Matrix Isolation Study Research Articles

Vibrational spectra of monoisotopic SiH4 and GeH4 in low-temperature matrices

IR absorption spectra of monoisotopic 28SiH4 and 76GeH4 are studied in Ar and N2 matrices at 10 K. It is shown that the absorption spectra of silane and germane are similar in the regions of the stretching ν3 and bending ν4 vibrations. Four groups of bands can be separated out in the spectra of each molecule: (1) narrow bands characteristic of the matrix isolation studies, (2) broad bands, (3) diffuse absorption with a large value of the spectral moment M2* the intensity of which increases upon annealing, and (4) bands of dimers the intensity of which increases quadratically with concentration. The spectra of 28SiH4 and 76GeH4 in nitrogen matrices contain a triplet in the stretching region and a doublet in the bending region, which is explained by the change in the molecular symmetry from Td to C3V on passage from the gas phase to solid nitrogen.

Photochemistry of Tetrasulfur Tetranitride: Laser Flash Photolysis and Quantum Chemical Study

The photochemistry of tetrasulfur tetranitride (1) was studied in hexane solution by the laser flash photolysis technique (LFP). The experimental findings were interpreted using the results of previous matrix isolation studies (Pritchina, E.A.; Gritsan, N.P.; Bally, T.; Zibarev, A.V. Inorg. Chem. 2009, 48, 4075) and high-level quantum chemical calculations. LFP produces two primary intermediates, one of which is the boat-shaped 8-membered cyclic compound (2) and the other is the 6-membered S(3)N(3) cyclic compound carrying an exocyclic (S)-N═S group (3). The primary products 2 and 3 absorb a second photon and undergo transformation to the 6-membered S(3)N(3) cycle carrying an exocyclic (N)-S≡N group (4), which is very unstable and converts back to intermediate 3. The quantum yield of the primary product formation is close to unity even though the quantum yield of photodegradation of 1 is low (~0.01). Thus, 1 is a photochromic compound undergoing in solution the thermally reversible photochemical isomerization. The mechanism of the photochromic process was established, and the rate constants of the elementary reactions were measured.

Comparison of the Conformational Behavior of Amino Acids and N-Acetylated Amino Acids: A Theoretical and Matrix-Isolation FT-IR Study of N-Acetylglycine

Within the structure determination task for peptides, which is of large interest due to the relation between structure and functionality, infrared spectra can provide detailed information on the conformational behavior. The conformational landscape ofN-acetylgycine has been studied by a combined theoretical and matrix-isolation FT-IR study. The acetylation simulates an amino acid a peptide bond. Four stable conformations were found at the MP2/6-31++G** level of theory. Among these, only one contains an intramolecular H-bond that has a small abundance at the considered temperature. Apart from this one, three other different conformations could be detected in an Ar matrix. The experimental rotamerization constants NAG2 ⇌ NAG1 and NAG3 ⇌ NAG1 could be estimated. The values of the rotamerization constants as well as the mean frequency deviation of N-acetylglycine were combined with previously obtained data of other N-acetylated amino acids and they appeared to be similar to the data for nonsubstituted amino acids. This suggests that the used methodology can be in the future applied to investigate small peptides. Analysis of H-bond frequency shifts and distance demonstrates that the intramolecular H-bonds in N-acetylated amino acids are stronger compared to those in nonsubstituted amino acids.

Infrared Matrix Isolation Study of the Thermal and Photochemical Reactions of Ozone with Dimethylcadmium

The matrix isolation technique has been combined with infrared spectroscopy and theoretical calculations to explore the reaction of (CH(3))(2)Cd with O(3) over a range of time scales and upon irradiation. During twin jet deposition, multiple novel product species were observed along with several stable "late" products. Following annealing of these matrices to 35 K, absorptions due to two novel product species increased in intensity. In addition, new bands appeared, indicating the formation of an additional product. Subsequent UV irradiation destroyed several of the initial products and produced a new photoproduct. On the basis of (18)O and (16,18)O spectroscopic data and theoretical calculations, the novel intermediates H(3)COCdCH(3), H(3)CCdCH(2)OH, H(3)COCdOOCH(3), and H(3)CCdCHO were identified. Merged jet deposition led to a number of stable "late" products, including H(2)CO, CH(3)OH, and C(2)H(6), identifications that were confirmed by (18)O substitution. Mechanistic inferences for this reaction are discussed.

