Low-temperature Matrix Isolation Research Articles

Experimental FTIR-MI and Theoretical Studies of Isocyanic Acid Aggregates.

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 (C3h). 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.

Activation of H2S by Atomic Cr, Mn, and Fe: Matrix Infrared Spectra and Quantum Chemical Calculations.

Hydrogen sulfide is toxic and corrosive gas abundantly available in nature. The activation of hydrogen sulfide to produce hydrogen and elemental sulfur is of great significance for possible applications in toxic pollutant control and hydrogen energy regeneration. The activation of H2S by transition metal atoms (M = Cr, Mn, and Fe) has been studied by low-temperature matrix isolation infrared spectroscopy and quantum chemical calculations. Experimental and theoretical results indicate that the reaction between ground-state M atoms and H2S is inhibited by the repulsive interactions between the reactants. After being excited upon photolysis, the corresponding excited-state M atoms react with H2S molecules spontaneously. The produced insertion product HMSH further decomposed to metal sulfides upon full-arc mercury lamp irradiation by the splitting of hydrogen.

Radicals and molecular products from the gas-phase pyrolysis of lignin model compounds: Coniferyl alcohol, theory and experiment

Coniferyl alcohol (CFA) is one of lignin’s main building blocks and a relevant model compound. Two aspects of CFA pyrolysis are studied in the current work to gain insight into the molecular mechanism of lignin thermolysis: (i) the nature of intermediate radicals generated from vacuum pyrolysis of CFA, and (ii) the product distribution from pyrolysis of CFA dispersed into the gas phase. Low temperature matrix isolation electron paramagnetic resonance (LTMI-EPR) spectroscopy is employed to investigate the intermediate radicals associated with gas phase low-pressure pyrolysis of CFA. An anisotropic EPR spectrum of radicals trapped at liquid nitrogen temperature was registered throughout the studied temperature range (400–700 °C) characterized by high g-value (≤ 2.0100), and broad EPR line-width (~13.5–13.0 G) prescribed to a mixture of O-centered radicals produced from cleavage of O−CH3 and O−H terminal bonds. Thermal annealing of frozen radicals suggested a major contribution of methylperoxyl (CH3O2) radicals produced by the liberated CH3 and trace amounts of oxygen. The pyrolysis of CFA, for the first time, was conducted in a dispersed, gas-phase conditions to minimize the heterogeneous secondary reactions mediated by particle surfaces typical for condensed-phase pyrolysis in conventional reactors. Several unique products including polyhydroxylated derivatives of mandelic acid and some carboxylic compounds were identified in addition to those normally produced during the conventional pyrolysis - coniferyl aldehyde, vanillin (and its oxidized form homovanilic acid), eugenol and its isomers. A comprehensive analysis of product formation mechanisms, particularly the pathways for generating abundant hydroxylated compounds is performed and the role of trace oxygen is studied at the wB97XD and M06-2X hybrid density functional theory levels.

Photoreaction of 2-chlorothiophenol studied by low-temperature matrix-isolation IR spectroscopy with DFT calculation

• Photoreaction of 2-chlorothiophenol isolated in cryogenic matrix. • Benzothiirene with HCl complex changes into five-membered-ring thioketene compound. • Isomerization of chloro-cyclohexadienethione compound. The photoreactions of 2-chlorothiophenol are investigated using matrix-isolation IR spectroscopy combined with density functional theory calculation at the B3LYP/6-311++G(3df,3pd) level. In a Ne matrix, 2,4-cyclopentadien-1-ylidenemethanethione, a five-membered-ring thioketene compound, is identified as the major photoproduct of a reaction similar to Wolff rearrangement. The reaction intermediate is spectroscopically detected as benzothiirene, which is produced from the reactant under UV ( λ ≥ 275 nm) irradiation and is then converted to the thioketene under λ ≥ 325 nm irradiation. Moreover, the complex of benzothiirene and HCl changes into 4-chloro-2,5-cyclohexadiene-1-thione under irradiation at λ ≥ 325 nm. By contrast, UV irradiation of 2-chlorothiophenol in an Ar matrix yields primarily 4-chloro-2,5-cyclohexadiene-1-thione via its isomer 2-chloro-2,4-cyclohexadiene-1-thione.

