Complexes of atmospheric alpha-dicarbonyls with water: FTIR matrix isolation and theoretical study

Probing microhydration-induced effects on carbonyl compounds

Characterizing the microhydration of organic molecules is a crucial step in understanding many phenomena relevant to atmospheric, biological, and industrial applications. However, its precise experimental and theoretical description remains a challenge. For four organic solutes containing a CO bond, and included in the recent HyDRA challenge [T. L. Fischer, M. Bödecker, A. Zehnacker-Rentien, R. A. Mata and M. A. Suhm, Phys. Chem. Chem. Phys., 2022, 24, 11442-11454.], we performed a detailed study of different monohydrate isomers and their properties; these were cyclooctanone (CON), 1,3-dimethyl-2-imidazolidinon (DMI), methyl lactate (MLA), and 2,2,2-trifluoroacetophenone (TPH) molecules. As reported in the literature, the O-H elongation shift of the water molecule appears to be a good candidate for characterizing complexation-induced effects. We also show that CO elongation shift and UV-vis spectroscopy can be successfully used for these purposes. Besides, we present a comparative analysis of the strengths of non-covalent interactions within these monohydrated complexes based on interpretative tools of quantum chemistry, including topological analysis of electron density (ρ), topological analysis of electron pairing function, and analysis of the core-valence bifurcation index (CVBI), which exhibits a close linear dependency on ρ. Accordingly, a classification of intermolecular water-solute interactions is proposed.

Photo-Induced Reactions between Glyoxal and Hydroxylamine in Cryogenic Matrices

In this paper, the photochemistry of glyoxal−hydroxylamine (Gly−HA) complexes is studied using FTIR matrix isolation spectroscopy and ab initio calculations. The irradiation of the Gly−HA complexes with the filtered output of a mercury lamp (λ > 370 nm) leads to their photoconversion to hydroxyketene−hydroxylamine complexes and the formation of hydroxy(hydroxyamino)acetaldehyde with a hemiaminal structure. The first product is the result of a double hydrogen exchange reaction between the aldehyde group of Gly and the amino or hydroxyl group of HA. The second product is formed as a result of the addition of the nitrogen atom of HA to the carbon atom of one aldehyde group of Gly, followed by the migration of the hydrogen atom from the amino group of hydroxylamine to the oxygen atom of the carbonyl group of glyoxal. The identification of the products is confirmed by deuterium substitution and by MP2 calculations of the structures and vibrational spectra of the identified species.

Complexes of Formaldehyde and α-Dicarbonyls with Hydroxylamine: FTIR Matrix Isolation and Theoretical Study

The interactions of formaldehyde (FA), glyoxal (Gly) and methylglyoxal (MGly) with hydroxylamine (HA) isolated in solid argon and nitrogen were studied using FTIR spectroscopy and ab initio methods. The spectra analysis indicates the formation of two types of hydrogen-bonded complexes between carbonyl and hydroxylamine in the studied matrices. The cyclic planar complexes are stabilized by O–H⋯O(C), and C–H⋯N interactions and the nonplanar complexes are stabilized by O–H⋯O(C) bond. Formaldehyde was found to form with hydroxylamine, the cyclic planar complex and methylglyoxal, the nonplanar one in both argon and nitrogen matrices. In turn, glyoxal forms with hydroxylamine the most stable nonplanar complex in solid argon, whereas in solid nitrogen, both types of the complex are formed.

