A gradient-corrected density functional study of indole self-association through N–H⋯π hydrogen bonding

A B3LYP/6-31++G(d,p) study of indole dimer was performed. The optimized geometry reveals the existence of N–H⋯π hydrogen bond in which the benzenoid ring of one subunit acts as a proton acceptor, the interplanar angle between the two monomeric units being 89.4° (a T-shaped structure), with the center-of-mass separation of 6.207 Å. The counterpoise-corrected interaction energy is 2.15 kcal mol−1 (9.00 kJ mol−1). Anharmonic vibrational frequencies of monomeric and dimeric N–H oscillators, their change upon dimerization and the intensity enhancement are excellently reproduced by one-dimensional B3LYP/6-31++G(d,p) vibrational potentials.