Argon Matrices Research Articles

Activation and Coordination of Ammonia at [Cp*Ir(H)2]: NMR and Matrix Isolation Studies

(1)H NMR exchange spectroscopy of a reaction mixture of [Cp*Ir(H)(4)] (1; Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) and ammonia suggests an exchange of hydrogen atoms between the hydrido ligands and ammonia. Treatment of 1 with ND(3) led to an H/D exchange between ND(3) and the hydrido ligands of 1. Subsequent studies showed that photolysis of 1 isolated in frozen argon matrices leads to the formation of the iridium compounds [Cp*Ir(H)(2)] (2) and [Cp*Ir(H)(3)] (4), as it was confirmed by IR spectroscopy. In the presence of water the aqua complex [Cp*Ir(H)(2)(OH(2))] (3) was generated simultaneously. Accordingly, photolysis of 1 in an argon matrix doped with ammonia gave rise to the ammine complex [Cp*Ir(H)(2)(NH(3))] (5). IR assignments were supported by calculations of the gas-phase IR spectra of 1-5 by DFT methods.

Tautomeric transitions of isocytosine isolated in argon and neon matrices induced by UV irradiation

The tautomeric equilibrium of the isocytosine molecules in the low temperature Ar and Ne matrices has been investigated by the FTIR matrix isolation spectroscopy and quantum-chemical calculations at the DFT, MP2, and CCSD(T) levels of theory. Broad-band UV irradiation of matrix isolated isocytosine results in an increase of the abundance of the minor isocytosine tautomer, KA1. After UV irradiation the intensity of the spectral bands assigned to this tautomer increases more than 1.5 times and the bands of other minor conformers are not observed. The values of ΔH (6.6kJ/mol) and ΔG (6.8kJ/mol) determined based on the experimental data are close to the ΔE value (8.3kJ/mol) calculated at the CCSD/aug-cc-pVDZ level of theory. Two Fermi resonances involving the νCO vibration of the minor tautomer KA1 (2-amino-4-oxo-pyrimidine or keto–amino) and four Fermi resonances involving the ring stretching vibrations of the most abundant tautomer EA1 (2-amino-4-hydroxy-pyrimidine or enol–amino) are found. The performance of the different computational methods in predicting the vibrational spectra of isocytosine is evaluated.

Thermolysis of 3,3,5,5‐Tetramethyl‐1,2,4‐trithiolane 1‐Oxide: First Matrix Isolation of the HOSS· Radical

AbstractFlash vacuum pyrolysis of 3,3,5,5‐tetramethyl‐1,2,4‐trithiolane 1‐oxide performed at 700 °C yields the 1‐oxatrisulfan‐3‐yl radical (HOSS·) along with disulfur monoxide (S2O) and diisopropyl sulfide, which were isolated in argon matrices at 10 K. Upon irradiation with UV light, the 1‐oxatrisulfan‐3‐yl radical undergoes isomerization to the 1‐oxatrisulfan‐1‐yl radical (HSSO·). Both radicals were identified by comparison of their computed and experimental IR and UV/Vis spectra. In addition, density functional theory (DFT) computations offer a plausible explanation of the most likely reaction mechanism, suggesting that the initial step is a 1,3‐H shift with simultaneous ring opening. A 1‐oxatrisulfane derivative formed thereby undergoes fragmentations via a radical and a competitive concerted pathway leading to the observed final products. The same mechanism also governs the thermal fragmentation of di‐tert‐butyl disulfide S‐oxide. Its pyrolysis at 700 °C affords an analogous set of products, including the 1‐oxatrisulfan‐3‐yl radical (HOSS·) as the key intermediate.

