Low-temperature Argon Matrix Research Articles

Thermal Decomposition of 2-Cyclopentenone.

The thermal decomposition of 2-cyclopentenone, a cyclic oxygenated hydrocarbon that occurs in the pyrolysis of biomass, has been studied in a combined experimental and theoretical approach. Gas-phase pyrolysis was performed at temperatures ranging from 1000 to 1400 K in a pulsed, microtubular reactor. Products were identified by FTIR spectroscopy following their isolation in a low-temperature argon matrix. The following products were identified: carbon monoxide, ketene, propenylketene, vinylacetylene, ethylene, propene, acrolein, acetylene, propyne, and propargyl radical. Computational results identify three different decomposition channels involving a H atom migration, and producing prop-2-enylketene (Pathway 1), prop-1-enylketene (Pathway 2), and a second conformation of prop-2-enylketene (Pathway 3). A fourth decomposition pathway involves simultaneous rupture of two C-C bonds forming a high energy cyclopropenone intermediate that further reacts to form ethylene, acetylene, and carbon monoxide. Finally, a fifth pathway to the formation of acrolein and acetylene was identified that proceeds via a multistep mechanism, and an interconversion from 2-cyclopentenone to 3-cyclopentenone was identified computationally, but not observed experimentally.

Infrared Spectroscopy and Photochemistry of Ethyl Maltol in Low-Temperature Argon Matrix

Ethyl maltol was investigated using matrix isolation infrared spectroscopy and DFT calculations. In an argon matrix (14.5 K), the compound was found to exist in a single conformer (form I), characterized by an intramolecular hydrogen bond with an estimated energy of ~17 kJ mol−1. The IR spectrum of this conformer was assigned, and the molecule’s potential energy landscape was explored to understand the relative stability and isomerization dynamics of the conformers. Upon annealing the matrix to 41.5 K, ethyl maltol was found to predominantly aggregate into a centrosymmetric dimer (2× conformer I) bearing two intermolecular hydrogen bonds with an estimated energy of ca. 28 kJ mol−1 (per bond). The UV-induced (λ > 235 nm) photochemistry of the matrix-isolated ethyl maltol was also investigated. After 1 min of irradiation, band markers of two rearrangement photoproducts formed through the photoinduced detachment-attachment (PIDA) mechanism, in which the ethyl maltol radical acts as an intermediate, were observed: 1-ethyl-3-hydroxy-6-oxibicyclo [3.1.0] hex-3-en-2-one and 2-ethyl-2H-pyran-3,4-dione. The first undergoes subsequent reactions, rearranging to 4-hydroxy-4-propanoylcyclobut-2-en-1-one and photofragmenting to cyclopropenone and 2-hydroxybut-1-en-1-one. Other final products were also observed, specifically acetylene and CO (the expected fragmentation products of cyclopropenone), and CO2. Overall, the study demonstrated ethyl maltol’s high reactivity under UV irradiation, with significant photochemical conversion occurring within minutes. The rapid photochemical conversion, with complete consumption of the compound in 20 min, should be taken into account in designing practical applications of ethyl maltol.

Formation of silyl metal hydride, HMnSiH3 and silylidyne, H3ReSiH in the activation of silane by manganese and rhenium atoms

The reactions of laser ablated manganese and rhenium metal atoms with silane molecules in low temperature argon matrix were studied. The experimental and theoretical results indicate that the reaction happened after the metal atoms were excited to Mn(6P) or Re(6P) excited states upon photolysis. Combining the isotopic shifts of the infrared absorptions and quantum chemical calculations, the silyl metal hydride HMnSiH3 and the silylidyne H3ReSiH were identified in the argon matrix. The vibronic coupling between the ground state X2A' and the first excited state A2A'' leads to a bond angle H-Si-Re of 130.4° in the doublet ground state H3ReSiH.

