Revisiting the microwave spectrum and molecular structure of 1-fluoronaphthalene

Abstract

Rotational spectra of 1-fluoronaphthalene isotopologues have been recorded using a chirped-pulse Fourier transform microwave spectrometer in the 2.0–8.0 GHz frequency range using neon as a carrier gas. Ten ¹³C isotopomers (each containing only a single ¹²C/¹³C substitution) of 1-fluoronaphthalene have been assigned in natural abundance for the first time. The rotational constants A₀, B₀, and C₀ and inertial defects are determined from experimentally measured transition frequencies. For all isotopologues, the measured values of inertial defects were observed to fall within the range from −0.142 to −0.145 u Ų. The negative inertial defects are attributed to the low frequency, out-of-plane bending mode of the 1-fluoronaphthalene ring, which is evidently of similar frequency in each isotopologue. The anharmonic frequency of this mode has been calculated to be 142.8 cm⁻¹ at the B3LYP-D3/cc-pVTZ level of theory, compared to 94 cm⁻¹ predicted from the inertial defect based on an empirical relation proposed by Oka. Recent, unpublished, THz Raman spectrum reveals a peak at 75 cm⁻¹, which is closer to the empirical prediction.