UV-induced isomerization of β-alanine isolated in argon matrices

We have employed low-temperature matrix-isolation FTIR spectroscopy, the density functional theory and ab initio calculations at the MP2 and CCSD(T) levels of theory to determine the conformational composition of the simplest β-amino acid, β-alanine. UV irradiation and thermal annealing of the samples together with the FTIR spectra of deuterated β-alanine were used to separate bands of different conformers. A detailed study of the potential energy surface of β-alanine obtained at the MP2/aug-cc-pVDZ level of theory reveals twenty β-alanine conformers, but only five of them may exist in matrices due to their sufficiently high relative stabilities and low energy barriers separating them from each other. An analysis of the FTIR spectra allows us to confirm the presence of four β-alanine conformers in argon matrices with certainty. Two of them, conformers I and II, have an NH⋯O intramolecular H-bond, the third, conformer V, has an N⋯HO H-bond, and the fourth, conformer IV, has no intramolecular H-bonds. The relative populations of the conformers determined using the relative Gibbs free energies calculated at the CCSD(T)/CBS level of theory at 420K are 48.1%, 23.7%, 16.8% and 3.2% for the conformers I, II, IV, and V, respectively. Some trace amount of conformer VII was also detected.

The Phenoxyl Radical–Water Complex—A Matrix Isolation and Computational Study

The phenoxyl radical 1 was generated in high yields by flash vacuum pyrolysis of allyl phenyl ether 2 with subsequent trapping of the products in argon at 3 K. In water-doped argon matrices, an OH···O complex between 1 and water is formed that could be characterized by IR spectroscopy. Several isotopomers of the complex were generated, and the IR spectra compared to results of density functional theory calculations. Other dimers between 1 and water were not found under these conditions. QM/MM calculations in simulated argon matrices reveal that an OH···π complex is unstable even at a time scale of picoseconds. This finding has implications on the related interaction between the tyrosyl radical and the water in biological systems.

Using heavy atom rare gas matrix to control the reactivity of 4-methoxybenzaldehyde: A comparison with benzaldehyde

Different patterns of photochemical behavior were observed for 4-methoxybenzaldehyde (p-anisaldehyde) isolated in xenon and in argon matrices. Monomers of the compound isolated in solid Xe decarbonylate upon middle ultraviolet irradiation, yielding methoxybenzene (anisole), and CO. On the other hand, p-anisaldehyde isolated in an Ar matrix and subjected to identical irradiation, predominantly isomerizes to the closed-ring isomeric ketene (4-methoxycyclohexa-2,4-dien-1-ylidene) methanone. Experimental detection of a closed-ring ketene photoproduct, generated from an aromatic aldehyde, constitutes a rare observation. The difference between the patterns of photochemical transformations of p-anisaldehyde isolated in argon and xenon environments can be attributed to the external heavy-atom effect, where xenon enhances the rate of intersystem crossing from the singlet to the triplet manifold in which decarbonylation (via p-methoxybenzoyl radical) takes place. The parent compound, benzaldehyde, decarbonylates (to benzene + CO) when subjected to middle ultraviolet irradiation in both argon and xenon matrices. This demonstrates the role of the methoxy p-anisaldehyde substituent in activation of the reaction channel leading to the formation of the ketene photoproduct.

First observation of infrared and UV–visible absorption spectra of adenine radical in low-temperature argon matrices

A UV-light induced photoreaction product of adenine in low-temperature argon matrices was identified as an adenine radical, where the hydrogen atom in the NH group was detached from adenine, by analyses of IR and UV–visible absorption spectra with aids of the density-functional-theory (DFT) and time-dependent DFT calculations. The adenine radical returned to adenine by recombination with the detached hydrogen atom upon longer-wavelength UV irradiation. A reversible isomerization to another adenine isomer via the adenine radical was also observed when the wavelength of UV irradiation was changed. The photoreaction mechanism of adenine in low-temperature argon matrices is discussed, including UV-light induced amine–imine tautomerism.

