Context. Astronomical spectra of hot molecular clouds in the wavelength range from centimeter to submillimeter show a huge number of rotational lines due to the emission of complex organic molecules, and a large fraction of these lines are unidentified. The assignment of these unidentified lines to new molecules, to known molecules in excited states, or to their isotopologues requires a good knowledge of the spectroscopic parameters of these molecules. Aims. We present the experimental study of the spectroscopic properties of 13 C-substituted ethyl cyanide 13 CH 3 CH 2 CN, CH 3 13 CH 2 CN, and CH 3 CH 2 13 CN. Methods. The rotational spectra of the three species in the ground state have been measured in the frequency ranges from 5 to 26 GHz using waveguide Fourier transform spectrometers and from 160 to 360 GHz using a source-modulated spectrometer employing backward-wave oscillators (BWOs). Results. A new accurate set of spectroscopic constants has been determined for each isotopic species. This permits prediction of the position of rotational lines that are best suited for detection with an accuracy of a few hundreds of kHz. The three isotopologues have been detected in an Orion IRc2 IRAM survey via several hundred of lines, illustrating that many "unidentified" bands are definitely due to isotopologues of known molecules.

Context. Ethyl cyanide is an abundant molecule in hot molecular clouds. Its rotational spectrum is very dense and several hundreds of rotational transitions within the ground state have been identified in molecular clouds in the 40−900 GHz frequency range. Lines from 13 C isotopically substituted ethyl cyanide were identified in Orion. Aims. To enable the search and the possible detection of other isotopologues of ethyl cyanide in interstellar objects, we have studied the rotational spectrum of deuterated ethyl cyanide: CH 2 DCH 2 CN (in-plane and out-of-plane) and CH 3 CHDCN and the spectrum of 15 N substituted ethyl cyanide CH 3 CH 2 C 15 N. Using these experimental data, we have searched for these species in Orion. Methods. The rotational spectrum of each species in the ground state was measured in the microwave and millimeter-submillimeter wavelength range using a waveguide Fourier Transform spectrometer (8−17 GHz) and a source-modulated spectrometer employing backward-wave oscillators (BWOs) (150−260 and 580−660 GHz). More than 300 lines were identified for each species, for J values in the range 71−80 and K a values in the range 28−31 depending on the isotopologues. The experimental spectra were analyzed using a Watson's Hamiltonian in the A-reduction. Results. From the fitting procedure, accurate spectroscopic constants were derived for each of the species. These new sets of spectroscopic constants enable us to predict reliably the rotational spectrum (lines frequencies and intensities) in the 4−1000 GHz frequency range and for J and K a up to 80 and 31, respectively. Combined with IRAM 30 m antenna observations of Orion, this experimental study allowed us to detect 15 N substituted ethyl cyanide CH 3 CH 2 C 15 N for the first time in Orion. The derived column density and rotational temperature are 10 13 cm −2 and 150 K for the plateau and 3 × 10 14 cm −2 and 300 K for the hot core. The deuterated species were searched for but were not detected. The upper limit to the column density of each deuterated isotopologues was 10 14 cm −2 .

The structures, relative stabilities, and vibrational wavenumbers of the two most stable conformers of serine, stabilized by the O−Hhydrogen bonds, have been evaluated by means of state-of-the-art composite schemes based on coupled-cluster (CC) theory. The socalled "cheap" composite approach (CCSD(T)/(CBS+CV) MP2 ) allowed determination of accurate equilibrium structures and harmonic vibrational wavenumbers, also pointing out significant corrections beyond the CCSD(T)/cc-pVTZ level. These accurate results stand as a reference for benchmarking selected hybrid and double-hybrid, dispersion-corrected DFT functionals. B2PLYP-D3 and DSDPBEP86 in conjunction with a triple-ζ basis set have been confirmed as effective methodologies for structural and spectroscopic studies of medium-sized flexible biomolecules, also showing intramolecular hydrogen bonding. These best performing double-hybrid functionals have been employed to simulate IR spectra by means of vibrational perturbation theory, also considering hybrid CC/DFT schemes. The best overall agreement with experiment, with mean absolute error of 8 cm −1 , has been obtained by combining CCSD(T)/(CBS +CV) MP2 harmonic wavenumbers with B2PLYP-D3/maug-cc-pVTZ anharmonic corrections. Finally, a composite scheme entirely based on CCSD(T) calculations (CCSD(T)/CBS+CV) has been employed for energetics, further confirming that serine II is the most stable conformer, also when zero-point vibrational energy corrections are included.

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