Vibrational modes of 2,6-, 2,7-, and 2,3-diisopropylnaphthalene. A DFT study

The IR and Raman spectra of three diisopropylnaphthalene (DIPN) isomers, 2,6-; 2,7-, and 2,3-DIPN, were calculated at the B3PW91/6-311G* level and were interpreted in terms of potential energy distribution (PED) analysis. Two types of internal coordinates were constructed to increase clarity of the PED interpretation: one expressing movements in which at least one H-atom participated and the other in which only the C-atoms were active (60 and 42 coordinates, respectively). The internal coordinates were optimized repeatedly for few hundred times to maximize the PED contributions. The similarities and differences between the vibrational spectra of the three molecules studied were pointed out. The IR and Raman bands useful for detection of the unknown 2,3-DIPN isomer were proposed. Comparison of the experimental vibrational spectra of the 2,6-DIPN isomer with the B3PW91/6-311G* theoretical wavenumbers shows very good agreement: usually the two wave numbers are within 10 cm−1 interval. Such an agreement allows for almost undoubtful assignment of the vibrational spectra.