Matrix Isolation FTIR Studies Research Articles

Photoinduced reaction of dichlorosilylene with acetylene: Matrix isolation FTIR spectroscopic study

A matrix-isolation FTIR study of the reaction between SiCl2 and acetylene showed that at low temperatures, it stops at the stage of complexation between the reactants. This made it possible to examine a photochemical version of this reaction, which led to the simultaneous formation of 1,1-dichloro-1-silacycloprop-2-ene and 3,3-dichloro-3-silaprop-1-yne in comparable yields in contrast to the thermal reaction, which, according to quantum chemical calculations carried out here, results in the formation of 1,1-dichloro-1-silacycloprop-2-ene as the only primary product.

Matrix Isolation FTIR and Theoretical Study of Weakly Bound Complexes of Isocyanic Acid with Nitrogen.

Weak complexes of isocyanic acid (HNCO) with nitrogen were studied computationally employing MP2, B2PLYPD3 and B3LYPD3 methods and experimentally by FTIR matrix isolation technique. The results show that HNCO interacts specifically with N2. For the 1:1 stoichiometry, three stable minima were located on the potential energy surface. The most stable of them involves a weak, almost linear hydrogen bond from the NH group of the acid molecule to nitrogen molecule lone pair. Two other structures are bound by van der Waals interactions of N⋯N and C⋯N types. The 1:2 and 2:1 HNCO complexes with nitrogen were computationally tracked as well. Similar types of interactions as in the 1:1 complexes were found in the case of the higher stoichiometry complexes. Analysis of the HNCO/N2/Ar spectra after deposition indicates that the 1:1 hydrogen-bonded complex is prevalent in argon matrices with a small amount of the van der Waals structures also present. Upon annealing, complexes of the 1:2 and 2:1 stoichiometry were detected as well.

Ammonia Activation by Ce Atom: Matrix-Isolation FTIR and Theoretical Studies.

The activation of ammonia by cerium atom has been investigated in solid argon using infrared spectroscopy and density functional theoretical calculations. The results reveal that the spontaneous formation of CeNH3 complex on annealing is the initial step in the reactions of cerium atoms with ammonia. The CeNH3 complexes rearrange to generate the inserted HCeNH2 molecules on irradiation. A "triplet-singlet" spin conversion occurs along the reaction path in which HCeNH2 (3A″) isomerizes into H2CeNH (1A'). The H2CeNH molecules finally decompose to yield HCeN + H2 upon photolysis. The periodic trend and differences for the M + NH3 (M = Ti, Zr, Hf, Ce, Th) systems are discussed on the basis of the present and previous works. DFT calculations predict that the most stable ground state for HHfNH2 and HThNH2 is singlet due to the stronger relativistic effects in Hf and Th atoms, while that for HTiNH2, HZrNH2, and HCeNH2 is triplet. Besides, the H2-elimination process is different for Ce and M (Ti, Zr, Hf, Th) cases.

Isocyano compounds newly recognized in photochemical reaction of thiazole: matrix-isolation FT-IR and theoretical studies

UV-induced photoreactions of thiazole isolated in low-temperature argon matrices have been investigated, leading to the conclusion that photoproducts identified newly, including three kind isocyano compounds, are produced by ring-opening reaction.

Matrix isolation FTIR study of hydrogen-bonded complexes of methanol with heterocyclic organic compounds

Hydrogen bonded complexes of heterocyclic compounds with methanol were studied using matrix isolation FTIR spectroscopy and theoretical calculations.

New aspects of UV photolysis of hydrogen peroxide. Nitrogen matrix isolation FTIR and theoretical studies

Abstract UV photochemistry of H 2 O 2 isolated in solid nitrogen at 15 K has been studied experimentally by FTIR matrix isolation technique and theoretically using DFT and MP2 methods. The major photoproducts were identified as (i) nitrous oxide and N 2 O⋯H 2 O complex and (ii) free and complexed to N 2 O hydroxyl radicals. Except for the OH radicals all observed species are unique and characteristic for H 2 O 2 photolysis in solid nitrogen. The identities of the products have been confirmed by comparison with results of quantum chemical calculations, and reference to the spectral data of related molecules. Differences in the photolysis course upon irradiation at 230 and 212 nm are discussed.

Theoretical DFT and matrix isolation FTIR studies of 2-(1,2,4-triazolyl)phenol isomers

The structure, isomerization pathways and vibrational spectra of the important heterocyclic 2-(1,2,4-triazolyl)phenol molecule were investigated by DFT calculations and matrix isolation FTIR spectroscopy. Among forty-five minima located on PES three isomers with intramolecular hydrogen bond OH⋯N, 2-TRP1, 1-TRP1 and 1-TRP2, are the most stable forms with the calculated abundance of 83.4%, 10.3% and 6.0%, respectively. The presented FTIR results allow identification and characterization of these species. Several hydrogen bond parameters such as OH bond distance, νOH wavenumber shift and occupancy of the antibonding σ∗(OH) orbital were found to be linearly related with the estimated interaction energy.

