Radiation-induced transformations of matrix-isolated formic acid: evidence for the HCOOH → HOCO + H channel

Abstract

The effect of X-ray irradiation on the isolated formic acid molecules (HCOOH) in solid noble gas matrices (Xe, Kr, Ar, and Ne) at very low temperatures (6 K) was first studied by FTIR spectroscopy. Carbon oxides (CO and CO2) and hydrocarboxyl radicals (HOCO) have been detected as the principal degradation products. The formation of HOCO radicals represents a primary dissociation channel for formic acid, which was not reported previously under UV photolysis in solids. This reaction can be explained by the involvement of the recombination-induced excited states, which are not populated in photolysis. The effects of the matrix and the absorbed dose on the product formation were studied in detail and possible mechanisms are discussed with particular attention to the difference between radiolysis and UV-photolysis of the matrix-isolated formic acid. The results obtained provide a new insight into the effects of high-energy impact on the simplest carboxylic acid with possible implications to the astrochemical problems, in particular, the prebiotic evolution in the interstellar medium.

VUV photochemistry of the H2O⋯CO complex in noble-gas matrices: formation of the OH⋯CO complex and the HOCO radical

“…A similar behavior was observed previously and may indicate the reaction of trans-HOCO with mobile H atoms. 56 As to the case of the Ne matrix, a slow decay (on a timescale of more than 10 h) of the OHÁ Á ÁCO complex accompanied by the formation of trans-HOCO was observed even in the dark at 4.5 K (the lowest temperature of our experiment) (see Fig. 7).…”

“…This irradiation resulted in gradual bleaching of the H 2 OÁ Á ÁCO bands, an increase of the band of the CO monomer (2140.9, 2138.5, 2135.5 and 2133.0 cm À1 in Ne, Ar, Kr, and Xe, respectively) 21 for Ne, Ar, Kr, and Xe respectively). 51,56 trans-HOCO (Table 2), Ng 2 H + ions (Ng = Ar, Kr, and Xe), 59 and trace amounts of HCO 35,51,60 and HCOÁ Á ÁH 2 O 35,56 were also detected. In addition, a new absorber with bands at 3546.5 and 2159.8 cm À1 in a Ne matrix, at 3526.5 and 2152.5 cm À1 in an Ar matrix, and at 3521.9 and 2147.9 cm À1 in a Kr matrix was observed.…”

“…). 56,62 In the Xe matrix, relatively intense bands of HXeH (1166.5 and 1181.4 cm À1 ) 63 are observed after annealing. Since the used annealing temperatures activate the mobility of H atoms, 51,64 these results clearly indicate that UV photolysis of FA produces considerable amounts of H atoms.…”

“…In these experiments, the H 2 OÁ Á ÁCO complex was generated in noble-gas matrices by photodecomposition of FA. The deposited FA/Ng matrices (Ng = Ne, Ar, Kr, and Xe) contained mostly FA monomers (in the more stable trans form) 14 with small amounts of FA dimers (tt1 and tt2) 55,56 and trans-FAÁ Á ÁH 2 O complexes. 56,57 The obtained spectra of trans-FA are in full agreement with the previous data.…”

“…The deposited FA/Ng matrices (Ng = Ne, Ar, Kr, and Xe) contained mostly FA monomers (in the more stable trans form) 14 with small amounts of FA dimers (tt1 and tt2) 55,56 and trans-FAÁ Á ÁH 2 O complexes. 56,57 The obtained spectra of trans-FA are in full agreement with the previous data. UV irradiation of the deposited matrices (B5000 pulses at 193 nm for Ng = Ne, Ar, and Kr and B20 000 pulses at 250 nm for Ng = Xe) resulted in efficient decomposition of FA (typically 470%).…”