Adriana Olbert-Majkut

Photolysis of the OC⋯HONO complex in low temperature matrices: Infrared detection and ab initio calculations of nitrosoformic acid, HOC(O)NO

Photochemistry of OC⋯HONO complexes in nitrogen matrices has been investigated using λ>340 nm radiation of a medium pressure mercury arc. Reaction of the OH radicals from HONO photolysis with CO within the nitrogen lattice site has led to formation of trans and cis-HOCO radicals. The HOCO radicals reacted further with NO via two reaction channels yielding trans and cis-HOC(O)NO or HNO+CO2. Comparison of the observed infrared frequencies and isotope shifts (13C, 18O, and 2H) with the computed frequencies confirmed the assignment of the cis and trans conformers of nitrosoformic acid that have been identified for the first time. In matrices with higher CO concentration the complexes between cis-HOC(O)NO molecule and carbon monoxide were also identified.

Ab initio calculations and matrix infrared spectra of the nitrous acid complexes with HF and HCl

Theoretical studies of the structure and spectral characteristics of the complexes formed by trans- and cis-HONO isomers with hydrogen fluoride and hydrogen chloride were carried out on the electron correlation level with the 6-311++G(2d,2p) basis set. The calculations demonstrate formation of three stable complexes between trans-HONO isomer and HX and five stable complexes between cis-HONO isomer and HX. In the most stable HX···trans-HONO, HX···cis-HONO complexes the HX subunit acts as a proton donor and interacts with an oxygen atom of the OH group which is a proton acceptor. The complexes predicted to be the most stable ones were identified and characterized in argon matrices. Six and five perturbed HONO vibrations of trans-HONO isomer were identified, respectively, for HF···trans-HONO and HCl···trans-HONO complexes whereas two perturbed vibrations of cis-HONO isomer were identified for HF···cis-HONO and HCl···cis-HONO complexes.

Infrared matrix isolation and quantum chemical studies of the nitrous acid complexes with water

The 1:1 and 2:1 complexes between water and trans- and cis-isomers of nitrous acid have been isolated in argon matrices and studied using FTIR spectroscopy and DFT(B3LYP) calculations with a 6-311++G(2d,2p) basis set. The analysis of the experimental spectra indicate that 1:1 complexes trapped in solid argon involve very strong hydrogen bond in which acid acts as the proton donor and water as the proton acceptor. The perturbed OH stretches are −248, −228 cm−1 red shifted from their free-molecules values in complexes formed by trans- and cis-HONO isomers, respectively. The calculated spectral parameters for the two complexes are in good agreement with experimental data. The calculations also predict stability of two more 1:1 weakly bound complexes formed by each isomer. In these the water acts as the proton donor and one of the two oxygen atoms of the acid as the acceptor. The experimental spectra demonstrate also formation of 2:1 complex between water and trans-HONO isomer in an argon matrix. The performed calculations indicate that the complex involves a seven-membered ring in which OH group of HONO forms very strong hydrogen bond with the oxygen atom of one water molecule and nitrogen atom acts as a weak proton acceptor for the hydrogen atom of the second water molecule of the water dimer. The observed perturbations of the OH stretch of trans-HONO (750 cm−1 red shift) is much larger than that predicted by calculations (556 cm−1 red shift); this difference is attributed to strong solvation effect of argon matrix on very strong hydrogen bond.

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.

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).

Visible light-driven chemistry of oxalic acid in solid argon, probed by Raman spectroscopy.

High-overtone induced chemistry of oxalic acid (OA) isolated in a low-temperature argon matrix was investigated using Raman spectroscopy. The Raman spectra of three conformers of OA are presented and discussed. Upon excitation of high overtone combination bands by 532 nm irradiation of the lowest energy cTc structure, the isomerization and unimolecular decomposition of OA were observed. Dissociation was induced presumably by absorption into the 5A(g) + B(u) vibrational state of the OH stretching mode of cTc. The photodecomposition leads to the formation of CO, CO(2), and H(2)O products. The experimental observations were supported by computational studies and vibrational anharmonic calculations.

Investigating isomerization reactions in solid state by using simultaneous high overtone pumping and Raman detection

Formation of the unstable cis-formic acid in solid argon matrix is induced by direct excitation of the 6<--0 transition of the nu(OH) vibration of the trans-formic acid. The experiment utilizes strongly focused laser beam that produces relatively high isomerization rate despite the low cross section of the absorption. Raman spectroscopy in a backscattering geometry is used for detection of the reactants and the products. This experimental arrangement allow us to use the same laser source for simultaneous pumping and Raman excitation, and it also guarantees that the excited and probed volumes overlap. The presented method has a high potential for solid state investigations of chemical reactions on the ground electronic state.

Librational motion of CO in solid Ar: Raman and IR spectra and quantum simulations

Rovibrational Raman spectra of CO molecules isolated in solid Ar are measured at temperatures of 9–30 K and compared to past and present IR spectra. The fundamental band appears as a triplet-split structure, while the center peak has completely different IR and Raman responses to temperature. The Raman peak is sharp and stable but broadens reversibly beyond recognition in the IR upon annealing. The red-shifted, intense line of the triplet is thermally inert in both spectroscopies. The third line is the weakest, and since it is concentration dependent, it is ascribed to a dimer, as before. The CO-H2O impurity complex is identified as a side band. We employ crystal field and quantum chemical modeling to interpret the disparity between the spectroscopies. The stable and broadened lines are assigned to double- and single-substitution sites, respectively. Thermal excitation is not effective in the former case of an angularly tight-confined, deep potential well. In the single-substitution case, the librational ...

Photolysis of the C2H4–HONO complex in low temperature matrices: formation of 2-nitrosoethanol

Abstract Photochemistry of C 2 H 4 ⋯HONO complexes in argon matrices has been investigated using λ>340 nm radiation of a medium pressure mercury arc. IR spectra were recorded after various periods of photolysis. The photolysis reaction was found to lead to formation of 2-nitrosoethanol that was identified for the first time. Three groups of bands showing different behavior during photolysis process and after matrix annealing were assigned to three different conformers of 2-nitrosoethanol molecule. The experimental data are supported by ab initio calculations.