Maria Wierzejewska

Photolysis of isothiocyanic acid HNCS in low-temperature matrices. Infrared detection of HSCN and HSNC isomers

Abstract A combination of matrix isolation technique and Fourier transform infrared spectroscopy has been used to study the low-temperature photochemistry of isothiocyanic acid HNCS. Near UV photolysis with a medium-pressure mercury lamp of the matrix isolated HNCS/DNCS precursor in nitrogen and argon led to the formation of two unobserved, so far, isomers: thiocyanic acid HSCN and isothiofulminic acid HSNC and their deuterated analogs. Both species are formed as primary products presumably via two reaction channels: formation of the [H⋯NCS] and hydrogen migration leading to the HSCN molecule and formation of the [HNC⋯S] followed by rearrangement to HSCN and HSNC species.

Infrared matrix isolation and ab initio studies on isothiocyanic acid HNCS and its complexes with nitrogen and xenon

Abstract The isothiocyanic acid HNCS (DNCS) and its complexes with nitrogen and xenon have been studied experimentally by FTIR matrix isolation technique and computationally with the use of ab initio calculations at the MP2 level. The spectra show that HNCS (DNCS) interacts specifically with nitrogen forming 1:1 hydrogen bonded complex in argon matrix while non-hydrogen bonded structure is probably formed in solid xenon. Two stable minima were localized on the potential energy surface. One of them involves an almost linear hydrogen bond from NH group of the acid molecule to nitrogen molecule lone pair (structure I) and has an interaction energy ΔECP equal to −6.85 kJ/mol. The second structure (II) where the nitrogen molecule interacts with the sulfur atom of the HNCS was found to be weaker bound and is characterized by ΔECP=−1.99 kJ/mol. A low energetic barrier of 5.86 kJ/mol between the structures I and II was found. Both experimental and theoretical results obtained for the Xe⋯HNCS system point to a structure with the NH group interacting with the xenon atom. An interaction energy ΔECP for this complex is equal to −3.64 kJ/mol.

First example of the ABC ν(OH) absorption structure for both gaseous and crystalline phase: infrared studies of dimethylphosphinic acid

Abstract Infrared spectra for both gaseous (4000-1000 cm −1 ) and polycrystalline (4000-400 cm −1 ) samples of dimethylphosphinic acid (CH 3 ) 2 POOH (DMPA) at various temperatures are reported and discussed. The main attention is focused on the hydrogen bond vibrations. For the first time, the characteristic ABC structure of the ν(OH) band, occurring in the spectra of solid samples of strongly hydrogen bonded complexes was observed in the gas phase spectra of DMPA dimers. In spite of different types of aggregates present in the solid state a similar shape of the ν(OH) absorption was found in the crystalline sample spectra. However, an inverse intensity distribution of the ABC subbands was observed. The presented results fully confirm the explanation of the ABC structure as being due to the Fermi resonances between ν(OH) and overtones of the deformation modes 2δ(OH) and 2γ(OH).

Conformational Behavior and Tautomer Selective Photochemistry in Low Temperature Matrices: The Case of 5-(1H-Tetrazol-1-yl)-1,2,4-triazole

The conformational properties and the photolysis behavior of one of the simplest N-C bonded bicyclic azoles, 5-(1H-tetrazol-1-yl)-1,2,4-triazole (T), were studied in argon and xenon matrices by infrared spectroscopy. Analysis of the experimental results was supported by extensive theoretical calculations carried out at the B3LYP/6-311++G(2d,2p) level of approximation. Out of the eight T minima located on the potential energy surface, the three most stable species were detected in low temperature matrices, namely, 5-(1H-tetrazol-1-yl)-1H-1,2,4-triazole (T1) and two conformers of 5-(1H-tetrazol-1-yl)-2H-1,2,4-triazole (T2a and T2b). With increase of the substrate temperature either during deposition of the matrices or during annealing the T2b → T2a conversion took place, in agreement with the predicted low energy barrier for this transformation (5.38 kJ mol(-1)). Both broad band and narrow band laser UV irradiations of T isolated in Xe and Ar matrices induce unimolecular decomposition involving cleavage of the tetrazole ring of T1 and T2a (T2b) that leads to the production of 1H-1,2,4-triazol-5-yl carbodiimide (P1) and 1H-1,2,4-triazol-3-yl carbodiimide (P2), respectively. When the laser is used, in addition to the main P1 and P2 photoproducts, several minor products could be successfully identified in the matrices: N-cyanocarbodiimide HNCNCN (detected for the first time) associated with nitrilimine HNNCH and HCN. An interesting phenomenon of tautomer-selective photochemistry was observed for the matrix-isolated compound. It could be explained by the different LUMO-HOMO energy gaps estimated for T1, T2a, and T2b, connected with different threshold energies necessary to start the photolysis of T1 and T2a (T2b).

FTIR matrix isolation studies of complexes of dimethylsulfide, dimethyldisulfide and hydrogen sulfide with nitric acid

Abstract Complexes formed by dimethylsulfide (DMS), dimethyldisulfide (DMDS) and hydrogen sulfide (H 2 S) with nitric acid have been observed and characterised in argon and nitrogen matrices. The spectra show that in all studied systems the HNO 3 molecule is hydrogen bonded to sulfur atom of the base molecule. The strength of interaction as evidenced by the shifts in the ν 1 OH modes of nitric acid decreases in the following order: DMS–HNO 3 , DMDS–HNO 3 , H 2 S–HNO 3 . The possible structure of the studied complexes is discussed.