Carboxylic Group and Its Tetrazolyl Isostere in One Molecule. Matrix Isolation FTIR and DFT Studies on Thermal Decomposition and Photochemistry of (Tetrazol-5-yl)acetic Acid

The title compound (tetrazol-5-yl)acetic acid (TA) is an interesting molecule that contains both a carboxylic group and its isostere tetrazolyl group. Out of nine theoretically predicted stable structures of TA, three appeared to be present in solid argon. Thermal decomposition of the species aided by water molecules was studied in detail both experimentally using FTIR matrix isolation technique and theoretically at the B3LYP/6-311++G(2d,2p) level. Experimentally, it was found that the decarboxylation process appeared at the presence of water traces in the system. Theoretically, it was shown that the energy barrier of the water assisted process was lower by ca. 30 kJ mol(-1) comparing to the process without water participation. The UV photolysis of the TA/Ar system was studied using both broad-band and narrow-band sources. The main photoproducts appeared to be carbodiimidylacetic acid and (1H-diaziren-3-yl)acetic acid. The progress of the reactions induced was followed by FTIR spectroscopy, whereas interpretation of the results was supported by quantum chemical calculations (DFT, TD-DFT).

Photochemical transformations of 5-methyltetrazole. Matrix isolation FTIR and DFT studies

Abstract In situ photolysis of 5-methyltetrazole (MT) isolated in low temperature argon matrices was induced by tunable UV laser radiation. The progress of the reactions was followed by FTIR spectroscopy that allowed for spectroscopic identification of three photoproducts resulting from the MT ring cleavage, namely C-methylnitrilimine CH3CNNH, N-methylcarbodiimide CH3NCNH, methylcyanamide CH3NHCN. The kinetic profiles obtained for these products show different behavior of the species in the course of irradiation. The N-methylcarbodiimide molecules were found to decompose further into two HCN or HNC molecules that interact in the matrix cages to form different hydrogen bonded dimers. Interpretation of the observed photoprocesses was supported by quantum chemical calculations (DFT, TD-DFT).

Simulating the Interaction between Amino Acids and DNA: A Combined Matrix-Isolation FT-IR and Theoretical Study of the 1-Methyluracil·Glycine H-Bond Complexes Using a Dual Sublimation Furnace

The H-bond complex formation between 1-methyluracil and glycine has been investigated by theoretical calculations and the most stable complex configurations have been identified by FT-IR spectroscopy in Ar matrices. The importance of this H-bonding system is huge since all DNA biological functions are dependent on the interactions with proteins. The theoretical optimizations have revealed six different closed H-bond complexes between 1-methyluracil and glycine. The obtained energies have demonstrated that the uracil C(4)═O site is a better H-acceptor site than the C(2)═O site. The stabilization energy of the most stable complex is -47.83 (MP2) or -54.14 kJ·mol(-1) (DFT). The DFT(B3LYP)/6-31G optimized geometries have been evaluated, and the obtained energies appeared to be in agreement with the results of the computational more expensive DFT(B3LYP)6-31++G** approach. In order to identify the 1:1 complexes in an argon matrix, a new dual miniature furnace has been developed which allows to sublimate both complex partners at their optimal temperature. The presence of three different glycine·1-methyluracil complexes has been demonstrated by analysis of the H-bond shifted modes. The H-bond parameters have been evaluated and previously obtained correlations for different H-bond complexes have been confirmed.

The conformational behavior and H-bond structure of asparagine: A theoretical and experimental matrix-isolation FT-IR study

Due to the high importance of the structural properties of peptides, the conformational behavior of one of their elementary building blocks, asparagine, has been investigated in this work. Matrix-isolation FT-IR spectroscopy is a suitable technique to investigate the intrinsic properties of small molecules. Asparagine has been subjected to matrix-isolation FT-IR spectroscopy supported with DFT and MP2 calculations. DFT optimization of asparagine resulted in 10 stable conformations with ∆EDFT<10kJ.mol−1. Compared to a previous study, one new conformation has been revealed. Further optimization at the MP2/6-31++G** level resulted in seven conformations with ∆EMP<10kJ.mol−1. A conformation containing the three intramolecular H-bonds, i.e. C=Osc…HNbb, C=Obb…HNsc and OHbb…Nbb appeared to be the most stable one at both levels despite the large negative entropy contribution due to these 3 H-bonds. At the sublimation temperature of 353K, the DFT method predicts four and the MP2 method six conformations to be present in the experimental matrix-isolation spectrum. These conformations have different intramolecular H-bonds, which has allowed to identify at least 4 low energy conformations in the FT-IR spectrum. Detailed comparison between theory and experiment resulted in a mean frequency deviation of 7.6cm−1.

Potential Energy Surface and Matrix Isolation FT-IR Study of Isoleucine

The conformational landscape of isoleucine was investigated by a theoretical DFT and MP2 study. This investigation has revealed new important conformations in comparison to a previous study. Five conformations have been predicted with an abundance larger than 5% at both levels of theory. Among these, three different types of amino acid backbone were present, each characterized by a typical intramolecular H-bond. Due to the H-bonding, the frequencies of the involved groups are strongly shifted, and these modes were probably observed with FT-IR spectroscopy in an Ar matrix. The experimental abundances could be estimated and were in good accordance with the MP2 stabilities, whereas the accordance with the DFT abundances was poor. In contrast to this, the correspondence of the DFT and the experimental frequencies was excellent, as demonstrated by the mean frequency deviation of only 4.7 cm(-1).

