Low-temperature Matrices Research Articles

PHOTOCHEMISTRY OF RADICAL CATIONS OF SOME ORGANIC OXYGEN-CONTAINING COMPOUNDS IN FROZEN MATRICES

The review is dedicated to the photochemical reactions of radical cations (RC) of various organic oxygen-containing compounds stabilized in low-temperature matrices. The nature of RC and the products of their photochemical reactions has been established via quantum chemistry, electron paramagnetic resonance (EPR) and low-temperature optical spectroscopy. Depending on the structure of the precursor molecule, various mechanisms of photoconversion arise for these RC: charge transfer to matrix molecules, hydrogen atom and proton transfer, isomerization, dissociation. This study allowed us to posit that there is no correlation between the structure of the molecule of the precursor molecule and the variety of available phototransformation channels for the corresponding RC in frozen matrices.

Hydrogen-atom tunneling in small thioamides: N-methylthiourea, thiobenzamide and 2-cyanothioacetamide

The most stable thione tautomeric forms of N-methylthiourea, thiobenzamide and 2-cyanothioacetamide were isolated in low-temperature argon matrices. The higher-energy thiol tautomers of these compounds were generated upon irradiation of matrix-isolated monomers with UV (λ > 270 nm) light. For N-methylthiourea and thiobenzamide, kept in the dark at 3.5 K for a long period of time, a spontaneous thiol → thione hydrogen atom tunneling transformation occurred. Only the thiol isomers with the favorably oriented hydrogen atom of the imino group underwent these hydrogen-atom tunneling processes. The other thiol isomers, with the hydrogen atom of the imino group oriented towards the sulfur atom, did not undergo the thiol → thione conversion. For the photogenerated thiol forms of 2-cyanothioacetamide, no spontaneous thiol → thione tautomeric transformation was detected. Instead, only the spontaneous conformational change of one S-H rotamer of the thiol 2-cyanothioacetamide tautomer into the other S-H rotamer was observed.

Energetic processing of thioacetamide in cryogenic matrices.

There is an ongoing debate on the apparent depletion of sulfur in the interstellar medium (ISM) compared to its universal abundance; therefore, the investigation of sulfurous compounds at low temperatures is of utmost importance. This work aims to study thioacetamide, H3C-C(=S)-NH2, in low-temperature inert Ar and para-H2 matrices by IR spectroscopy. The samples have been exposed to various sources of irradiation, such as Lyman-α or laser UV photons as well as energetic electrons. Using different host materials enabled assessing the matrix's impact on precursor decomposition. The response of the molecule to different types of irradiation has also been evaluated. The existence of three main decomposition channels were deduced: formation of (i) CH3, CH4, and HNCS; (ii) H2S and H2C=C=NH; and (iii) NH3 and H2C=C=S. The H3C-CN and H3C-NC isomers of H2C=C=NH could also be identified. Secondary products such as HNC and HCN were also detected in the quantum solid para-H2 in contrast to the more rigid Ar matrix. The listed decomposition products have been observed in the ISM, with the exception of H2C=C=NH and H3C-NC. The results point to the potential sensitivity of the precursor molecule to energetic radiation in space environments. Finally, the findings of this work will serve as a foundation for future irradiation experiments using the astrochemically more relevant pure thioacetamide ice.

Hydrogen-atom-assisted processes on thioacetamide in para-H2 matrix - formation of thiol tautomers.

