Low-energy vibrational spectra of flexible diphenyl molecules: biphenyl, diphenylmethane, bibenzyl and 2-, 3- and 4-phenyltoluene

Gas phase absorption far-infrared (FIR) spectra of six flexible hydrocarbon molecules containing two phenyl groups –biphenyl, diphenylmethane, bibenzyl and 2-, 3-, 4-phenyltoluene– are reported for the first time, allowing an accurate determination of most of their active low-frequency vibrational modes. DFT calculations have been carried out at the harmonic and perturbative anharmonic levels to predict the vibrational spectra of these molecules and unambiguously assign observed vibrational modes.