Gas-Phase Raman Spectra of s-trans- and s-gauche-1,3-Butadiene and Their Deuterated Isotopologues

The gas-phase Raman spectra of 1,3-butadiene and its 2,3-d(2),1,1,4,4-d(4) and d(6) isotopologues have been recorded using intense (6 W) green laser excitation and sensitive CCD detection. Hundreds of bands have been observed and assigned for each isotopologue. These spectra provide the best data to date for the s-trans conformer and also provide the first direct observation of the gas-phase Raman bands of the s-gauche conformer. Spectra recorded at elevated temperatures up to 250 °C for the d(0) and d(6) species help confirm the assignment of bands for the gauche rotamer. DFT computations were utilized to complement the studies. For the most part, the observed gas-phase gauche bands are in good agreement with previous matrix isolation studies. A best set of frequencies are reported for the fundamentals of the gauche rotamer of the d(0) and d(6) species.

Matrix isolation study of the thermal and photochemical reaction of ozone with Trimethyl Indium

The matrix isolation technique has been combined with infrared spectroscopy and theoretical calculations to explore the reaction of (CH 3) 3In with O 3 over a range of time scales. Upon twin jet deposition (short reaction time), formation of the novel H 3COIn(CH 3) 2 species along with a low yield of CH 2O was observed. Subsequent UV irradiation greatly increased the yield of H 3COIn(CH 3) 2 while the intensities of the CH 2O bands were not affected. An extensive set of bands were seen for H 3COIn(CH 3) 2 after irradiation and 18O spectroscopic data was obtained as well. The identification of this species was supported by theoretical calculations at the B3LYP/lanl2dz and B3LYP/dgdzvp levels of theory. Merged jet deposition (longer reaction times) led to high yield of H 2CO, CH 3OH and C 2H 6, identifications that were confirmed by 18O substitution. Mechanistic inferences for the initial steps of this reaction are discussed.

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.

Estimation of the rotamerization constants of different conformations of N-acetylalanine: A theoretical and matrix-isolation FT-IR study

The conformational landscape of N-acetylalanine has been investigated by a theoretical and matrix-isolation FT-IR study. Optimizations of N-acetylalanine structures has been conducted at successive higher levels of theory HF/3-21G, DFT(B3LYP)/6-31++G** and MP2/6-31++G**. This resulted in three stable conformations. Among these, one conformation contains an intramolecular H-bond. The vibrational properties of these conformations were calculated and used to identify the conformations in a cryogenic argon matrix. The intensities of some bands assigned to a particular conformation were used to estimate the rotamerization constants K r12 and K r13 for the equilibria NAA1 NAA2 and NAA1 NAA3, respectively. The obtained experimental values were in agreement with the theoretical predictions.

The Pyrolysis of Isoxazole Revisited: A New Primary Product and the Pivotal Role of the Vinylnitrene. A Low-Temperature Matrix Isolation and Computational Study

