Argon Matrix Research Articles

ИК спектроскопия высокого разрешения в низкотемпературных матрицах. Структура фундаментальных полос поглощения SiH-=SUB=-4-=/SUB=- в азотной и аргоновой матрицах

The features of high resolution IR spectra of the SiH4 molecule in N2 and Ar low-temperature matrices at 6.6-20 K were analyzed depending on experimental conditions. It was found, that in the nitrogen matrix in region of stretching vibration of 28SiH4 three narrow bands are observed, in the region of bending vibration (ν4) two bands are observed instead of one, which is explained by a change in the symmetry of the molecule from Td in the gas phase to C3v in solid nitrogen. In the argon matrix spectra change even more, where, in addition to narrow bands, the broad enough components are also recorded. The calculations of the SiH4 spectrum in the Ar matrix were performed based on the QM/MM approach, which confirm the reality of the change in the symmetry of the molecule as a result of its interaction with the matrix environment.

Experimental Tests of Natural Gas Samplers Prior to Mercury Concentration Analysis

Mercury (Hg) is a natural, trace component of natural gas. Corrosion of aluminum heat exchangers by liquid metallic Hg can lead to dramatic issues. The quantification of gaseous Hg concentration in natural gas streams is therefore crucial prior to the implementation of Hg removal units for preventing the formation of liquid Hg. Different methodologies exist for the determination of Hg concentration in natural gas, one of which relies on the sampling of natural gas at high pressure using stainless-steel cylinders prior to off-site Hg measurement. An inert internal coating is supposed to hamper Hg adsorption, presumably making the Hg analysis reliable. Here, we challenge this statement by showing that even silicon-coated cylinders are inefficient for preventing Hg adsorption on internal walls. Different cylinders were tested for gaseous Hg concentration stability over time in a clean Argon matrix. We find that gaseous Hg concentration sharply declines in almost all tested cylinders (uncoated, PTFE-coated an...

Quenched tunneling in ammonia

Quantum Mechanics Inversion between two energetically equivalent umbrella-spaced configurations in ammonia (NH3) is a classic textbook example of tunneling in a symmetric double-minimum potential. Park et al. report on how a very strong direct-current electric field, as powerful as intermolecular interactions, can suppress this quantum mechanical phenomenon in the system. They observed that NH3 molecules isolated in a solid argon matrix at 10 kelvin strongly orient along the external field direction and that, as the field is increased up to 200 million volts per meter, their energy states shift, eventually quenching the tunneling from the normal state to the inverted one. The observed changes are reversible and open up opportunities for manipulating tunneling dynamics in many other molecules. Proc. Natl. Acad. Sci. U.S.A. 116 , 23444 (2019).

Alpha-Carbonic Acid Revisited: Carbonic Acid Monomethyl Ester as a Solid and its Conformational Isomerism in the Gas Phase.

In this work, earlier studies reporting α‐H2CO3 are revised. The cryo‐technique pioneered by Hage, Hallbrucker, and Mayer (HHM) is adapted to supposedly prepare carbonic acid from KHCO3. In methanolic solution, methylation of the salt is found, which upon acidification transforms to the monomethyl ester of carbonic acid (CAME, HO‐CO‐OCH3). Infrared spectroscopy data both of the solid at 210 K and of the evaporated molecules trapped and isolated in argon matrix at 10 K are presented. The interpretation of the observed bands on the basis of carbonic acid [as suggested originally by HHM in their publications from 1993–1997 and taken over by Winkel et al., J. Am. Chem. Soc. 2007 and Bernard et al., Angew. Chem. Int. Ed. 2011] is inferior compared with the interpretation on the basis of CAME. The assignment relies on isotope substitution experiments, including deuteration of the OH‐ and CH3‐ groups as well as 12C and 13C isotope exchange and on variation of the solvents in both preparation steps. The interpretation of the single molecule spectroscopy experiments is aided by a comprehensive calculation of high‐level ab initio frequencies for gas‐phase molecules and clusters in the harmonic approximation. This analysis provides evidence for the existence of not only single CAME molecules but also CAME dimers and water complexes in the argon matrix. Furthermore, different conformational CAME isomers are identified, where conformational isomerism is triggered in experiments through UV irradiation. In contrast to earlier studies, this analysis allows explanation of almost every single band of the complex spectra in the range between 4000 and 600 cm−1.