Coronene-uracil complexes embedded in argon matrices: FTIR spectroscopy and quantum-mechanical calculations

We employ low-temperature matrix-isolation FTIR spectroscopy and quantum chemical calculations to study the interaction between nucleobase uracil and coronene which models the graphene surface. To observe the dimer FTIR spectrum, we use a quartz microbalance that allows us to produce matrix samples with precisely determined concentrations of coronene and uracil (with the concentration ratio of 2.5:1:1000 for coronene:uracil:argon). The interaction between coronene and uracil results in spectral shifts of uracil spectral bands. These shifts do not exceed 10 cm−1. The maximum shifts are observed for the C=O stretching and NH out-of-plane vibrations of uracil. The structures and interaction energies of stacked and H-bonded coronene-uracil complexes are calculated at the DFT/B3LYP(GD3BJ)/aug-cc-pVDZ and MP2/aug-cc-pVDZ levels of theory. In total, 19 stable stacked and two H-bonded coronene-uracil dimer structures are found in the calculations. The interaction energy obtained for the most stable stacked dimer is −12.1 and −14.3 kcal/mol at the DFT and MP2 levels, respectively. The interaction energies of the H-bonded dimers do not exceed − 3 kcal/mol. The IR spectra of the studied monomeric molecules and of all the dimers are calculated at the DFT/B3LYP(GD3BJ)/aug-cc-pVDZ level of theory. The spectral shifts of the most stable stacked coronene-uracil dimer obtained in the calculations are in good agreement with the experimental results.

Silylated silicon–carbonyl complexes as mimics of ubiquitous transition-metal carbonyls

Transition-metal-carbonyl complexes are common organometallic reagents that feature metal-CO bonds. These complexes have proven to be powerful catalysts for various applications. By contrast, silicon-carbonyl complexes, organosilicon reagents poised to be eco-friendly alternatives for transition-metal carbonyls, have remained largely elusive. They have mostly been explored theoretically and/or through low-temperature matrix isolation studies, but their instability had typically precluded isolation under ambient conditions. Here we present the synthesis, isolation and full characterization of stable silyl-substituted silicon-carbonyl complexes, along with bonding analysis. Initial reactivity investigations showed examples of CO liberation, which could be induced either thermally or photochemically, as well as substitution and functionalization of the CO moiety. Importantly, the complexes exhibit strong Si-CO bonding, with CO→Si σ-donation and Si→CO π-backbonding, which is reminiscent of transition-metal carbonyls. This similarity between the abundant semi-metal silicon and rare transition metals may provide new opportunities for the development of silicon-based catalysis.

Experimental (Raman and IR) and computational (DFT, MP2) studies of conformational diversity of 1-chloromethyl-1-fluorosilacyclohexane

Since 1-chloromethyl-1-fluorosilacyclohexane is a newly synthesized molecular compound its structural parameters and conformational stability is unknown. Raman and infrared vibrational spectroscopy methods were employed for analysis of this molecule. IR spectra were recorded for both gas phase and liquid sample, whereas the Raman experiments were performed in the liquid state. Additionally, low temperature matrix isolation infrared spectra were recorded after isolating the molecule in argon and nitrogen matrices. For the assignment of the experimental spectral bands, theoretical DFT/B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ calculations were performed. From the calculations it was found that 1-chloromethyl-1-fluorosilacyclohexane may exist in twelve different conformational forms out of which the chair axial trans conformer is the most stable form. However, there are three more chair type conformers – equatorial trans, equatorial gauche and axial gauche that are stable enough to be observed in the experimental spectra.

