Theoretical study on the π-π interactions in stacked benzene molecules

In this paper,the parallel-stacked benzene molecules has been studied by DFT B3LYP method using 3-21G* basis set. The calculated system is composed of five benzene molecules,which stand face to face in a straight line,and this system looks like a stack of pennies. When the distance (d) between the centers of every next benzene molecules is arranged with a certain value ranging from 0.40 nm to 0.46 nm,it is found that the energy of the system varies with the changing of the angle (A) between the normal of benzene planes and the line through the centers of every benzene molecules. The existence of the energy minimums indicates the most favorable correlation between d and A is as follows: d = 0.40 nm,A = 25 degrees; d = 0.42nm,A = 29 degrees; d = 0.44 nm,A = 34 degrees; d = 0.46 nm,A = 40 degrees (Shown in Fig.3). The calculated system may find its application in the field of the pi-pi interaction in polymer. A typical example is the interaction between the phenyl groups of polystyrene,where the polymer chain skeleton adopts a unique conformation, such as TGT (G) over bar. The calculated result is in a good agreement with experimental evidence from the observation of polystyrene micro - particles by scanning tunneling microscopy (STM). The STM image shows that the phenyl groups of polystyrene stack up as a parallel-displaced structure,the measured d is 0.42 similar to 0.44 nm,while the measured A is 25 degrees similar to 30 degrees.