This paper describes the pyrolysis of parent isoxazole and of its 5-methyl and 3,5-dimethyl derivatives by the high-pressure pulsed pyrolysis method, where activation of the precursor molecules occurs predominantly by collisions with the host gas (Ar in our case), rather than with the walls of the pyrolysis tube, where catalyzed processes may occur. The products were trapped at 15 K in Ar matrices and were characterized by vibrational spectroscopy. Thereby, hitherto unobserved primary products of pyrolysis of isoxazole and of its 5-methyl derivative, 3-hydroxypropenenitrile or 3-hydroxybutenenitrile, respectively, were observed. E-Z photoisomerization could be induced in the above hydroxynitriles. On pyrolysis of isoxazole, ketenimine and CO were observed as decomposition products, but this process did not occur when the 5-methyl derivative was pyrolyzed. Instead, the corresponding ketonitrile was formed. In the case of 3,5-dimethylisoxazole, 2-acetyl-3-methyl-2H-azirine was detected at moderate pyrolysis temperatures, whereas at higher temperatures, 2,5-dimethyloxazole was the only observed rearrangement product (next to products of dissociation). These findings are rationalized on the basis of quantum chemical calculations. Thereby it becomes evident that carbonyl-vinylnitrenes play a pivotal role in the observed rearrangements, a role that had not been recognized in previous theoretical studies because it had been assumed that vinylnitrenes are closed-shell singlet species, whereas they are in fact open-shell singlet biradicaloids. Thus, the primary processes had to be modeled by the multiconfigurational CASSCF method, followed by single-point MR-CISD calculations. The picture that emerges from these calculations is in excellent accord with the experimental findings; that is, they explain why some possible products are observed while others are not.

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.

Vibrational Spectrum and Structure of CoO6: A Model Compound for Molecular Oxygen Reversible Binding on Cobalt Oxides and Salts; A Combined IR Matrix Isolation and Theoretical Study

The formation and structure of a novel species, a disuperoxo-cobalt dioxide complex (CoO(6)), has been investigated using matrix isolation in solid neon and argon, coupled to infrared spectroscopy and by quantum chemical methods. It is found that CoO(6) can be formed by successive complexation of cobalt dioxide by molecular oxygen without activation energy by diffusion of ground state O(2) molecules at 9K in the dark. The IR data on one combination and seven fundamentals, isotopic effects, and quantum chemical calculations are both consistent with an asymmetrical structure with two slightly nonequivalent oxygen ligands complexing a cobalt dioxide subunit. Evidence for other, metastable states is also presented, but the data are not complete. The electronic structure and formation pathway of this unique, formally +VI oxidation state, complex has been investigated using several functionals of current DFT within the broken-symmetry unrestricted formalism. It has been shown that the M06L pure local functional well reproduce the experimental observations. The ground electronic state is predicted to be an open shell (2)A'' doublet with the quartet states above by more than 9 kcal/mol and the sextet lying even higher in energy. The ground state has a strong and complex multireference character that hinders the use of more precise multireference approaches and requires caution in the methodology to be used. The geometrical, energetic, and vibrational properties have been computed.

The influence of the peptide bond on the conformation of amino acids: A theoretical and FT-IR matrix-isolation study of N-acetylproline

A combined experimental matrix-isolation FT-IR and theoretical study has been performed to investigate the conformational behavior of N-acetylproline. The conformational landscape of N-acetylproline was explored using successively higher computational methods, i.e. HF, DFT(B3LYP) and finally MP2. The exploration resulted in 10 conformations with a relative energy difference smaller than 22 kJ.mol −1 at the HF/3-21G level of theory. These conformations led to six different conformations after DFT(B3LYP) optimizations. Further optimization at the MP2/6-31++G** level of theory resulted in the same six conformations, all of them with an energy difference smaller than 11.5 kJ.mol −1. One conformation with an intramolecular H-bond was found which was energetically the most favorable conformation. The vibrational and thermodynamical features were calculated using the DFT and MP2 methodologies. In the experimental matrix-isolation FT-IR spectrum, the most stable conformation was dominant and at least two non-H-bonded conformations could be identified. An experimental rotamerization constant between the H-bonded and the other non-H-bonded conformations was estimated and appeared to agree reasonably well with the theoretical MP2 predictions. Some new spectral features of N-acetylproline compared to proline were discovered which might be used to discriminate between the acetylated and non-acetylated form.

Matrix isolation study of the formation of methanol cluster structures in the spectral region of C–O and O–H stretch vibrations

Infrared absorption spectra of methyl alcohol isolated in an argon matrix are recorded experimentally. The transformation of the structure of the spectral bands with rising matrix temperature is analyzed in the spectral region of the C–O and O–H stretch deformation vibrations (1000–1100 and 3000–3800 cm−1). The experimental data are interpreted using quantum-chemical modelling of an optimal spatial structure and of the vibrational spectra of different nano-sized methyl alcohol clusters. The structure of the clusters changes gradually as the temperature of the argon matrix is raised from 10 to 50 K.