Transient species in the ozonolysis of tetramethylethene

The reaction of alkenes with ozone has great effect on atmospheric oxidation, its transient species can produce OH radicals and contribute to the formation of secondary organic aerosols (SOA). In the present study, the reaction of tetramethylethene (TME) with ozone was investigated using self-assembled low temperature matrix isolation system. The TME and ozone were co-deposited on a salt plate at 15 K, and then slowly warmed up the plate. The first transient species primary ozonide (POZ) was detected, indicating that the reaction followed Criegee mechanism. Then POZ began to decompose at 180 K. However, secondary ozonide (SOZ) was not observed according to Criegee mechanism. Probably, Criegee Intermediate (CI) did not react with inert carbonyl of acetone, but with remaining TME formed tetra-methyl epoxide (EPO).

Study on ozonolysis of asymmetric alkenes with matrix isolation and FT-IR spectroscopy

O3 and alkenes are important reactants in the formation of SOA in the atmosphere. The intermediates and reaction mechanism of ozonation of alkene is an important topic in atmospheric chemistry. In this study, the low-temperature matrix isolation was used to capture the intermediates such as Primary ozonides (POZs), Criegee Intermediates (CIs), and Secondary ozonides (SOZs) generated from ozonation of 2-methyl-1-butene (2M1B) and 2-methyl-2-butene (2M2B). The results have been identified by the vacuum infrared spectroscopy and theoretical calculation. Our results show that during the ozonation of asymmetric alkenes, two kinds of CIs and more than two kinds of SOZs were generated due to the different decomposition modes of POZs. The infrared absorption peaks of (CH3)2COO and CH3CH2C(CH3)OO for O–O telescopic vibration was determined to be 889 cm−1 and 913 cm−1, respectively. Using the merged jet method, it was found that a large amount of HCHO was produced during the ozonation of 2M1B, and glyoxal and methylglyoxal were produced in the ozonation of 2M2B. Our findings highlight the importance of asymmetric alkene ozonolysis reactions in producing CIs, further improving the understanding of the generation of CIs from ozonation of alkenes.

Insights into the Photodecomposition of Azidomethyl Methyl Sulfide: A S2/S1 Conical Intersection on Nitrene Potential Energy Surfaces Leading to the Formation of S-Methyl-N-sulfenylmethanimine.

UV photodecomposition of azidomethyl methyl sulfide (AMMS) yields a transient S-methylthiaziridine which rapidly evolves to S-methyl-N-sulfenylmethanimine at 10 K. This species was detected by infrared matrix isolation spectroscopy. The mechanism of the photoreaction of AMMS has been investigated by a combined approach, using low-temperature matrix isolation FTIR spectroscopy in conjunction with two theoretical methods, namely, complete active space self-consistent field and multiconfigurational second-order perturbation. The key step of the reaction is governed by a S2/S1 conical intersection localized in the neighborhood of the singlet nitrene minimum which is formed in the first reaction step of the photolysis, that is, N2 elimination from AMMS. Full assignment of the observed infrared spectra of AMMS has been carried out based on comparison with density functional theory and second-order perturbation Møller-Plesset methods.

Nucleoside conformers in low-temperature argon matrices: Fourier transform IR spectroscopy of isolated thymidine and deuterothymidine molecules and quantum-mechanical calculations

The conformational equilibrium of thymidine and deuterothymidine molecules in low-temperature Ar matrices has been studied using low-temperature matrix-isolation Fourier IR spectroscopy and quantum-chemical calculations by the DFT/B3LYP and MP2 methods. It has been found that two anti-conformers ta2_0 and ta3_0 with different structures of the sugar ring, C2′-endo and C3′-endo, predominate in low-temperature matrices. In isolated state, each of these conformers has a few low-barrier satellites that can fully pass into more stable structures when a molecule enters the matrix. The main syn conformer ts2_0 is stabilized by an intramolecular hydrogen bond between the O5′H group of the sugar and the C2O group of the base (O5′H⋅⋅⋅O2), while C2′-endo is the predominant conformation of the deoxyribose ring. The considerably lower population of ts2_0 compared to the anti-conformers ta2_0, ta3_0 can be explained by the smaller population of satellite conformations. It has been shown that the absorption band of νN3D stretching vibration is split by the Fermi resonance.

