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Article Cite This: J. Am. Chem. Soc. 2019, 141, 11614−11620

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Hydrogen Abstraction/Addition Tunneling Reactions Elucidate the Interstellar H2NCHO/HNCO Ratio and H2 Formation Karolina A. Haupa,*,† György Tarczay,*,§,∥ and Yuan-Pern Lee*,†,‡,⊥

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Department of Applied Chemistry and Institute of Molecular Science and ‡Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu, 30010, Taiwan § HAS-ELTE Laboratory Astrochemistry Lendület Research Group, Pázmány P. S. 1/a, Budapest 1117, Hungary ∥ Laboratory of Molecular Spectroscopy, Institute of Chemistry, ELTE Eötvös Loránd University, Pázmány P. S. 1/a, Budapest 1117, Hungary ⊥ Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: Formamide (H2NCHO) is the smallest molecule possessing the biologically important amide bond. Recent interstellar observations have shown a strong correlation between the abundance of formamide and isocyanic acid (HNCO), indicating that they are likely to be chemically related, but no experiment or theory explains this correlation satisfactorily. We performed H + H2NCHO reactions in a para-hydrogen quantum-solid matrix host and identified production of H2NCO and HNCO from hydrogen-abstraction reactions. We identified also D2NCO, DNCO, HDNCO, and HDNCHO from the reaction H + D2NCHO, indicating the presence of hydrogen-addition reactions of DNCO and HDNCO. From the observed temporal profiles of H2NCHO, H2NCO, HNCO, and their deuterium isotopologues, we showed that a dual-cycle consisting of hydrogen abstraction and hydrogen addition can satisfactorily explain the quasi-equilibrium between H2NCHO and HNCO and explain other previous experimental results. Furthermore, this mechanism also indicates that the catalytic formation of H2 from H atoms might occur in interstellar ice grains.

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INTRODUCTION A central question in astrochemistry is whether biomolecular building units are formed under extraterrestrial conditions. Both the observation of relevant species in the interstellar medium (ISM) and laboratory experiments on its possible mechanisms of formation and destruction are hence extremely important. Molecules with a peptide [−NH−C(O)−] link play an important role in the synthesis of amino acid and proteins, which are essential for living systems.1−3 Formamide (H2NCHO) is the smallest molecule that contains an amide structure; it is hence considered as a possible precursor of both metabolic and generic material in the formation of nucleic polymers under prebiotic or abiotic conditions.4 Formamide was first detected in Sgr B2 in 19715 and later in Orion KL and other massive star-forming regions.6−9 It was also observed in various prestellar and protostellar objects,10−12 protostellar shock regions,13,14 and comets.15−17 Several possible gaseous sources of H2NCHO, including reactions of NH2 + H2CO → H2NCHO + H18 and H2 + HNCO → H2NCHO have been proposed.19 Many laboratory experiments also showed that H2NCHO can be formed from various solid precursors, including HCN/H2O,20 NH3/CO,21 CH4/HNCO,22 H2O/CH4/N2,23 HNCO,24 and CN/H2O,25 under conditions of interstellar ice analogues, typically in lowtemperature ice and under vacuum−ultraviolet (VUV) irradiation or bombardment of protons or electrons or ions. © 2019 American Chemical Society

Astronomical observations revealed that a nearly linear correlation between the abundances of isocyanic acid (HNCO) and H2NCHO exists in pre- and protostellar environments over 6 orders of magnitude in luminosity, indicating a close chemical relation between these two species. 9,10 The DNCO/HNCO ratio (∼1%) is also comparable with the D/H ratio of formamide (∼2%) in the ALMA-PILS survey,26 supporting such a relation. Several investigations of possible reaction paths connecting HCNO and H2NCHO were reported to rationalize this relation. Ferus et al. detected H2NCHO in a gaseous mixture of HNCO and H2 upon exposure to a glow-discharge and proposed that the most likely reaction channel is HNCO + H2 → H2NCHO.19 Raunier et al. irradiated solid HNCO at 10 K with VUV light and observed H2NCHO;24 they proposed that photodissociation of HNCO yields hot H atoms that react subsequently with HNCO to form H2NCHO. On the basis of this result and other reports, López-Sepulcre et al. proposed that hydrogenation of HNCO on an icy grain mantle is a likely route of formation of H2NCHO.10 Contrary to this hypothesis, Noble et al. observed no detectable amount of H2NCHO when they bombarded hydrogen atoms onto HCNO ice near 10 K; otherwise, they observed formation of DNCO after Received: April 26, 2019 Published: June 27, 2019 11614

