Photo-Induced Reactions between Glyoxal and Hydroxylamine in Cryogenic Matrices

In this paper, the photochemistry of glyoxal–hydroxylamine (Gly–HA) complexes is studied using FTIR matrix isolation spectroscopy and ab initio calculations. The irradiation of the Gly–HA complexes with the filtered output of a mercury lamp (λ > 370 nm) leads to their photoconversion to hydroxyketene–hydroxylamine complexes and the formation of hydroxy(hydroxyamino)acetaldehyde with a hemiaminal structure. The first product is the result of a double hydrogen exchange reaction between the aldehyde group of Gly and the amino or hydroxyl group of HA. The second product is formed as a result of the addition of the nitrogen atom of HA to the carbon atom of one aldehyde group of Gly, followed by the migration of the hydrogen atom from the amino group of hydroxylamine to the oxygen atom of the carbonyl group of glyoxal. The identification of the products is confirmed by deuterium substitution and by MP2 calculations of the structures and vibrational spectra of the identified species.


Introduction
Glyoxal, HCOHCO (Gly), the simplest α-dicarbonyl, plays a significant role in atmospheric photochemistry. It is formed in the atmosphere as a product of the oxidation of volatile organic compounds (VOCs) [1], and the direct photolysis of glyoxal is its main removal route from the atmosphere [2,3]. The gas-phase photodissociation of glyoxal involves several fragmentation channels, whose relative contributions depend on the excitation wavelength and other factors such as the occurrence of intermolecular collisions [4][5][6][7][8][9][10]. Moreover, glyoxal is also absorbed by cloud droplets, where it participates in complex photochemical transformations [11][12][13][14][15].Our group has reported systematic study of the photochemical behavior of molecular complexes of glyoxal with atmospherically relevant molecules such as water, methanol and hydrogen peroxide isolated in cryogenic matrices [16][17][18]. For the glyoxal-methanol complex, a very interesting photochemical behavior was observed, namely, under irradiation in the visible range, glyoxal complexed with methanol isomerized to the rather exotic hydroxyketene molecule, H(OH)C=C=O (Equation (1)) [18].
This reaction only occurred for the complex with methanol. The complex of glyoxal with H 2 O 2 shows a different photochemical behavior, in which the hydrogen peroxide molecule undergoes addition to glyoxal [17]. In contrast, the complex of glyoxal with water does not undergo a photochemical isomerization reaction [16]. The complex of methanol with glyoxal derivative, methylglyoxal, shows similar behavior to the glyoxal-methanol complex [19]. Irradiation of this complex leads to the formation of methylhydroxyketene. The UV photochemistry of hydroxylamine has not yet be Gericke et al. report that the end product of the photolysis of hyd nm is H2 + HNO [32]. The main dissociation channel leads to H + H efficiency of 1.7 for hydrogen atoms. According to Luckhauset irradiation of hydroxylamine at 240 nm produces NH2 and OH vibrational ground state) [33]. The photolysis of glyoxal monom an argon matrix leads to the formation of formaldehyde and carb are excited in the S2←S0 absorption region (320 nm > λ > 260 nm were observed under excitation in the S1←S0 absorption region ( study concerning the photolysis of glyoxal in argon and nitroge only trace amounts of formaldehyde and carbon monoxide we was irradiated with a wavelength of λ ≥ 370 nm. Irradiation w medium-pressure mercury lamp led to the appearance of a smal dioxide in addition to HCHO and CO. The exposure of the NH2O λ ≥ 370 nm radiation led to the appearance of trace amounts of ni as a result of hydroxylamine degradation, but no HNO was dete As can be seen in Figure 2, the exposure of Gly/HA/Ar(N2) nm led to a decrease in bands due to the glyoxal-hydroxy simultaneously a set of new bands appeared. The new band matrices were additionally irradiated with the full output of a me lamp. The bands that appeared after irradiation were separated and 2, based on their behavior in all the performed experiments. bands assigned to groups 1a, 1b and 2 are listed in Table 1 and T The UV photochemistry of hydroxylamine has not yet been studied in matrices. Gericke et al. report that the end product of the photolysis of hydroxylamine vapor at 193 nm is H 2 + HNO [32]. The main dissociation channel leads to H + H + HNO with a quantum efficiency of 1.7 for hydrogen atoms. According to Luckhauset et al., the singlephoton irradiation of hydroxylamine at 240 nm produces NH 2 and OH radicals (mostly in their vibrational ground state) [33]. The photolysis of glyoxal monomers and "cage" dimers in an argon matrix leads to the formation of formaldehyde and carbon monoxide when they are excited in the S 2 ← S 0 absorption region (320 nm > λ > 260 nm) [34]. No photoproducts were observed under excitation in the S 1 ← S 0 absorption region (λ~445 nm). Our present study concerning the photolysis of glyoxal in argon and nitrogen matrices showed that only trace amounts of formaldehyde and carbon monoxide were formed when glyoxal was irradiated with a wavelength of λ ≥ 370 nm. Irradiation with the full output of a medium-pressure mercury lamp led to the appearance of a small concentration of carbon dioxide in addition to HCHO and CO. The exposure of the NH 2 OH/Ar(N 2 ) matrices to the λ ≥ 370 nm radiation led to the appearance of trace amounts of nitrogen oxide dimers [35] as a result of hydroxylamine degradation, but no HNO was detected [36].