Conformational-Dependent Photodissociation of Glycolic Acid in an Argon Matrix

Ultraviolet-induced photodissociation and photo-isomerization of the three most stable conformers (SSC, GAC, and AAT) of glycolic acid are investigated in a low-temperature solid argon matrix using FTIR spectroscopy and employing laser radiation with wavelengths of 212 nm, 226 nm, and 230 nm. The present work broadens the wavelength range of photochemical studies of glycolic acid, thus extending the understanding of the overall photochemistry of the compound. The proposed kinetic model for the photodissociation of glycolic acid proceeds from the lowest energy conformer (SSC). The model suggests that ultraviolet light induces isomerization only between the SSC and GAC conformers and between the SSC and AAT conformers. The relative reaction rate coefficients are reported for all proposed reactions. These results suggest that the direct photodissociation of GAC and AAT conformer does not occur in an argon matrix. The main photodissociation channel via the SSC conformer produces formaldehyde–water complexes. The proposed photodissociation mechanism emphasizes that the conformers’ relative abundancies can significantly affect the photodissociation rate of the molecule. For example, in the case of high relative GAC and AAT concentrations, the ultraviolet photodissociation of glycolic acid requires the proceeding photo-isomerization of GAC and AAT to SSC.

Conformational composition of propanol in gaseous state and in matrix isolation

Conformational analysis of the experimentally recorded IR absorption spectra of propanol in gaseous state and in a low-temperature argon matrix at 20 K and at 35 K was carried out for different spectral ranges. It showed that the conformational composition of the samples in gas and in matrix isolation is different. In gaseous propanol Gt conformers predominate, while in matrix isolation the most energetically favorable form is Tg conformer, which prevails in percentage at both considered temperatures of an Ar matrix.

Conformational Space, IR-Induced, and UV-Induced Chemistry of Carvacrol Isolated in a Low-Temperature Argon Matrix.

In this work, monomers of carvacrol (5-isopropyl-2-methylphenol), a natural monoterpene exhibiting wide range bioactivity, were trapped in a cryogenic argon matrix and characterized by infrared spectroscopy, while quantum chemical calculations at the B3LYP and MP2 levels were employed to characterize the conformational landscape of the isolated molecule. Four conformers have been localized on the potential energy surface, and the factors accounting for their relative stability were analyzed. The two most stable conformers of carvacrol, differing in the relative orientation of the isopropyl group and both having the OH group pointing away from the vicinal methyl fragment, were identified in the cryomatrix for the first time. The individual spectral signatures of the two conformers were distinguished based on the change in their relative abundance induced by exposing the matrix to broadband infrared light. Matrix-isolated carvacrol was also irradiated with broadband UV light (λ > 200 nm), which resulted in the cleavage of the OH group. Recombination of the released H atom at the ortho- or para-position of the ring resulted in the formation of alkyl-substituted cyclohexadienones. These were found to undergo subsequent valence and open-ring isomerizations, leading, respectively, to the formation of a Dewar isomer and open-chain conjugated ketenes. Decarbonylation of the photoproducts was also observed for longer irradiation times. A mechanistic analysis of the observed photochemical transformations is presented.

Molecular structure, spectroscopy and photochemistry of alprazolam

In this article, a comprehensive study of the molecular structure, spectroscopy, and photochemistry of alprazolam (Xanax®; 8-chloro-1-methyl-6-phenyl-4H-[1,2,4]triazolo[4,3-a][1,4]benzodiazepine) is reported. The structure of the isolated molecule of the compound was investigated using density functional theory (DFT), revealing that the molecule exists in a single conformer, which is associated with 12 equivalent-by-symmetry minima. The molecules of the compound were trapped from the gas phase into a low temperature (10 K) argon matrix, and the infrared (IR) spectrum of the matrix-isolated monomers was obtained and assigned. The matrix-isolated molecules were then subjected to in situ ultraviolet (UV) irradiation. It was concluded that alprazolam is photostable under these experimental conditions, contrarily to what is known to happen for the compound in solution or in solid state in the presence of excipients used in the pharmaceutical formulations. An explanation for the photostability of the matrix-isolated compound is provided, based on rapid recombinations of the biradical formed from the UV-induced diazepine ring cleavage or the chlorine atom and the complementary radical resulting from the scissoring of the C–Cl bond, which are favored by the cage confinement of the matrix-isolated molecules. The major fragmentation channels of the alprazolam molecule upon electron bombardment (70 eV) were determined by analysis of its electron ionization mass spectrum, which reveals that the major primary fragmentation processes lead to formation of cyanobenzene, N2, HCl (Cl2), and benzene.The compound was also investigated in solution by multinuclear (1H, 13C and 15N) nuclear magnetic resonance (NMR) and ultraviolet (UV) spectroscopies, and in crystalline phase (P-1 polymorph) through IR and Raman spectroscopies. In addition, the structure of the crystal, previously reported in the literature [M. R. Caira, B. Easter, S. Honiball, A. Horne and L. R. Nassimbeni, Structure and Thermal Stability of Alprazolam and Selected Solvates, J. Pharm. Sci., 1995, 84, 1379–1384], was revisited in order to evaluate the relative importance of the different types of intermolecular interactions, using Hirshfeld surface analysis, the CE-B3LYP energy decomposition model, and the harmonic oscillator model of aromaticity (HOMA) index. Finally, the enthalpy of sublimation of the crystal was estimated from the CE-B3LYP calculated lattice energy.