DOI: 10.1021/jacs.9b04491 J. Am. Chem. Soc. 2019, 141, 11614−11620

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Journal of the American Chemical Society bombardment of D atoms onto HNCO.27 On the basis of these results, they proposed that the reaction of an H atom with HNCO produces carbamoyl (H2NCO) radical or OCN + H2; subsequent attack by a second H atom leads to the back formation of HNCO and H2, instead of H2NCHO. Existing experimental evidence failed to explain satisfactorily the correlation between HNCO and H2NCHO. Theoretical investigations of the conversion between HNCO and H2NCHO are also incomplete. Nguyen et al. investigated the gaseous reaction of H + HNCO and reported that formation of the stabilized adduct H2NCO cannot compete successfully with its decomposition to NH2 + CO at all temperatures and pressures.28 Ferus et al. employed the ab initio molecular dynamics method and proposed the gaseous reaction H2 + HNCO → H2NCHO to explain their experimental results discussed previously,19 but according to the calculations of Nguyen,29 this reaction has a barrier ∼250 kJ mol−1, making it unlikely to occur. Rimola et al. simulated possible routes for the formation of H2NCHO in ice with 33 H2O molecules in a cluster.25 They found that CN reacts rapidly with H2O in ice to form H2NCHO via multiple steps. They also indicated that both H-addition to H2NCO to form H2NCHO and H-abstraction of H2NCO to form HNCO + H2 are barrierless. The reaction of H with H2NCHO has not been explored. In this work, we took advantage of the unique characteristics of the quantum-solid para-hydrogen (p-H2) matrix host30 to investigate the reactions of H atoms with H2NCHO (or D2NCHO) and found that both H-abstraction and H-addition play important roles in connecting HNCO, H2NCO, and H 2 NCHO; a quasi-equilibrium between HNCO and H2NCHO in the dual-cyclic hydrogen reactions can consequently explain satisfactorily the observed tight relation between the astronomical abundances of HNCO and H2NCHO and previous experimental results.

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Figure 1. PES of the H-addition and H-abstraction reactions of H2NCHO. (a) All possible channels of H + H2NCHO and (b) the most feasible channels of H + H2NCHO, H2NCO, and HNCO computed at the B3LYP/aug-cc-pVTZ (listed in gray, bottom) and CCSD(T)/cc-pVTZ (blue, top) levels. Zero-point vibrational energy (ZPVE) corrected energies are in kJ mol−1. In the CCSD(T) calculations, g-HNCHO is not a minimum.

RESULTS AND DISSCUSSION Computed Potential-Energy Scheme (PES). Although some possible reaction paths of H-abstraction and H-addition reactions with H2NCO and HNCO have been reported,25,28 they are incomplete. We computed energies of all possible reactions of H atoms with H2NCHO and H2NCO at the B3LYP/aug-cc-pVTZ level; the energies were corrected with the zero-point vibrational energy (ZPVE) using harmonic vibrational frequencies. Because the first step of the reactions, H + H2NCHO, is previously uninvestigated, we computed the energies at both the B3LYP/aug-cc-pVTZ and the CCSD(T)/ cc-pVTZ levels, as presented in Figure 1a. The predicted structures of all species including transition states are collected in Table S1. The PES of the following steps H + H2NCO and H + HNCO are shown in Figure S1; the corresponding structures are collected in Table S2. As shown in Figure 1a, the reactants H + H2NCHO with relative energy 0.0 kJ mol−1 are shown in the middle. Two hydrogen-addition channels, to O atom and C atom (in green), and one addition−elimination to form HCO + NH3 (in orange) are presented on the right-hand side. Three hydrogen-abstraction channels involving two H atoms on N (in red and green) and the aldehyde H atom (in blue) are presented on the left-hand side. Among all these six channels, the H-abstraction from the aldehyde moiety leading to H2NCO + H2 has the smallest barrier and is the most exothermic; as other channels have much greater barriers, they cannot compete with this reaction at low temperature.

