Ne Matrices Research Articles

The Trifluorooxygenate Anion [OF3]-: Spectroscopic Evidence for a Binary, Hypervalent Oxygen Species.

Experimental evidence for hypervalent compounds of second-row elements is still scarce in literature. Here, we present the first report of the long-sought binary hypervalent trifluorooxygenate anion [OF3]-. It was isolated in solid Ne matrices under cryogenic conditions after reacting oxygen difluoride with free fluoride ions from laser ablation of alkali metal fluorides MF (M = Li-Cs). As predicted by VSEPR theory and calculations at the CCSD(T) level, and confirmed by FTIR spectroscopy as well as isotopic labeling, [OF3]- exhibits a C2v-symmetric T-shaped structure with one short and two long O-F bond distances. Analysis of the natural local molecular orbitals shows the presence of 2c-2e bond and one 3c-4e bond. Natural resonance theory indicates the importance of the stability of [OF2]•- for the stability of [OF3]-. Although free [OF2]•- was not detected, the species MOF2 (M = Na-Cs) could be observed in the same experiments, which are best described as contact ion pairs of M+[OF2]•-.

The Trifluorooxygenate Anion [OF3]−: Spectroscopic Evidence for a Binary, Hypervalent Oxygen Species

Experimental evidence for hypervalent compounds of second‐row elements is still scarce in literature. Here, we present the first report of the long‐sought binary hypervalent trifluorooxygenate anion [OF3]−. It was isolated in solid Ne matrices under cryogenic conditions after reacting oxygen difluoride with free fluoride ions from laser ablation of alkali metal fluorides MF (M = Li–Cs). As predicted by VSEPR theory and calculations at the CCSD(T) level, and confirmed by FTIR spectroscopy as well as isotopic labeling, [OF3]− exhibits a C2v‐symmetric T‐shaped structure with one short and two long O‐F bond distances. Analysis of the natural local molecular orbitals shows the presence of 2c‐2e bond and one 3c‐4e bond. Natural resonance theory indicates the importance of the stability of [OF2]•− for the stability of [OF3]−. Although free [OF2]•− was not detected, the species MOF2 (M = Na–Cs) could be observed in the same experiments, which are best described as contact ion pairs of M+[OF2]•−.

Pyridine Dimers and Their Low-Temperature Isomerization: A High-Resolution Matrix-Isolation Spectroscopy Study.

The bonding between two neutral aromatic compounds, especially small ones, has been controversially debated in the last decades, and terms like "π-stacking" had to be revised. Surprisingly, despite of many experimental and computational work, there is still no clear consensus about the structure of and the bonding in the pyridine dimer. In this work, for different isomeric forms of the pyridine dimer, the structures and bonding were elucidated by combining high-resolution matrix-isolation spectroscopic results with quantum-chemical calculations. High-resolution IR spectra of Ne matrices at 4 K containing pyridine were recorded for different concentrations and upon annealing to 10 and 12 K, relying on three isotopologues of pyridine. The spectra show the presence of hydrogen-bonded, T-shaped, and stacked forms of weakly-bound pyridine dimers. Among these, the hydrogen-bonded isomer is identified as the lowest-energy form. The results provide for the first time conclusive information about the interaction between two pyridine dimers.

Pyridine Dimers and Their Low‐Temperature Isomerization: A High‐Resolution Matrix‐Isolation Spectroscopy Study

AbstractThe bonding between two neutral aromatic compounds, especially small ones, has been controversially debated in the last decades, and terms like “π‐stacking” had to be revised. Surprisingly, despite of many experimental and computational work, there is still no clear consensus about the structure of and the bonding in the pyridine dimer. In this work, for different isomeric forms of the pyridine dimer, the structures and bonding were elucidated by combining high‐resolution matrix‐isolation spectroscopic results with quantum‐chemical calculations. High‐resolution IR spectra of Ne matrices at 4 K containing pyridine were recorded for different concentrations and upon annealing to 10 and 12 K, relying on three isotopologues of pyridine. The spectra show the presence of hydrogen‐bonded, T‐shaped, and stacked forms of weakly‐bound pyridine dimers. Among these, the hydrogen‐bonded isomer is identified as the lowest‐energy form. The results provide for the first time conclusive information about the interaction between two pyridine dimers.

Curtius-Type Rearrangement of Sulfinyl Azides: A Matrix Isolation and Computational Study.

