Context. The primary alcohol n-propanol (i.e., normal-propanol or propan-1-ol; C3H7OH) occurs in five different conformers: Ga, Gg, Gg', Aa, and Ag. All rotational spectra of the three conformers of the G family are well described, making astronomical search of their spectroscopic signatures possible, as opposed to those of the Aa and Ag conformers. Aims. Our goal is to facilitate the astronomical detection of Aa and Ag conformers of n-propanol by characterizing their rotational spectra. Methods. We recorded the rotational spectra of n-propanol in the frequency domain of 18-505 GHz. Additional double-modulation double-resonance (DM-DR) measurements were performed, more specifically with the goal to unambiguously assign weak transitions of the Aa conformer and to verify assignments of the Ag conformer. Results. We derived a spectroscopic quantum mechanical model with experimental accuracy (with Jmax = 70 and Ka,max = 6) for Aa n-propanol. Furthermore, we unambiguously assigned transitions (with Jmax = 69 and Ka,max = 9) of Ag n-propanol; in doing so, we prove the existence of two tunneling states, Ag+ and Ag−. Conclusions. The astronomical search of all five conformers of n-propanol is now possible via their rotational signatures. These are applied in a companion article on the detection of n-propanol toward the hot molecular core Sgr B2(N2).

The high resolution vibrational spectrum of ethyl cyanide (C 2 H 5 CN) has been investigated in the far-IR using synchrotron-based Fourier transform spectroscopy. The assignment was performed using the Automated Spectral Assignment Procedure (ASAP) allowing accurate rotational energy levels of the four lowest fundamental vibrations of the species, namely the v 13 = 1 @ 205.934099(8) cm −1 , and v 21 = 1 @ 212.141101(8) cm −1 , v 20 = 1 @ 372.635293(15) cm −1 , v 12 = 1 @ 532.699617(16) cm −1 states, to be determined. The analysis not only confirms the applicability of the ASAP in the treatment of (dense) highresolution infrared spectra but also reveals some of its limitations. Complementary to the infrared study, the pure rotational spectrum of C 2 H 5 CN was also studied in selected frequency ranges from 75 to 255 GHz. New observations of a prototypical high-mass star-forming region, G327.3−0.6, performed with the Atacama Large Millimeter Array show that vibrational satellites of C 2 H 5 CN can be very intense, of order several tens of Kelvin in units of brightness temperature.

Complex organic molecules (COMs) show regularly very rich spectra, sometimes at a line density close to the confusion limit. The plethora of lines often originates from the presence of different conformers and/or low-lying vibrational states. The analysis of spectra may considerably be further complicated by vibration-rotation or other interactions. However, accurate spectroscopic predictions in the millimeter-wave (MMW) region are essential for identifying molecules in space. Double-resonance techniques can help to solve these challenges.We present first MMW−MMW double-resonance (DR) spectra to unambiguously assign weak and perturbed rotational lines of propanal (C 2 H 5 CHO). As already shown in chirped pulse Fourier transform microwave (CP-FTMW) a and in MW−MMW DR experiments, b the Autler-Townes splitting c allows for distinguishing between regressive or progressive energy level schemes. The Autler-Townes splitting is clearly visible in our 2D spectra. Furthermore, implementation of a double-modulation double-resonance (DM-DR) technique, pulse modulation of the pump and frequency modulation of the probe source, allows for confusion-and baseline-free spectra containing only the line(s) of interest. We discuss details of the observed Autler-Townes splitting, benefits of simplified spectra, and possible future applications, such as automatization.

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