Similarly, we presented the H-addition and addition− elimination reactions on the right-hand side and the Habstraction reactions on the left-hand side of Figure S1 for reactions H + H2NCO and H + HNCO. For H + H2NCO, all paths are barrierless and the addition reaction to form H2NCHO is the most exothermic. For H + HNCO, we used the results from Ngyuen et al.28 who employed the PUMP4/6311++G(d,p) level of theory; the addition reaction to form H2NCO has the smallest barrier (39 kJ mol−1), other channels has barriers greater than 66 kJ mol−1. According to these computations, we present only the most accessible channels of H + H2NCHO, H2NCO, and HNCO in Figure 1b in an integrated form to include all H atoms and H2 involved; results from both B3LYP/aug-cc-pVTZ and CCSD(T)/cc-pVTZ levels are listed. H + H2NCHO leads to H2NCO + H2 with a barrier of 26 (2) kJ mol−1; the number in parentheses is the B3LYP energy. Subsequently, both the Habstraction and H-addition of H2NCO leading to HNCO + H2 and H2NCHO, respectively, are barrierless, in agreement with Rimola et al.25 H-addition to HNCO to form H2NCO has a barrier ≈42 (21) kJ mol−1. Larger barriers (126 kJ mol−1) for 11615


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cm−1 (not shown)35 and several groups of new lines appeared. After irradiation, the matrix was maintained in darkness with its spectrum recorded at time intervals 30 min for more than 10 h. The difference spectra after 6 and 10 h in darkness are presented in Figures 2c and 2d, respectively. The intensities of lines of H2NCHO decreased significantly after 6 h in darkness and increased slightly after further 4 h in darkness. Lines in a group with intense ones at 3542.9, 1809.5, and 1565.9 cm−1 are readily assigned to the H2NCO radical, according to comparison with lines reported in a Xe matrix32 and those predicted from computation (Table S4). These lines appeared upon irradiation; their intensities decreased continuously in darkness. Lines in a second group with intense ones observed at 2264.0/2263.1 cm−1 are readily assigned to HNCO after comparison with those reported in Ar and Xe matrices32,36 and those predicted quantum-chemically (Table S5). These lines appeared upon irradiation; their intensities increased significantly after 6 h in darkness, and increased slightly after further 4 h in darkness. Observation of both H2NCO and HNCO upon UV/IR irradiation of the Cl2/H2NCHO/p-H2 matrix indicates that the following reactions occurred after hydrogen atoms were generated:

H atom tunneling reactions were reported to be permeable at 11 K.31 Furthermore, the formation of H2NCO from H + HNCO was observed at 45 K in an UV-irradiated HNCO/Xe matrix.32 H-abstraction of HNCO to form NCO + H2 (Figure S1b) is unlikely to occur because it was predicted to be endothermic with a barrier ∼274 kJ mol−1.28 Predicted vibrational wavenumbers and IR intensities of H2NCHO, H2NCO, and HNCO are listed in Tables S3−S5. The corresponding PES’s for the reactions of H + D2NCHO or D2NCO or DNCO are presented in Figure S2. For the formation of DNCHO and HDNCO, two isomers are possible. Predicted vibrational wavenumbers and IR intensities of D2NCHO, DNCO, D2NCO, (Z)- and (E)-HDNCO, and cis- and trans-HDNCHO are listed in Tables S6−S12. Spectral Studies. The experiments were conducted in solid p-H2 at 3.3 K. Although this environment is atypical of the surface of interstellar grains, it is an ideal media to understand the reactions of hydrogen atoms at low temperature. To investigate the reaction of formamide (H2NCHO) with H atoms, we deposited a gaseous mixture of H2NCHO/ Cl2/p-H2 (0.2/8/10 000) at 3.3 K for 7 h; a representative IR spectrum is presented in Figure 2a. IR spectra covering other spectral regions are shown in Figure S3.

H + H 2NCHO → H 2 + H 2NCO

(1)

H + H 2NCO → H 2 + H 2NCO

(2)