Photodynamics of [FeFe]-Hydrogenase Model Compounds with Bidentate Heterocyclic Ligands.

Two asymmetrically structured model compounds for the hydrogen-generating [Fe-Fe]-hydrogenase active site were investigated to determine the ultrafast photodynamics, structural intermediates, and photoproducts compared to more common symmetric di-iron species. The bidentate-ligand-containing compounds studied were Fe2(μ-S2C3H6)(CO)4(bipy), 1, and Fe2(μ-S2C3H6)(CO)4(phen), 2, in dilute room temperature acetonitrile solution and low-temperature 2Me-THF matrix isolation using static FTIR difference and time-resolved infrared spectroscopic methods (TRIR). Ultraviolet-visible spectra were also compared to time-dependent density functional theory (TD-DFT) to ascertain the orbital origins of long wavelength electronic absorption features. The spectroscopic evidence supports the conclusions that only a propyl-bridge flip occurs in low-temperature matrix, while early time CO ejection leads to the formation of solvated isomeric species on the 25 ps time scale in room temperature solution.

Conformational behaviour, photochemistry and flash vacuum pyrolysis of a 2-(1H-tetrazol-1-yl)thiophene

The conformational and photochemical behaviour of a 2-(1H-tetrazol-1-yl)thiophene was studied using low-temperature matrix isolation infrared spectroscopy and theoretical calculations. FVP of 2-(1H-tetrazol-1-yl)thiophene afforded new thieno-fused heterocycles.

Matrix isolation infrared spectroscopic study of 4-Pyridinecarboxaldehyde and of its UV-induced photochemistry

The structure, infrared spectrum, barrier to internal rotation, and photochemistry of 4-pyridinecarboxaldehyde (4PCA) were studied by low-temperature (10K) matrix isolation infrared spectroscopy and quantum chemical calculations undertaken at both Moller-Plesset to second order (MP2) and density functional theory (DFT/B3LYP) levels of approximation. The molecule has a planar structure (Cs point group), with MP2/6-311++G(d,p) predicted internal rotation barrier of 26.6kJmol−1, which is slightly smaller than that of benzaldehyde (~30kJmol−1), thus indicating a less important electron charge delocalization from the aromatic ring to the aldehyde moiety in 4PCA than in benzaldehyde. A complete assignment of the infrared spectrum of 4PCA isolated in an argon matrix has been done for the whole 4000–400cm−1 spectral range, improving over previously reported data. Both the geometric parameters and vibrational frequencies of the aldehyde group reveal the relevance in this molecule of the electronic charge back-donation effect from the oxygen trans lone electron pair to the aldehyde CH anti-bonding orbital. Upon in situ UV irradiation of the matrix-isolated compound, prompt decarbonylation was observed, leading to formation of pyridine.

Radicals and molecular products from the gas-phase pyrolysis of lignin model compounds. Cinnamyl alcohol

The experimental results on detection and identification of intermediate radicals and molecular products from gas-phase pyrolysis of cinnamyl alcohol (CnA), the simplest non-phenolic lignin model compound, over the temperature range of 400–800°C are reported. The low temperature matrix isolation – electron paramagnetic resonance (LTMI-EPR) experiments along with the theoretical calculations, provided evidences on the generation of the intermediate carbon and oxygen centered as well as oxygen-linked, conjugated radicals. A mechanistic analysis is performed based on density functional theory to explain formation of the major products from CnA pyrolysis; cinnamaldehyde, indene, styrene, benzaldehyde, 1-propynyl benzene, and 2-propenyl benzene. The evaluated bond dissociation patterns and unimolecular decomposition pathways involve dehydrogenation, dehydration, 1,3-sigmatropic H-migration, 1,2-hydrogen shift, CO and CC bond cleavage processes.

ChemInform Abstract: Advances in the Development of Complexes That Contain a Group 13 Element Chalcogen Multiple Bond.

AbstractReview: 75 refs.