As can be seen in Figure 2, the exposure of Gly/HA/Ar(N 2 ) to irradiation at λ ≥ 370 nm led to a decrease in bands due to the glyoxal-hydroxylamine complexes, and simultaneously a set of new bands appeared. The new bands diminished when the matrices were additionally irradiated with the full output of a medium-pressure mercury lamp. The bands that appeared after irradiation were separated into three groups, 1a, 1b and 2, based on their behavior in all the performed experiments. The wavenumbers of the bands assigned to groups 1a, 1b and 2 are listed in Tables 1 and 2.

Formation of Hydroxyketene-Hydroxylamine Complexes
The sets of bands belonging to groups 1a and 1b (see Figure 2) involve a very intense band in the 2140-2090 cm −1 region that is characteristic of a ketene group, which suggests that the HCOHCO· · · NH 2 OH complex undergoes a similar photoconversion reaction to that of HCOHCO· · · CH 3 OH, during which double hydrogen transfer occurs and then a hydroxyketene-hydroxylamine complex is formed (Equation (2)).
For the H(OH)C=C=O· · · CH 3 OH complex, the ν as (C=C=O) band was observed at ca. 2105 cm −1 .
Three new bands assigned to groups 1a and 1b appear in the ν(OH) region, one of which has a close wavenumber to the νOH of HK in the H(OH)C=C=O· · · CH 3 OH complex (ca. 3400 cm −1 ) [18]. The other absorptions occur in the vicinity of δNOH, ωNH 2 of HA and δCH + ν s CCO(νCO) vibrations of HK (see Table 1). These spectroscopic data support the conclusion that during the irradiation of the HCOHCO· · · NH 2 OH complexes, their photoconversion to H(OH)C=C=O· · · NH 2 OH complexes (HK-HA) takes place.     The bands assigned to groups 1a and 1b are attributed to two different types of HK-HA complexes as discussed below. Bands of group 1a are only identified in an argon matrix, while bands of group 1b are observed in both solid argon and nitrogen.
The structures of HK-HA complexes of 1:1 stoichiometry were optimized by the MP2/6-311++G(2d,2p) method. The calculations resulted in twelve stationary points (I HKH -XII HKH ), whose structures and ∆E CP (ZPE) binding energies are shown in Figure S2 in the Supplementary Materials. The six most stable structures, I HKH -VI HKH , are also presented in Figure 3. The geometrical parameters for these structures are given in Table  S2 in the Supplementary Materials. The three most stable structures, I HKH -III HKH (with similar binding energies from −26.16 to −24.10 kJ mol −1 ), are stabilized by the OH . . . N hydrogen bond between the hydroxyl group of HK and the N atom of the amino group of HA. Structures II HKH and III HKH are additionally stabilized by OH . . . O interactions, where the hydroxylamine OH group serves as a proton donor and the O atom of the hydroxyl group of hydroxyketene serves as a proton acceptor. The next three structures, IV HKH -VI HKH , are stabilized by the OH . . . O hydrogen bond that occurs between the hydroxyl group of HK and the oxygen atom of the hydroxyl group of HA. Complex V HKH is additionally stabilized by the NH . . . O interaction of the amino group of HA and the hydroxyl group of HK. Structures IV HKH -VI HKH are predicted to be about 6-7 kJ mol −1 less stable than structures I HKH -III HKH .
