Near-Infrared Radiation Induced Conformational Change and Hydrogen Atom Tunneling of 2-Chloropropionic Acid in Low-Temperature Ar Matrix

Former assignments of the matrix-isolation infrared (MI-IR) spectrum of 2-chloropropionic acid are revised with the help of near-infrared (NIR) laser irradiation induced change in conformer ratios. This method allows not only the unambiguous assignment of each band in the MI-IR spectrum to the two <i>trans</i> (<i>Z</i>) and the <i>cis</i> (<i>E</i>) conformers but also the assignment of the spectral bands to different matrix sites. The tunneling decay of the higher-energy <i>cis</i> conformer prepared from both <i>trans</i> conformers in different sites is also investigated. It is shown that the tunneling decay time is very sensitive to the matrix site, especially if the in situ prepared high-energy conformer has a strained geometry in the matrix cage. The analysis shows that the kinetics of some <i>cis</i> → <i>trans</i> back conversion processes cannot be fitted by a single exponential decay. The possible reasons of this observation are examined and discussed. The present and former results clearly show that, in addition to tunneling processes, the decay rates strongly depend on solid-state effects. Therefore, simple theoretical predictions of decay rates, which do not take into account the solid-state effects, can only be compared to experimental observations only if experimentally proven that these effects do not significantly affect the experimentally measured tunneling rates

Electron Radiolysis of Ammonium Perchlorate: A Reflectron Time-of-Flight Mass Spectrometric Study

Thin films of ammonium perchlorate (NH₄ClO₄) were exposed to energetic electrons at 5.5 K to explore the radiolytic decomposition mechanisms. The effects of radiolysis were monitored on line and in situ via Fourier transform infrared spectroscopy (FTIR) in the condensed phase along with electron impact ionization quadrupole mass spectrometry (EI-QMS) and single-photon photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) during the temperature-programmed desorption (TPD) phase to probe the subliming molecules. Three classes of molecules were observed: (i) nitrogen bearing species [ammonia (NH₃), hydroxylamine (NH₂OH), molecular nitrogen (N₂), nitrogen dioxide (NO₂)], (ii) chlorine carrying molecules [chlorine monoxide (ClO), chlorine dioxide (ClO₂), dichlorine trioxide (Cl₂O₃)], and (iii) molecular oxygen (O₂). Decay profiles of the reactants along with the growth profiles of the products as derived from the infrared data were fit kinetically to obtain a reaction mechanism with the initial steps involving a proton loss from the ammonium ion (NH₄⁺) yielding ammonia (NH₃) and the decomposition of perchlorate ion (ClO₄^–) forming chlorate ion (ClO₃^–) plus atomic oxygen. The latter oxidized ammonia to hydroxylamine and ultimately to nitrogen dioxide. Molecular oxygen and nitrogen were found to be formed via recombination of atomic oxygen and multistep radiolysis of ammonia, respectively

Exploring the Conformational Space of Cysteine by Matrix Isolation Spectroscopy Combined with Near-Infrared Laser Induced Conformational Change

Six conformers of α-cysteine were identified by matrix isolation IR spectroscopy combined with NIR laser irradiation. Five of these conformers are identical with the five out of six conformers that have recently been identified by microwave spectroscopy. The sixth conformer observed in the present study is a short-lived conformer, which decays by H-atom tunneling; its half-life in a 12 K N₂ matrix is (1.1 ± 0.5) × 10³ s. This study proves that matrix isolation IR spectroscopy combined with NIR laser irradiation is a suitable method to identify conformers of a complex system for which computations predict several dozens of conformers, and that the reliability of this method for conformational assignment is comparable to that of microwave spectroscopy

Formation of Hydroxylamine in Low-Temperature Interstellar Model Ices

We irradiated binary ice mixtures of ammonia (NH₃) and oxygen (O₂) ices at astrophysically relevant temperatures of 5.5 K with energetic electrons to mimic the energy transfer process that occurs in the track of galactic cosmic rays. By monitoring the newly formed molecules <i>online</i> and <i>in situ</i> utilizing Fourier transform infrared spectroscopy complemented by temperature-programmed desorption studies with single-photon photoionization reflectron time-of-flight mass spectrometry, the synthesis of hydroxylamine (NH₂OH), water (H₂O), hydrogen peroxide (H₂O₂), nitrosyl hydride (HNO), and a series of nitrogen oxides (NO, N₂O, NO₂, N₂O₂, N₂O₃) was evident. The synthetic pathway of the newly formed species, along with their rate constants, is discussed exploiting the kinetic fitting of the coupled differential equations representing the decomposition steps in the irradiated ice mixtures. Our studies suggest the hydroxylamine is likely formed through an insertion mechanism of suprathermal oxygen into the nitrogen–hydrogen bond of ammonia at such low temperatures. An isotope-labeled experiment examining the electron-irradiated D3-ammonia–oxygen (ND₃–O₂) ices was also conducted, which confirmed our findings. This study provides clear, concise evidence of the formation of hydroxylamine by irradiation of interstellar analogue ices and can help explain the question how potential precursors to complex biorelevant molecules may form in the interstellar medium