The stable geometries and binding energies of the benzene–naphthalene complex were studied by the point-by-point method using ab initio calculations at the MP2/6-31G*(0.25) and MP2/6-31+G* levels. Medium-size basis sets were employed not only to save computational time but also to compensate for the tendency of the MP2 method to overestimate the electron correlation energy of aromatic clusters. The use of the 6-31G*(0.25) and 6-31+G* basis sets in the test calculation for the benzene dimer yielded results very similar to those from the CCSD(T) calculation. As for the benzene–naphthalene complex, four stable geometries were found: one parallel-displaced type and three T-shaped ones, with each type similar to the case of the benzene dimer. The global minimum was found to be the parallel-displaced structure whose energy was −4.88 kcal/mol at the MP2/6-31G*(0.25) level and −3.94 kcal/mol at the MP2/6-31+G* level. These values are ∼1.9 times that of the benzene dimer on the same level of calculation. The energy of the most stable T-shaped structure was −4.17 and −3.47 kcal/mol at the MP2/6-31G*(0.25) and MP2/6-31+G* levels, respectively, which are ∼1.8 times that of the corresponding T-shaped structure of the benzene dimer.

The role of ring torsion in the enhancement of intramolecular vibrational energy redistribution (IVR) in aromatic molecules was investigated by conducting excitation and dispersed fluorescence spectroscopy of 1,1'-binaphthyl (1,1'-BN) and 2,2'-BN. The dispersed fluorescence spectra of 1,1'-BN in the origin region of S(1)-S(0) were well resolved, which presented 25-27 cm(-1) gaps of torsional mode in the ground state. The overall profile of the dispersed spectra of 1,1'-BN is similar to that of naphthalene. In contrast, the spectra of 2,2'-BN were not resolved due to the multitude of the active torsional modes. In both cases, dissipative IVR was observed to take place with a relatively small excess vibrational energy: 237.5 cm(-1) for 1,1'-BN and 658 cm(-1) for 2,2'-BN, which clearly shows that ring torsion efficiently enhances the IVR rate. Ab initio and density functional theory calculations with medium-sized basis sets showed that the torsional potential of 1,1'-BN has a very flat minimum over the range of torsional angles from ca. 60° to 120°, whereas that of 2,2'-BN showed two well-defined potential minima at ca. 40° and 140°, in resemblance to the case of biphenyl. In this work, we propose that aromatic molecules be classified into "strong" and "weak" torsional hindrance cases: molecules with strong hindrance case show shorter torsional progressions and more effective IVR dynamics than do those with weak hindrance.