Wang Weizhou, Pitonák Michal, Hobza Pavel
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of Czech Republic and Centre for Biomolecules and Complex Molecular Systems, Flemingovo nám. 2, 166 10 Prague 6, Czech Republic.
Chemphyschem. 2007 Oct 8;8(14):2107-11. doi: 10.1002/cphc.200700415.
Three low-energy structures of the benzene dimer are investigated by several theoretical procedures (RI-MP2, CCSD(T), RI-DFT-D, DFT/BH&H) covering London dispersion energy. The RI-DFT-D and CCSD(T) calculations are used to verify the DFT/BH&H dimer characteristics, as only at this level can anharmonic calculations be performed. It is ascertained that the T-shaped (C(2v)) structure, in which the C-H stretching frequency of the proton donor shows a significant blue shift, is not stable at any level of theory. It is either a transition structure or a minimum which is easily transformed into a parallel-displaced structure or a T-shaped (C(s)) structure, even at low temperature. Consequently, no blue shift can be detected. On the other hand, the calculated anharmonic IR spectra of the two most stable structures of benzene dimer, namely, the T-shaped (C(s)) and the parallel-displaced ones, give rise to a small red (and no blue) shift of the C-H stretching vibration. This finding is fully consistent with the experimental results.
通过涵盖伦敦色散能的几种理论方法（RI-MP2、CCSD(T)、RI-DFT-D、DFT/BH&H）研究了苯二聚体的三种低能结构。RI-DFT-D和CCSD(T)计算用于验证DFT/BH&H二聚体的特性，因为只有在此级别才能进行非谐计算。已确定质子供体的C-H伸缩频率出现显著蓝移的T形（C(2v)）结构在任何理论级别上都不稳定。它要么是过渡结构，要么是即使在低温下也很容易转变为平行位移结构或T形（C(s)）结构的极小值。因此，无法检测到蓝移。另一方面，苯二聚体的两种最稳定结构，即T形（C(s)）和平行位移结构的计算非谐红外光谱，导致C-H伸缩振动出现小的红移（且无蓝移）。这一发现与实验结果完全一致。