Photoinduced Double Proton Transfer in the Glyoxal-Methanol Complex Revisited: The Role of the Excited States

Under irradiation in the visible range, the glyoxal-methanol complex in a cryogenic argon matrix undergoes a double proton transfer (DPT) reaction through which the glyoxal molecule isomerizes into hydroxyketene. In this work, we employ electronic structure simulations in order to shed more light on the underlying mechanism. Rewardingly, we find that the lowest singlet excited state (S₁) of the complex acts as a gateway to two previously unknown isomerization pathways, of which one takes place entirely in the singlet manifold and the other also involves the lowest triplet state (T₁). Both of these pathways are fully compatible with the available experimental data, implying that either or both are operative under experimental conditions. In either pathway, the methanol molecule acts as a proton shuttle between the proton-donating and proton-accepting sites of glyoxal, resulting in a dramatic lowering of the potential energy barrier to isomerization with respect to the case of isolated glyoxal. The occurrence of DPT in the singlet manifold is demonstrated directly with the use of nonadiabatic molecular dynamics simulations at the spin-flip time-dependent density functional theory level.

Model Behavior: Characterization of Hydroxyacetone at the Air-Water Interface Using Experimental and Computational Vibrational Sum Frequency Spectroscopy

Small atmospheric aldehydes and ketones are known to play a significant role in the formation of secondary organic aerosols (SOA). However, many of them are difficult to experimentally isolate, as they tend to form hydration and oligomer species. Hydroxyacetone (HA) is unusual in this class as it contributes to SOA while existing predominantly in its unhydrated monomeric form. This allows HA to serve as a valuable model system for similar secondary organic carbonyls. In this paper the surface behavior of HA at the air-water interface has been investigated using vibrational sum frequency (VSF) spectroscopy and Wilhelmy plate surface tensiometry in combination with computational molecular dynamics simulations and density functional theory calculations. The experimental results demonstrate that HA has a high degree of surface activity and is ordered at the interface. Furthermore, oriented water is observed at the interface, even at high HA concentrations. Spectral features also reveal the presence of both cis and trans HA conformers at the interface, in differing orientations. Molecular dynamics results indicate conformer dependent shifts in HA orientation between the subsurface (∼5 Å deep) and surface. Together, these results provide a picture of a highly dynamic, but statistically ordered, interface composed of multiple HA conformers with solvated water. These results have implications for HA's behavior in aqueous particles, which may affect its role in the atmosphere and SOA formation.

Conformational Landscape of the 1/1 Diacetyl/Water Complex Investigated by Infrared Spectroscopy and ab Initio Calculations

The complexes of diacetyl with water have been studied experimentally by Fourier transform infrared (FTIR) spectroscopy coupled to solid neon matrix and supersonic jet, and anharmonic ab initio calculations. The vibrational analysis of neon matrix spectra over the 100-7500 cm^-1 infrared range confirms the existence of two nearly isoenergetic one-to-one (1/1) diacetyl-water S₁ and S₂ isomers already evidenced in a previous argon matrix study. A third form (S₃) predicted slightly less stable ( J. Mol. Mod. 2015 , 21 , 214 ) is not observed. The correct agreement obtained between neon matrix and anharmonic calculated vibrational frequencies is exploited in several cases to derive band assignments for the vibrational modes of a specific isomer. Thereafter, theoretical x_ij anharmonic coupling constants are used for the attribution of combination bands and overtones relative to the 1/1 dimer. Finally, the most stable isomer of the one-to-two (1/2) diacetyl-water complex is identified in the OH stretching region of water on the grounds of comparison of experimental and calculated vibrational shifts between water dimer and the three most stable 1/2 isomers.

Topological insights into the 1/1 diacetyl/water complex gained using a new methodological approach

The 1/1 diacetyl/water complex is of atmospheric relevance. Previous experimental and theoretical studies have focused on two isomeric forms, and geometry optimizations were carried out on them. Herein, we propose a six-step methodological approach based on topological properties to search for and characterize all of the isomeric forms of the 1/1 noncovalent diacetyl/water complex: (1) a molecular electrostatic potential (MESP) study to get an overview of the V min and V max regions on the molecular surfaces of the separate molecules (diacetyl and water); (2) a topological (QTAIM and ELF) study allowing thorough characterization of the electron densities (QTAIM) and irreducible ELF basins of the separate molecules; (3) full optimization of the predicted structures based on the interaction between complementary reaction sites; (4) energetic characterization based on a symmetry-adapted perturbation theory (SAPT) analysis; (5) topological characterization of the optimized complexes; (6) analysis of the complexes in terms of orbital overlaps (natural bond orbitals, NBO analysis). Using this approach, in addition to achieving the topological characterization of the two isomeric forms already reported, a third possible isomer was identified and characterized. Graphical Abstract Topological tools to study monohydrated complexes.