The interplay of VSCF/VCI calculations and matrix-isolation IR spectroscopy – Mid infrared spectrum of CH3CH2F and CD3CD2F

We present the first matrix-isolation infrared (MI-IR) spectra of CH3CH2F and its isotopologue CD3CD2F in Neon and Argon matrix, together with new gas-phase IR spectra. Extensive vibrational self-consistent field and configuration interaction (VSCF/VCI) calculations are performed, based on an ab initio potential energy surface at ae-CCSD(T)-F12a/cc-pCVTZ-F12 level of electronic structure theory. We encounter an excellent agreement between VCI calculated transitions and the experimental MI-IR and gas-phase IR spectra. Mean absolute deviations are scattering between 1 and 4 cm−1. The interplay of accurate vibrational structure calculations and high-resolution infrared experiments enables unprecedented insights in the CH respectively CD stretch region, providing the first rigorous assignment of the energetically very close ν1,ν12 and ν13 fundamental transitions.

Conformer-Specific Heavy-Atom Tunneling in the Rearrangement of Benzazirines to Ketenimines.

5-Methoxy-2H-benzazirine was prepared via irradiation of the corresponding phenyl azide, isolated in an argon matrix at cryogenic temperatures. It undergoes ring expansion to the corresponding ketenimine in the dark at T < 30 K despite a calculated activation barrier of 4.9 kcal mol-1 [B3LYP/6-311++G(d,p)]. Since this rearrangement proceeds with a rate constant in the order of 10-4 s-1, exhibiting only a shallow temperature dependence, the results are interpreted in terms of heavy-atom tunneling. Of the four isomeric benzazirines resulting from the initial photolysis, only one can be observed to rearrange; this conformer specificity is explained by the other potentially observable rearrangements being either too fast or too slow to be detected due to the differences in heights and widths of their respective activation barriers.

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.

Niobium isocyanide complexes, Nb(CNAr)6, with Ar= 2,6-dimethylphenyl (Xyl), a diamagnetic dimer containing four reductively coupled isocyanides, and Ar= 2,6-diisopropylphenyl (Dipp), a paramagnetic monomer analogous to the highly unstable hexacarbonylniobium(0).

Treatment of bis(mesitylene)niobium(0) with 6-7 equivalents of 2,6-dimethylphenyl isocyanide (CNXyl) affords two products with the empirical formula Nb(CNXyl)n (n= 7 or 6), which have been shown to be the diamagnetic dimers bis[μ-N,N',N'',N'''-tetrakis(2,6-dimethylphenyl)squaramidinato(2-)]bis[pentakis(2,6-dimethylphenyl isocyanide)niobium(I)], [Nb2(C9H9N)10(C36H36N4)] or [Nb(CNXyl)5]2[μ-C4(NXyl)4]·xSolvent, 1, and bis[μ-N,N',N'',N'''-tetrakis(2,6-dimethylphenyl)squaramidinato(2-)]bis[tetrakis(2,6-dimethylphenyl isocyanide)niobium(I)] tetrahydrofuran trisolvate, [Nb2(C9H9N)8(C36H36N4)]·3C4H8O or [Nb(CNXyl)4]2[μ-C4(NXyl)4]·3THF (THF= tetrahydrofuran), 2. Each contains NbI bound to either five or four terminal isocyanides, respectively, and to an unprecedented bridging tetraarylsquaramidinate(2-) unit, coordinated as a bidentate ligand to each niobium center, symmetrically due to the crystallographic inversion center that coincides with the centroid of the central C4 unit. Thus, in the presence of CNXyl, the bis(mesitylene)niobium(0) is oxidized to niobium(I), resulting in the facile loss of both mesitylene groups and the reductive coupling of two CNXyl groups per niobium to provide the first examples of tetraarylsquaramidinate(2-) ligands, [cyclo-C4N4Ar4]2-, coordinated to metals. In contrast, bis(mesitylene)niobium(0) reacts with the more crowded 2,6-diisopropylphenyl isocyanide (CNDipp) to afford the paramagnetic monomer hexakis(2,6-diisopropylphenyl isocyanide)niobium(0), [Nb(C13H17N)6] or Nb(CNDipp)6, 3, the first zero-valent niobium isocyanide analog of the highly unstable Nb(CO)6, which is presently only known to exist in an argon matrix at 4.2 K.

