Experimental FTIR-MI and Theoretical Studies of Isocyanic Acid Aggregates

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Experimental FTIR-MI and Theoretical Studies of Isocyanic Acid Aggregates © 2023 by the authors. Licensee MDPI, Basel, Switzerland. Published version Krupa, Justyna; Wierzejewska, Maria; Lundell, Jan Krupa, J., Wierzejewska, M., & Lundell, J. (2023). Experimental FTIR-MI and Theoretical Studies of Isocyanic Acid Aggregates. Molecules, 28(3), Article 1430. https://doi.org/10.3390/molecules28031430 2023 Citation: Krupa, J.; Wierzejewska, M.; Lundell, J. Experimental FTIR-MI and Theoretical Studies of Isocyanic Acid Aggregates. Molecules 2023,28, 1430. https://doi.org/10.3390/ molecules28031430 Academic Editors: Weiwei Han, Lihong Hu and Juexin Wang Received: 6 January 2023 Revised: 29 January 2023 Accepted: 31 January 2023 Published: 2 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). molecules Article Experimental FTIR-MI and Theoretical Studies of Isocyanic Acid Aggregates Justyna Krupa 1,* , Maria Wierzejewska 1and Jan Lundell 2,* 1Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland 2Department of Chemistry, University of Jyväskylä, FI-40014 Jyväskylä, Finland *Correspondence: [email protected] (J.K.); [email protected] (J.L.); Tel.: +358-40-744-5270 (J.L.) Abstract: Homoaggregates of isocyanic acid (HNCO) were studied using FTIR spectroscopy combined with a low-temperature matrix isolation technique and quantum chemical calculations. Computationally, the structures of the HNCO dimers and trimers were optimized at the MP2, B3LYPD3 and B2PLYPD3 levels of theory employing the 6-311++G(3df,3pd) basis set. Topological analysis of the electron density (AIM) was used to identify the type of non-covalent interactions in the studied aggregates. Five stable minima were located on the potential energy surface for (HNCO) 2 , and nine were located on the potential energy surface for (HNCO) 3 . The most stable dimer (D1) involves a weak, almost linear N-H· · · N hydrogen bond. Other structures are bound by a N-H· · · O hydrogen bond or by O · · · C or N · · · N van der Waals interactions. Similar types of interactions as in (HNCO) 2 were found in the case of HNCO trimers. Among nine stable (HNCO) 3 structures, five represent cyclic forms. The most stable T1 trimer structure is characterized by a six-membered ring formed by three N-H · · · N hydrogen bonds and representing high symmetry (C 3h ). The analysis of the HNCO/Ar spectra after deposition indicates that the N-H · · · O hydrogen-bonded dimers are especially prevalent. Upon annealing, HNCO trimers were observed as well. Identification of the experimentally observed species relied on previous experimental data on HNCO complexes as well as computed data on HNCO homoaggregates’ vibrational spectra. Keywords: HNCO; hydrogen bond; Fourier transform infrared (FTIR); matrix isolation (MI); solid argon; vibrational spectroscopy; intermolecular interaction; computational chemistry; molecular complex; atmospheric chemistry 1. Introduction Isocyanic acid (HNCO) is an atmospheric pollutant emitted into the air during different processes such as fossil fuel combustion and biomass burning [ 1 – 5 ]. As a trace gas in ambient air, HNCO poses a health risk for humans since, when inhaled, the molecule participates in protein carbamylation reactions that lead to development of serious diseases [ 6 ]. In addition to research related to the chemistry of the atmosphere and the impact of HNCO on human health, properties and photochemical reactivity of isocyanic acid and its isomers have been extensively studied both theoretically and experimentally [ 7 – 25 ]. Four open-chain isomers were previously identified for CHNO species: fulminic acid (HCNO), isofulminic acid (HONC), cyanic acid (HOCN) and the most stable form, isocyanic acid (HNCO). Teles et al. [ 14 ] reported infrared spectra of all these four CHNO isomers together with their isotopomers isolated in low-temperature argon matrices. Theoretically, it has been shown by Shapley and Bacskay [ 23 ] that in addition to chain forms, several cyclic and branched CHNO structures of much higher energies are stable. Photolysis of