AUTOFIT, an automated fitting tool for broadband rotational spectra, and applications to 1-hexanal

Broadband chirped-pulse Fourier transform microwave (CP-FTMW) spectrometers have increased the sensitivity for molecular rotational spectroscopy. The measurement dynamic range is often large enough that isotopologues of the molecular species with the highest transition strength will also be detectable. In order to analyze the complex spectra from these broadband measurements, an automated spectral assignment program called AUTOFIT has been developed. The algorithm of AUTOFIT is described and its performance is illustrated by the analysis of the CP-FTMW spectrum of 1-hexanal obtained over the spectral range 6-40 GHz. The rotational spectra of a total of 12 conformers of 1-hexanal have been assigned using AUTOFIT to automatically identify the spectrum of conformers predicted by ab initio calculations. In addition, the rotational spectra of the C-13 and O-18 isotopologues for the two lowest energy conformers and the C-13 isotopologues for the third and fourth lowest energy conformers are assigned. The effect of the quality of the input theoretical estimates of the rotational spectroscopy parameters on AUTOFIT performance is discussed. It is shown that the use of B3LYP-D3/aug-cc-pVTZ optimized structures can improve AUTOFIT search speeds by a factor of 10-60x compared to B3LYP structures. (C) 2015 Elsevier Inc. All rights reserved.