TA has been isolated in low-temperature para-H2 matrices and it has been exposed to H atoms. In accordance with previous experimental results, TA exclusively exists in its more stable thione tautomeric form in the freshly deposited matrix. However, upon H atom generation, the bands belonging to the precursor start decreasing with the simultaneous appearance of new bands. By comparing the position of these new peaks with earlier results obtained in para-H2, they can be undoubtedly attributed to the presence of the higher-energy thiol tautomeric forms of TA. No other products could be observed, except for NH3. Quantum-chemical computations have been invoked to understand the mechanism behind the observed thione-thiol tautomerization assisted by H atoms. Accordingly, tautomerization starts with barrierless H-atom addition on the S atom of the thione resulting in the formation of the intermediate 1-amino-1-mercapto-ethyl radical (H3C-Ċ(-SH)-NH2), which has been detected tentatively for the first time. The second step is a barrierless H-atom induced H-abstraction from the -NH2 moiety of the H3C-Ċ(-SH)-NH2 radical. A comprehensive mechanism for tautomerization is proposed based on the experimental and theoretical results. Although earlier studies showed the possibility of TA thione-thiol tautomerization in cryogenic matrices achieved by energetic UV irradiation, the present study points out that this can also take place through a barrierless pathway by simply exposing the TA molecules to H atoms. As such, this is the first evidence for the occurrence of such a reaction in a matrix-isolated environment. The current results also help us better understand the mystery behind thione-thiol tautomerization in low-temperature environments.

Stability of monomeric indigo in the electronic ground state: An experimental matrix isolation infrared and theoretical study

Monomers of indigo were investigated using experimental (matrix-isolation) and theoretical methods. For the first time, the infrared spectrum of indigo molecules isolated in low-temperature Ar matrices was recorded and subjected to a detailed theoretical analysis. An excellent agreement between the experimental spectrum of indigo monomers and the spectrum theoretically predicted for the highly-symmetric (C2h) most stable dioxo form of the compound allowed a very reliable assignment of the observed infrared absorption bands. Matrix-isolated monomers of indigo were exposed to UV–vis radiation of different wavelengths. The conducted experiments demonstrated that molecules of indigo do not undergo any structural changes, even under irradiation with short-wavelength UV (λ > 200 nm) light. Theoretical calculations concerning the stability of photoproducts that could be potentially generated from indigo were conducted. The calculations carried out for the species generated by hydrogen-atom transfer from N1′ to O atom attached to the opposite ring revealed that both the oxo-hydroxy and dihydroxy tautomers of indigo should be unstable in the ground electronic state. The calculations carried out for the C2H tautomer (which may be generated by a hydrogen atom shift from N1′ to C2 of the opposite ring) led to optimization of the structure that is potentially stable, but very much distorted from planarity.

Influence of heteroatoms and substituents on structural and spectroscopic parameters of saturated six-member ring heterocycles: experimental and theoretical study of 1-methyl-1-germacyclohexane

The conformational analysis of newly synthesized compound, 1-methyl-1-germacyclohexane, is performed by means of a combined vibrational spectroscopy and theoretical methods. The liquid 1-methyl-1-germacyclohexane at room temperature is investigated using conventional ATR-FTIR, FT-IR (gas phase) and Raman methods. FT-IR spectra of the isolated in low-temperature neon and nitrogen matrices are registered. The further detailed vibrational conformational analysis is performed utilizing density functional theory and perturbation theory based methods. All possible 12 canonical ring conformations considering axial/equatorial CH3 group position are analyzed by utilizing DFT/B3LYP, DFT/M06-2X and MP2 at aug-cc-pVTZ theory level.The most stable conformer with the global energy minimum structure in the chair axial conformation is investigated in detail. Analysis of the potential energy surface reveals transition states (TS) and energy barriers. The conformational path is found to be as follows: chair (1C4)→ envelope/half-chair (TS)→ skew-boat (1S3 - C1 symmetry)→ boat (TS)→ skew-boat (1S5 - C2 symmetry). The energy barrier for 1C4 (chair) to 1S3 (skew-boat) conversion is 4.8 kcal/mol while for the reverse process equals to 0.5 kcal/mol.A similar conformational analysis is performed for different configurations of germacyclohexane, silacyclohexane and cyclohexane with mono- and disubstituents: CH2Cl, CH3, Cl and F. Such approach enables investigation of influence of the heteroatoms and substituents of various size and type on the structural parameters and conformational diversity.