The highly unstable methyl sulfinyl azide, CH3S(O)N3, has been synthesized and characterized for the first time. In the gas phase, CH3S(O)N3 decomposes quickly at room temperature (300 K) with an estimated half-life (t1/2) of 7 min. Upon irradiation at 266 nm in cryogenic Ar (10 K) and Ne (3 K) matrices, the azide extrudes molecular nitrogen by yielding the novel sulfinyl nitrene intermediate CH3S(O)N in the closed-shell singlet ground state, which has been characterized with matrix-isolation IR and UV-vis spectroscopy. Prolonged irradiation at 266 nm causes Curtius rearrangement of the nitrene to form N-sulfinylamine CH3NSO and S-nitrosothiol CH3SNO. By high-vacuum flash pyrolysis (HVFP) at 800 K, CH3S(O)N3 also decomposes and furnishes CH3S(O)N with minor fragmentation products HNSO and CH2 in the gas phase. A similar photo-induced Curtius-type rearrangement of trifluoromethyl sulfinyl azide CF3S(O)N3 to CF3NSO and CF3SNO has also been observed in matrices. According to the theoretical calculations at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G (3df,3pd) level of theory, the rearrangement of CH3S(O)N3 prefers a stepwise pathway by initial formation of the nitrene intermediate CH3S(O)N. In line with the thermal persistence of CH3S(O)N in the gas phase, the barriers for its subsequent rearrangement are higher than 30 kcal mol-1.

Unique O═N...O Pnicogen Interactions in Nitromethane Dimers: Evidence Using Matrix Isolation Infrared Spectroscopy and Computational Methodology.

Geometries of nitromethane homodimers have been revisited using ab initio and density functional methodologies, following their formation within cryogenic matrices, confirmed using infrared spectroscopy. In contrast to the claim that the intermolecular interactions are due to dispersion forces or very weak hydrogen bonds, in the present work, concrete evidence for the prevalence of O═N...O pnicogen bonding has been presented. The pnicogen bonds have been found to be primarily responsible for the characteristic geometry of a homodimer. The formation of a nitromethane dimer with pnicogen bonding stabilization is evidenced using matrix isolation infrared spectroscopy with Ne and Ar matrices and computations. The interactions within homodimers have been characterized using quantum theory of atoms in molecules, natural bond orbital, energy decomposition, electrostatic potential mapping, and noncovalent interaction analyses. The larger intermolecular separations in liquid nitromethane indicated by previous molecular dynamics-based studies alongside the supposed invariance of infrared spectra in the gas phase and liquid phase could have led to the assumption of a lack of intermolecular interactions. However, the prevalence of hydrogen and pnicogen bonds across these larger than usual separations is affirmed by the geometries presented here.

Fluoromethylsulfinyl radicals: spectroscopic characterization and photoisomerization via intramolecular hydrogen shift.

Two new sulfinyl radicals, CHF2SO˙ and CH2FSO˙, have been generated in the gas phase through homolytic cleavage of the weak S-S bonds in disulfane oxides CHF2S(O)SCF3 and CH2FS(O)SCF3 by high-vacuum flash pyrolysis (HVFP) at ca. 500 °C. The IR spectroscopy characterization of the two fluoromethylsulfinyl radicals in solid N2 (10 K), Ar (10 K), and Ne (3 K) matrices reveals the presence of two conformers for CHF2SO˙ (gauche and cis) and one conformer for CH2FSO˙ (gauche). Upon 266 nm laser irradiation, these radicals undergo both isomerization and decomposition in the matrices. In addition to the dominant formation of the elusive oxathiyl radicals CHF2OS˙ (gauche and cis) and CH2FOS˙ (gauche) via 1,2-alkyl migration, two higher-energy carbon-centered radicals ˙CF2SOH and ˙CHFSOH bearing similar molecular structures to hydroperoxyalkyl radicals (˙QOOH) form via intramolecular 1,3-hydrogen shift in the two sulfinyl radicals. Additionally, the involvement of 1,3-hydrogen shift in CHF2OS˙ and CH2FOS˙ is also indicated by the observation of the fragmentation species. The identification of these radicals by matrix-isolation IR and UV-vis spectroscopy is aided by the quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level of theory. The stability of the isomers of the two sulfinyl radicals CHF2SO˙ and CH2FSO˙ has been discussed according to the experimental observations and also based on the CCSD(T)-F12a/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) calculated energy profiles.