Degradation mechanism of γ-irradiated polytetrafluoroethylene (PTFE) powder by low-temperature matrix-isolation infrared spectroscopy and chemiluminescence spectroscopy

In order to elucidate the mechanism of degradation of γ-irradiated polytetrafluoroethylene (PTFE), gases evolved by isothermal heating under vacuum were trapped in low-temperature argon matrices. Infrared spectra of the evolved gases were assigned to COF2 and CF4 by comparison with the corresponding literatures. Kinetic analyses of the IR absorbance and chemiluminescence intensity changes against heating time were carried out to determine the rate constants of the thermal reactions, showing that CF4 was produced more rapidly than COF2. By comparison between the rate constants obtained by infrared spectroscopy and chemiluminescence spectroscopy, we propose a new thermal reaction mechanism of γ-irradiated PTFE at the molecular level. Thermal reaction mechanism of γ-irradiated polytetrafluoroethylene (PTFE) is elucidated at the molecular level by kinetic analysis, where a new multichannel Fourier-transform chemiluminescence spectrometer was used.

Advances in the development of complexes that contain a group 13 element chalcogen multiple bond.

Inorganic group 13 element (M) chalcogenides (E) based on the general formular M2E3 are ubiquitous in synthesis, catalysis and material science. The parent ME fragment which aggregates to form three dimensional networks in the condensed phase can be expected to exhibit multiple bond character between the elements. Low temperature matrix isolation techniques are required to investigate the nature of this elusive species. An alternate approach for respective studies is the synthesis of electron-precise molecular complexes that contain the ME entity and for which isolation at ambient temperature is possible. This is realized by kinetic stabilization with bulky ligands and thermodynamic stabilization using electron donor, as well as acceptor groups attached to the ME functionality (i.e. donor-acceptor stabilization). In this article we revise the literature on complex compounds that exhibit a bonding interaction between a group 13 element atom and a chalcogen atom that is reasonably to be interpreted in terms of a double- or triple bond.

Speciation of Iron (III) Oxide Nanoparticles and Other Paramagnetic Intermediates during High-Temperature Oxidative Pyrolysis of 1-Methylnaphthalene

Low Temperature Matrix Isolation - Electron Paramagnetic Resonance (LTMI-EPR) Spectroscopy was utilized to identify the species of iron oxide nanoparticles generated during the oxidative pyrolysis of 1-methylnaphthalene (1-MN). The otherwise gas-phase reactions of 1--MN were impacted by a polypropylenimine tetra-hexacontaamine dendrimer complexed with iron (III) nitrate nonahydrate diluted in air under atmospheric conditions. The EPR fine structure of Fe (III)2O3 nanoparticles clusters, characterized by g-factors of 2.00, 2.28, 3.76 and 4.37 were detected on a cold finger maintained at 77 K after accumulation over a multitude of experiments. Additionally, a high valence Fe (IV) paramagnetic intermediate and superoxide anion-radicals, O2•- adsorbed on nanoparticle surfaces in the form of Fe (IV) --- O2•- were detected from the quenching area of Zone 1 in the gas-phase.

UV-light induced conformational changes of 2-pyridinecarboxylic acid in low-temperature argon matrices

Conformational changes of 2-pyridinecarboxylic acid (2-picolinic acid) upon UV irradiation have been investigated by low-temperature matrix-isolation infrared spectroscopy. Only one conformer having an intramolecular hydrogen bond of COOH⋯N in the pyridine ring is found to exist in argon matrices after deposition before UV irradiation, in contrast to a reported feature in the CCl4 solution. The hydrogen bond is broken by UV irradiation, resulting in two less stable conformers; one increases immediately at the early irradiation stage while the other increases slowly during the prolonged irradiation time. The conformations around the OCOH and CCOOH bonds of the photoproducts are identified by assignments of the IR bands measured in an argon matrix with an aid of the density-functional-theory calculation. The proposed photoreaction pathways of the conformational changes based on this observation are supported by a kinetic analysis of the IR absorbance changes with the reactants and the photoproducts against the irradiation time.