N-H•••π interactions in indole•••benzene-h6,d6 and indole•••benzene-h6,d6 radical cation complexes.: Mass analyzed threshold ionization experiments and correlated ab initio quantum chemical calculations

Indole-benzene complexes in the neutral and cationic forms were investigated by resonance enhanced two-photon ionization (REMPI) and mass analyzed threshold ionization (MATI) experiments and nonempirical ab initio quantum chemical methods. The experiment yields vibrational frequencies of the ionized complex in its ionic ground state. In addition, by observing the breakdown of the MATI signal at the cluster ion mass for a certain internal energy and its simultaneous appearance at the fragment mass, the dissociation energy of the ionic complex is found with high precision. Using a thermochemical cycle from this and the also measured adiabatic ionization energies of indole-benzene and indole, the dissociation energy of the neutral complex is found. The data were recently presented for indole-benzene-h(6) and are shown for indole-benzene-d(6) for the first time in this work. Stacked and N-H...pi H-bonded structures of the neutral dimer were optimized using the approximative resolution of identity MP2 (RI-MP2) method combined with extended basis. The RI-MP2 treatment showed the preferential stability of the stacked structure while the CCSD(T) calculations favor the N-H...pi H-bonded structure. The final stabilization enthalpy estimate (5.3 kcal/mol) agreed nicely with the experimental value of 5.2 kcal/mol (1823 +/- 15 cm(-1)) and points clearly to a N-H...pi bounded structure of the complex. In the case of a radical cation, the stacked structure was shown not to be stable and was converted during optimization to the N-H...pi H-bonded structure. The final stabilization enthalpy estimate (12.8 kcal/mol) agreed reasonably well with the experimental value of 13.1 kcal/mol (4581 10 cm(-1)). The theoretical harmonic intermolecular stretch frequency obtained for the neutral and cationic complexes (78 and 105 cm(-1)) agreed fairly well with the experimental values (70 and 95 cm(-1)). A surprisingly large N-H...pi stabilization energy calculated for the benzene...indole complex supports speculation about the role of these interactions in the biological environment. The excellent agreement of the experimental and theoretical binding energies gives for the first time direct evidence that the theoretical treatment used can yield stabilization energies (enthalpies) of large molecular clusters differing from the experimental values by less than 0.5 kcal/mol.