Photoinduced Long-Distance Hydrogen-Atom Transfer in Molecules with a 7-Hydroxyquinoline Frame and a Carbaldehyde or Aldoxime Group as the Intramolecular Hydrogen Transporting Crane.

The photochemical properties of monomeric 7-hydroxyquinoline substituted at position 8 with carbaldehyde or aldoxime groups were studied for the molecules isolated in solid Ar low-temperature matrices (at 10 K). It was experimentally demonstrated that upon UV excitation, both carbaldehyde and aldoxime groups act as intramolecular cranes transmitting hydrogen atoms from the hydroxyl group to the remote nitrogen atom of the quinoline ring. Furthermore, in the case of 7-hydroxyquinoline-8-aldoxime (and its derivatives), the second photochemical channel was activated upon UV (λ > 360 nm) excitation. This process involves syn-anti isomerization around the double C═N bond in the aldoxime group. The structures of the reactant hydroxy tautomeric form and the photoproduced isomers of the studied molecules were unequivocally determined by means of IR spectroscopy combined with theoretical predictions of the IR spectra of the candidate structures.

Gas-phase detection of oxirene

Oxirenes-highly strained 4π Hückel antiaromatic organics-have been recognized as key reactive intermediates in the Wolff rearrangement and in interstellar environments. Predicting short lifetimes and tendency toward ring opening, oxirenes are one of the most mysterious classes of organic transients, with the isolation of oxirene (c-C2H2O) having remained elusive. Here, we report on the preparation of oxirene in low-temperature methanol-acetaldehyde matrices upon energetic processing through isomerization of ketene (H2CCO) followed by resonant energy transfer of the internal energy of oxirene to the vibrational modes (hydroxyl stretching and bending, methyl deformation) of methanol. Oxirene was detected upon sublimation in the gas phase exploiting soft photoionization coupled with a reflectron time-of-flight mass spectrometry. These findings advance our fundamental understanding of the chemical bonding and stability of cyclic, strained molecules and afford a versatile strategy for the synthesis of highly ring-strained transients in extreme environments.

UV Laser-Induced Phototransformations of Matrix-Isolated 5-Chloro-3-nitro-2-hydroxyacetophenone

Conformational changes of 5-chloro-3-nitro-2-hydroxyacetophenone were studied by experimental and theoretical methods. Phototransformations of the compound were induced in low-temperature argon matrices by using UV radiation, which was followed by FT-IR measurements. Two types of changes within the molecule were detected: rotations of the hydroxyl and acetyl groups. A new conformer without an intramolecular hydrogen bond was generated upon irradiation with λ = 330 nm, whereas the reverse reaction was observed at 415 nm.

NIR and UV induced transformations of indazole-3-carboxylic acid isolated in low temperature matrices

The molecular structure and NIR and UV induced phototransformations of indazole-3-carboxylic acid were studied in low temperature argon and nitrogen matrices by FTIR spectroscopy and B3LYP/6–311++G(2d,2p) calculations. Eleven minima of IC were located on the S0 potential energy surface. The three most stable structures were detected experimentally in both matrices after deposition. Upon NIR irradiation a dominant process was rotamerization within the carboxylic group leading to changes in the population of the trans 1HIC1 and cis 1HIC2 forms. In turn, at UV irradiation at λ = 260 nm two new tautomers (2HIC2 and 2HIC3) were generated indicating that the hydrogen atom transfer in pyrazole ring took place. Based on the obtained kinetic curves, differences in the phototransformation rates in different matrices were indicated.

