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

In this paper,the parallel-stacked benzene molecules has been studied by DF T 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 thi s system looks like a stack of pennies. When the distance (d) between the c enters of every next benzene molecules is arranged with a certain value ran ging from 0.40 nm to 0.46 nm,it is found that the energy of the system vari es 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 a nd 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). Th e calculated system may find its application in the field of the pi-pi inte raction in polymer. A typical example is the interaction between the phenyl groups of polystyrene,where the polymer chain skeleton adopts a unique con formation, such as TGT (G) over bar. The calculated result is in a good agr eement with experimental evidence from the observation of polystyrene micro - particles by scanning tunneling microscopy (STM). The STM image shows th at the phenyl groups of polystyrene stack up as a parallel-displaced struct ure,the measured d is 0.42 similar to 0.44 nm,while the measured A is 25 de grees similar to 30 degrees.