FTIR Matrix-Isolation Study of the Reaction Products of Vanadium Atoms with Propene: Observation of Allylvanadium Hydride As a Precursor to Sacrificial Hydrogenation of Propene

Vanadium atoms have been reacted with different partial pressures of propene in Ar under matrix-isolation conditions, and the products have been observed using Fourier transform infrared (FTIR) spectroscopy. Under dilute propene in Ar conditions, new features are observed in the IR spectra corresponding to a C-H insertion product, identified here as H-V-(η(3)-allyl). Use of d(3)-propene (CD(3)-CH═CH(2)) demonstrates that the initial V-atom insertion occurs at the methyl group of the propene molecule, and DFT calculations have been used to support the identity of the initial product. Upon increasing the partial pressure of propene, additional features corresponding to propane (C(3)H(8)) are observed, with the hydrogen-atom source for the observed hydrogenation demonstrated to be additional propene units. Analysis of a systematic increase in the partial pressure of propene in the system demonstrates that the yield of propane correlates with the decrease of the allyl product, demonstrating the H-V(allyl) species as a reactive intermediate in the overall hydrogenation process. An overall mechanism is proposed to rationalize the formation of the insertion product and ultimately the products of hydrogenation, which agrees with previous gas-phase and matrix-isolation work involving propene and the related system, ethene.

Matrix Isolation Studies on Sulfur Heterocycles and Related Species

The matrix isolation technique in combination with vacuum pyrolysis, photochemistry, and quantum mechanical computations was applied to study the structures and reactivities of dimethoxycarbene (DMC) and 1,3-dithian-2-ylidene. 1,2,4-Trithiolane was used for the generation of thioformaldehyde S-sulfide and the isomeric dithiirane. Irradiation of thioformaldehyde S-oxide in argon matrix at 10 K led to the first isolation of parent oxathiirane as well as HCSOH, a remarkable species with a rare formal carbon–sulfur triple bond.

ChemInform Abstract: C—F Bond Activation of Highly Fluorinated Molecules at Rhodium: From Model Reactions to Catalysis

AbstractReview: ca. 100 refs.

Electron‐bombardment matrix isolation and PEPICO studies as complementary techniques in ion chemistry: an FTIR study of the decomposition of the vinyl fluoride cation

FTIR spectra have been obtained for matrices formed following electron bombardment of gas mixtures containing varying amounts of vinyl fluoride (VF) in Ar (1:400 to 1:25,600; VF/Ar). The major matrix-isolated products are a π-complex of HF/C(2)H(2) , fluoroacetylene (HC≡CF) and two isomers of C(2)H(2)F(•). These products correspond well with the products of photoionization of VF near 15.8 eV. These observations support the dominant mechanism of ionization in the EB-MI environment as charge transfer of the substrate molecule to Ar(•+). Some differences are noted between the observed EB-MI products and the results from PEPICO studies, primarily in that the EB-MI products are observed as neutralized forms. The close correlation in EB-MI and photoionization results allows the EB-MI technique to be utilized as an ion structural analysis tool in complement to PEPICO studies, and allows the use of PEPICO studies to help predict and elucidate high-pressure chemistry mechanisms through EB-MI studies. The differences in the EB-MI results and ions observed using the PEPICO technique are rationalized in terms of the differences in the experimental techniques. Using VF as the test system, reagent partial pressure conditions that best complement PEPICO studies are determined. Although the major results are observed for all VF partial pressures, dilute samples give rise to further ionization of the primary products, and more concentrated samples give rise to radical-radical reaction chemistry. As a result, a nominal range of 1:3200 (VF/Ar) is demonstrated to provide the best correlation with the gas-phase PEPICO measurements.