The influence of low-temperature argon matrix on embedded water clusters. A DFT theoretical study

Computer simulations of an argon fcc crystal fragment with embedded water clusters of different sizes are performed using the quantum mechanical DFT/M06-2X method. The effect of the argon matrix on the structural, energy, and spectral parameters of individual water clusters are investigated. The formation energies of (H2O)n@Arm complexes, as well as deformation energies of water clusters and of the argon crystal involved in the embedment, are computed for n = 1–7. Matrix shifts of the IR vibrational frequencies of water clusters isolated in argon matrices are predicted based on the results of the calculations. The predictions indicate a possibility of the formation of small stable water complexes in low-temperature argon matrices.

CH⋯O H-bond mediated tautomerization of 2-methyl-1,3-cyclohexanedione: A combined IR spectroscopic and theoretical study

Molecular association and its impact on the keto-enol tautomerization of 2-methyl-1,3-cyclohexanedione (MCHD) have been investigated in low temperature argon matrix and thin solid film. The system exists exclusively in diketo tautomeric form in argon matrix. The CH⋯O H-bonded homodimers of the diketo tautomer are produced by annealing the matrix at 28 K. No trace of the keto-enol tautomer is observed in matrix isolated homodimers in the temperature range of 8–28 K. However, tautomeric conversion initiates in a thin film of pure diketo tautomer when the temperature of the film is raised to ~170 K. Transition state calculations on the monomeric and dimeric MCHD demonstrate that CH⋯O H-bond formations between diketo tautomers play a vital role in lowering the tautomerization barrier. However, the extent of CH⋯O H-bonded dimer formation in matrix isolation, as well as extent of tautomerization in the neat sample are found to be smaller than that for the previously reported 1,3-cyclohexanedione (CHD) under similar experimental conditions (J. Phys. Chem. A 2012, 116, 3836–3845). Electronic structure calculations suggest that formation of the CH⋯O H-bonded dimer is less feasible in presence of the bulky 2-methyl groups of MCHD, as compared to CHD. Additionally, the transition state geometry of the dimeric keto-enol product of MCHD, as compared to the same for CHD, is more strained and offers a weaker CH---O H-bond that contributes to lesser tautomeric conversion in the former.

Conformational Landscape and Polymorphism in 5-Acetic Acid Hydantoin.

The conformational space of 5-acetic acid hydantoin {5AAH; [2-(2,5-dioxoimidazolidin-4-yl)acetic acid]} was investigated by quantum chemical calculations performed at the DFT(B3LYP)/6-311++G(d,p) level of theory. A total of 13 conformers were located in the potential energy surface of the molecule, six of them bearing the carboxylic group in the cis arrangement (O═C-O-H dihedral equal to ∼0°) and the other seven possessing this group in the trans configuration (O═C-O-H dihedral equal to ∼180°). The most stable conformer (cis-I) was trapped from the gas phase into a low temperature argon matrix (10 K), and its infrared spectrum was fully assigned, also with help of results of normal coordinates' analysis based on the DFT computed vibrational data. The electronic structure of this conformer was analyzed by using the natural bond orbital (NBO) method. The investigation of the thermal properties of 5AAH was undertaken by differential scanning calorimetry (DSC), polarized light thermal microscopy (PLTM) and Raman spectroscopy, allowing identification of five different polymorphs. Very interestingly, in the room temperature stable polymorph the molecular units of 5AAH assume the geometry of the highest-energy conformer predicted by the calculations for the isolated molecule.

Vibrational Bands of the 2-Butyn-1-yl Radical.

