Solid-State Isomerization and Infrared Band Strengths of Two Conformational Isomers of Cyclopropanecarboxaldehyde, a Candidate Interstellar Molecule

Abstract

At least a dozen of the known interstellar molecules possess a formyl group (HCO), suggesting that other such species exist and await discovery in the interstellar medium. Here we examine the mid-infrared (mid-IR) spectrum and selected physical properties of one such candidate, cyclopropanecarboxaldehyde, in amorphous ices. Mid-IR transmission spectra of solid cyclopropanecarboxaldehyde are presented for the first time and used to determine the cis-to-trans ratio of conformational isomers present in amorphous samples. The measured ratio is compared to one from an electron-diffraction study of the gas-phase compound. The cis-to-trans isomerization in the amorphous compound is followed and the activation energy is determined. The first IR band strengths for solid cyclopropanecarboxaldehyde are reported. Also presented are refractive indices and densities at 15 K for amorphous forms of two related compounds, cyclopropane and cyclopropanemethanol. Two low-temperature reactions for the interstellar formation of cyclopropanecarboxaldehyde are briefly described.

Comprehensive rotational study and astronomical search for cyclopropanecarboxaldehyde

“…This result can be compared to those previously reported for the relative stability of CPCA isomers, which seem fairly ambiguous. Durig & Little (1988), Durig et al (1992) and Bartell & Guillory (1965) in their respective experiments found that the cis isomer was the most stable species in the gas phase, with an energy difference of around 40-60 cm −1 , while Hudson & Coleman (2019) showed that the cis/trans ratio was 1:1. On the other hand, Volltrauer & Schwendeman (1971) proposed that in the gas phase the trans species was slightly more stable (10±20 cm −1 ) than the cis one, but with large uncertainties.…”

“…In addition, CPCA was investigated by infrared (IR) and Raman spectroscopy (Durig & Little 1988;Durig et al 1992;Durig & Shen 2000;Hudson & Coleman 2019) and electron diffraction (Bartell & Guillory 1965). In these works the abundance ratios for both cis and trans isomers were derived and it was found that the cis isomer is more stable in the gas phase while the trans is the only species present in liquid phase, which is in contrast with the rotational spectroscopy study (Volltrauer & Schwendeman 1971).…”

Conformational stability of cyclopropanecarboxaldehyde is ruled by vibrational effects

“…solid, liquid or gas, which can favour one conformer with respect to the others. For example, intermolecular interactions that are present in the solid state can preferentially stabilise one conformer with respect to another [3,4].…”

“…However, in the liquid phase, the stability appears to be reversed and the anti species is the most stable [13]. This only apparent difference is justified by the stronger dipole moment of the anti conformer with respect to syn, which allows the formation of stronger intermolecular interactions in the condensed phase [3,4,13]. The stability of CPCA conformers was also analysed by NMR spectroscopy in a freon matrix at T=103 K and the results clearly point out a larger stability of the anti conformer with respect to syn, with an energy barrier of 21.0 kJ mol −1 [18].…”