In Tables S3-S5 in the Supplementary Materials, the harmonic and anharmonic wavenumbers predicted at the MP2/6-311++G(2d,2p) level for HA and HK monomers and HA-HK complexes (I HKH -VI HKH ) are listed. In Table 1, the experimental wavenumbers observed for the photoproducts 1a and 1b are compared with their corresponding calculated wavenumbers of structures I HKH and IV HKH , respectively. Because the wavenumbers and intensities of the vibrational bands of complexes I HKH -III HKH are quite similar (see Table S5 in the Supplementary Materials), in Table 1 we only present the data calculated for the most stable structure in the group, I HKH . The same applies for complexes IV HKH -VI HKH , as in Table 1 we only present the data for the structure IV HKH . Comparison of the experimental and calculated wavenumbers suggests that the bands of group 1a are due to HA-HK complexes stabilized mainly by an OH· · · N bond and represented by structure I HKH , while the bands assigned to group 1b correspond to complexes stabilized by an OH· · · O bond and represented by structure IV HKH . The bands of the ν(OH) and τ(OH) vibrations provide the most information on the structure of the complexes. For complex I HKH , anharmonic MP2 calculations predict intense bands of ν(OH) at 3533 cm −1 (calculated intensity I = 365 km mol −1 ) originating from disturbed hydroxylamine as well as ν(OH) at 3260 cm −1 (I = 697 km mol −1 ) and τ(OH) at 752 cm −1 (I = 119 km mol −1 ) due to perturbed hydroxyketene. In the spectra of the argon matrices, we observed two bands of group 1a at 3537.2 cm −1 and 792.2 cm −1 corresponding to the disturbed ν(OH) vibration of HA and τ(OH) vibration of HK, respectively. The ν(OH) absorption of disturbed HK was not identified. This is most probably due to the fact that the absorption is broad and diffuse, which is a commonly observed feature for the ν(OH) stretch of the relatively strong O-H· · · O or O-H· · · N bonds [37]. Moreover, possible overlapping of this band with the ν(OH) absorption of the HA dimer that occurs in this region [38] makes identification of the band even more difficult. Two additional bands are identified for I HKH at 1339.6 and 1153.7 cm −1 . The first one is attributed to the δCH + ν s CCO mode of HK and the second one to the perturbed ωNH 2 mode of HA, in accordance with the calculations (see Table 1). The calculations predict that for complex IV HKH , the ν(OH) bands due to disturbed HA and HK are located at 3667 cm −1 (I = 70 km mol −1 ) and 3488 cm −1 (I = 548 km mol −1 ), respectively. These bands are identified at 3608.0, 3607.1 cm −1 (HA) and 3400.0, 3394.5 cm −1 (HK) in the spectra of the argon and nitrogen matrices, respectively. The ν(OH) band of HK in IV HKH is observed at a very similar wavenumber to the corresponding band of the HK-Me complex (3395.7 cm −1 ) [18], which confirms that group 1b belongs to a complex in which HK plays the role of proton donor toward the oxygen atom of the OH group of HA, forming an O-H· · · O bond (which is the case for structures IV HKH , V HKH and VI HKH ).  In Tables S3-S5 in the Supplementary Materials, the wavenumbers predicted at the MP2/6-311++G(2d,2p) level for HA-HK complexes (IHKH-VIHKH) are listed. In Table 1, the observed for the photoproducts 1a and 1b are compared calculated wavenumbers of structures IHKH and IVHKH, wavenumbers and intensities of the vibrational bands of com similar (see Table S5 in the Supplementary Materials), in Tab The shape of the very broad band of the ν as C=C=O vibration of HK with few subpeaks (or folds) on the band (see Figure 4) suggests that different structures of the HA-HK complex with similar wavenumbers of the ν as C=C=O mode are probably isolated in the argon and nitrogen matrices. This band is broader in the Ar than in the N 2 matrix, which is in accordance with the fact that in solid argon all six structures I HKH -VI HKH of HA-HK may contribute to