The 2-butyn-1-yl radical is an isomer of C4H5 and is structurally similar to the propargyl radical, which is the simplest resonance-stabilized hydrocarbon radical. The C4H5 radical is likely to be important to astrochemistry and combustion, similar to propargyl, yet little research has been done on its spectroscopic properties. In this work, seven vibrational bands of the 2-butyn-1-yl radical are reported. The radical was formed by pyrolysis of 1-bromo-2-butyne at 800 K and isolated in a low-temperature argon matrix. The experimentally observed frequencies and intensities of the seven vibrational bands were found to be consistent with QCISD predictions from the literature and with new B3LYP calculations in this work.

Quantum-chemical simulation of low-temperature argon matrix with embedded water clusters

Quantum-chemical simulation of low-temperature argon matrix with embedded water clusters

Complexes of Glycolic Acid with Nitrogen Isolated in Argon Matrices. I. Structures and Thermal Effects.

Molecular complexes between glycolic acid and nitrogen were studied in a low-temperature argon matrix with FTIR spectroscopy, and supported by MP2 and BLYPD3 calculations. The calculations indicate 11 and 10 stable complex structures at the MP2 and BLYPD3 levels of theories, respectively. However, only one hydrogen-bonded complex structure involving the most stable SSC conformer of glycolic acid was found experimentally, where the nitrogen molecule is bound with the carboxylic OH group of the SSC conformer. The complex shows a rich site structure variation upon deposition of the matrix in different temperatures and upon annealing experiments, which provide interesting prospects for site-selective chemistry.

Photoinduced Wolff rearrangement and tautomerization of 3-chloro-2-hydroxypyridine isolated in an Ar matrix

The photoreaction mechanism of 3-chloro-2-hydroxypyridine isolated in a low-temperature argon matrix was investigated by IR spectroscopy and density functional theory (DFT) calculations. Two tautomeric forms, keto and enol I, with an intramolecular N⋯H–O hydrogen bond, existed in the matrix before UV irradiation. Tautomerization from the keto form to enol I occurred completely upon the first UV irradiation (λ ≥ 300 nm), while the second light irradiation (λ ≥ 250 nm) induced the reverse tautomerization from enol I to the keto form as well as photodissociation to yield a five-membered-ring ketene, 2H-pyrrol-2-ylidenemethanone, and HCl. The latter is similar to the photolysis of 2-chlorophenol to yield its corresponding five-membered-ring ketene by Wolff rearrangement after dissociation of HCl by UV irradiation. Kinetic analysis indicates the existence of a short-lifetime precursor, which is expected to be the less stable enol tautomer with an intramolecular Cl⋯H–O hydrogen bond. A trace of the intermediate was spectroscopically detected only during UV irradiation.

Effect of argon environment on small water clusters in matrix isolation

The influence of cryogenic argon environment on small water clusters was investigated by quantum-chemical simulation of structure and vibrational spectra of water clusters consisting of different numbers of molecules. Comparison of calculation results for vacuum and argon environment shows a red shift of spectral bands in argon. Obtained IR frequencies and intensities for water clusters in argon are compared with experimentally registered FTIR spectra of water trapped in a low-temperature argon matrix.

Effects of Entangled IR Radiation and Tunneling on the Conformational Interconversion of 2-Cyanophenol.

The conformers of 2-cyanophenol (2CP) and their interconversions were studied by infrared (IR) spectroscopy after trapping the monomers of the gaseous compound into low-temperature (15 K) argon (Ar) and nitrogen (N2) matrixes. To assist in the interpretation of the experimental results, B3LYP, MP2, and QCISD electronic structure calculations were carried out for the 2CP molecule. Two planar conformers, cis and trans (orientation of OH with respect to the cyano group), are predicted with gas-phase populations at the sublimation temperature of ∼98 and ∼2%, respectively. The most stable form (cis) was experimentally identified in both cryomatrixes, whereas the less stable one (trans) was not detected in Ar but could be observed in the N2 matrix with an abundance of ∼15%. Selective and bidirectional conversion between the two identified conformers was achieved upon irradiating the compound trapped in N2 matrix with near-infrared (NIR) laser light tuned at the wavenumbers of the 2ν(OH) transitions of the respective conformers. The conformational composition of 2CP was also found to be affected by the broad-band IR radiation emitted by the spectrometer source. This effect could be suppressed, partially or completely, by using different long-pass IR filters, with cutoff values of approximately 2200, 1590, and 1170 cm-1. The observed conformational changes are rationalized in terms of a competition between the over-the-barrier (light-induced) and through-the-barrier (hydrogen atom tunneling) effects. Very interestingly, both effects occur on the same time scale.