Photochemical transformations of 4,6-dihydroxypyrimidine and 2-methyl-4,6-dihydroxypyrimidine isolated in low-temperature Ar, Ne and H2 matrices

Monomers of 4,6-dihydroxypyrimidine and 2-methyl-4,6-dihydroxypyrimidine were trapped from the gas phase into low-temperature Ar, Ne and normal-H2 matrices. Dihydroxy and oxo-hydroxy tautomers were identified. The isolated monomers were exposed to UV (285 > λ > 270 nm) radiation. For the molecules isolated in Ar and Ne matrices, such UV excitation led to conversion of the oxo-hydroxy tautomer into three products: the dihydroxy form, the Dewar isomer and the open-ring ketene. In solid-hydrogen matrices, UV-induced oxo-hydroxy → dihydroxy hydrogen-atom-transfer conversion did not occur; the UV-excited oxo-hydroxy form of the investigated compounds transformed only to the Dewar and open-ring ketene products.

Effects of trapping site on the spectroscopy of 1P1 excited group 12 metal atoms in rare gas matrices

A molecular dynamics deposition model has been used to simulate the growth of rare gas matrices doped with atoms of the group 12 elements zinc, cadmium and mercury. This study investigates the sites occupied by Zn, Cd and Hg metal atoms when isolated in the solid rare gases. To probe the results, the resonance 1P1←1S0 transitions of the matrix-isolated metal atoms were calculated and compared with the recorded spectra of the M/RG solids. The theoretical spectroscopy obtained in this work was generated using the molecular dynamics with quantum transitions method. In Ne matrices the metal atoms preferably occupy tetra- and hexa-vacancy sites while in the case of Xe matrices, only the single vacancy site is formed. For Ar and Kr matrices Zn but especially Cd can be trapped in tetra- and hexa-vacancy sites in addition to single-vacancy sites, while Hg atoms show exclusive occupancy in single vacancy sites.

Structures and infrared spectra of 5-chlorouracil molecules in the low-temperature inert Ar, Ne matrices and composite films with oxide graphene

The structures and vibrational spectra of biologically active molecules of 5-chlorouracil (5CU) in various states were studied. The IR Fourier spectra of 5CU molecules isolated in the low-temperature Ar and Ne matrices were obtained in the infrared range of 3800–500 cm–1. The populations of 5CU tautomers were estimated, and vibrational spectra were calculated using the DFT/B3LYP method with a basis of 6-311++G(df,pd). In the Ar matrix, 7 Raman modes of 5CU amplified by Fermi resonance were detected in the spectral range of 1900–500 cm–1. In the region of fundamental vibrations νCO and in the vibration region of the ring, triple Fermi resonances were observed. Thin 5CU films on the surface of graphene oxide (GO) and composite films 5CU/GO obtained from an aqueous solution were grown and investigated at cryogenic temperatures. It was shown that, as the temperature rises from the helium temperature to room temperature, the 5CU film crystallizes on the GO surface. Comparison of the spectra of the 5CU films obtained enabled identification of vibrations whose absorption bands were most sensitive to changes in the crystal structure of the film. Most of these fundamental modes were associated with vibrations of the NH group. With an increase in the concentration of 5CU in the 5CU/GO composite films, a nonuniform distribution of the 5CU micro-crystals over the sample area was observed.

How Inert, Perturbing, or Interacting Are Cryogenic Matrices? A Combined Spectroscopic (Infrared, Electronic, and X-ray Absorption) and DFT Investigation of Matrix-Isolated Iron, Cobalt, Nickel, and Zinc Dibromides.

The interactions of FeBr2, CoBr2, NiBr2, and ZnBr2 with Ne, Ar, Kr, Xe, CH4, and N2 matrices have been investigated using IR, electronic absorption, and X-ray absorption spectroscopies as well as DFT calculations. ZnBr2 is linear in all of the matrices. NiBr2 is linear in all but N2 matrices, where it is severely bent. For FeBr2 and CoBr2 there is a more gradual change, with evidence of nonlinearity in Xe and CH4 matrices as well as N2. In the N2 matrices, the presence of νNN modes blue-shifted from the "free" N2 values indicates the presence of physisorbed species, and the magnitude of the blue shift correlates with the shift in the ν3 mode of the metal dibromide. In the case of NiCl2 and NiBr2, chemisorbed species are formed after photolysis, but only if deposition takes place below 10 K. There was no evidence for chemisorbed species for NiF2 and FeBr2, and in the case of CoBr2 the evidence was not strong.