Photochemical study and vibrational spectra of propiolamide isolated in low-temperature Ar, Xe, and N2 matrices

Propiolamide (HCCCONH2, PA) has been isolated in low-temperature matrices (Ar, Xe, and N2) and studied by matrix isolation infrared (IR) spectroscopy and quantum chemical calculations undertaken at the DFT(B3LYP)/6-311++G(d,p) level of theory. In situ broadband UV light (λ > 235 nm) irradiation of matrix-isolated PA led to the observation of new bands ascribed to different photoproducts resulting from three distinct photoreaction pathways: (i) tautomerization to propiolimidic acid (HCCC(OH)NH), (ii) CC bond cleavage, with formation of acetylene (HCCH) and isocyanic acid (HNCO), and (iii) decarbonylation, leading to generation of ethynamine (HCCNH2). Pathways (i) and (ii) were observed in all matrices, while (iii) was only observed in the matrices of the rare gases. Observation of characteristic bands due to the different photoproducts has been established, and the assignment of the IR spectra for all the species was carried out. As a whole, the detailed structural, vibrational, and photochemical data reported in this article appear relevant to the rational practical use of PA in its multiple application areas (in particular in the pharmaceutical and materials sciences domains) as well as for its analytical determination, including its search in the interstellar space.

Structure and IR spectroscopic properties of complexes of 1,2,4-triazole and 3-amino-1,2,4-triazole with dinitrogen isolated in solid argon

Complexes of 1,2,4-triazole (TR) and 3-amino-1,2,4-triazole (AT) with N2 were studied computationally employing MP2 and B3LYPD3 methods and experimentally by FTIR matrix isolation technique. The results show that both triazoles interact specifically with dinitrogen in several different ways. For the 1:1 complexes of 1,2,4-triazole five stable minima were located on the potential energy surface. The most stable of them comprises a weak hydrogen bond formed between the NH group of the ring and the lone pair of the nitrogen molecule. The second most stable structure is bound by the N⋯π bond formed between one of the N atoms of the N2 molecule and the triazole ring. Three other complexes are stabilized by the C-H⋯N and N⋯N van der Waals interactions. In the case of 3-amino-1,2,4-triazole, the two most stable dinitrogen complexes are analogous to those found for the 1,2,4-triazole and involve N-H⋯N and N⋯π bonds. In other structures amino or CH groups act as proton donors to the N2 molecule. The N⋯N van der Waals interactions are also present. The analysis of the infrared spectra of low temperature matrices containing TR or AT and dinitrogen indicates that in both systems mostly 1:1 hydrogen-bonded complexes with the NH group interacting with N2 are present in solid argon.

Low-Temperature Infrared Spectra and Ultraviolet-Induced Rotamerization of 5-Chlorosalicylaldehyde.

5-Chlorosalicylaldehyde (abbreviated as 5CSA) is an important chemical used in the synthesis of fragrances, dyes, and pharmaceuticals. In this investigation, 5CSA isolated in solid N2, at 10 K, and in its neat amorphous and crystalline phases, at 50 and 190 K, respectively, were investigated by infrared spectroscopy and DFT(B3LYP)/6-311++G(d,p) calculations. The systematic theoretical analysis of the 5CSA conformational landscape showed that the compound exhibits four different conformers, which were structurally characterized in detail. In the as-deposited low-temperature matrices of 5CSA, only the most stable conformer, the intramolecularly hydrogen-bonded form I, was found. The same was observed in the case of the investigated low-temperature amorphous and crystalline phases of 5CSA. Conformer I was successfully converted into a higher-energy conformer(II), where both aldehyde and hydroxyl groups are rotated by 180° relative to their position in the initial conformer, through narrowband ultraviolet (UV) (λ = 308 nm) in situ irradiation of the as-deposited N2 matrix of 5CSA. The infrared spectra of both matrix-isolated conformers, as well as those of the neat amorphous and crystalline phases of 5CSA, were assigned and interpreted in comparative terms, allowing us to elucidate structurally and vibrationally relevant effects of the main intra- and intermolecular interactions operating in the different studied phases. Very interestingly, the observed UV-induced I → II rotamerization was found to take place in an exclusive basis, with no other photochemical processes being observed to occur upon UV irradiation, under the experimental conditions used in the present investigation.

