CCL: Pi complex vrs reactants

 Sent to CCL by: Tanja van Mourik [tanja.vanmourik ..]
 Hi Richard,
> Is it reasonable for a pi-complex to be higher in energy than the reactants from which it is formed? What will be the likely reason for such an occurrence?
 > Or am  I doing something wrong in my calculations( DFT B3LYP/6-31G*)?
 > I'll be glad if someone can offer me some insight.
B3LYP does not describe dispersion, and therefore underestimates the interaction energies of pi-bonded systems. This may well result in a repulsive interaction. See for example:
A critical note on density functional theory for studies on rare-gas dimers, J. Chem. Phys. 116, 9620 (2002)
 Chem. Phys. 304, 317 (2004) (on indole-water)
Assessment of density functional for intramolecular disperison-rich interactions, J. Chem. Theor. Comp. 4, 1610 (2009)
You may want to consider density functionals that are better for dispersion interactions, such as the Truhlar functionals (M05-2X, M06-2X, ...), double hybrid functionals (like B2-PLYP) or DFT-D (DFT augmented with an empirical dispersion term). If you can afford ab initio methods, SCS-MP2 may be a good option (as regular MP2 tends to overestimate dispersion interactions).
 Hope this helps,
   Tanja van Mourik
   Senior Lecturer in Chemistry
   School of Chemistry, University of St. Andrews
   North Haugh, St. Andrews, Fife KY16 9ST, Scotland (UK)
   email: tanja.vanmourik()
   The University of St Andrews is a charity registered
   in Scotland: No SC013532