A theoretical study of propynal under interstellar conditions and beyond, covering low-frequency infrared spectra, spectroscopic constants, and hot bands

Abstract

From an astrochemical point of view propynal is a complex organic molecule. Moreover, it is a potential candidate for the formation of prebiotic propanal and propenal. Therefore, this molecule is of particular interest for astrochemical investigations. As it has been detected in the interstellar medium, it is of high relevance in this field of research. Although experimental data are available for the vibrational fundamental bands and rotational constants, experimental data for vibrational overtones and combination bands are scarce and fairly old. Additionally, high level ab initio calculations are also not reported. In this work, we provide accurate quantum chemical calculations as well as a detailed analysis of vibrational and rovibrational properties for this molecule. The low frequency spectrum up to 350 cm−1 has been studied for temperatures between 10 and 300 K. For the same temperature range partition functions are provided. Furthermore, the impact of hot bands up to room temperature has been investigated. A comparison of our results with experimental data is provided for the rotational constants, geometrical parameters and a rovibrational spectrum. The underlying potential energy surface within these calculations is based on explicitly correlated coupled-cluster theory and includes up to 4-mode coupling terms within an n-mode expansion. The vibrational and rovibrational calculations rely on vibrational respectively rovibrational configuration interaction theory.

Spectroscopic Characterization of Diazophosphane—A Candidate for Astrophysical Observations

“…However, it is subject to intensity borrowing due to coupling with the much weaker overtones 2ν 5 and 2ν 6 . As discussed in Tschöpe et al (2021) and Tschöpe & Rauhut (2023), this leads to the typical compression of the branch facing the borrowing bands, i.e., toward higher energies, in this case. Another typical example of an A-type pattern arises for ν 2 , which was to be expected since it is assigned to the N≡N stretching mode.…”

“…Moreover, even in cases where hot bands contribute more strongly to the overall appearance of the rotational spectrum, i.e., at 300 K, this does not lead to a shift in intensities toward higher or lower frequencies. This is in contrast to other molecules (Tschöpe & Rauhut 2023) and other parts of the diazophosphane spectrum (see Section 3.6).…”

“…Compared to these previous works, this effect is smaller for diazophosphane than for other molecules. This is not surprising, as the largest differences were observed for CH bond lengths (Tschöpe et al 2021;Tschöpe & Rauhut 2023), which are not present in diazophosphane. However, the largest absolute difference between r a and r g was again found for the bond involving hydrogen, i.e., Δ r(PH) = 0.008 Å.…”

“…The latter uses the expectation value of the bond lengths expressed in normal coordinates to calculate instantaneous internuclear distances. It is well known Tschöpe & Rauhut 2023) that these two approaches can yield significantly different results. Compared to these previous works, this effect is smaller for diazophosphane than for other molecules.…”

“…For this reason, the PESs of HPN 2 and DPN 2 have been transformed to a quartic force field to obtain the force constants needed for VPT2 calculations. For diazophosphane, we have chosen the A-reduced Hamiltonian over its alternative, the S-reduced Hamiltonian similar to our previous studies, see Dinu et al (2022) and Tschöpe & Rauhut (2023). Moreover, we followed the convention to use the I r orientation for this near-prolate asymmetric top molecule.…”