HNCO has also been the subject of extensive studies both in argon and xenon matrices leading to isomerization and photodecomposition processes [8,17,18,26,27]. Although the properties of HNCO and its isomers are relatively well understood, much less data is available on molecular complexes of isocyanic acid. Such studies are Molecules 2023,28, 1430. https://doi.org/10.3390/molecules28031430 https://www.mdpi.com/journal/molecules Molecules 2023,28, 1430 2 of 14 useful to obtain information on how spectral properties and chemical reactivity of the molecule change upon intermolecular interactions. Raunier et al. [ 28 ] investigated the thermal reactivity of HNCO with water ice and the 1:1 HNCO complex with H 2 O isolated in an argon matrix using infrared spectroscopy. It was concluded, based on both experimental findings and the results of MP2/6-31G(d,p) calculations, that HNCO acts as a proton donor in the complex with H 2 O to form a N-H · · · O hydrogen bond. In turn, photo-induced decomposition of formohydroxamic acid (HCONHOH) isolated in solid argon led to the formation of two different forms of the 1:1 HNCO complex with water in which HNCO acted either as a proton donor or proton acceptor [ 29 ]. Molecular complexes of HNCO with NH 2 OH or CH 3 OH have been identified upon photodecomposition of N-hydroxyurea and acetohydroxamic acid, respectively [ 30 , 31 ]. Keresztes et al. [ 32 ] performed photolysis using 220 nm light on 1,2,5- and 1,3,4-oxadiazoles, and HCN · · · HNCO and HCN · · · HOCN complexes were produced upon photolysis. Subsequent photoreactions using excitation with a hydrogen lamp led to the formation of three complexes of isocyanic acid: HCNO · · · HNC, HNCO · · · HNC and HNC · · · HOCN. Two papers on HNCO complexes with dinitrogen and sulfur dioxide isolated in argon matrices have been published [ 33 , 34 ]. In both cases, 1:1 complexes exhibited either N-H · · · N or N-H · · · O hydrogen bonds. Along with them, complexes bound only by van der Waals interactions were detected. Very recently, Zhao et al. [35] reported computational studies on various hydrogen-bonded HNCO complexes, including the HNCO dimer, related to atmospheric chemistry. Non-covalent interactions are considered to have a significant influence on properties of chemical and biological systems. They also have an impact on atmospheric chemistry, affecting existing reactions and contributing to new reaction channels. Molecular complexes formed in dense environments at low temperature can affect chemistry of interstellar media and some planetary atmospheres. Here we report results of isocyanic acid aggregation in low-temperature argon matrices. The formation of dimers and trimers was experimentally followed by FTIR spectroscopy, and it was thereafter identified with the help of quantum chemical calculations. To our knowledge, such aggregates have not been the subject of experimental studies and could be of interest for better understanding of chemical reactivity of tropospheric gases, especially via intermolecular interactions of N-H-containing molecules. In our studies on HNCO aggregates, an interesting question arose as to which atoms in the acid are the best proton acceptor positions for hydrogen bonds involving the N-H group in the HNCO molecule. 2. Results and Discussion 2.1. Structure and Energetics of HNCO Dimers Five minima (D1-D5) were found on the potential energy surface for the HNCO dimer at the MP2 and B3LYPD3 levels of theory. At the B3LYP and B2PLYPD3 levels, one of the structures denoted D4 converged to D2. All MP2-computed energy minima structures are shown in Figure 1together with the atom numbering used in this work. Table 1shows the values of two topological AIM parameters: the electron density ρ (r) and its Laplacian ∇2ρ (r) at the bond critical points. Table 1also contains the MP2 calculated values of intermolecular distances and angles. Table 2presents energetic parameters obtained for HNCO dimers using the MP2, B3LYP, B3LYPD3 and B2PLYPD3 methods. Based on the computed structures shown in Figure 1and the values of the AIM parameters in Table 1, it was found that three structures (D1, D2 and D4) comprised hydrogen bonds of the N-H · · · N or N-H · · · O type, and all three possessed C 1 symmetry. Two other structures (D3 and D5) were connected by van der Waals interactions and had C 2h symmetry. The most stable structure with the largest interaction energy (E int ) appeared to be an open dimer D1 with two subunits bound by an almost linear N-H · · · N hydrogen bond, with the N · · · N distance of ca. 3.1 Å and the N-H· · · N angle of ca. 174◦. Molecules 2023,28, 1430 3 of 14 Molecules 2023, 28, x FOR PEER REVIEW 3 of 14 with two subunits bound by an almost linear N-H…N hydrogen bond, with the N…N distance of ca. 3.1 Å and the N-H…N angle of ca. 174°. Figure 1. The MP2-optimized structures of the HNCO dimers. The H, N, C and O atoms are shown in white, blue, gray and red, respectively. The positions of the bond (3,−1) and ring (3,+1) critical points derived from AIM calculations are shown by small green and red dots, respectively. Table 1. Interatomic distances (Å), angles (degree) and electron density parameters of the intermolecular bond critical points BCP (Atomic units) and ring critical points RCP (Atomic units) of the HNCO dimers computed at the MP2/6-311++G(3df,3pd) level. Complex Geometric Parameters 1 AIM Parameters Interatomic Distance Angle BCP ρ(r) ∇2ρ(r) H...Y X…Y X–H...Y D1 2.047 3.055 173.5 H1…N6 0.0211 +0.0663 D2 2.039 3.037 169.7 H1…O8 0.0170 +0.0698 D3 3.036 C3…O8 0.0071 +0.0291 3.036 C7…O4 0.0071 +0.0291 (4 at.) 2 0.0070 +0.0299 D4 2.303 3.062 131.2 H1…O8 0.0110 +0.0437 D5 3.196 N2…N6 0.0073 +0.0257 1 X: N or C; Y: N or O. 2 RCP. Figure 1. The MP2-optimized structures of the HNCO dimers. The H, N, C and O atoms are shown in white, blue, gray and red, respectively. The positions of the bond (3, − 1) and ring (3,+1) critical points derived from AIM calculations are shown by small green and red dots, respectively. Table 1. Interatomic distances (Å), angles (degree) and electron density parameters of the intermolecular bond critical points BCP (Atomic units) and ring critical points RCP (Atomic units) of the HNCO dimers computed at the MP2/6-311++G(3df,3pd) level. Complex Geometric Parameters 1AIM Parameters Interatomic Distance Angle BCP ρ(r) ∇2ρ(r) H...Y X· · ·Y X–H...Y D1 2.047 3.055 173.5 H1· · · N6 0.0211 +0.0663 D2 2.039 3.037 169.7 H1· · · O8 0.0170 +0.0698 D3 3.036 C3· · · O8 0.0071 +0.0291 3.036 C7· · · O4 0.0071 +0.0291 (4 at.) 20.0070 +0.0299 D4 2.303 3.062 131.2 H1· · · O8 0.0110 +0.0437 D5 3.196 N2· · · N6 0.0073 +0.0257 1X: N or C; Y: N or O. 2RCP. The two molecular moieties in D1 did not lie in the same plane but were twisted with respect to each other. The MP2 method gave a C3N2N6C7 dihedral angle of 62.0 ◦ , whereas B3LYPD3 and B2PLYPD3 predicted the D1 structure to be deviated from planarity by 78.3 ◦ and 70.7 ◦ , respectively. The D2 and D3 dimers were of almost equal stability compared with each other. The D2 form was an elongated structure in which the N-H group of one molecular moiety interacts with the terminal oxygen atom of the second subunit. The N-H · · · O hydrogen bond formed was close to linearity with the N-H · · · O angle of ca. 170 ◦ . The NCO backbones of the two moieties in D2 were twisted by ca. 25 ◦ from one another. A cyclic planar D3 structure with C 2h symmetry had, according to the AIM results, Molecules 2023,28, 1430 4 of 14 HNCO subunits connected by two C · · · O van der Waals interactions, and their backbones were anti-parallel to each other. The D4 dimer was also planar and contained, like D2, the N-H · · · O type hydrogen bond. However, in this case, the hydrogen bond appeared strongly bent with the N-H · · · O angle of ca. 131 ◦ and the backbones of the two moieties almost perpendicular to each other (the N2C7N6 angle of ca. 115 ◦ ). From the energy point of view, the least stable D5 dimer of C 2h symmetry was characterized by anti-parallel NCO backbones, and hydrogen atoms were oriented away from the center of symmetry of the dimer. The two HNCO moieties were bound by one identified N · · · N van der Waals interaction. Table 2. BSSE-corrected interaction energies E int and relative energies ∆ E (kJ mol −1 ) of the HNCO dimers calculated at MP2, B3LYP, B3LYPD3, B2PLYPD3 and CCSD(T) levels with 