Empirically Modified Potentials of Interaction between Rare Gases for Matrix Isolation Problems

Simple ways of modeling the gas-phase potentials of rare gas Ne, Ar, Kr, and Xe dimers are proposed to describe solid matrices and atomic and molecular systems isolated in them. The fitting parameters for Ne, Ar, Kr, and Xe, based on the reproduction of the lattice structural parameter and energy of atomization, were calculated. The resulting effective solid-state potentials of inert gas dimers were applied to the problem of modeling the sodium atom captured by argon matrix. The effect of solid-state modification on the geometries of stable trapping sites and on the shifts and shapes of the calculated electronic absorption spectrum of the Na@Ar system was investigated. It is shown that using the effective solid-state potentials can improve the agreement with experimental values.

Infrared Spectroscopic and Theoretical Studies of the 3d Transition Metal Oxyfluoride Molecules

A collection of 3d transition metal (V, Mn, Fe, Co, and Ni) oxyfluorides were prepared via the reactions of laser-ablated metal atoms and OF2 in an argon matrix, and the products were identified by infrared spectroscopy together with 18OF2 substitution. OMF2 is the major product from the reactions of metal atoms and OF2. The tetravalent metal center is coordinated to two fluorine atoms and one oxygen atom. Triatomic OMF molecules were observed in the reactions of V, Mn, Fe, and Co with OF2. In addition to OMF and OMF2, OMnF3 and OFeF3 were also formed presumably via the reactions of OMnF and OFeF with F2 resulting from photodecomposition of OF2. The seldom observed OF radical was produced in all of these experiments. Electronic structure calculations at the density functional theory and molecular orbital theory including electron correlation effects (CCSD(T) and CASPT2) levels are used to aid in the assignment of the structures. For OMF (M = Sc-Mn), the structures are bent and those for M = Fe-Cu are linear. The OMF2 molecules are optimized to be C2v structures. Both OMF and OMF2 have a high spin ground state, with the exception of OCoF2 in which the ground state quartet is the lower energy structure. The M-O stretching frequency is a sensitive measure of the computational method in terms of the bond angle, the coupling of the M-O and M-F stretches, and the amount of spin on the oxygen. A bonding analysis in terms of the CAS orbitals shows that a number of the structures have a multireference character after M = Cr. Oxidation states of the metal are given based on the CASPT2 results. Heats of formation for the OMF and OMF2 are reported.

Infrared spectroscopy of the n-propyl and i-propyl radicals in solid para-hydrogen

We report infrared spectra of the n- and i-propyl radicals isolated in solid para-hydrogen (p-H2). Propyl radicals were produced via 248 nm irradiation of matrices formed by co-deposition of p-H2 and either 1-iodopropane (forming n-propyl) or 2-iodopropane (forming i-propyl). Secondary photolysis was used to group spectral bands arising from the same species. In addition to those observed previously in either argon matrix or helium droplet spectra, we assign approximately 20 previously unreported bands. The assignment of features to propyl radicals is made via comparisons to VPT2+K anharmonic frequency computations. In separate measurements, propyl radicals were generated via the reaction between propene and hydrogen atoms produced within the p-H2 matrix. Because the barriers are between 700 and 1500 cm−1, the H + propene reaction is necessarily under tunneling control when carried out in solid p-H2 at 3.3 K. i-Propyl radicals were produced in ∼5–20 times greater abundance than n-propyl radicals, indicating that H atom addition to the terminal carbon is faster than addition to the central carbon. The Wentzl-Kramers-Brillouin approximation is applied to one-dimensional abinitio potentials to estimate the tunneling time constants for formation of n- and i-propyl. This model predicts the rate of addition to the terminal carbon (forming i-propyl) to be 17 times faster, in qualitative agreement with the experimental branching ratio.

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.

