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

最近，我们在CASSCF和MRSDCI水平上用从头算方法证明了苯二聚体阳离子(C_6H_6)_2^+的全局最小值是扭曲的夹心C_lt；2h&gt；几何构型，比夹心D_lt；6h&gt；和T形C_&lt；2v&gt；结构分别低0.016和0.162 eV。在这项研究中，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&lt；2v&gt；夹心结构，比D&lt；6h&gt；夹心结构低0.032 eV。解离能(D_E)。相对于(C_6H_6)_2^++C_6H_6的计算值为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者的猜测形成了鲜明对比，即电荷定位于二聚体核心或分散在所有三个本征…上我们还研究了苯酚二聚体及其阳离子的可能的几何结构。计算的苯酚二聚体的结合能与实验值和其他理论值符合得很好。在MP2和CASSCF水平上，计算出二聚体阳离子的结合能约为28kcal/mol，比中性二聚体大三倍以上。PAP在苯酚二聚体阳离子中的OH伸缩频率(3616~3618 cm~(-1))与观测值(3620±3 cm~(-1))基本一致。计算出二聚阳离子中的PDP比裸酚小1210~1223 cm~(-1)~(-1)。PDP在苯酚二聚体阳离子中的OH伸缩频率的大幅降低可以归因于参与氢键的0-H键的延长，从而在阳离子中形成了比中性二聚体更强的氢键。这些PDP在苯酚二聚体阳离子中的OH伸缩频率值可供将来参考。较少

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