6-311++G(3df,3pd) basis set. DIMER MP2 CCSD(T)/ MP2 2B3LYPD3 CCSD(T)/ B3LYPD3 2B2PLYPD3 1CCSD(T)/ B2PLYPD3 2B3LYP 1 ∆E Eint ∆E Eint ∆E Eint ∆E Eint ∆E Eint ∆E Eint ∆E Eint D1 0.00 − 15.95 0.00 − 15.07 0.00 − 16.87 0.00 − 15.11 0.00 − 16.37 0.00 − 15.07 0.00 − 12.48 (26%) D2 2.42 − 13.36 1.77 − 13.65 1.56 − 15.11 1.74 − 13.82 1.91 − 14.28 1.77 − 13.69 0.84 − 11.43 (24%) D3 2.47 − 13.19 1.06 − 13.82 0.74 − 15.91 0.90 − 14.03 1.85 − 14.28 0.98 − 13.98 4.05 −8.04 (49%) D4 3.14 − 12.43 2.45 − 12.52 2.16 − 14.28 2.34 − 12.64 D5 7.92 −7.54 7.66 −6.91 9.17 −7.16 7.63 −6.99 8.88 −6.99 7.63 −6.95 11.03 −0.92 (87%) 1 D4 structure is not energetically stable at B2PLYP and B3LYP levels. 2 CCSD(T) calculations are single-point calculations at the MP2, B3LYPD3 or B2PLYPD3 optimized geometries, respectively. Based on computational data on energetics for the HNCO dimers presented in Table 2, some observations can be made. At all levels of calculations employed, the D1 dimer was the most stable one and was characterized by the highest interaction energy. The three dimers D2, D3 and D4 were of similar energetic stability with respect to each other, and they were only slightly less stable than the global minimum D1. The D5 form was the least energetically favored, as this structure was bound only by van der Waals interactions. Very recently, four HNCO dimers of similar structures optimized at B3LYPD3/aug-cc-pVTZ level were reported [ 35 ]. These structures resemble the planar structures D1, D2 and D3 in this study. The two structures D4 and D5 were identified for the first time in this work and were not considered in the previous study [35]. It is interesting to compare the geometry of (HNCO) 2 with the results obtained earlier for dimers of the sulfur analogue of isocyanic acid, HNCS [ 36 ]. Computational studies revealed three structures to be stable for (HNCS) 2 (see Figure S1 in Supplementary Materials). Two of them exhibited N-H · · · N and N-H · · · S hydrogen bonds corresponding to the binding patterns found for D1 and D4 isocyanic acid dimers. Although the mutual arrangement of the two molecular moieties in these dimers is slightly different, the type of interaction is analogous. The other (HNCO) 2 structures considered here do not have their counterparts in the case of isothiocyanic acid. There was a cyclic structure similar to structure D3 here with anti-parallel HNCS subunits, but the interaction was through N-H · · · S hydrogen bonds opposite to the C · · · O van der Waals interactions appearing in the HNCO dimer. Moreover, for the HNCS dimers [ 36 ] the contribution of dispersion to stabilize the system is crucial, and computational methods taking it into account are the right choice, while the results obtained at the more traditional B3LYP level are less reliable. To check whether this is also the case for HNCO dimers, i.e., if the dispersion effects have a significant contribution to the total energy of the interaction, we performed calculations using the B3LYP functional with the 6-311++G(3df,3pd) basis set. These data, shown in Table 2, indicated the highest contribution of dispersion one should expect for the cyclic D3 and D5 dimers characterized by two O · · · C or one N · · · N van der Waals interactions. The Molecules 2023,28, 1430 5 of 14 numbers in parentheses in Table 2indicate the percentage contribution of dispersion to the total interaction energy. 2.2. Structure and Energetics of HNCO Trimers Nine structures were found to be stable at all three levels of theory for HNCO trimers. Their geometries are shown in Figure 2, and their energetic parameters are presented in Table 3. The values of the topological AIM parameters and values of intermolecular distances and angles calculated at the MP2/6-311++G(3df,3pd) level for the trimers are gathered in Table S1 in the Supplementary Materials. Molecules 2023, 28, x FOR PEER REVIEW 6 of 14 Figure 2. The MP2-optimized structures of the HNCO trimers. The H, N, C and O atoms are shown in white, blue, gray and red, respectively. The positions of the bond (3,−1) and ring (3,+1) critical points derived from AIM calculations are shown by small green and red dots, respectively. The geometry of (HNCO) 3 