According to computations, the H-abstraction channels is the most favored path for H + H2NCHO but has been overlooked previously in reactions involving hydrogen atoms in either experiments or modeling; typically only hydrogen addition reactions were considered. We performed several experiments with varied ratios of [H]0/[H2NCHO]0. We monitored the variations of mixing ratios of H2NCHO, H2NCO, and HNCO after UV and IR irradiation over 10 at 0.5 h intervals. With the p-H2 matrix at 3.3 K and in darkness, we expect that only reactions involving H atoms can take place. Two representative temporal profiles of the mixing ratios of H2NCHO, H2NCO, and HNCO are presented in Figure 3; trace a is from an experiment of [H]0/ [H2NCHO]0 ≈ 12.5 and trace b is from [H]0/[H2NCHO]0 ≥ 18. In trace a, H2NCHO decreased monotonically while the matrix was maintained in darkness, whereas H2NCO and HNCO increased and attained a plateau, likely because of the depletion of H atoms. Under H-rich conditions (trace b), the mixing ratio of H2NCHO decreased significantly upon UV/IR irradiation and that of HNCO increased significantly. The anticorrelation of the mixing ratios of HNCO and H2NCHO was clearly observed, indicating the close chemical relation under such conditions. The mixing ratio of H2NCO remained much smaller and decreased slightly over the period in darkness, behaving like a reaction intermediate. This behavior is consistent with theoretical computations indicating that reaction 1 has a barrier, whereas reaction 2 is barrierless and expected to be rapid so that H2NCO becomes an intermediate. Furthermore, the decrease of HNCO and increase of H2NCHO after 6 h in darkness indicate that further reactions of HNCO to form H2NCHO occurred. To clearly show that these H-additions to convert HNCO to H2NCHO indeed occurred, we performed experiments on partially deuterated formamide, D2NCHO. Representative IR spectra of the D2NCHO/Cl2/p-H2 (0.2/8/10 000) matrix after deposition, upon UV/IR irradiation, and after maintain-

Figure 2. Partial spectra of a H2NCHO/Cl2/p-H2 (0.2/8/10 000) matrix recorded at various stages of an experiment. (a) Absorption spectrum after deposition at 3.3 K for 7 h. (b) Difference spectrum of this matrix on irradiation with light at 365 nm for 20 min and full IR light for 25 min. (c) Difference spectrum of this matrix 6 h in darkness after irradiation ceased. (d) Difference spectrum of this matrix 10 h in darkness.

The observed lines of H2NCHO with intense ones at 3556.9, 3433.9, 2861.5, 1742.2, and 1268.1 cm−1 agree with those reported in the gaseous phase,33 in argon34 and xenon32 matrices, and the scaled harmonic vibrational wavenumbers and relative IR intensities predicted quantum-chemically (Table S3). After the matrix was irradiated with light at 365 nm for 20 min to generate Cl atoms, followed by irradiation with unfiltered IR light for 25 min to produce hydrogen atoms from Cl + H2 (v = 1), the difference spectrum is presented in Figure 2b; the difference spectrum indicates the spectrum obtained on subtracting the spectra recorded before the experimental step from that recorded after the designated step. The intensities of lines of H2NCHO decreased; intense lines of HCl near 2894.1 11616


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are previously unreported, but they are unambiguously identified according to comparison with quantum-chemical computations (Tables S8−S12); the structures of the latter four are shown in Figure S5. The observation of these mixed isotopic species clearly indicates that the H-addition reactions occurred so that DNCO was converted to HDNCO and HDNCHO, to be discussed later. Representative temporal profiles of the mixing ratios of all observed species are presented in Figure S6. Under H-rich conditions (trace b), the mixing ratio of D2NCHO decreased significantly upon irradiation and that of DNCO significantly increased over that of D2NCO. The anticorrelation of the mixing ratios of DNCO and D2NCHO was also clearly observed. Furthermore, the mixing ratios of HDNCO and HDNCHO increased, indicating that these H-addition reactions occurred more readily under H-rich conditions. According to computations and previous reports, hydrogenaddition reactions of HNCO and H2NCO are expected to occur in experiments with H + HNCO in solid p-H2, H + HNCO → H 2NCO

(3)

H + H 2NCO → H 2NCHO

(4)

We are, however, unable to distinguish whether observed H2NCO in experiments with H + H2NCHO was produced from both reactions 1 and 3 or from only reaction 1. The experiments with H + D2NCHO provides clear information. After the initial H- and D-abstraction reactions (reactions 5 and 6), H-addition reactions to DNCO and, subsequently, to HDNCO are expected to occur (reactions 7 and 8).

Figure 3. Estimated mixing-ratio profiles of H2NCHO, H2NCO, and HNCO after UV/IR photolysis as a function of period in darkness. (a) [H]0/[H2NCHO]0 ≈ 12.5 with [H2NCHO]0 = 17 ppm. (b) [H]0/[H2NCHO]0 ≥ 18 with [H2NCHO]0 = 18 ppm.

ing in darkness are presented in Figure 4; IR spectra covering other spectral regions are shown in Figure S4. Lines of D2NCHO and DNCO were identified on comparison with those reported in matrices37,38 and predicted quantumchemically (Tables S6 and S7). Lines of D2NCO, (Z)HDNCO, (E)-HDNCO, trans-HDNCHO, and cis-HDNCHO

H + D2 NCHO → H 2 + D2 NCO

(5)

H + D2 NCO → HD + DNCO

(6)

