ИСТИНА

Hydrocarboxyl radical (HOCO) is a key intermediate in atmospheric and combustion processes and it is probably involved in extraterrestrial prebiotic chemistry. Basic importance of this radical has stimulated extensive theoretical and experimental studies over several decades. On the other hand, the application of matrix isolation to investigation of this species is rather limited. Recently we have demonstrated the formation of hydrocarboxyl radical upon X-ray irradiation of the H2O…CO2 complexes isolated in noble-gas matrices. In the present work, we investigate the formation of HOCO upon irradiation of matrix-isolated formic acid (HCOOH) by X-rays and VUV light. Particular attention is drawn to the trans⇄ cis conformational changes of HOCO. The experiments were carried out with HCOOH/M (1/1000÷2500) samples (M = noble gas or N2) in closed-cycle helium cryostats using FTIR spectroscopy. An X-ray tube with a tungsten anode (Eeff. ~ 20 keV), microwave discharge lamps (Kr and Xe) and an ArF excimer laser (193 nm) were used as radiation sources. Vibrational excitation of HOCO was performed with a narrow-band optical parametric oscillator. The experiments on X-ray radiolysis of matrix-isolated HCOOH have revealed the involvement of the HCOOH → HOCO + H primary channel. It is the first observation of this channel of the radiation-driven decomposition of formic acid in the solid phase. The mechanism involving population of specific triplet states (inaccessible upon direct photoexcitation) was proposed for the explanation of this result. HOCO also appears as a result of photolysis of HCOOH/M matrices with VUV lamps. The corresponding mechanism probably involves the formation and radiation-driven evolution of the H2O…CO intermolecular complexes. To remind, 193-nm photolysis of matrix-isolated formic acid does not lead to the formation of hydrocarboxyl radical. For HOCO (with an H atom), the higher-energy cis conformer is found only in a nitrogen matrix. In contrast, for the deuterated species DOCO, the higher-energy cis conformer appears in both argon and nitrogen matrices. The trans and cis conformers of hydrocarboxyl radical can be interconverted by selective vibrational excitation as it has been demonstrated for a number of other species. Similarly to carboxylic acids, cis-HOCO convertes to the trans conformer as a result of hydrogen-atom tunneling through the torsional barrier and the kinetics of this process has been studied in detail, including isotope and matrix effects. Finally, the trans ⇄ cis transformation of hydrocarboxyl radical induced by broad-band IR radiation of the spectrometer has been investigated in a nitrogen matrix. The obtained results may have implications for various aspects of low temperature chemistry and physics including astrochemically-relevant studies. This work was partially supported by the Russian Science Foundation (grant № 14-13-01266).