CCL: Pi complex vrs reactants

 Sent to CCL by: Tanja van Mourik [tanja.vanmourik .. st-andrews.ac.uk]
 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
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