its broadness whereas in solid nitrogen it is only the three IV HKH -VI HKH . The MP2 calculations predict that the position of this band in each particular structure differs by a few wavenumbers (from 2114 cm −1 to 2123 cm −1 , see Table S5 in the Supplementary Materials). may contribute to its broadness whereas in solid nitrogen it is The MP2 calculations predict that the position of this band differs by a few wavenumbers (from 2114 cm −1 to 2123 Supplementary Materials). To obtain some information about the mechanism of pho HK-HA, we performed experiments with deuterated hydroxy isotopic enrichment was ca. 90% as estimated from the m predicted intensities of the bands due to the ν(OH) and ν(OD the matrices doped with Gly and HAd indicate that the com solid argon and nitrogen have the same structures as those o hydroxylamine. The spectra are presented in Figure S1, an bands of Gly-HAd complexes are listed in Table S1 in the Sup irradiation (λ > 370 nm) of the Gly/HAd/Ar(N2) matrices leads to the Gly-HAd complexes and due to the formation o photoproducts. Like in the experiments with non-deuterate bands of photoproducts can be separated into three groups: 1 spectra presented in Figure 5 show the bands attributed to all 1a and 1b bands evidence the formation of the HKd-HAd co with hydroxylamine, with structures analogical to those obser the non-deuterated HA (IHKH and IVHKH). The wavenumbe assigned for the HKd-HAd complexes are listed in Table 3. To obtain some information about the mechanism of photoconversion of Gly-HA to HK-HA, we performed experiments with deuterated hydroxylamine, ND 2 OD (HA d ). The isotopic enrichment was ca. 90% as estimated from the measured and theoretically predicted intensities of the bands due to the ν(OH) and ν(OD) vibrations. The spectra of the matrices doped with Gly and HA d indicate that the complexes Gly-HA d formed in solid argon and nitrogen have the same structures as those observed for non-deuterated hydroxylamine. The spectra are presented in Figure S1, and the wavenumbers of the bands of Gly-HA d complexes are listed in Table S1 in the Supplementary Materials. The irradiation (λ > 370 nm) of the Gly/HA d /Ar(N 2 ) matrices leads to a decrease in bands due to the Gly-HA d complexes and due to the formation of the new bands of the photoproducts. Like in the experiments with non-deuterated Gly-HA complexes, the bands of photoproducts can be separated into three groups: 1a, 1b and 2. The difference spectra presented in Figure 5 show the bands attributed to all three groups. The identified 1a and 1b bands evidence the formation of the HK d -HA d complexes of hydroxyketene with hydroxylamine, with structures analogical to those observed in the experiment with the non-deuterated HA (I HKH and IV HKH ). The wavenumbers of the identified bands assigned for the HK d -HA d complexes are listed in Table 3.
The spectra recorded after irradiation of the matrices with deuterated hydroxylamine involve numerous bands due to a number of species. The HK d -HA d complexes are formed with relatively low yield, and the identification of their absorptions is quite a difficult task. However, we were able to unequivocally identify in the photolyzed CHOCHO/ND 2 OD/Ar matrices four bands corresponding to group 1a (3536.1, 2614.  Table 3. The spectra recorded after irradiation of the matrices with deuterated hydroxylamine involve numerous bands due to a number of species. The HKd-HAd complexes are formed with relatively low yield, and the identification of their absorptions is quite a difficult task. However, we were able to unequivocally identify in the photolyzed CHOCHO/ND2OD/Ar matrices four bands corresponding to group 1a (3536.1, 2614.  