Investigation of phenyl azide photochemistry by conventional and time-resolved spectroscopy. Elucidation of intermediates and reaction mechanisms

Early studies of the photochemistry of parent phenyl azide concerned the products obtained under specific experimental conditions. Depending on the products, various mechanisms and intermediates were proposed. Later work involving conventional spectroscopic studies in low temperature matrices allowed the direct detection and identification of several intermediates. Electron Spin Resonance spectroscopy allowed the determination of a ground triplet phenyl nitrene and IR spectroscopy allowed the direct observation of ketenimine and bicyclic azirine. However, low temperature spectroscopy studies gave contradictory results with the formation of secondary photochemical intermediates and products. Recent studies using-time resolved spectroscopy allowed the direct observation of singlet phenyl nitrene. Furthermore, modern computational tools helped to corroborate experimental spectra of intermediates and to determine electronic structure of other possible intermediates, relative energies and mechanistic pathways. A unified picture of the photochemistry of phenyl azide has emerged from experimental, spectroscopic and theoretical studies. In this review, the results presented in each methodology are discussed.

Formation of the Elusive Silylenemethyl Radical (HCSiH2; X2B2) via the Unimolecular Decomposition of Triplet Silaethylene (H2CSiH2; a3A″).

We investigated the formation of small organosilicon molecules─potential precursors to silicon-carbide dust grains ejected by dying carbon-rich asymptotic giant branch stars─in the gas phase via the reaction of atomic carbon (C) in its 3P electronic ground state with silane (SiH4; X1A1) using the crossed molecular beams technique. The reactants collided under single collision conditions at a collision energy of 13.0 ± 0.2 kJ mol-1, leading to the formation of the silylenemethyl radical (HCSiH2; X2B2) via the unimolecular decomposition of triplet silaethylene (H2CSiH2; a3A″). The silaethylene radical was formed via hydrogen migration of the triplet silylmethylene (HCSiH3; X3A″) radical, which in turn was identified as the initial collision complex accessed via the barrierless insertion of atomic carbon into the silicon-hydrogen bond of silane. Our results mark the first observation of the silylenemethyl radical, where previously only its thermodynamically more stable methylsilylidyne (CH3Si; X2A″) and methylenesilyl (CH2SiH; X2A') isomers were observed in low-temperature matrices. Considering the abundance of silane and the availability of atomic carbon in carbon-rich circumstellar environments, our results suggest that future astrochemical models should be updated to include contributions from small saturated organosilicon molecules as potential precursors to pure gaseous silicon-carbides and ultimately to silicon-carbide dust.

Tautomers of 6-thiopurine in low-temperature Ar matrices: FTIR spectroscopy analysis and quantum mechanical calculations

The structures and vibrational spectra of 6-thiopurine (6TP) molecules in an isolated state were studied by the spectroscopic and computational methods. FTIR spectra of 6TP molecules isolated in low-temperature Ar matrices (at 11 K) were obtained in the infrared range 3800–200 cm–1. The optimized structures of tautomers, model clusters and the population of tautomers were estimated by the DFT, MP2 and CCSD(T) methods. The vibrational spectra were calculated by the DFT/B3LYP method with different basis sets [6–311++G(df,pd), aug-cc-pVDZ, aug-cc-pVTZ] and the MP2/aug-cc-pVDZ/anharmonic method. In the spectral range of 1700–200 cm–1 of the experimental FTIR spectra, five combination modes enhanced by the Fermi resonance were observed. Fermi resonances with the participation of librational modes of domestic molecules were found in the 600–500 cm–1 region. It was revealed that the incorporation of 6TP between the closest packing planes of Ar lattice leads to a significant increase in the frequency of two out-of-plane “butterfly” modes.

Photochromism of a Spiropyran in Low-Temperature Matrices: Unprecedented Bidirectional Switching between a Merocyanine and an Allene Intermediate.