Magnetically Bistable Nitrenes: Matrix Isolation of Furoylnitrenes in Both Singlet and Triplet States and Triplet 3-Furylnitrene

Two simple acylnitrenes, 2-furoylnitrene (2) and 3-furoylnitrene (6), were generated through 266 nm laser photolysis of the corresponding azides. Both are magnetically bistable in cryogenic matrices, as evidenced by the direct observation of the closed-shell singlet state with IR spectroscopy in solid Ne, Ar, Kr, Xe, and N2 matrices (3-40 K) and the triplet state in toluene (10 K) with EPR spectroscopy (32: |D/hc| = 1.48 cm-1 and |E/hc| = 0.029 cm-1; 36: |D/hc| = 1.39 cm-1 and |E/hc|c = 0.039 cm-1). Subsequent visible-light and UV laser irradiations led to the formation of furyl isocyanates (3 and 7) and ring-opening product 3-cyanoacrolein (9-E and 9-Z), respectively, in which the elusive 3-furylnitrene (38) was also identified by IR and EPR spectroscopy (|D/hc| = 1.12 cm-1 and |E/hc| = 0.005 cm-1).

Fast Heavy-Atom Tunneling in Trifluoroacetyl Nitrene.

Chemical reactions involving quantum mechanical tunneling (QMT) increasingly attract the attention of scientists. In contrast to the hydrogen-tunneling as frequently observed in chemistry and biology, tunneling solely by heavy atoms is rare. Herein, we report heavy-atom tunneling in trifluoroacetyl nitrene, CF3 C(O)N. The carbonyl nitrene CF3 C(O)N in the triplet ground state was generated in cryogenic matrices by laser (193 or 266 nm) photolysis of CF3 C(O)N3 and characterized by IR and EPR spectroscopy. In contrast to the theoretically predicted activation barriers (>10 kcal mol-1 ), CF3 C(O)N undergoes rapid rearrangement into CF3 NCO with half-life times of less than 10 min and unprecedentedly large 14 N/15 N kinetic isotope effects (1.18-1.33) in solid Ar, Ne, and N2 matrices even at 2.8 K. The tunneling disappearance of CF3 C(O)N becomes much slower in the chemically active toluene and in 2-methyltetrahydrofuran at 5 K.

Investigation of Dissolution Behavior HPMC/Eudragit®/Magnesium Aluminometasilicate Oral Matrices Based on NMR Solid-State Spectroscopy and Dynamic Characteristics of Gel Layer

Burst drug release is often considered a negative phenomenon resulting in unexpected toxicity or tissue irritation. Optimal release of a highly soluble active pharmaceutical ingredient (API) from hypromellose (HPMC) matrices is technologically impossible; therefore, a combination of polymers is required for burst effect reduction. Promising variant could be seen in combination of HPMC and insoluble Eudragits® as water dispersions. These can be applied only on API/insoluble filler mixture as over-wetting prevention. The main hurdle is a limited water absorption capacity (WAC) of filler. Therefore, the object of this study was to investigate the dissolution behavior of levetiracetam from HPMC/Eudragit®NE matrices using magnesium aluminometasilicate (Neusilin® US2) as filler with excellent WAC. Part of this study was also to assess influence of thermal treatment on quality parameters of matrices. The use of Neusilin® allowed the application of Eudragit® dispersion to API/Neusilin® mixture in one step during high-shear wet granulation. HPMC was added extragranularly. Obtained matrices were investigated for qualitative characteristics, NMR solid-state spectroscopy (ssNMR), gel layer dynamic parameters, SEM, and principal component analysis (PCA). Decrease in burst effect (max. of 33.6%) and dissolution rate, increase in fitting to zero-order kinetics, and paradoxical reduction in gel layer thickness were observed with rising Eudragit® NE concentration. The explanation was done by ssNMR, which clearly showed a significant reduction of the API particle size (150-500nm) in granules as effect of surfactant present in dispersion in dependence on Eudragit®NE amount. This change in API particle size resulted in a significantly larger interface between these two entities. Based on ANOVA and PCA, thermal treatment was not revealed as a useful procedure for this system.

Methoxysulfinyl Radical CH3OSO: Gas-Phase Generation, Photochemistry, and Oxidation.

Methylsulfoxide radicals CH3SOx (x = 1-4) are key reactive sulfur species (RSS) in the atmospheric oxidation of volatile organic sulfur compounds (VOSCs). Through flash vacuum pyrolysis (FVP) of trifluoromethanesulfinic acid methyl ester CF3S(O)OCH3 at 1000 K, the methoxysulfinyl radical CH3OSO has been generated in the gas phase and subsequently characterized in cryogenic N2, Ar, and Ne matrices by IR spectroscopy. Upon 266 nm laser irradiation, CH3OSO efficiently isomerizes to the less stable methylsulfonyl radical CH3SO2 in matrices without noticeable decomposition. In the gas phase, CH3OSO reacts with O2 and yields sulfinyl peroxyl radical CH3OS(O)OO, a new member in the CH3SOx (x = 1-4) family. This radical dissociates into SO3 and CH3O with the intermediacy of the sulfonyoxyl radical CH3OSO3 under the 266 nm laser irradiation. Additionally, the photoisomerization of CF3S(O)OCH3 to sulfenic ester CF3OSOCH3 was also observed.

