Ab initio investigations of the pyrrole dimer: a direct observation of the π-facial hydrogen bond

The structure and energetics of the pyrrole dimer are investigated by ab initio calculations. The full geometry optimization of the complex at MP2/6-31G∗∗ level shows that the angle formed by the planes of two monomers is 63.19° with the center-of-mass separation 4.060 Å. The N–H hydrogen of one monomer points toward the ring plane of the other monomer with a distance 1.909 Å, which is a clear manifestation of hydrogen bond formation in this simple model of aromatic–aromatic interactions. The binding energy, corrected for the basis set superposition error with counterpoise methods, is 5.11 kcal/mol. The electrostatic and charge-transfer interactions play an essential role in stabilizing the complex.