H + DNCO → HDNCO

(7)

H + HDNCO → HDNCHO

(8)

The observation of intense lines of D2NCO and DNCO in experiments with D2NCHO indicates that H-abstraction dominates (reactions 5 and 6). The observation of continuously increasing weaker lines of HDNCO and HDNCHO clearly indicates that H-addition subsequently occurred (reactions 7 and 8). Our observations support a dual-cycle mechanism to connect H2NCHO, H2NCO, and HNCO, as presented in Figure 5. H-abstraction (reactions 1 and 2) removes one

Figure 4. Partial spectra of a D2NCHO/Cl2/p-H2 (0.2/8/10 000) matrix recorded at various stages of an experiment. (a) Absorption spectrum after deposition at 3.3 K for 7 h. (b) Difference spectrum of this matrix on irradiation with UV light at 365 nm for 20 min and full IR light for 25 min. (c) Difference spectrum of this matrix 6 h in darkness after irradiation ceased. (d) Difference spectrum of this matrix 10 h in darkness.

Figure 5. Dual-cyclic mechanism of H-abstraction and H-addition reactions connecting H2NCHO, H2NCO, and HNCO 11617


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recorded at each stage of experiments. To avoid the reaction of vibrationally excited H2 with produced radicals during data acquisition, light of wavenumber above 4000 cm−1 from the spectrometer was blocked with a filter (cutoff wavelength 2.4 μm). We added a small fraction of Cl2 to the sample and employed UV irradiation at 365 ± 6 nm from a light-emitting diode to produce Cl atoms; because of the diminished cage effect of p-H2, Cl atoms became isolated and stabilized in solid p-H2. The matrix sample was subsequently irradiated with IR light so that Cl reacted with vibrationally excited H2 to form HCl + H.35 The H atoms thus produced can move efficiently through the lattice on continuously breaking and formation of neighboring H−H bonds via quantum tunneling.41 This chemical diffusion makes H atoms very mobile; when an H atom is adjacent to a reactant (Cl or formamide or others), a reaction might occur. Applying this method, we have demonstrated the excellent efficiency in reactions of H with pyridine,42 quinoline,43 and HONO.44 The gaseous mixture of H2NCHO/p-H2/Cl2 (or D2NCHO/p-H2/ Cl2) in the range 0.2/5−10/10 000 was deposited at total flow rate ∼6.5 STP cm3 min−1 (STP indicates standard temperature 273 K and pressure 760 Torr) over a period of 5−7 h. Commercially available H2NCHO (Sigma-Aldrich, 99.5%) and D2NCHO (CDN Isotopes, 99% isotopic purity) were used without further purification except degassing. The vapor (∼0.1 Torr at 298 K) of H2NCHO was mixed with gaseous p-H2 before deposition. Gaseous Cl2 was codeposited from a separate line. The mixing ratios of H2NCHO, H2NCO, and HNCO were estimated according to the method of Ruzi and Anderson45 using the thickness of the matrix and IR intensities of some intense lines of these species computed at the CCSD(T)/ccpVTZ level of theory. Because of the large error in the predicted IR intensities, the error in estimated mixing ratios might be as large as a factor of 2, but their relative changes are reliable. The initial mixing ratio of H atoms, [H]0, were similarly estimated by the intensities of the coproduct HCl produced from the reaction Cl + H2 → HCl + H. p-H2 was prepared on passing n-H2 through an iron(III)-oxide catalyst cooled to ∼12.9 K, as described previously.30 Quantum-Chemical Computations. Quantum-chemical computations were performed using Gaussian 09 and CFOUR program packages.46,47 Geometry optimizations and harmonic vibrational analysis were performed using density-functional theory (DFT) with the B3LYP density-functional and coupled-cluster CCSD(T) methods. The standard Dunning correlation-consistent basis set with or without augmented diffuse functions, aug-cc-pVTZ and cc-pVTZ, were used in B3LYP and CCSD(T) methods, respectively. In the CCSD(T) computations the core-electrons were not correlated. Computed harmonic frequencies were scaled with a linear scaling factor, which was obtained on least-squares fitting of the unscaled harmonic wavenumbers with the observed wavenumbers of H2NCHO according to a linear relation y = a × x + b, in which y is the observed wavenumber and x is the computed harmonic vibrational wavenumber. Fitted parameters of a and b are listed in the captions of corresponding tables of the Supporting Information.