Investigation of the thermal and photochemical reactions of ozone with styrene in argon and krypton matrices

The matrix isolation technique, combined with infrared spectroscopy and twin jet codeposition, has been used to characterize intermediates formed during the ozonolysis of styrene. Absorptions assigned to early intermediates grew in after warming the matrix from 19K to 68K in the twin jet krypton matrix experiments. A number of these absorptions have been assigned to the primary ozonide, formaldehyde-O-oxide Criegee intermediate, and secondary ozonide of styrene, transient species not previously observed for this system. In contrast, the room temperature reaction of ozone with styrene led to the observation of “late,” stable products of this ozonolysis reaction. These product absorptions were observed after merged jet deposition, followed by cryogenic trapping in solid argon. Irradiation with λ⩾220nm of merged and twin jet argon matrices involving ozone led to O atom production and subsequent reaction with styrene. Identification of intermediates formed during the ozonolysis of styrene was further supported by 18O isotopic labeling experiments as well as theoretical density functional calculations at the B3LYP/6-311G++(d,2p) level.

Conformers, Infrared Spectrum and UV-Induced Photochemistry of Matrix-Isolated Furfuryl Alcohol

The infrared spectra of furfuryl alcohol (2-furanmethanol, FFA) were investigated for FFA monomers isolated in low-temperature argon matrices. The structural interpretation of the obtained experimental spectra was assisted by analysis of the molecule's conformational landscape. According to the DFT(B3LYP)/6-311++G(d,p) calculations, five different minimum energy structures were found on the potential energy surface of the molecule. They can be defined by the orientation of the OCCO and CCOH dihedral angles: GG', GG, TG, TT, GT (G = +gauche, G' = -gauche, T = trans) and have a symmetry equivalent configuration: GG' = G'G, GG = G'G', TG = TG', GT = G'T. When zero-point energies are taken into account, only three (GG', GG, and TT) out of the five unique minima correspond to stable structures. The most stable conformer GG' (OCCO, 72.7°; CCOH, -59.3°), which in gas phase at room temperature accounts for ∼65% of the total population, was the only form isolated in the argon matrices at 14 K. The other two relevant forms convert into conformer GG' during matrix deposition. The low temperature glassy and crystalline states of FFA were also obtained and their infrared spectra assigned, suggesting the sole existence of the GG' conformer also in these phases. The photochemical behavior of FFA induced in situ, by tunable UV-laser, was also studied. The longest wavelength resulting in photochemical changes in the structure of the irradiated sample was found to be λ = 229 nm. Such UV irradiation of the matrix-isolated FFA led to production of formaldehyde and different isomeric C(4)H(4)O species. Cycloprop-2-ene-1-carbaldehyde and buta-2,3-dienal (two conformers) are the main initial C(4)H(4)O photoproducts formed upon short-time excitation at λ = 229 nm. But-3-ynal (two conformers) was the principal photoproduct resulting from prolonged excitation at λ= 229 nm, being consumed upon irradiation at shorter wavelengths (λ < 227.5 nm). Vinyl ketene is produced from FFA in the trans conformation and undergoes isomerization to the cis form upon irradiation at λ < 227.5 nm. Cyclopropene, propyne, allene, and CO were also identified in the irradiated matrices (in particular at the later stages of irradiation), suggesting that the photoproduced aldehydes partially decarbonylate during the performed photochemical experiments.

Intramolecular hydrogen-atom tunneling and photoreaction mechanism of 4-bromo-2-chloro-6-fluorophenol in low-temperature argon matrices

Infrared spectra of 4-bromo-2-chloro-6-fluorophenol in low-temperature argon matrices were measured with an FTIR spectrometer. Despite that an F-type geometrical isomer, which has an intramolecular hydrogen bond of OH⋯F, is as stable in the CCl4 solution at room temperature as a Cl-type geometrical isomer, which has that of OH⋯Cl, no infrared bands of the F-type were observed in an argon matrix at 20K. This implied that the F-type changed to the more stable Cl-type by hydrogen-atom tunneling in the argon matrix. To confirm the tunneling isomerization, similar experiments on the deuterated species were performed, resulting in that an OD stretching band of the F-type appeared as well as that of the Cl-type. This finding suggested that a deuterium is inhibited from the tunneling migration. The UV-induced photoreaction pathways of 4-bromo-2-chloro-6-fluorophenol were additionally examined, resulting in that 2-fluoro-4-bromocyclopentadienylidenemethanone was produced from the Cl-type by Wolff rearrangement after dissociation of the H atom in the OH group and the Cl atom. However no cyclohexadienone derivative, which is analogous to a photoproduct of a parent molecule, 2-chloro-6-fluorophenol, was produced upon UV irradiation. It is concluded that the bromine atom located at the 4th position disturbs the intramolecular migration of the Cl atom.