Table 3. One may expect that the hydrogen and deuterium exchange between glyoxal and deuterated hydroxylamine in the CHOCHO···ND2OD complexes (both non-planar, IGHd, and planar, IIGHd, ones) would lead to the formation of H(OD)CCO···ND2OH and H(OD)CCO···NHDOD characterized by similar structures to the non-deuterated complexes IHKH and IVHKH. The additional subscript d4 or d2 is applied to mark whether the complex formed involves ND2OH, IHKH-d4, IVHKH-d4 or NHDOD, IHKH-d2, IVHKH-d2 (i.e., whether in the exchange process the OD or ND2 group of hydroxylamine participates). Among the four identified bands for group 1a in solid argon, the most informative are those observed at 3536.1 and 2614.2 cm −1 . The first one appears in the OH stretching region of hydroxylamine and shows very good agreement with the wavenumber calculated for complex IHKH-d4 (3533 cm −1 ), which provides strong evidence that in the exchange reaction with the hydrogen atom of CH, the deuterium of the OD group of hydroxylamine One may expect that the hydrogen and deuterium exchange between glyoxal and deuterated hydroxylamine in the CHOCHO· · · ND 2 OD complexes (both non-planar, I GHd , and planar, II GHd , ones) would lead to the formation of H(OD)CCO· · · ND 2 OH and H(OD)CCO· · · NHDOD characterized by similar structures to the non-deuterated complexes I HKH and IV HKH . The additional subscript d 4 or d 2 is applied to mark whether the complex formed involves ND 2 OH, I HKH-d4 , IV HKH-d4 or NHDOD, I HKH-d2 , IV HKH-d2 (i.e., whether in the exchange process the OD or ND 2 group of hydroxylamine participates). Among the four identified bands for group 1a in solid argon, the most informative are those observed at 3536.1 and 2614.2 cm −1 . The first one appears in the OH stretching region of hydroxylamine and shows very good agreement with the wavenumber calculated for complex I HKH-d4 (3533 cm −1 ), which provides strong evidence that in the exchange reaction with the hydrogen atom of CH, the deuterium of the OD group of hydroxylamine participates, leading to the formation of H(OD)CCO· · · ND 2 OH, complex I HKH-d4 . In turn, the wavenumber of the identified 2614.2 cm −1 band has practically the same value as that calculated for the OD stretch of NHDOD in complex I HKH-d2 (2615 cm −1 ). The appearance of this band indicates that one of the deuterium atoms of the ND 2 group participates in the exchange reaction and the H(OD)CCO· · · NHDOD complex I HKHd-2 is formed. The broad, intense ν as C=C=O absorption at ca. 2113 cm −1 probably involves the corresponding band for both complex I HKH-d2 and complex I HKH-d4 . The band observed at 1000 cm −1 is attributed to the δCOD vibration of I HKH-d4 in accordance with calculations, and the corresponding band is also observed for the 1b complex. Table 3. Comparison of the observed and calculated anharmonic wavenumbers (cm −1 ) for the H(OD)CCO-ND 2 OH, H(OD)CCO-NHDOD complexes. Numerous bands were identified for the 1b complexes in both solid argon and nitrogen, and they have close wavenumbers in the spectra of the two matrices. Comparison of the identified wavenumbers with the calculated ones for various types of deuterated complexes shows good agreement with the wavenumbers calculated for complex IV HKH-d2 (Table 3). Strong evidence for IV HKH-d2 formation is provided by the band observed at 2661.4, 2664.5 cm −1 in the spectra of solid argon and nitrogen, respectively, which is due to the νOD vibration of perturbed hydroxylamine. Its wavenumber is ca. 45 cm −1 higher than the ν(OD) of complex I HKH-d2 (2614.2 cm −1 ), as expected when the OD group of HA d serves as a proton acceptor. The appearance of this band also shows that in the process of exchange with the H atom of glyoxal, one of the deuterium atoms of the ND 2 group participates. The bands identified at 3349.6, 1155.6 cm −1 (solid nitrogen) and at 1153.6 cm −1 (solid argon) are attributed to the νNH and γNHD vibrations according to calculations and are direct evidence for the formation of NHDOD. The other four bands identified for complex IV HKH-d2 are attributed to perturbed HK d vibrations (see Table 3).

