimer and its cation. The calculated binding energy of the phenol dimer reasonably agrees with experimental and other theoretical values. The calculated binding energies of the dimer cation are about 28 kcal/mol at the MP2 and CASSCF levels ; this is more than three times larger than in the neutral dimer. The OH stretching frequencies (3616〜3618 cm^<-1>) of the PAP in the phenol dimer cation are in good agreement with the observed value (3620±3 cm^<-1>). Those of the PDP in the dimer cation are calculated to be 2434〜2447 cm^<-1>, which are 1210〜1223 cm^<-1> smaller than that of bare phenol. The large reduction of the OH stretching frequency of the PDP in the phenol dimer cation can be attributed to elongation of the 0-H bond involved in the hydrogen bond on the formation of a stronger hydrogen bond in the cation than in the neutral dimer. These values for the OH stretching frequencies of the PDP in the phenol dimer cation will serve for future reference. Less