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.

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 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 FT-IR study of (CsBr) n and (CsI) n ( n = 1–3)

The species present in the vapours of CsBr and CsI have been isolated in krypton and xenon matrices and were investigated by FT-IR spectroscopy in the range of 180–30 cm −1. The recorded spectra revealed the presence of substantial amounts of dimer beside the alkali halide monomers. The assignment of the bands and characterization of the fundamentals was carried out on the basis of DFT quantum chemical calculations. The applied B3P86 exchange-correlation functional in conjunction with the Stuttgart–Cologne relativistic effective core potentials and polarized triple-zeta valence basis set reproduced the accurate experimental monomer Cs–Br and Cs–I bond distances within 0.001 Å, and the harmonic stretching frequency within 1.1 cm −1. Based on calculations at this level we report reliable geometrical parameters and fundamental frequencies for the dimer and trimer species (CsBr) n and (CsI) n . Utilizing the computed data we propose the identification of the E′ asymmetric stretching mode of the trimer (CsI) 3 trapped in a Xe matrix and that of (CsBr) 3 in Kr and Xe matrices.

Conformational behavior of the simplest dipeptide: Formylglycine. Quantum chemical and matrix isolation FTIR study

The conformational landscape of the simplest dipeptide formylglycine (FG) and its vibrational spectra were studied by B3LYP and MP2 methods and matrix isolation FTIR spectroscopy. Sixteen minima have been found on the potential energy surface. The most stable conformer is characterized by the planar structure, a stretched backbone and a cis COOH group. The calculated gas phase abundance at 333 K for two most stable FG1c and FG4a equals ca. 88% and 8%, and the remaining conformers have a negligible population. The presented FTIR results allow unequivocal identification and characterization of FG1c in nitrogen and argon matrices while other species are not experimentally observed.

Theoretical and matrix isolation FTIR studies of 3-amino-1,2,4-triazole and its isomers

The structure, isomerization pathways and vibrational spectra of the important heterocyclic 3-amino-1,2,4-triazole (AT) molecule were studied by MP2 and B3LYP methods. Among 11 minima located on PES, 3-amino-1,2,4- 1H-triazole (1-AT) and 3-amino-1,2,4- 2H-triazole (2-AT) are the most stable with the calculated abundance at 298 K of 79.4% and 20.6%, respectively. The presented FTIR results allow unequivocal identification and characterization of two AT tautomers isolated in an argon matrix. The performed studies reveal that conformational cooling does not take place in the studied matrices. This observation is consistent with the predicted high energy barrier of ca. 45 kcal/mol for a direct 1,2-hydrogen transfer between 1-AT and 2-AT species.

Matrix-isolation FTIR study of carbon dioxide: Reinvestigation of the CO 2 dimer and CO 2⋯N 2 complex

Carbon dioxide isolated in argon and nitrogen matrices was reinvestigated through FTIR spectroscopy. The study of the variation of the IR absorptions with the concentration of CO 2 in the matrix, and also with the addition of different amounts of N 2 (present as an impurity in the Ar gas) was performed. Some of the bands were reassigned to the CO 2 dimer and CO 2⋯N 2 complex and some others, like the symmetric stretching mode for the CO 2 dimer and the ( ν 1 + ν 3) and (2 ν 2 + ν 3) combination bands for the CO 2 dimer and that of the CO 2⋯N 2 complex, were reported for the first time. The bonding properties of the (CO 2) 2 and CO 2⋯N 2 complexes have been interpreted by a Natural Bond Orbital (NBO) analysis in terms of “donor–acceptor” interactions.

UV-induced generation of rare tautomers of allopurinol and 9-methylhypoxanthine — A matrix isolation FTIR study

Monomers of allopurinol and 9-methylhypoxanthine were studied using the matrix isolation technique combined with Fourier transform infrared spectroscopy. The oxo tautomeric forms of both compounds were found to dominate in freshly deposited low-temperature argon matrices. For 9-methylhypoxanthine, a small amount of the hydroxy tautomer was also detected in an Ar matrix before any irradiation. Upon exposure of the matrices to the UV ( λ > 230 nm or λ > 270 nm) light, a proton transfer photoreaction converting the oxo forms of both compounds into the corresponding hydroxy tautomers occurred. Generation of conjugated ketenes as minor photoproducts was also observed. For 4( 3H)-pyrimidinone (a model compound for both allopurinol and 9-methylhypoxanthine), photoreversibility of the UV-induced oxo → hydroxy transformation was experimentally proven by direct observation of the back hydroxy → oxo photoreaction. The substrates (oxo tautomers) and products (hydroxy tautomers) of the observed phototransformations were identified by comparison of their IR spectra with the spectra theoretically predicted at the DFT(B3LYP)/6-31++G(d,p) level. The IR bands in the experimental spectra were assigned to the calculated normal modes.