5-Methylhydantoin: From Isolated Molecules in a Low-Temperature Argon Matrix to Solid State Polymorphs Characterization.

The molecular structure, vibrational spectra and photochemistry of 5-methylhydantoin (C4H6N2O2; 5-MH) were studied by matrix isolation infrared spectroscopy and theoretical calculations at the DFT(B3LYP)/6-311++G(d,p) theory level. The natural bond orbital (NBO) analysis approach was used to study in detail the electronic structure of the minimum energy structure of 5-MH, namely the specific characteristics of the σ and π electronic systems of the molecule and the stabilizing orbital interactions. UV irradiation of 5-MH isolated in argon matrix resulted in its photofragmentation through a single photochemical pathway, yielding isocyanic acid, ethanimine, and carbon monoxide, thus following a pattern already observed before for the parent hydantoin and 1-methylhydantoin molecules. The investigation of the thermal properties of 5-MH was undertaken by differential scanning calorimetry (DSC), polarized light thermal microscopy (PLTM) and Raman spectroscopy. Four different polymorphs of 5-MH were identified. The crystal structure of one of the polymorphs, for which it was possible to grow up suitable crystals, was determined by X-ray diffraction (XRD). Two of the additional polymorphs were characterized by powder XRD, which confirmed the molecules pack in different crystallographic arrangements.

The acetylacetone-water complex in a low-temperature solid argon matrix

A mixture of acetylacetone and water was condensed in low-temperature Ar matrices at 5K, and infrared spectra were observed. It was found that the H2O molecules formed hydrogen bonds with the acetylacetone. The structure of the H2O-C5H8O2 complex was estimated with the aid of a density functional calculation. A similar sample was prepared using deuterium-substituted water to confirm the structure of the complex. Upon annealing the matrix-isolated sample, aggregation of H2O molecules was observed and the amount of the complex remained unchanged.

First observation of methane photochemical generation from an N,N-dimethylamino-substituted arene: the case of 4-(N,N-dimethylamino)benzaldehyde (DMABA)

The first observation of methane photochemical production from an N,N-dimethylamino-substituted arene (DMABA: 4-(N,N-dimethylamino)benzaldehyde) is described. UV irradiation (λ>298 nm) of DMABA isolated in a solid low temperature (10 K) argon matrix was found to lead to major unimolecular fragmentation of the compound to methane and 4-(methylideneamino)benzaldehyde (4MAB). This reaction channel was found to be strongly preferred over the photochemical decarbonylation, which is usually found as the major photochemical transformation undergone by matrix-isolated benzaldehydes. New structural and spectroscopic data for both the reactant molecule (DMABA) and its major observed photoproduct (4MAB) are also presented. The experimentally obtained results are supported by an extensive set of quantum chemical calculations undertaken using both ab initio (MP2) and density functional theory (DFT) methods.

Conformers of Kojic Acid and Their Near-IR-Induced Conversions: Long-Range Intramolecular Vibrational Energy Transfer.

Conformational transformations were investigated for molecules of kojic acid trapped in low-temperature argon and nitrogen matrixes. Two conformers, differing from each other by 120° rotation of the hydroxymethyl (-CH2OH) moiety, were found to be populated in freshly deposited matrixes, prior to any irradiation. Matrixes containing isolated monomers of kojic acid were irradiated with narrowband, tunable near-infrared (near-IR) laser light. Excitations at wavenumbers corresponding to the overtone of the stretching vibration of the OH bond of the hydroxymethyl group led to conversion of one of the observed conformers into another. The direction of this conformational transformation depended on the wavenumber (within the 7126-7115 cm(-1) range) used for irradiation. The same conformational photoconversion was also observed to occur upon narrowband irradiation at much lower wavenumbers (from the 6468-6447 cm(-1) range). Near-IR light from this range selectively excites overtone vibrations of the OH group directly attached to the heterocyclic ring. Such an observation provides a convincing evidence of a long-range vibrational energy transfer from the initially excited OH group (directly attached to the ring) to the remote hydroxymethyl fragment which changes its orientation. Structural changes, occurring in matrix-isolated molecules of kojic acid upon near-IR excitation, were monitored by FTIR spectroscopy.