hydrogen atom from H2NCHO and H2NCO, respectively, to form H2NCO and HNCO, whereas H-addition (reactions 3 and 4) add one hydrogen atom to HNCO and H2NCO, respectively, to form H2NCO and H2NCHO. All these four reactions are the most feasible among all possible channels in each reaction. The observation that under rich-H conditions in experiments with D2NCHO the mixing ratios of HDNCO and HDNCHO were slightly enhanced is explicable by the fact that reaction 7, and similarly reaction 3, has a small barrier. Nevertheless, reaction 3 occurs at 3.3 K, likely via hydrogenatom tunneling, as supported by the observation of HDNCO via reaction 7 in the experiments with D2NCHO. The observation of HNCO, likely from D-abstraction of HDNCO, and the slight increase of its mixing ratio with time in experiments with H + D2NCHO also support the dualcycle mechanism. Under H-rich conditions in experiments with H2NCHO, we observed that the mixing ratio of HNCO attained a maximum, whereas that of NH2CHO attained a minimum and subsequently increased slowly after approximately 6 h in darkness. This effect is consistent also with the dual-cycle mechanism according to which reactions 3 and 4 became more important in the later reaction period.

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CONCLUSION In addition to the H-addition reactions, we observed the previously ignored H-abstraction reactions of H + H2NCHO. The dual-cycle mechanism including both hydrogen addition and hydrogen abstraction satisfactorily connects H2NCHO and HNCO through feasible chemical reactions so that the quasi-equilibrium between H2NCHO and HNCO becomes attainable. Because reaction 3 has a small barrier, one would expect that HNCO would be more abundant than H2NCHO under H-rich conditions, consistent with the astronomical observations.10 This rate-determining step and the barrierless reactions of H + H2NCO to form H2NCHO and HNCO also explain why H2NCO was unobserved in hydrogenation experiments of HNCO ∼ 10 K in the previous report.27 The mechanism depicted in Figure 5 also suggests an additional route for the formation of interstellar H2. As is generally accepted, H2 formation from H + H in the gaseous ISM is inefficient, and the formation of H2 on interstellar grains has been proposed and demonstrated to account for the production of interstellar H2.39,40 According to the mechanism presented in Figure 5, HNCO, H2NCO, and H2NCHO act as catalysts in the reaction of H + H → H2. Taking into account of this mechanism, and likely other potential and similar Haddition/H-abstraction catalytic cycles, in the astronomical model might increase the rate of formation of H2 in interstellar ice.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b04491. Geometries and Cartesian coordinates; comparison of observed vibrational wavenumbers and IR intensities; PES of possible channels of H + H2NCO and H + HNCO and of the H-addition and D-abstraction reactions of D2NCHO; spectra of a H2NCHO/Cl2/pH2 matrix and D2NCHO/Cl2/p-H2 matrix recorded at various stages of an experiment; structures of isomers of HDNCO and HDNCHO predicted with the B3LYP/ aug-cc-pVTZ method; estimated mixing-ratio profiles of D2NCHO, D2NCO, (Z)-HDNCO, (E)-HDNCO, transHDNCHO, cis-HDNCHO, DNCO and HNCO after

EXPERIMENTAL SECTION

Experimental Details. The p-H2 matrix-isolation system for IR absorption is described elsewhere.30 A gold-coated copper plate at 3.3 K served as both a substrate for the matrix sample and a mirror to reflect the IR beam to the detector. We employed infrared (IR) absorption spectra to characterize reaction products. Because p-H2 is soft and has weak interactions with the guest molecules, the IR lines of species isolated in solid p-H2 typically have narrow widths and small matrix shifts, allowing unambiguous identification of molecules, including their isomers. IR absorption spectra covering the spectral region of 550−4000 cm−1 were recorded with a Fourier-transform infrared (FTIR) spectrometer equipped with a KBr beam splitter and an HgCdTe detector at 77 K. Typically 500 scans at resolution 0.25 cm−1 were 11618


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UV/IR photolysis as a function of period in darkness (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.A.P.). *E-mail: [email protected] (G.T.). *E-mail: [email protected] (Y.-P.L.). ORCID

Karolina A. Haupa: 0000-0002-6604-4730 György Tarczay: 0000-0002-2345-1774 Yuan-Pern Lee: 0000-0001-6418-7378 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Ministry of Science and Technology, Taiwan (grants MOST106-2745-M009-001-ASP and MOST107-3017-F009-003) and Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. The National Center for High-Performance Computation provided computer time. G.T. gratefully acknowledges support from the Lendület Program of the Hungarian Academy of Sciences.

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