The Fragmentation Dynamics of Simple Organic Molecules of Astrochemical Interest Interacting with VUV Photons

An experimental investigation on the fragmentation dynamics following the double photoionization of simple organic molecules of astrochemical interest, propylene oxide and N-methylformamide molecules, induced by VUV photons has been reported. Falcinelli (2019) ACS Earth Space Chem

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Experiments used linearly polarized light in the 18–37 eV (propylene oxide) and 26–45 eV (N-methylformamide) photon energy range at the ELETTRA Synchrotron Facility of Trieste (Italy), coupling ion imaging and electron–ion–ion coincidence techniques with time-of-flight mass spectrometry. In the case of propylene oxide, six different two-body fragmentation processes have been recorded with the formation of CH₂⁺/C₂H₄O⁺, CH₃⁺/C₂H₃O⁺, O⁺/C₃H₆⁺, OH⁺/C₃H₅⁺, C₂H₃⁺/CH₃O⁺, C₂H₄⁺/CH₂O⁺ ion pairs. On the other hand, the double photoionization of N-methylformamide occurs producing two main fragmentation reactions, forming CH₃⁺ + CH₂NO⁺ and H⁺ + C₂H₄NO⁺. The relative cross sections and the threshold energies for all fragmentation channels are recorded as a function of the photon energy. Furthermore, in the case of the double photoionization of propylene oxide, the measure of the kinetic energy released distribution for the CH₃⁺/C₂H₃O⁺ final ions with their angular distributions allowed the identification of a bimodal behavior indicating the possible formation of two different stable isomers of C₂H₃O⁺: acetyl and oxiranyl cations. The obtained results are important to clarify the physical chemistry of the elementary processes induced by the interaction of ionizing radiations with simple organic molecules of astrochemical interest: propylene oxide and N-methylformamide.

This paper reports on an experimental study concerning the dissociative double photoionization of two simple organic molecules of astrochemical and basic interest, in order to investigate their interaction with UV and EUV ionizing radiations with subsequent direct ejection of two valence electrons. The studied molecules are (i) propylene oxide, being the first chiral molecule recently detected in space; and (ii) N-methylformamide, being one of the simplest organic molecules containing the peptide bond, discovered in 2017 in interstellar medium and of great interest in order to obtain a deeper understanding of the degradation mechanism of proteins by ionizing radiations.
Such a study has been performed by using linearly polarized synchrotron radiation in order to identify in each case the leading two-body dissociation channels and to measure (i) the threshold energy for the different ionic products formation; (ii) the related branching ratios; and (iii) the kinetic energy released distribution of fragment ions at different photon energies.

This preliminary study is important in providing new data on C₃H₆O²⁺ and C₂H₅NO²⁺ dication energetics, and nuclear dissociation dynamics, being mandatory information for further experimental and theoretical investigations of the interaction between chiral and peptide bond containing molecules and linearly or circularly polarized light.
Therefore, we are planning during the next available beamtimes to employ circularly polarized light, as available at the “CiPo” Beamline of ELETTRA Synchrotron Radiation Facility (Trieste, Italy), to selectively photoionize the two enantiomers of propylene oxide, probing possible differences on the angular and energy distribution of either fragment ions and/or ejected photoelectrons as a function of the radiation energy.
In the case of N-methylformamide, preliminary reported results indicate that UV radiation in the energy range of 26–45 eV is not able to cleave the C–N bond related to the carbonyl functionality.

In fact, the equation representing the two-body fragmentation process forming the CH₃⁺/CH₂NO⁺ ion pair, involves the cleavage of one C–N bond of the molecular precursor, but it is not the bond to the carbonyl carbon atom. Furthermore, this could be useful for investigations aimed at studying the formation/destruction routes of formamide, which is a well-known abundant simple organic molecule in astrochemical environments.

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The Fragmentation Dynamics of Simple Organic Molecules of Astrochemical Interest Interacting with VUV Photons Falcinelli S., Vecchiocattivi F., Pirani F., Alagia M., Schio L., Richter R., Stranges S., Zhaunerchyk V., Balucani N., Rosi M. ACS Earth and Space Chemistry, Vol. 3 - 9, pp. 1862-1872 (2019)
doi: 10.1021/acsearthspacechem.9b00115

Ultima modifica il Venerdì, 09 Settembre 2022 17:58