is, as in the case of dimers, determined by the presence of hydrogen bonds and/or van der Waals interactions. Of the nine structures, the most stable form was a cyclic trimer T1 characterized by three N-H…N hydrogen bonds and C 3h symmetry. Trimers T2, T3 and T4 exhibited cyclic structures with a C s , C s and C 3 symmetry, respectively. They had very similar relative energies and were slightly less stable than T1. These trimers included N-H…O or both N-H…N and N-H…O hydrogen bonds. Another cyclic structure T7 characterized by C 1 symmetry was the least stable of all hydrogenbonded forms. There were also four trimers that represent open, elongated structures with three moieties connected by hydrogen bonds (T5 and T6, both with C 1 symmetry) or van der Waals contacts (T9, C s ) or both (T8, C 1 ). Optimized trimer structures indicated the existence of energetic co-operativity in the species. This was also illustrated by the calculated Figure 2. The MP2-optimized structures of the HNCO trimers. The H, N, C and O atoms are shown in white, blue, gray and red, respectively. The positions of the bond (3, − 1) and ring (3,+1) critical points derived from AIM calculations are shown by small green and red dots, respectively. Molecules 2023,28, 1430 6 of 14 Table 3. BSSE-corrected interaction energies E int and relative energies ∆ E (kJ mol −1 ) of the HNCO trimers calculated at MP2, B3LYPD3, B2PLYPD3 and CCSD(T) levels with 6-311++G(3df,3pd) basis set. TRIMER MP2 CCSD(T)/ MP2 1B3LYPD3 CCSD(T)/ B3LYPD3 1B2PLYPD3 CCSD(T)/ B2PLYPD3 1 ∆E Eint ∆E Eint ∆E Eint ∆E Eint ∆E Eint ∆E Eint T1 0.00 −47.90 0.00 −44.76 0.00 −49.70 0.00 −45.30 0.00 −48.73 0.00 −44.88 T2 3.74 −45.64 3.09 −43.96 0.48 −50.79 3.03 −44.76 2.51 −47.69 2.99 −44.21 T3 4.63 −43.84 3.32 −41.91 2.45 −48.06 3.31 −42.50 3.84 −45.59 3.27 −42.20 T4 6.50 −43.25 5.26 −42.91 2.80 −48.82 5.05 −43.75 4.80 −45.76 5.09 −43.21 T5 15.50 −33.08 13.70 −32.28 14.37 −35.96 13.69 −32.53 14.83 −34.50 13.66 −32.28 T6 18.26 −29.94 15.76 −30.61 16.35 −33.66 15.75 −31.02 17.03 −31.99 15.70 −30.69 T7 17.61 −29.56 15.44 −28.55 16.70 −32.15 15.47 −28.60 17.84 −30.06 15.43 −28.55 T8 19.11 −28.64 16.70 −27.80 18.37 −31.02 16.79 −27.63 19.08 −29.39 16.77 −27.72 T9 26.31 −20.85 23.28 −20.77 25.84 −23.15 23.11 −21.10 26.63 −21.35 23.12 −21.02 1 CCSD(T) calculations are single-point calculations at the MP2, B3LYPD3 or B2PLYPD3 optimized geometries, respectively. The geometry of (HNCO) 3 is, as in the case of dimers, determined by the presence of hydrogen bonds and/or van der Waals interactions. Of the nine structures, the most stable form was a cyclic trimer T1 characterized by three N-H · · · N hydrogen bonds and C 3h symmetry. Trimers T2, T3 and T4 exhibited cyclic structures with a C s , C s and C 3 symmetry, respectively. They had very similar relative energies and were slightly less stable than T1. These trimers included N-H · · · O or both N-H · · · N and N-H · · · O hydrogen bonds. Another cyclic structure T7 characterized by C 1 symmetry was the least stable of all hydrogen-bonded forms. There were also four trimers that represent open, elongated structures with three moieties connected by hydrogen bonds (T5 and T6, both with C 1 symmetry) or van der Waals contacts (T9, C s ) or both (T8, C 1 ). Optimized trimer structures indicated the existence of energetic co-operativity in the species. This was also illustrated by the calculated interaction energies. For example, for the most stable trimer (T1), the total interaction energy was about three times the interaction energy associated with a single hydrogen bond found for the D1 dimer. Other trimers with lower symmetry (T2 and T3), representing cyclic structures and containing both N-H · · · N and N-H · · · O hydrogen bonds, had similar energies as T1 and T4 structures. When the cyclic structures were open, the cooperativity effect mediated by the linked hydrogen bonds was lost, and the trimer structures appeared less stable and more weakly bound. 