Infrared spectra and ultraviolet-tunable laser induced photochemistry of matrix-isolated phenol and phenol-d5

Monomers of phenol and its ring-perdeuterated isotopologue phenol-d(5) were isolated in argon matrices at 15 K. The infrared (IR) spectra of these species were recorded and analyzed. In situ photochemical transformations of phenol and phenol-d(5) were induced by tunable UV laser light. The photoproducts have been characterized by IR spectroscopy supported by theoretical calculations of the infrared spectra. The primary product photogenerated from phenol was shown to be the phenoxyl radical. The analysis of the progress of the observed phototransformations led to identification of 2,5-cyclohexadienone as one of the secondary photoproducts. Spectral indications of other secondary products, such as the Dewar isomer and the open-ring ketene, were also detected. Identification of the photoproducts provided a guide for the interpretation of the mechanisms of the observed photoreactions.

Solvation effects and stabilization of multicharged ions: a case study of ArmBeOq+ complexes

Using first-principle methods, we characterized Ar(m)BeO(q+) (0 ≤ m ≤ 3 and 0 ≤ q ≤ 2) and BeO(q+) (3 ≤ q ≤ 5) multicharged ions (MCIs). This includes their structural parameters, relative stabilities and vibrational wavenumbers. Our calculations confirm the existence of the ArBeO complex. In addition, we found (meta-)stable neutral and ionic complexes such as ArBeO(+), ArBeO(2+), Ar(2)BeO, Ar(2)BeO(+), Ar(2)BeO(2+), Ar(3)BeO, Ar(3)BeO(+) and Ar(3)BeO(2+). The analysis of the structural parameters and of the electron density differences shows that a strong perturbation in the electronic structure of the BeO(q+) (q = 1,2) moiety occurs upon complexation, resulting in a major increase in covalency of these multicharged ions (MCIs). The consequences of solvation of MCIs in argon matrices are pointed out. More generally, the effects on the spectroscopy of MCIs trapped on cold matrices are discussed.

Ethane cation decomposition characterization by EBMI spectroscopy: gas‐phase dissociative recombination as a source of secondary products

The decomposition products of the d(6) -ethane cation following charge-transfer ionization with Ar(+) , under conditions of varying ionization electron current, have been isolated in solid argon matrices at 18 K and examined using Fourier transform infrared spectroscopy. Gas samples containing 1 : 1600 d(6) -ethane : Ar were subjected to electron bombardment by using either a high (pin) or a low (plate) ionization density anode configuration with ionization currents between 20 and 150 μA. Under high ionization density conditions, the observed major products were d(4) -ethene (C(2) D(4) ) and d(2) -acetylene (C(2) D(2) ), with smaller yields of C(2) D(5) , C(2) D(3) , and C(2) D. The yield of each dehydrogenation product was enhanced with increased current. Analogous experiments employing the low ionization density plate anode resulted in reduced C(2) D(6) destruction and the formation of only C(2) D(4) and C(2) D(2) . The results suggest the onset of dissociative recombination processes under high ion density conditions. In this context, the results can be interpreted as a dissociative recombination of primary ion products, which gives rise to further dehydrogenation, and appearance of additional neutral radical products.

Low Temperature Matrix Isolation IR-Spectroscopy and Quantum Chemical Study of β-Alanine Structure