Electron-induced delayed desorption of solid argon doped with methane

The total yield of particles desorption from solid Ar doped with CH4 under irradiation with an electron beam was studied at 5 K. The measurements were carried out at a CH4 concentration of 1 and 5%. The effect of explosive delayed desorption from the surface of argon matrix was discovered in both mixtures. With a higher concentration of CH4, it appeared at lower doses and was more pronounced. Two types of self-oscillations were observed: long-period bursts (on a time scale of about 25 min) and short-period oscillations (of about 10 s). In pure solid Ar delayed desorption was not observed despite the accumulation of a significant number of excess electrons, exceeding their number in mixtures of Ar and CH4 as it was found by measurements of thermally stimulated exoelectron emission. This finding discards the model of Coulomb explosion for the phenomenon detected. In this paper we focused on the role of hydrogen (one of the radiolysis products) in delayed desorption. The formation of atomic hydrogen in the matrix was traced via cathodoluminescence by the emission band of the excimer Ar2H* at 166 nm. Desorption of excited hydrogen atoms in the excited state was detected by the Ly-α emission line. A decrease of the Ar2H* band intensity at higher concentration of CH4 was found evidencing bleaching these centers likely due to recombination of H atoms with energy release and formation of molecular hydrogen. The data obtained give additional evidence in favor of the hypothesis that the exothermic reactions of radiolysis products serve as a stimulating factor for delayed desorption.

Modeling and spectral simulation of formic acid dimer in Ar matrix using ONIOM calculations

The supermolecule approach has been used to model molecules embedded in a solid argon matrix. The interaction between the guest and the host atoms in the first solvation shell (FSS) are evaluated using density functional calculations while that beyond the FSS is incorporated using ONIOM calculations to extend the results of our previous study [F. Ito, J. Chem. Phys. 133 (2010) 214502]. The vibrational spectra of a formic acid dimer (FAD) in an Ar matrix was simulated using (FAD)-Arn clusters (n = 94–1095). Structural relaxation in the first solvation shell is crucial to quantitatively reproduce the matrix shifts of fundamental bands, according to calculations at the B971/6-31++G(3df,3pd) level. Moreover, the influence of the outer Ar atoms is negligible.

New possible route of HC3N formation in Titan’s atmosphere

The structures of the C2N2/C2H2 complex in solid argon matrices have been investigated using FTIR spectroscopy and ab initio calculations, at the aug-cc-pVTZ level of theory. Predicted frequency shifts for the linear structure, characterized by a strong hydrogen bond between the nitrogen of C2N2 and the acetylenic proton were found in good agreement with the ones from the experiment. The photodissociation of C2N2–C2H2 complex trapped in argon matrix at 10 K has been performed at 120 nm. The FTIR measurement and ab initio calculations pointed to the formation of HC3N, HCN and probably HC2NC and HNC as final reactional products. This mechanism route is potentially important for chemical models of the Titan’s atmosphere.

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.

Argon matrix Raman spectroscopic and aggregation model density function theoretical study of methyl isothiocyanate

AbstractThe Raman spectra for methyl isothiocyanate (MTC) isolated in an argon matrix at different annealing temperatures and MTC in carbon tetrachloride at different concentrations were collected separately. Concentration‐dependent properties were characterized, and no noncoincidence effect (NCE) was observed. Argon matrix isolated Raman spectra shows that, the SCNC stretching at 6K presents as one distinct peak, it separates into two peaks at 25–35 K and merges as one peak again above 40 K, whereas its frequency shifted to a lower wavenumber. Concentration‐dependent experiments showed the peak frequency of SCNC stretching shifted to a higher wavenumber when the concentrations decreased. Aggregated dimer structures were proposed to explain the MTC SCNC and CS vibrational concentration effects with no NCE phenomenon. Density functional theories were performed to calculate the monomer and dimer of MTC; the dimer structure can explain the experimental data well. The strong antiparallel head‐to‐tail vibrational coupling of the SCNC oscillators in neighbouring molecules accounts for the frequency shifts of the SCNC stretching.

Photochemistry of XCH2CN (X = -Cl, -SH) in Argon Matrices.

We report infrared spectra and photochemical behavior of the potentially astrochemically significant species, mercaptoacetonitrile (HS-CH2C≡N) and, for comparison purposes, chloroacetonitrile (Cl-CH2C≡N), both suspended in an argon matrix at 6 K. Photolytic formation of the isocyano products HS-CH2-NC and Cl-CH2-NC were observed as well as CH3NSC and CH3SCN (in HS-CH2CN photolysis). While no dissociation products were observed for Cl-CH2-CN, photolysis of HS-CH2-CN produced compounds necessitating the loss of the CN group to form CH2═S, the SH group to form H2C-CN and HC-CN, or both CN and SH to form CH3 and CH4. Observation of emission spectra upon annealing indicates the presence of free sulfur atom in matrices of photolyzed HS-CH2-CN.

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.