Infrared matrix isolation and theoretical studies of SO2–HNO3 and SO2–HONO systems

Abstract Argon matrix infrared spectra of sulphur dioxide complexes with nitric or nitrous acid indicate formation of hydrogen-bonded structures. The red shifts of the OH stretching modes are equal to ca. 179, 51 and 40 cm −1 in SO 2 –HNO 3 , SO 2 –HONO-trans and SO 2 –HONO-cis complexes, respectively. Theoretical studies of the structure and spectral characteristics of the title complexes were carried out on the electron correlation level with 6-31G(d) basis set. For all studied systems only one stable structure was found with the OH group interacting with one oxygen atom of the sulphur dioxide molecule. The calculated infrared spectra reproduce well the frequencies and the intensities of the measured spectra.

Light-Induced Opening and Closing of the Intramolecular Hydrogen Bond in Glyoxylic Acid

The isomerization process of glyoxylic acid (GA) conformers and their complexes with a water molecule were studied in a low temperature argon matrix. The research target was to understand how starting conformation and complexation affects the near-IR (NIR) induced conformer interconversion. The most stable GA conformer (Tc) is characterized by an intramolecular hydrogen bond, and it is found to undergo light-induced conformer interconversion slower than the open (Tt) conformer. Upon complexation with water, the isomerization processes slow down in the case of the Tc conformer, whereas for the Tt-based complex the influence of water is negligible on the isomerization process.

Conformational study of eugenol by density functional theory method and matrix-isolation infrared spectroscopy.

The B3LYP/6-311++G(2d,2p) study of the potential energy surface of eugenol (4-allyl-2-methoxyphenol, 2-methoxy-4-pro-2-emyl-phenol) was performed with the aim of finding all possible conformers of the molecule. Twelve conformers were found belonging to one of three groups differing in the relative orientation of the OH and OCH 3 moieties: SA (syn-anti), AA (antianti) and AG (antigauche). The lowest-energy conformers of eugenol (SAA+, SAA- and SAS) stabilized by the intramolecular hydrogen bond differ only in the arrangement of the allyl group with respect to the aromatic ring. The calculated abundance of all three SA species equals 99.8% whereas the remaining AA and AG show the negligible population of 0.2%. In consonance with theoretical predictions, only syn-anti conformers are present in the low temperature matrices studied. The presented FTIR results allow, for the first time, unequivocal identification and spectral characterization of three SA conformers of the eugenol molecule isolated in solid argon and xenon. The performed studies reveal that conformational cooling (upon increasing the substrate temperature during deposition) takes place in the studied matrices and that the less stable SAA- and SAS species convert into SAA+. This observation appears to be consistent with the theoretically predicted energy barriers of 6.70 and 10.45 kJ/mol for the SAA- --> SAA+ and SAS --> SAA+ interconversions which are low enough to be surpassed during deposition at higher temperatures.

Crystal structure and vibrational spectra of bis(betaine) sulfamate

The crystal structure of bis(betaine) sulfamate (BBAS) has been determined by X-ray diffraction method at 250 and 100 K as monoclinic, space group P21/c, Z=4. The crystal comprises of monoprotonated betaine dimers and sulfamate anions NH2SO3−. Two kinds of hydrogen bonds are present in the crystal: strong, symmetric and linear O···H···O hydrogen bonds (R(O···O)=2.448(2)–2.442(2) A) and weak N–H···O hydrogen bonds (R(N···O)=2.983(2)–3.028(2) A). At low temperature (100 K) the BBAS structure is completely ordered. At 250 K 5% of sulfamate anions have their orientation changed through inversion of the NH2 groups. Infrared transmission (for both BBAS and the deuterated from, DBBAS), reflection and Raman spectra of the title compound have been recorded and discussed in relation to the X-ray crystal structure. Additionally, temperature dependence of the BBAS infrared spectra in the 12–300 K range was investigated on purpose to find a possible relation between structural and spectroscopic changes in the studied crystal.

Ultraviolet-Tunable Laser Induced Phototransformations of Matrix Isolated Isoeugenol and Eugenol

In situ photochemical transformations of monomers of 2-methoxy-4-(prop-1-enyl)phenol (isoeugenol) and 2-methoxy-4-(prop-2-enyl)phenol (eugenol) isolated in low temperature matrices were induced by tunable UV laser light, and the progress of the reactions was followed by FTIR spectroscopy. Conformer-selective E ↔ Z geometrical isomerizations could be successfully induced by irradiation at different wavelengths from the 310-298 nm range in the isoeugenol molecule, contains an asymmetrically substituted exocyclic C═C bond. Photolysis of both studied compounds was also observed, with H-atom shift from the OH group and formation of two types of long-chain conjugated ketenes. The photoproduced ketenes were found to undergo subsequent photodecarbonylation. Interpretation of the observed photoprocesses was supported by quantum chemical calculations undertaken at different levels of theory (DFT, MP2, QCISD).