Quantitative infrared intensity studies of vapor-phase glyoxal, methylglyoxal, and 2,3-butanedione (diacetyl) with vibrational assignments

Glyoxal, methylglyoxal, and 2,3-butanedione (diacetyl) are all known biomass burning effluents and suspected aerosol precursors. Pressure-broadened quantitative infrared spectra of glyoxal, methylglyoxal, and diacetyl vapors covering the 520-6500 cm(-1) range are reported at 0.112 cm(-1) resolution, each with a composite spectrum derived from a minimum of 10 different sample pressures for the compound, representing some of the first quantitative intensity data for these analytes. Many vibrational assignments for methylglyoxal are reported for the first time, as are some near-IR and far-IR bands of glyoxal and diacetyl. To complete the vibrational assignments, the far-infrared spectra (25-600 cm(-1)) of all three vapors are also reported, those of methylglyoxal for the first time. Density functional theory and ab initio MP2 theory are used to help assign vibrational modes. Potential bands for atmospheric monitoring are discussed.

Hydrated complexes of atmospheric interest: rotational spectrum of diacetyl-water

The rotational spectrum of the molecular complex diacetyl-water has been measured by pulsed jet Fourier transform microwave (FTMW) spectroscopy. The water molecule acts as an electrofore and induces a dipole moment when combined with nonpolar diacetyl. Only one conformer has been observed, corresponding to the global minimum. Its shape is unambiguously established: the water moiety is linked asymmetrically to the organic molecule; it behaves as a proton donor to one of its oxygen atoms and interferes with the internal rotation of adjacent methyl group through a C-H...O interaction. Each rotational transition appears as a quintuplet, due to the internal rotation of the two methyl groups, which are nonequivalent in the adduct. From the analysis of the observed splittings, the V(3) barriers to the internal rotation of the two methyl groups have been determined to be 3.81(2) and 4.11(2) kJ/mol, respectively.

Photo-induced hydrogen exchange reaction between methanol and glyoxal: formation of hydroxyketene

We study the structure and photochemistry of the glyoxal-methanol system (G-MeOH) by means of FTIR matrix isolation spectroscopy and ab initio calculations. The FTIR spectra show that the non-hydrogen-bonded complex, G-MeOH-1, is present in an inert environment of solid argon. MP2/aug-cc-pVDZ calculations indicate that G-MeOH-1 is the most stable complex among the five optimized structures. The interaction energy partitioned according to the symmetry-adapted perturbation theory (SAPT) scheme demonstrates that the dispersion energy gives a larger contribution to the stabilization of a non-hydrogen-bonded G-MeOH-1 complex than compared to the hydrogen-bonded ones. The irradiation of G-MeOH-1 with the filtered output of a mercury lamp (lambda>370 nm) leads to its photo-conversion into the hydroxyketene-methanol complex HK-MeOH-1. The identity of HK-MeOH-1 is confirmed by both FTIR spectroscopy and MP2/aug-cc-pVDZ calculations. An experiment with deuterated methanol (CH(3)OD) evidences that hydroxyketene is formed in a photo-induced hydrogen exchange reaction between glyoxal and methanol. The pathway for the photo-conversion of G-MeOH-1 to HK-MeOH-1 is studied by a coupled-cluster method [CR-CC(2,3)]. The calculations confirm our experimental findings that the reaction proceeds via hydrogen atom exchange between the OH group of methanol and CH group of glyoxal.