Competitive fragmentation pathways of acetic acid dimer explored by synchrotron VUV photoionization mass spectrometry and electronic structure calculations

Abstract

In present study, photoionization and dissociation of acetic acid dimers have been studied with the synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. Besides the intense signal corresponding to protonated cluster ions (CH(3)COOH)(n)·H(+), the feature related to the fragment ions (CH(3)COOH)H(+)·COO (105 amu) via β-carbon-carbon bond cleavage is observed. By scanning photoionization efficiency spectra, appearance energies of the fragments (CH(3)COOH)·H(+) and (CH(3)COOH)H(+)·COO are obtained. With the aid of theoretical calculations, seven fragmentation channels of acetic acid dimer cations were discussed, where five cation isomers of acetic acid dimer are involved. While four of them are found to generate the protonated species, only one of them can dissociate into a C-C bond cleavage product (CH(3)COOH)H(+)·COO. After surmounting the methyl hydrogen-transfer barrier 10.84 ± 0.05 eV, the opening of dissociative channel to produce ions (CH(3)COOH)(+) becomes the most competitive path. When photon energy increases to 12.4 eV, we also found dimer cations can be fragmented and generate new cations (CH(3)COOH)·CH(3)CO(+). Kinetics, thermodynamics, and entropy factors for these competitive dissociation pathways are discussed. The present report provides a clear picture of the photoionization and dissociation processes of the acetic acid dimer in the range of the photon energy 9-15 eV.

Laser desorption time-of-flight mass spectrometry of vacuum UV photo-processed methanol ice

“…Reference spectra of acetic acid show the appearance at m/z = 61 assigned to protonated acetic acid (CH 3 COOH)H + . This mass peak has also been observed in VUV photoionisation studies of acetic acid clusters (Guan et al 2012). An additional intensity decrease is observed at m/z = 45 and 46, which can be explained by the desorption of the remaining ethanol.…”

A Photoionization Reflectron Time-of-flight Mass Spectrometric Study on the Formation of Acetic Acid (CH3COOH) in Interstellar Analog Ices

“…from two fragments of the dimer: (CH 3 COOH)•H + (m/z=61) and (CH 3 COOH)•COOH + (m/z=105), as will be demonstrated. Fragments of trimers were not detected either because of the relatively low conversion of carbon dioxide-methane to acetic acid or because the energy used to photoionize the subliming molecules in our experiments is insufficient to produce trimer fragments such as (CH 3 COOH)•CH 3 CO + (m/z=103), which has an appearance energy of 12.41 eV (Guan et al 2012).…”

“…In principle, since the IE of all C 2 H 4 O 2 isomers is below the photoionization energy of 10.84 eV used in the present experiments, the specific assignment of the signal at m/z= 60 to one species in particular would not be possible. However, laboratory studies have revealed that gas-phase acetic acid (CH 3 COOH) forms a strongly bound dimer (CH 3 COOH) 2 involving two hydrogen bonds (Forysinski et al 2011;Guan et al 2012), while the hydrogen bonding in the trimer (CH 3 COOH) 3 was shown to be less favorable. Using supersonic beams of acetic acid monomers, dimers, and trimers, coupled to tunable vacuum ultraviolet light, to detect and count the resulting ions, Guan et al (2012), Hu et al (2006), and Zielke et al (2009) revealed that upon soft photoionization ionized acetic acid dimers and trimers undergo reaction and fragment in a pattern characterized by signal from acetic acid (CH 3 COOH + ) (m/z= 60), by protonated clusters (…”

“…We already established that m/z=60 (Figure 7(a)) belongs to C 2 H 4 O 2 isomers, and among them is acetic acid. The signal at m/z=61 ((CH 3 COOH)H + ; Figure 7(b)) represents the most common fragment of acetic acid dimers (Hu et al 2006;Zielke et al 2009;Forysinski et al 2011;Guan et al 2012), with the (CH 3 COOH)•COOH + fragment detected at m/z=105 (Figure 7(c)) being the second most intense (Hu et al 2006;Zielke et al 2009;Forysinski et al 2011). Notably, the signals from the isotopologues of (CH 3 COOH)H + and (CH 3 COOH)•COOH + were also confirmed (Figures 7(b) and (c)), thus supporting the detection of acetic acid dimers.…”

“…In recent years, considerable attention from experimental physical chemists to observational astrophysicists has been devoted to untangling the formation routes of three structural isomers of C 2 H 4 O 2 : acetic acid (CH 3 COOH), methyl formate (HCOOCH 3 ), and glycolaldehyde (HCOCH 2 OH) (Bennett & Kaiser 2007aPuletti et al 2010;Shiao et al 2010;Guan et al 2012;Burke et al 2014;Favre et al 2017;Zhu et al 2018a) ( Figure 1). Among these structural isomers-molecules with the same chemical formula but distinct connectivity of the atoms-acetic acid (CH 3 COOH) is considered as a critical prebiotic molecule, since it shares the C-C=O(OH) backbone with the simplest amino acid glycine (NH 2 CH 2 COOH), from which acetic acid differs only by an amino group (-NH 2 ).…”