Vibrational spectra of fluorene, 1-methylfluorene and 1,8-dimethylfluorene

In this paper, we report the gas phase infrared spectra of fluorene and its methylated derivatives using a heated multipass cell and argon as a carrier gas. The observed spectra in the 4000–400 cm−1 range have been fitted using the modified scaled quantum mechanical force field (SQMFF) calculation with the 6-311G** basis. The advantage of using the modified SQMFF method is that it scales the force constants to find the best fit to the observed spectral lines by minimizing the fitting error. In this way we are able to assign all the observed fundamental bands in the spectra. With consecutive methyl substitutions two sets of bands are found to shift in a systematic way. The set of four aromatic CH stretching vibrations around 3000 cm−1 shifts toward lower frequencies while the single most intense aromatic CH out-of-plane bending mode around 750 cm−1 shifts toward higher frequencies. The reason for shifting of aromatic CH stretching frequency toward lower wave numbers with gradual methyl substitution has been attributed to the lengthening of the CH bonds due to the +I effect of the methyl groups to the ring current as revealed from the calculations. While the unexpected shifting of the aromatic CH out-of-plane bend toward higher wave numbers with increasing methyl substitution is ascribed to the lowering of the number of adjacent aromatic CH bonds on the plane of the benzene ring with gradual methyl substitutions.