Infrared absorption spectra of partially deuterated methoxy radicals CH2DO and CHD2O isolated in solid para-hydrogen

Abstract

The investigation of partially deuterated methoxy radicals is important because the symmetry lowering from C to C provides new insights into the couplings between rovibronic states via Jahn-Teller and spin-orbit interactions. The vibrational spectrum of the partially deuterated methoxy radical CHDO in a matrix of p-H has been recorded. This species was prepared by irradiating a p-H matrix containing deuterated d-nitritomethane (CHDONO) at 3.3 K with laser light at 355 nm. The identification of the radical is based on the photochemical behavior of the precursor and comparison of observed vibrational wavenumbers and infrared (IR) intensities with those predicted from a refined quartic, curvilinear, internal coordinate force field calculated with the coupled-cluster singles and doubles with perturbative triples/cc-pVTZ method. CHDO reacts with H with a rate coefficient (3.5 ± 1.0) × 10 s. Predominantly c-CHDOH and a negligibly small amount of t-CHDOH were produced. This stereoselectivity results from the reaction H + C-CHDOH, which was demonstrated by an additional experiment on irradiation of a CHDOH/Cl/p-H matrix with ultraviolet and IR light to induce the H + CHDOH reaction; only c-CHDOH was observed from this experiment. Even though the energies of transition states and products for the formation of c-CHDOH and t-CHDOH differ by only ∼10 cm, the selective formation of c-CHDOH can be explained by tunneling of the hydrogen atom via an optimal tunneling path. Similarly, the vibronic spectrum for the partially deuterated specie d-methoxy radical (CHDO) was obtained upon irradiation of d-nitritomethane (CHDONO) at 355 nm. Lines associated with the fundamental vibrational modes were observed and assigned; line positions agree with theoretically predicted vibrational wavenumbers. CHDO reacts with H with a rate coefficient (6.0 ± 1.4) × 10 s; CDOH was produced as a major product because the barrier for the formation of CHDOH from H + CHDOH is greater by ∼400 cm. Rate coefficients of the decays of CHO, CHDO, CHDO, and CDO and their corresponding potential energy surfaces are compared.

Hydrogen Abstraction of Acetic Acid by Hydrogen Atom to Form Carboxymethyl Radical •CH2C(O)OH in Solid para-Hydrogen and Its Implication in Astrochemistry

“…Although the H-abstraction of the hydroxyl moiety of CD 3 C­(O)­OH can have a tunneling reaction, it is unlikely to occur because of the large barrier and the endothermicity; no product CH 3 C­(O)­O• was observed in the reaction H + CH 3 C­(O)­OH either. This negative result of H-abstraction of the deuterated species CD 3 C­(O)­OH is analogous to our previous reports of the H + CH 2 DOH reaction to form CHDOH + H 2 but not CH 2 OH and of the H + DC­(O)­OCH 3 reaction to form DCO­(O)­CH 2 • (tentative assignment) but not •C­(O)­OCH 3 …”

“…The PES indicates that the reaction H + CH 3 C­(O)­OH → •CH 2 C­(O)­OH + H 2 has a relatively large barrier, 43 kJ mol –1 ; our experimental observation of •CH 2 C­(O)­OH at 3.3 K even in darkness indicates that this reaction occurred via tunneling. Similar H atom tunneling reactions have been observed for the reactions of H atoms with HONO (∼21 kJ mol –1 ), CH 3 OH (∼35 kJ mol –1 ), , H 2 NC­(O)H (∼26 kJ mol –1 ), C 5 H 5 N (∼8 kJ mol –1 ), HC­(O)­OCH 3 (∼41 and 46 kJ mol –1 for the formation of HC­(O)­CH 2 and C­(O)­OCH 3 radicals, respectively), and CH 3 CONH 2 (∼41 kJ mol –1 ); predicted barriers are provided in parentheses. Paulson et al photolyzed HC­(O)­OH in solid p -H 2 at 1.7 K with laser light at 193 nm and observed tunneling reaction H + HC­(O)­OH because some H atoms were produced from photolysis; the reaction proceeded even after photolysis was terminated, resulting in the formation of HOCO from abstraction of the hydrogen on the carbon atom rather than the hydroxyl hydrogen.…”

“…These experiments are not meant to mimic the conditions in interstellar ices but rather serve as a model system to study the tunneling reactions of H atoms. In our previous investigations on reactions of H atoms with methanol CH 3 OH, formamide HC­(O)­NH 2 , methyl formate HC­(O)­OCH 3 , and acetamide CH 3 C­(O)­NH 2 , we found that the previously neglected H-abstraction plays an important role. For example, in the reaction H + HC­(O)­NH 2 , the dual-cyclic hydrogen-abstraction and hydrogen-addition connecting HC­(O)­NH 2 , H 2 NCO, and HNCO can explain the tight relation between astronomical abundances of HC­(O)­NH 2 and HNCO …”

“…These experiments are not meant to mimic the conditions in interstellar ices but rather serve as a model system to study the tunneling reactions of H atoms. In our previous investigations on reactions of H atoms with methanol CH 3 OH, 49 In this work, we investigated the reaction of hydrogen atoms with acetic acid, CH 3 C(O)OH, an isomer of methyl formate, in solid p-H 2 at 3.3 K. We observed exclusively the formation of the carboxymethyl radical, •CH 2 C(O)OH, produced after the hydrogen-abstraction from the methyl group of CH 3 C-(O)OH. This finding provides a previously neglected route for the formation of •CH 2 C(O)OH that is expected to play an important role in the production of interstellar glycine.…”

First Experimental Confirmation of the CH3O + H2CO → CH3OH + HCO Reaction: Expanding the CH3OH Formation Mechanism in Interstellar Ices

“…The radical CH 3 O is also not observed in the H 2 CO + H experiments. It should be noted, though, that the most intense IR features of its isotopologue CHD 2 O overlap with those of the other reactants and products in the spectra (see, e.g., Haupa et al 2017), and therefore we cannot rule out its presence in the D 2 CO + H experiments. Moreover, we observe a clear CO peak (ν = 2138 cm −1 ) in the H 2 CO + H experiment, as well as HDCO (ν = 1694 cm −1 ) and H 2 CO (ν = 1727 cm −1 ) peaks in the D 2 CO + H counterpart, in agreement with the results by, e.g., Hidaka et al (2009) and Chuang et al (2016) that H-atom-induced abstraction reactions take place for the studied conditions (see, e.g., Figure 5 in Hidaka et al 2009).…”