Examination of cooperative effects in π-stacking

CHED 981

Macey L. Renault, mlrenaul@olemiss.edu and Gregory S. Tschumper, tschumpr@olemiss.edu. Department of Chemistry and Biochemistry, University of Mississippi, University, MS 38677-1848

Cooperative effects are largely considered to be negligible in π-stacking interactions based on an impressive computational study of benzene trimers and tetramers by Tauer and Sherrill [J. Phys. Chem. Phys. A, 109, 10475-10478 (2005)] where, for example, the non-additivity of the tetramer is less than 2% of the interaction energy for the configurations examined.   Recently, our group identified two smaller prototypes for delocalized π-stacking (HCCCCH and NCCN) that mimic the interactions between aromatic molecules like benzene [Phys. Chem. Chem. Phys., 9, 1550-1558 (2007)].  In this work, similar computational procedures have been employed to examine the non-additivity in the trimers, tetramers, pentamers and hexamers of diacetylene and cyanogen.  The cooperative effects are much larger in these systems, in both an absolute and relative sense.  In the hexamer of cyanogen, the non-additivity exceeds 1 kcal/mol while it exceeds 2 kcal/mol in the diacetylene hexamer.