Fourier transform microwave spectra and ab initio calculation of N-ethylformamide

Broad-band high-resolution rotational spectroscopy for laboratory astrophysics

“…Ohba et al (2005) have found that the lines they observed in the microwave domain do not show any evidence for splitting due to internal rotation of the methyl group, CH 3 , and derived a lower limit of 1000 cm −1 to the barrier height of this internal rotation which is compatible with the estimated value derived from their ab initio calculations (1400 cm −1 ). Hence, no internal rotation splittings are expected for the rotational transitions of this molecule.…”

“…CH 3 CH 2 NHCHO (N-Ethylformamide, hereafter referred to as NEFA) is the next potential member of this series of peptide molecules to be present in space. Its rotational spectrum has been studied in the laboratory up to 13.4 GHz by Ohba et al (2005), and up to 20.5 GHz by Vaquero-Vara et al (2017). However, it is difficult to extrapolate these measurements up to the millimeter domain to search for it in the 3, 2, and 1 mm line surveys of astrophysical sources showing a rich molecular content.…”

“…The three dipole moment components of the molecule, µ a , µ b , and µ c , are non-zero (Ohba et al (2005); Vaquero-Vara et al (2017)) which will produce a large number of rotational transitions for a gas at ambient temperature. Consequently, we performed two frequency switching observations with different frequency throws, Δ ν FS , of 37 and 25 MHz.…”

“…NEFA exhibits indeed a very dense spectrum in the W band, with more than 100 lines per GHz. In addition to the fact that the molecule has a −, b −, and c -type transitions (Ohba et al 2005; Vaquero-Vara et al 2017), there are several low-lying vibrational excited states (Bermúdez et al 2019), and other conformers, trans − ap and cis − ap , that are 137 and 482 cm −1 above the trans − sc ground state conformer (Vaquero-Vara et al 2017) that will contribute to the spectrum. The observed intensities are really low due to the large pressure broadening of the rotational lines of the molecule (which imposes a rather low working pressure of 3×10 −3 mbar -see above), to the large values of the rotational and vibrational partition functions of the molecule ( Q rot (300K) ≃8.5×10 4 ; Q vib (300K) ≃13.1, Q conf (300K) ≃1.47), and to its partial decomposition when in contact with the walls of the chamber.…”

“…The K =0 and 1 components of the same transition do not show any measurable splitting, while the K =2 exhibits an emerging shoulder (clearly seen in that Figure) due to its hyperfine structure. Using the diagonal elements of the 14 N quadrupole coupling tensor in NEFA (Ohba et al 2005), we have computed the expected hyperfine splitting of all lines in the W band. Most of the lines we have identified have the hyperfine splitting concentrate around ±10 kHz of the central frequency.…”