Photochromism of spiropyrans has attracted much attention due to its potential in many light-controlled system applications. However, several fundamental aspects regarding the structure, energetics, and mechanistic details of the transformations of spiropyrans are still not well understood. Here, we report the study of the photochromism of a 6-hydroxy-spiropyran (HBPS) under conditions of matrix isolation, where monomers of the compound are frozen in a solidified noble gas (krypton, at 15 K). The structure of the matrix-isolated HBPS was first elucidated by infrared (IR) spectroscopy supported by density functional theory computations. Then, the photochromism of HBPS, from the colorless spiropyran to the colored merocyanine, was induced by ultraviolet (UV) irradiation at 310 nm. The analysis of the IR spectrum of the photoproduced species revealed the exclusive formation of the most stable merocyanine MC-TTC stereoisomer. Subsequent visible-light (550 nm) irradiation of MC-TTC generated a new colorless allenic isomeric species ALN, where the UV irradiation (310 nm) of ALN was found to convert this species back to MC-TTC. This constitutes an unprecedented bidirectional transformation between a colored merocyanine and a colorless allene species. The newly observed photoswitching reaction (or photochromism) occurs along an intramolecular hydrogen bond existing in both merocyanine and allenic species, thus suggesting that it might be generally feasible in the chemistry of spiropyrans. On the other hand, the usual assumption that, as a general rule, merocyanines photochemically revert to spiropyrans is not supported in this work.

Water Clusters in Interaction with Corannulene in a Rare Gas Matrix: Structures, Stability and IR Spectra

The interaction of polycyclic aromatic hydrocarbons (PAHs) with water is of paramount importance in atmospheric and astrophysical contexts. We report here a combined theoretical and experimental study of corannulene-water interactions in low temperature matrices and of the matrix’s influence on the photoreactivity of corannulene with water. The theoretical study was performed using a mixed density functional based tight binding/force field approach to describe the corannulene-water clusters trapped in an argon matrix, together with Born-Oppenheimer molecular dynamics to determine finite-temperature IR spectra. The results are discussed in the light of experimental matrix isolation FTIR spectroscopic data. We show that in the solid phase, π isomers of (C20H10)(H2O)n, with n = 2 or 3, are energetically favored. These π complexes are characterized by small shifts in corannulene vibrational modes and large shifts in water bands. These π structures, particularly stable in the case of the water trimer where the water cluster is trapped “inside” the corannulene bowl, may account for the difference in photoreactivity of non-planar–compared to planar–PAHs with water. Indeed, planar PAHs such as pyrene and coronene embedded in H2O:Ar matrices form σ isomers and react with water to form alcohols and quinones under low energy UV irradiation, whereas no photoreactivity was observed for corannulene under the same experimental conditions.

Detection of 1H-Triphosphirene (c-HP3) and 2-Triphosphenylidene (HP3): The Isovalent Counterparts of 1H-Triazirine (c-HN3) and Hydrazoic Acid (HN3).

The hitherto elusive 1H-triphosphirene (c-HP3) and 2-triphosphenylidene (HP3) molecules were prepared in low-temperature matrices and detected isomer selectively through photoionization coupled with reflectron time-of-flight mass spectrometry (PI-ReTOF-MS). Our results reveal a thermodynamically preferred cyclic isomer (c-HP3) compared to the acyclic structure (HP3) in contrast to the isovalent HN3 system favoring hydrazoic acid (HN3) compared to 1H-triazirine (c-HN3). Theoretical computations suggest a ring strain energy of 1H-triphosphirene (c-HP3) of only 35 kJ mol-1, which is significantly lower than the tetrahedral phosphorus molecule (P4) of 74 kJ mol-1. This work provides a fundamental benchmark to understand the electronic structure and chemical bonding of cyclic molecules and offers an unconventional approach to preparing highly strained, still elusive molecules such as 1H-triazirine and tetrahedral tetranitrogen (N4) in the near future involving progressive nonequilibrium chemistries.

Photochemical Study and Vibrational Spectra of Propiolamide Isolated in Low-Temperature Ar, Xe, and N2 Matrices

Photochemical Study and Vibrational Spectra of Propiolamide Isolated in Low-Temperature Ar, Xe, and N2 Matrices