H(OD)CCO-NHDOD/ H(OD)CCO-ND 2 OH H(OD)CCO-NHDOD
As discussed above, the experiments with ND 2 OD prove that the irradiation of CHOCHO· · · ND 2 OD leads to the formation of H(OD)CCO· · · NHDOD and H(OD)CCO· · · ND 2 OH complexes as bands due to perturbed NHDOD and ND 2 OH molecules can be clearly identified in the spectra. This fact demonstrates that in the hydrogen and deuterium exchange reaction between glyoxal and hydroxylamine, the deuterium atom of the OD group or one of the deuterium atoms of the ND 2 group participates: (i) CHOCHO· · · ND 2 OD → H(OD)CCO· · · ND 2 OH or (ii) CHOCHO· · · ND 2 OD → H(OD)CCO· · · NHDOD. In the argon matrix, three complexes are probably formed, namely I HKH-d2 , I HKH-d4 and IV HKH-d2 , but in the nitrogen matrix only the latter. Whereas the formation of IV HKH-d2 is very well documented by a number of bands identified for this complex, the presence of I HKH-d2 and I HKH-d4 is evidenced by one band characteristic of the particular complex (νOD HA d2 at 2614.2 cm −1 for I HKH-d2 and νOH HA d3 at 3536.1 cm −1 for I HKH-d4 ). The other two identified bands of group 1a (2113, 1000 cm −1 ) attributed to ν as C=C=O and δCOD vibrations may be due to both complex I HKH-d2 and complex I HKH-d4 .
The obtained data show that the non-planar complexes of glyoxal with hydroxylamine, Gly-HA d , I GH (stabilized by the OH· · · O bond), that are formed in the argon matrices may photoconvert to the I HKH-d2 , I HKH-d4 and IV HKH-d2 HK d -HA d complexes of hydroxyketene with hydroxylamine. In the nitrogen matrices, both the non-planar, I GH , and planar, II GH (main product), Gly-HA d complexes are present (the latter stabilized by OH . . . O and CH . . . N bonds), and after conversion the IV HKH-d2 HK d -HA d complexes are formed. These data suggest that irradiation of the non-planar Gly-HA, HA d complexes stabilized by the OH . . . O hydrogen bond, I GH , leads to double hydrogen exchange (or hydrogen and deuterium exchange) between glyoxal and hydroxylamine in which the hydroxyl or the amino group of HA, HA d may be involved. The irradiation of the planar complex, II GH , stabilized by OH . . . O and CH . . . N hydrogen bonds, proceeds by double hydrogen exchange between the CH group of glyoxal and the amino group of HA only.
In the case of the CHOCHO-CH 3 OD complex, the photoconversion resulted in an exchange of proton and deuterium between the CH group of glyoxal and the OD group of methanol, and finally the H(OD)CCO-CH 3 OH complex was formed. According to the coupled-cluster calculations, the non-hydrogen-bonded (non-planar) glyoxal-methanol complex photoconverted initially to a planar hydrogen-bonded complex, which then converted to the final photoproduct (hydroxyketene-methanol complex) [18,20].

Formation of Hydroxy(hydroxyamino)acetaldehyde (Hemiaminal)
Irradiation of Gly/HA/Ar(N 2 ) matrices with a wavelength of λ ≥ 370 nm also leads to the appearance of the group of bands labeled as group 2 (see Figure 2 and Table 4). This group is assigned to different conformers of the product of the addition of hydroxylamine to one of the CHO groups of glyoxal, namely, hydroxy(hydroxyamino)acetaldehyde, HHA. The formation of HHA proceeds via the addition of the nitrogen atom of the amino group of hydroxylamine to the carbon atom of the carbonyl group of glyoxal and the subsequent migration of the hydrogen atom (from the NH 2 group of HA) to the oxygen atom of the carbonyl group of glyoxal, as presented in Scheme 1. This molecule is formed readily in an argon matrix, but we only observe trace amounts of this photoproduct in a nitrogen matrix. The molecule has not been characterized so far, and its infrared spectrum is unknown. The three most stable structures of this compound predicted at the MP2 level are presented in Figure 6. All forms and their geometrical parameters are given in the Supplementary Materials ( Figure S3 and Table S7). Table 4.
Comparison of the observed and calculated anharmonic wavenumbers for CHOCH(OD)NDOD.   (43) a the wavenumber of the most intense line is in bold; b the MP2-calculated harmonic intensities (km mol −1 ) are given in parentheses.

CHOCH(OD)NDOD
Tables S8 and S9 in the Supplementary Materials show the harmonic and anharmonic wavenumbers and band intensities predicted at the MP2/6-311++G(2d,2p) level and the potential energy distribution (PED) for the two most stable forms of HHA. In Table S10, the calculated wavenumbers and band intensities of IHHA-VIHHA structures are given for comparison. In Table 2, the experimental wavenumbers observed for photoproduct 2 are compared with the calculated wavenumbers (and intensities) of the bands predicted for the three most stable conformers of HHA (IHHA-IIIHHA).
