Theoretical study on the structure and the frequency of isomers of the naphthalene dimer

The structures of the naphthalene monomer and dimer were investigated with performing vibrational analysis. The MP2 optimization showed that the naphthalene monomer has a nonplanar geometry in the 6-31G, 6-31G*, 6-31+G*, and 6-311G basis sets, while it has a planar geometry in the 6-31G*(0.25) and Dunning's correlation consistent basis sets. The MP2/cc-pVDZ calculation showed the presence of the four stable isomers, which were part of the isomers in the previous MP2/6-31G* calculation ( Walsh, T. R. Chem. Phys. Lett. 2002, 363, 45). The presence of extra structures in the MP2/6-31G* calculation is attributed to a poor description of the potential energy surface, which is evident from the nonplanar structure of the monomer in the MP2/6-31G* calculation. The relative stability among the isomers in the MP2/cc-pVDZ calculation without counterpoise correction was maintained in both the single-point calculation at the MP2/aug-cc-pVDZ//MP2/cc-pVDZ level and the counterpoise-corrected optimization at the MP2/cc-pVDZ level. The relative stability among the isomers suggested an enhancement of the pi-pi interaction in the structure with lower symmetry, which could be explained using a molecular-orbital model. The vibrational analysis in MP2/cc-pVDZ without the counterpoise correction suggested that the isomers of the naphthalene dimer were distinguishable by the observation of the infrared spectrum in the low-frequency region (150-600 cm(-1)).