Formation of interstellar cyanoacetamide: a rotational and computational study

Abstract

Context: Cyanoacetamide is a -CN bearing molecule that is also an amide derivative target molecule in the interstellar medium.

Aims: The aim of our investigation is to analyze the feasibility of a plausible formation process of protonated cyanoacetamide under interstellar conditions and to provide direct experimental frequencies of the ground vibrational state of the neutral form in the microwave region in order to enable its eventual identification in the interstellar medium.

Methods: We used high-level theoretical computations to study the formation process of protonated cyanoacetamide. Furthermore, we employed a high-resolution laser-ablation molecular beam Fourier transform spectroscopic technique to measure the frequencies of the neutral form.

Results: We report the first rotational characterization of cyanoacetamide, and a precise set of the relevant rotational spectroscopic constants have been determined as a first step to identifying the molecule in the interstellar medium. We fully explored the potential energy surface to study a gas-phase reaction on the formation process of protonated cyanoacetamide. We found that an exothermic process with no net activation barrier is initiated by the high-energy isomer of protonated hydroxylamine, which leads to protonated cyanoacetamide.

Keywords: ISM: molecules; astrochemistry; molecular data; molecular processes.

Fig. 1

Energy profile in kcal/mol for the reactions of protonated hydroxilamine with cyanoacetaldehyde producing O-protonated cyanoacetamide computed at the B2PLYPD3/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ (in parentheses) levels of theory. The zero-point vibrational energy computed at the B2PLYPD3/aug-cc-pVTZ level is included.

Fig. 2

Intermediates, transition states, and products involved on the reaction of formation of protonated cyanoacetamide. Geometries are optimized at the B3PLYPD3/aug-cc-pVTZ level of theory. Bond lengths are given in Angstroms for the intermediates and TS.

Fig. 3

Structure of trans cyanoacetamide, showing the two different ¹⁴N nuclei: an amide (N_a) and a cyano (N_c) nitrogen atom.

Fig. 4

Rotational transitions 1₁₁ ← 0₀₀ and 2₁₂ ← 1₀₁ of trans cyanoacetamide observed during the LA-MB-FTMW experiment along with the B2PLYPD3/aug-cc-pVTZ predictions, displaying the high resolution of our spectrometer as well as the great agreement between the experimental and theoretically predicted frequencies. The resonance frequency is determined by the arithmetic mean of two Doppler components. The LA-MB-FTMW spectrum was obtained by averaging 200 experimental cycles (six free induction decay signals per cycle).