The appearance of several bands in the νC=O region and other distinct spectral regions indicates that more than one form of HHA is trapped in the argon matrix. The data suggest that one form probably dominates, and another few are isolated in smaller amounts. In the νOH region, we only observe two bands assigned to this group located at 3623.8 and 3558.0 cm −1 , which, according to the calculations, most suit the IIHHA form. Other identified bands in group 2 also mostly agree with the wavenumbers calculated for the conformer IIHHA. However, we are not able to unequivocally assign the experimental bands to a specific structure because all the structures have similar vibrational characteristics and the amounts of the photoproducts in the matrices are small. Furthermore, the result of the experiment using deuterated hydroxylamine does not give clear evidence as to which conformers of HHA are formed in the matrices. The results of this experiment are presented in Figure 5 and Table 2 and in Tables S8, S9 and S11 in the Supplementary Materials.
A photoaddition reaction was also observed as a result of the irradiation (at λ > 345 nm) of the glyoxal-hydrogen peroxide complex in an argon matrix [17], in which 2hydroxy-2-hydroperoxyethanal was formed. The formation of this photoproduct proceeds via the addition of one of the oxygen atoms of hydrogen peroxide to the carbon atom of the carbonyl group of glyoxal and the subsequent migration of the hydrogen atom (from H2O2) to the oxygen atom of the carbonyl group of glyoxal. Scheme 1. Schematic reaction path for the formation of CHOCH(OH)NHOH, HHA, from the CHOCHO-NH2OH complex.

Experimental and Computational Methods
Gaseous hydroxylamine (NH2OH, HA) was prepared from hydroxylamine phosphate salt (95%, Fluka) by heating the salt powder (at 50-65 °C) directly in the Scheme 1. Schematic reaction path for the formation of CHOCH(OH)NHOH, HHA, from the CHOCHO-NH 2 OH complex. subsequent migration of the hydrogen atom (from the NH2 group of HA) to the oxygen atom of the carbonyl group of glyoxal, as presented in Scheme 1. This molecule is formed readily in an argon matrix, but we only observe trace amounts of this photoproduct in a nitrogen matrix. The molecule has not been characterized so far, and its infrared spectrum is unknown. The three most stable structures of this compound predicted at the MP2 level are presented in Figure 6. All forms and their geometrical parameters are given in the Supplementary Materials ( Figure S3 and Table S7).  Tables S8 and S9 in the Supplementary Materials show the harmonic and anharmonic wavenumbers and band intensities predicted at the MP2/6-311++G(2d,2p) level and the potential energy distribution (PED) for the two most stable forms of HHA. In Table S10, the calculated wavenumbers and band intensities of I HHA -VI HHA structures are given for comparison. In Table 2, the experimental wavenumbers observed for photoproduct 2 are compared with the calculated wavenumbers (and intensities) of the bands predicted for the three most stable conformers of HHA (I HHA -III HHA ).
The appearance of several bands in the νC=O region and other distinct spectral regions indicates that more than one form of HHA is trapped in the argon matrix. The data suggest that one form probably dominates, and another few are isolated in smaller amounts. In the νOH region, we only observe two bands assigned to this group located at 3623.8 and 3558.0 cm −1 , which, according to the calculations, most suit the II HHA form. Other identified bands in group 2 also mostly agree with the wavenumbers calculated for the conformer II HHA . However, we are not able to unequivocally assign the experimental bands to a specific structure because all the structures have similar vibrational characteristics and the amounts of the photoproducts in the matrices are small. Furthermore, the result of the experiment using deuterated hydroxylamine does not give clear evidence as to which conformers of HHA are formed in the matrices. The results of this experiment are presented in Figure 5 and Table 2 and in Tables S8, S9 and S11 in the Supplementary Materials.