2.3. Results of Matrix Isolation FTIR Studies Figure 3presents the selected regions of the infrared spectra of the HNCO/Ar matrices obtained upon deposition at 15 K (10 K for measurement) at different sample concentrations. Trace (e) in this figure shows the corresponding spectral ranges of the difference spectrum obtained by subtracting the HNCO/Ar = 1/12,000 spectrum from the HNCO/Ar = 1/2000 spectrum . When the HNCO/Ar mixtures were deposited on a cold window, the HNCO monomer bands appeared in the spectra, as previously described [ 12 , 14 ]. Note, that in the ν NH stretching region of HNCO, a doublet is present due to the Fermi resonance [ 14 ]. In addition, even at high dilutions of the gaseous mixtures, several other absorptions were observed. The intensity of these bands increased relative to the monomer absorptions as the concentration of isocyanic acid in the matrix increased. Molecules 2023,28, 1430 7 of 14 Molecules 2023, 28, x FOR PEER REVIEW 7 of 14 interaction energies. For example, for the most stable trimer (T1), the total interaction energy was about three times the interaction energy associated with a single hydrogen bond found for the D1 dimer. Other trimers with lower symmetry (T2 and T3), representing cyclic structures and containing both N-H…N and N-H…O hydrogen bonds, had similar energies as T1 and T4 structures. When the cyclic structures were open, the cooperativity effect mediated by the linked hydrogen bonds was lost, and the trimer structures appeared less stable and more weakly bound. 2.3. Results of Matrix Isolation FTIR Studies Figure 3 presents the selected regions of the infrared spectra of the HNCO/Ar matrices obtained upon deposition at 15 K (10 K for measurement) at different sample concentrations. Trace (e) in this figure shows the corresponding spectral ranges of the difference spectrum obtained by subtracting the HNCO/Ar = 1/12,000 spectrum from the HNCO/Ar = 1/2000 spectrum. When the HNCO/Ar mixtures were deposited on a cold window, the HNCO monomer bands appeared in the spectra, as previously described [12,14]. Note, that in the νNH stretching region of HNCO, a doublet is present due to the Fermi resonance [14]. In addition, even at high dilutions of the gaseous mixtures, several other absorptions were observed. The intensity of these bands increased relative to the monomer absorptions as the concentration of isocyanic acid in the matrix increased. Figure 3. Bottom panels: the νNH, νasNCO and δNCO regions in the spectra of HNCO/Ar matrices with 1/12,000 (a), 1/6000 (b), 1/3000 (c) and 1/2000 (d) concentrations. Top panels: the difference spectrum obtained by subtracting the HNCO/Ar = 1/12,000 spectrum from the HNCO/Ar = 1/2000 spectrum (e). The numbers show the red (minus) and blue (plus) wavenumber shifts compared to the HNCO monomer values. Letter M denotes HNCO monomer bands. Figure 3. Bottom panels: the ν NH, νas NCO and δ NCO regions in the spectra of HNCO/Ar matrices with 1/12,000 ( a ), 1/6000 ( b ), 1/3000 ( c ) and 1/2000 ( d ) concentrations. Top panels: the difference spectrum obtained by subtracting the HNCO/Ar = 1/12,000 spectrum from the HNCO/Ar = 1/2000 spectrum ( e ). The numbers show the red (minus) and blue (plus) wavenumber shifts compared to the HNCO monomer values. Letter M denotes HNCO monomer bands. Analyzing the spectra after the subsequent annealing processes of the matrices at 26, 28, 30, 33 and 35 K (the highest temperature possible for argon matrices annealing) revealed interesting changes in the bands observed. The temperature increase in the range of 26–30 K did not cause significant changes in the spectra, apart from a slight general increase in the observed intensities of the bands. More pronounced changes were observed upon annealing the matrix at 33 and 35 K. Figure 4shows selected regions of the spectra of the HNCO/Ar = 1/6000 matrices obtained after deposition at 15 K and subsequent 10 min annealing at 33 K. The trace (c) in Figure 4shows the corresponding ranges of the difference spectrum obtained by subtracting the spectrum after deposition from the spectrum after annealing at 33 K. As can be seen in Figure 4(trace (b)) there are bands which were not present or were relatively weak in the spectrum recorded after deposition (see trace (a)). Moreover, these additional bands grew in intensity when the matrix was annealed up to 33 K. Therefore, they must be related to dimers or