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In order to simulate different altitudes in the atmosphere of Titan, we investigated using infrared spectrometry and mass spectrometry the photochemistry of ethyl cyanide (CH 3 CH 2 CN) ices at different temperatures. Heating experiments of the solid phase until complete desorption showed up three phase transitions with a first one appearing to be approximately at the temperature of Titan's surface (94 K), measured by the Huygens probe. Ethyl cyanide, whose presence has been suggested in solid phase in Titan, can be considered as another nitrile for photochemical models of the Titan atmosphere after our first study (Toumi et al., 2016) concerning vinyl cyanide (CH 2 CHCN). The desorption energy of ethyl cyanide has been calculated to be 36.75 ( ± 0.55) kJ mol − 1 using IRTF and mass spectroscopical techniques. High energetic photolysis ( λ > 120 nm) have been performed and we identified ethyl isocyanide, vinyl cyanide, cyanoacetylene, ethylene, acetylene, cyanhydric acid and a methylketenimine form as photoproducts from ethyl cyanide. The branching ratios of the primary products were determined at characteristic temperatures of Titan thanks to the value of the ν CN stretching band strength of ethyl cyanide that has been calculated to be 4.12 × 10 −18  cm molecule − 1 . We also report here for the first time the values of the photodissociation cross sections of C 2 H 5 CN for different temperatures. [ABSTRACT FROM AUTHOR]

Abstract: The quadratic, cubic and semi-diagonal quartic force field of ethyl cyanide has been calculated at the B3LYP level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quadruple-ζ quality and a core correlation correction. The empirical structures are also determined and their accuracy is discussed. The potential barrier V 3 hindering internal rotation of the methyl group has been calculated from 23 rotational transitions of CH3CH2C15N which were found split into doublets, giving V 3 =3074(27)calmol−1. [Copyright &y& Elsevier]

• First high-resolution infrared study of ethyl cyanid. • Four energetically lowest vibrational fundamentals studied. • Global fits of infrared and pure rotational spectra performed. • Strong vibrational satellites towards high mass star forming region assigned. 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, v 21 = 1 @ 212.141101(8) cm−1, v 20 = 1 @ 372.635293(15) cm−1, and 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) high-resolution 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. [ABSTRACT FROM AUTHOR]