In this study we have focused on conformational structure of the β-alanine molecule isolated in low-temperature argon matrices. The main purpose was to determine the set of the β-alanine conformers, which can occur in the matrices. Matrix isolation FTIR-spectroscopy was combined with quantum chemical calculations performed by the MP2 and DFT(B3LYP) methods using the aug-cc-pVDZ and aug-cc-pVTZ basis sets. Gibbs free energies of the β-alanine conformers also were calculated at the CCSD(T)/CBS level of theory.Totally 21 β-alanine conformers were found at the MP2/aug-cc-pVDZ level of theory. But 10 of them are separated from lower energy conformers by low energy barriers and they can not present in the matrixes. High-resolution FTIR spectra were registered for the samples immediately after deposition as well as for the samples which were UV irradiated or annealed to 25K. Both the UV irradiation and the matrix annealing result in redistribution of the band intensities in the FTIR spectra. It allows us to distinguish spectral bands of different β-alanine conformers. Assignment of the spectral bands was performed based on calculated vibrational spectra of the β-alanine conformers. As a result we detected presence of 4 β-alanine conformers in the Ar matrices.Further data about β-alanine conformational structure can be used in molecular dynamics simulations and also they can be useful for searching of β-alanine in the interstellar space.Key words: β-alanine, matrix isolation, quantum chemical calculations.

Complexation of formaldoxime with water. Infrared matrix isolation and theoretical studies

The 1:1, 1:2 and 2:1 formaldoxime-water complexes isolated in the argon matrices have been studied by help of FTIR spectroscopy and MP2/6-311++G(2d,2p) method. The calculations predicted the stability of the three CH 2NOH⋯H 2O isomeric complexes, three CH 2NOH⋯(H 2O) 2 ones and one (CH 2NOH) 2⋯H 2O complex. The analysis of the experimental spectra and their comparison with theoretical ones indicated that both the 1:1 and 1:2 complexes trapped in solid argon have the most stable cyclic structures stabilized by the O–H⋯O and O–H⋯N bonds between the formaldoxime and water molecules. In the 1:2 complex formaldoxime interacts with the water dimer, one H 2O molecule acts as a proton acceptor for the OH group of formaldoxime whereas the second H 2O molecule acts as a proton donor toward the nitrogen atom of the formaldoxime molecule. In the (CH 2NOH) 2⋯H 2O complex the OH group of the water molecule acts as a proton donor toward one of the oxygen atoms of the formaldoxime cyclic dimer.

Nitrene-carbene rearrangement during photolysis of 2-azido-1,3,5-triazines in argon matrices

It was shown using IR spectroscopy and ESR spectroscopy that UV irradiation of 2-azido-4,6-dichloro-1,3,5-triazine isolated in solid argon resulted in triplet 4,6-dichloro-1,3,5-tri-azinyl-2-nitrene (D = 1.384 cm−1, E = 0.004 cm−1), whose further photochemical transformation included the consecutive formation of 3-didehydro-1,2,4,6-tetraazepine, 2-chloro-1-diazochloromethyl-2-isocyanocarboimide, and presumably triplet 2-chloro-1-chloromethyl-idene-2-isocyanocarboimide and isocyanodichloroacetonitrile. The photolysis of 2-azido-4,6-dimethoxy-1,3,5-triazine and 2-azido-4,6-di(dimethylamino)-1,3,5-triazine affords photo-chemically stable triplet 4,6-dimethoxy-1,3,5-triazinyl-2-nitrene (D = 1.436 cm−1, E = 0.0044 cm−1) and 4,6-bis(dimethylamino)-1,3,5-triazinyl-2-nitrene (D = 1.468 cm−1, E = 0.0042 cm−1) as the final products.

The S–H⋯N versus O–H⋯N hydrogen bonding in the ammonia complexes with CH3OH and CH3SH

Abstract The methanol and methanethiol complexes with ammonia were studied by means of infrared matrix isolation spectroscopy and ab initio MP2/6-311++G(3df,3pd) calculations. The calculations resulted in the stability of two similar structures for each of the two complexes: CH 3 OH–NH 3 , CH 3 SH–NH 3 , and in addition, in the third structure for the latter complex. In the argon matrices the most stable complexes stabilized by the O–H⋯N or S–H⋯N bond were formed. The OH, SH stretching frequencies exhibited 266, 84 cm −1 red shifts in the complexes with respect to the monomers which correlate well with the calculated frequency shifts for the most stable CH 3 OH–NH 3 , CH 3 SH–NH 3 structures and their energies of interactions (Δ E = −18.8 kJ mol −1 for CH 3 OH–NH 3 and Δ E = −7.1 kJ mol −1 for CH 3 SH–NH 3 ). Both theoretical and experimental spectra demonstrate the large increase of the intensity of the SH stretching vibration after formation of the complex (more than 300 times), as compared to the OH stretching vibration (less than 20 times). The small intensity of the SH band in the CH 3 SH spectrum and its large growth in CH 3 SH–NH 3 one are explained by atypical behavior of the dipole moment function of the SH proton donor.