Modelling the matrix shift on the vibrational frequency of ThO by DFT-D3 calculations.

Benchmark calculations with a goal to find dispersion-corrected DFT-D3 methods suitable for a reliable estimation of matrix shifts on the vibrational frequency were carried out on the ThO molecule in three rare gas (Rg = Ne, Ar, and Kr) matrices. The matrices were modelled by the explicit approach, in which a single and a double shell of Rg atoms around ThO was considered. The selection of exchange-correlation functionals was based on test calculations on triatomic ThO⋯Rg models. The B3LYP, PBE0, CAM-B3LYP, and LC-ωPBE functionals were found to be the best suited for the estimation of matrix shifts. The single shell of Rg's around ThO accounted for a major part of the shifts; the addition of a second Rg shell resulted only in a minor improvement. Continuum solvation models considerably overestimated the effect of Rg matrices both when the whole matrix was treated by the model and when the first shell was treated explicitly and the rest with a continuum solvation model.

ChemInform Abstract: A Matrix Isolation and Computational Study of Molecular Palladium Fluorides: Does PdF6 Exist?

AbstractBinary palladium fluorides from PdF to PdF6 are investigated by matrix‐isolation methods using thermal evaporation and laser ablation to generate Pd atoms for reaction with F2‐doped Ar and Ne matrices as well as neat F2 matrices.

Defect creation via dissociative recombination of ionic centers in solid Ne matrices

Recombination of the intrinsic ionic centers Ne2+ (self-trapped holes) with the detrapped electrons in solid Ne matrices and relaxation channels have been studied. The experiments were performed employing combination of the cathodoluminescence (CL) with current and optical activation spectroscopy techniques. CL spectra were recorded simultaneously in the VUV and visible range. Yields of spectrally resolved thermally and photon-stimulated luminescence (TSL, PSL) and thermally and photon-stimulated exoelectron emission (TSEE, PSEE) were measured in the time-correlated manner. It was found that the recombination reaction proceeds with irreversible dissociation of the transient Ne2∗∗ centers and the dissociative recombination (DR) products exit the matrix cage. Products of the DR reaction are found to be in 3s and 3p states. The detection of “defect” components in the TSL and PSL points to the defect formation via DR in Ne matrices. The temperature range of the electron traps stability is elucidated. A long-lasting “afteremission” of electrons and afterglow of VUV photons observed on switching off the irradiation suggest the accumulation of the uncompensated negative charge.

The electronic structure of VO in its ground and electronically excited states: A combined matrix isolation and quantum chemical (MRCI) study.

The electronic ground and excited states of the vanadium monoxide (VO) molecule were studied in detail. Electronic absorption spectra for the molecule isolated in Ne matrices complement the previous gas-phase spectra. A thorough quantum chemical (multi-reference configuration interaction) study essentially confirms the assignment and characterization of the electronic excitations observed for VO in the gas-phase and in Ne matrices and allows the clarification of open issues. It provides a complete overview over the electronically excited states up to about 3 eV of this archetypical compound.

Ultraviolet photolysis studies on XeO4 in noble-gas and F2 matrices and the formation and characterization of a new Xe(VIII) oxide, (η(2) -O2 )XeO3.

The photolytic behavior of the thermochemically unstable xenon(VIII) oxide XeO4 was investigated by UV irradiation in noble-gas and F2 matrices. Photolysis of Xe(16) O4 or Xe(18) O4 in noble-gas matrices at 365 nm yielded XeO3 and a new xenon(VIII) oxide, namely, (η(2) -O2 )XeO3 , which, along with XeO4 , was characterized by matrix-isolation IR spectroscopy and quantum-chemical calculations. Calculations of the UV spectrum showed that the photodecomposition is induced by an n→σ* transition, but the nature of the excitation differs when different light sources are used. There is strong evidence for the formation of mobile (1) D excited O atoms in the case of excitation at 365 nm, which led to the formation of (η(2) -O2 )XeO3 by reaction with XeO4 . Matrix-isolation IR spectroscopy in Ne and Ar matrices afforded the natural-abundance xenon isotopic pattern for the ν3 (T2 ) stretching mode of Xe(16) O4 , and (18) O enrichment provided the (16) O/(18) O isotopic shifts of XeO4 and (η(2) -O2 )XeO3 .