C–F Bond Activation of Highly Fluorinated Molecules at Rhodium: From Model Reactions to Catalysis

AbstractRhodium complexes are excellent tools for the activation of aromatic and olefinic carbon–fluorine bonds. C–F bond cleavage reactions are key steps for the derivatization of highly fluorinated compounds via stoichiometric or catalytic reaction pathways. The reaction routes involve hydrodefluorinations, but also the selective introduction of functional groups at distinctive positions, to provide access to new fluorinated building blocks. One can identify two general types of rhodium precursors that can induce the C–F activation step. Cyclopentadienyl compounds have been applied for thermal and photochemical C–F bond cleavage reactions of aromatics in solution, but oxidative addition steps have also been investigated in matrix isolation studies at low temperature. Square‐planar hydrido, silyl, and boryl complexes are often involved in unique stoichiometric and catalytic processes for the derivatization of aromatics and olefins.

CH stretching vibration of N-methylformamide as a sensitive probe of its complexation: infrared matrix isolation and computational study

The complexes between trans-N-methylformamide (t-NMF) and Ar, N(2), CO, H(2)O have been studied by infrared matrix isolation spectroscopy and/or ab initio calculations. The infrared spectra of NMF/Ne, NMF/Ar and NMF/N(2)(CO,H(2)O)/Ar matrices have been measured and the effect of the complexation on the perturbation of t-NMF frequencies was analyzed. The geometries of the complexes formed between t-NMF and Ar, N(2), CO and H(2)O were optimized in two steps at the MP2/6-311++G(2d,2p) level of theory. The four structures, found for every system at this level, were reoptimized on the CP-corrected potential energy surface; both normal and CP corrected harmonic frequencies and intensities were calculated. For every optimized structure the interaction energy was partitioned according to the SAPT scheme and the topological distribution of the charge density (AIM theory) was performed. The analysis of the experimental and theoretical results indicates that the t-NMF-N(2) and CO complexes present in the matrices are stabilized by very weak N-H···N and N-H···C hydrogen bonds in which the N-H group of t-NMF serves as a proton donor. In turn, the t-NMF-H(2)O complex present in the matrix is stabilized by O-H···O(C) hydrogen bonding in which the carbonyl group of t-NMF acts as a proton acceptor. Both, the theoretical and experimental results indicate that involvement of the NH group of t-NMF in formation of very weak hydrogen bonds with the N(2) or CO molecules leads to a clearly noticeable red shift of the CH stretching wavenumber whereas engagement of the CO group as a proton acceptor triggers a blue shift of this wavenumber.

Formation and Structure of Co2O4: A Combined IR Matrix Isolation and Theoretical Study

The formation and structure of dicobalt tetroxide (Co₂O₄) has been investigated using matrix isolation in solid neon and argon coupled to infrared spectroscopy and quantum chemical methods. It is found that Co₂O₄ can be formed by dimerization of cobalt dioxide without activation energy by diffusion of ground state CoO₂ molecules at 9 K in the dark. The IR data on eight fundamentals, isotopic effects and quantum chemical calculations are both consistent with a centro-symmetrical structure with two pairs of equivalent oxygen atoms, engaged in a stronger terminal Co-O bond and in a weaker bridging Co-O-Co position. Evidence for other, metastable states is also presented, but the data are not conclusive. The electronic structure and formation pathway has been investigated using the Tao-Perdew-Staroverov-Scuseria/triple-zeta valence polarived basis set (TPSS/TZVP) and broken symmetry unrestricted density functional theory (BS-UDFT) approach and the ground electronic state is predicted to be an open shell ¹Ag singlet with the quintet, triplet, septet, and nonet states above by 3.3, 4.9, 9.3, and 27.7 kcal/mol, respectively, but certainly has a complex multireference character that hinders the use of more precise multireference approaches. Different formation pathways have been considered, and the 2(O═Co═O) → Co₂O₄ dimerization reaction is found to be the only barrierless channel and to be strongly exothermic. Comparisons with another transition metal (TM) oxide system (V₂O₄) suggests that the difference in predicted ground state geometries in TM₂O₄ systems might be due in HOMO-LUMO shapes of the isolated dioxide subunits and optimal overlap configurations.