A photoaddition reaction was also observed as a result of the irradiation (at λ > 345 nm) of the glyoxal-hydrogen peroxide complex in an argon matrix [17], in which 2-hydroxy-2-hydroperoxyethanal was formed. The formation of this photoproduct proceeds via the addition of one of the oxygen atoms of hydrogen peroxide to the carbon atom of the carbonyl group of glyoxal and the subsequent migration of the hydrogen atom (from H 2 O 2 ) to the oxygen atom of the carbonyl group of glyoxal.

Experimental and Computational Methods
Gaseous hydroxylamine (NH 2 OH, HA) was prepared from hydroxylamine phosphate salt (95%, Fluka) by heating the salt powder (at 50-65 • C) directly in the deposition line. Deuterated hydroxylamine (ND 2 OD, HA d ) was prepared by heating hydroxylamine phosphate salt in deuterated water (D 2 O) solution and then evaporating off the water under a vacuum. This procedure was repeated several times until the deuteration degree was about 90%. Monomeric glyoxal (CHOCHO, Gly) was obtained from solid trimer dihydrate (98%, Sigma) topped with phosphorus pentoxide (P 4 O 10 ) powder by heating the materials to 120 • C under a vacuum and then collecting the gaseous glyoxal in a liquid nitrogen trap.
The glyoxal/hydroxylamine/argon (or nitrogen) matrices were prepared by simultaneous deposition of CHOCHO/Ar(N 2 ) and NH 2 OH vapor on a cold gold mirror kept at 11-17 K by a closed cycle helium refrigerator (Air Products, Displex 202A). The CHOCHO/Ar(N 2 ) matrix concentration was varied in the range of 1/200-1/2000. The absolute concentration of hydroxylamine in the matrices could not be determined, but its concentration was varied by changing the rare gas flow rate and the heating temperature of the hydroxylamine salt. Infrared spectra (4000-600 cm −1 ) with a resolution of 0.5 cm −1 were recorded in reflection mode using a Bruker 113v FTIR spectrometer and a liquid N 2 -cooled MCT detector.
The samples deposited in matrices were subjected to the radiation of a 200 W mediumpressure mercury lamp (Philipps CS200W2). A 5 cm water filter (to reduce the infrared light) and a glass long-wavelength pass filter (WG370) were used.
The MP2 method with 6-311++G(2d,2p) basis set was used for geometry optimization of all the monomers and complex structures and calculation of harmonic and anharmonic vibrational spectra of the monomers, complexes and photoproducts [39,40]. Glyoxal under standard conditions exists in a trans form [34,41,42], so this conformer was considered exclusively in our study in terms of the nature of its interaction with hydroxylamine. Binding energies were corrected using the Boys-Bernardi full counterpoise procedure [43]. The calculations were performed using the Gaussian 03 program (version C.02) for geometry optimization and Gaussian 16 (version B.01) for frequency calculations [44,45]. The potential energy distribution (PED) of the normal modes was computed using the Gar2ped program [46].
The HK-HA complexes are formed in the exchange reaction of two hydrogens between two complex subunits, namely, the CH group of Gly and the NH or OH group of HA. The experiments with deuterated hydroxylamine, ND 2 OD, provide evidence that hydrogen atoms from both the OH and NH 2 groups of hydroxylamine may participate in the exchange process. The H(OH)CCO . . . NH 2 OH complexes exist in two forms in the matrices. The first type is stabilized by the OH . . . N interaction between the hydroxyl group of HK and the amino nitrogen of HA. In the second type, the OH . . . O hydrogen bond is formed between the hydroxyl group of HK and the oxygen atom of the hydroxyl group of HA. These two types of HK-HA complex are formed in an argon matrix in noticeable amounts, but in nitrogen only HK-HA complexes bonded by the OH . . . O interaction are created.
Hydroxy(hydroxyamino)acetaldehyde, HHA, belonging to the hemiaminals family, is formed by the addition of HA to the carbon atom of one aldehyde group of Gly and the subsequent migration of the hydrogen atom from the NH 2 group of HA to the oxygen atom of the carbonyl group of Gly. In an argon matrix, the HHA hemiaminal conformers are observed in noticeable amounts, whereas in a nitrogen matrix the yield of HHA is low. The difference in the yield of the products, both HK-HA complexes and the HHA adduct, in the argon and nitrogen matrices may relate to the fact that different types of glyoxal-hydroxylamine (Gly-HA) complexes exist in argon and nitrogen before irradiation.