The HRuCCH, RuCCH2, and Ru-η2-C2H2 Molecules: Infrared Spectra and Density Functional Calculations

Laser-ablated ruthenium atoms undergo reaction with acetylene during condensation in excess neon and argon matrices to form a metallacycle complex, insertion into the C-H bond, and rearrangement to the vinylidene complex. The subject molecules were identified by (13)C(2)H(2) and C(2)D(2), isotopic substitutions and density functional theory (DFT) frequency calculations. The HRuCCH molecule is described by Ru-H, CH, and CC stretching modes and CCH deformation modes. A very strong CC double bond stretching, weak CH stretching, and CCH deformation frequencies were observed for the Ru═C═CH(2) complex. The metallacycle Ru-η(2)-(C(2)H(2)) is characterized through CC double bond stretching, CH stretching and CCH deformation modes. The reaction mechanism for formation of the Ru═C═CH(2) complex was investigated by B3LYP internal reaction coordinate calculations, and the hydrido-alkyny complex is the rate-determining step. The delocalized three-center-four-electron π bond using the Ru 4d(xz) electron pair contributes to the C-C π* orbital and provides stabilization energy (ΔE((2)), second-order perturbation) for the vinylidene Ru═C═CH(2) complex.

Matrix-isolation study of the phenol–water complex and phenol dimer

The FTIR experiments in argon matrices provide a large number of bands of the phenol–water complex and phenol dimer. The assignment of the bands is done by using calculations at the DFT B3LYP level of theory. For the phenol–water system, three structures are obtained computationally but only the strongest complex is found in the experiments. The spectral effect due to hydrogen bonding is somewhat larger in the phenol–water complex compared to the phenol dimer in both experiment and theory. The formation of the hydrogen bonded phenol dimer and phenol–water complex is very efficient and always should be taken into account in matrix-isolation experiments with phenol.

Noble Gas Atoms as Electron Donors: Is the Stabilization of Strongly Electrophilic Borylnitrenes Feasible?

AbstractThe possible interaction between an electrophilic borylnitrene, CatBN (Cat = Catecholato), with the noble gas atoms Rg (Rg = Kr, Xe) is probed by computational chemistry techniques including hybrid density functional (B3LYP), second‐order Møller–Plesset perturbation (MP2), and coupled‐cluster theory with singles, doubles, and a perturbative estimate of triple excitations [CCSD(T)] in conjunction with aug‐cc‐pVDZ and cc‐pVTZ and the corresponding small‐core pseudopotential correlation consistent basis sets for krypton and xenon. In the triplet manifold the interaction between CatBN and Rg is of the van der Waals type with a small N–Rg bond dissociation energy. In contrast, the singlet CatBNRg(1A′) has a short N–Rg distance (Kr: 1.962 Å; Xe: 2.029 Å) in agreement with a single bond character. The N–Xe bond dissociation energy is 17 kcal·mol–1 at the highest level of theory, while the N–Kr bond is significantly weaker. Experiments in xenon or xenon doped argon matrices do not give any indication for the formation of the CatBNXe(1A′) compound in spite of the strong N–Xe interaction predicted by theory. The experimental observation is explained by the existence of a crossing of singlet and triplet potential energy surfaces. At the geometry of triplet CatBNRg (large N–Rg distance) the singlet state is higher in energy whereas at the geometry of singlet CatBNRg (short N–Rg distance) the triplet is higher. Fast intersystem crossing to the lower lying triplet surface in the presence of Xe is assumed to preclude the formation of CatBNXe(1A′).