higher aggregates of HNCO. Additionally, these bands can be grouped into two sets since they behave differently when the matrix temperature increases from 33 K to 35 K (see Figure S2 in Supplementary Materials). The group of bands denoted as A was characterized by a slight increase in intensity in the temperature range 26–30 K, with a strong intensity increase upon annealing at 33 K and a decrease in intensity upon annealing at 35 K. On the other hand, the intensities of the B-type bands increased over the entire temperature range, but the increase was much stronger between 33 and 35 K. This different behavior of the A and B bands allowed us to assign them to the HNCO dimer and HNCO trimer structures, respectively. Molecules 2023,28, 1430 8 of 14 Molecules 2023, 28, x FOR PEER REVIEW 8 of 14 Analyzing the spectra after the subsequent annealing processes of the matrices at 26, 28, 30, 33 and 35 K (the highest temperature possible for argon matrices annealing) revealed interesting changes in the bands observed. The temperature increase in the range of 26–30 K did not cause significant changes in the spectra, apart from a slight general increase in the observed intensities of the bands. More pronounced changes were observed upon annealing the matrix at 33 and 35 K. Figure 4 shows selected regions of the spectra of the HNCO/Ar = 1/6000 matrices obtained after deposition at 15 K and subsequent 10 min annealing at 33 K. The trace (c) in Figure 4 shows the corresponding ranges of the difference spectrum obtained by subtracting the spectrum after deposition from the spectrum after annealing at 33 K. Figure 4. Bottom panels: the νNH, νasNCO and δNCO vibrational spectral regions for HNCO/Ar = 1/6000 after deposition (a, blue line) and after subsequent 10 min annealing at 33 K (b, grey line). Top panels: the difference spectrum (c, pink line) obtained by subtracting the spectrum (a) from the spectrum (b). As can be seen in Figure 4 (trace (b)) there are bands which were not present or were relatively weak in the spectrum recorded after deposition (see trace (a)). Moreover, these additional bands grew in intensity when the matrix was annealed up to 33 K. Therefore, they must be related to dimers or higher aggregates of HNCO. Additionally, these bands can be grouped into two sets since they behave differently when the matrix temperature increases from 33 K to 35 K (see Figure S2 in Supplementary Materials). The group of bands denoted as A was characterized by a slight increase in intensity in the temperature range 26–30 K, with a strong intensity increase upon annealing at 33 K and a decrease in intensity upon annealing at 35 K. On the other hand, the intensities of the B-type bands increased over the entire temperature range, but the increase was much stronger between 33 and 35 K. This different behavior of the A and B bands allowed us to assign them to the HNCO dimer and HNCO trimer structures, respectively. In Table 4, the selected wavenumber shifts calculated for the five most plausible dimer structures using B3LYPD3 and MP2 methods are compared to the corresponding experimental values. Those obtained at the B2PLYPD3/6-311++G(3df,3pd) level are listed in Table S2 in the Supplementary Materials. The computed intensities of the bands are Figure 4. Bottom panels: the ν NH, νas NCO and δ NCO vibrational spectral regions for HNCO/Ar = 1/6000 after deposition ( a , blue line) and after subsequent 10 min annealing at 33 K ( b , grey line). Top panels: the difference spectrum ( c , pink line) obtained by subtracting the spectrum (a) from the spectrum (b). In Table 4, the selected wavenumber shifts calculated for the five most plausible dimer structures using B3LYPD3 and MP2 methods are compared to the corresponding experimental values. Those obtained at the B2PLYPD3/6-311++G(3df,3pd) level are listed in Table S2 in the Supplementary Materials. The computed intensities of the bands are shown in parentheses in these Tables. Analogous data for HNCO trimers, for the ν NH spectral region, are presented in Table S3 in the Supplementary Materials. These computational data were used to support the assignment of the HNCO dimer and trimer species in the following.