Rotational spectroscopy of methyl benzoylformate and methyl mandelate: structure and internal dynamics of a model reactant and product of enantioselective reduction

Phys. Chem. Chem. Phys.

Zenchyzen

Jäger

2016

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o-Toluic acid, a photo-oxidation product in the atmosphere, and its monohydrate were characterized in the gas phase by pure rotational spectroscopy. High-resolution spectra were measured in the range of 5-14 Hz using a cavity-based molecular beam Fourier-transform microwave spectrometer. Possible conformers were identified computationally, at the MP2/6-311++G(2df,2pd) level of theory. For both species, one conformer was identified experimentally, and no methyl internal rotation splittings were observed, indicative of relatively high barriers to rotation. In the monomer, rocking of the carboxylic acid group is a large amplitude motion, characterized by a symmetrical double-well potential. This and other low-lying out-of-plane vibrations contribute to a significant (methyl top-corrected) inertial defect (-1.09 amu Å(2)). In the monohydrate, wagging of the free hydrogen atom of water is a second large amplitude motion, so the average structure is planar. As a result, no c-type transitions were observed. Water tunneling splittings were not observed, because the water rotation coordinate is characterized by an asymmetrical double-well potential. Since the minima are not degenerate, tunneling is precluded. Furthermore, a concerted tunneling path involving simultaneous rotation of the water moiety and rocking of the carboxylic acid group is precluded, because the hilltop along this coordinate is a virtual, rather than a real, saddle-point. Inter- and intramolecular non-covalent bonding is discussed in terms of the quantum theory of atoms in molecules. The percentage of o-toluic acid hydrated in the atmosphere is estimated to be about 0.1% using statistical thermodynamics.

Section: Inter-and Intramolecular Non-covalent Bondingmentioning

confidence: 99%

Rotational spectroscopy of the atmospheric photo-oxidation product o-toluic acid and its monohydrate

Phys. Chem. Chem. Phys.

Zenchyzen

Jäger

2016

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“…The setup has been described recently. 40,41 A small amount of water (0.5%) was added to about 1−2 bar of neon backing gas. All geometry optimizations and frequency analyses were done using the MP2 method 42 with the 6-311++G(d,p) basis set, 43 as implemented in Gaussian 09.…”

Section: The Journal Of Physical Chemistry Lettersmentioning

confidence: 99%

“…The sample container and pulsed nozzle were heated to 373 and 463 K, respectively. The setup has been described recently. , A small amount of water (0.5%) was added to about 1–2 bar of neon backing gas.…”

Section: Experimental Sectionmentioning

confidence: 99%

Contrasting Effects of Water on the Barriers to Decarboxylation of Two Oxalic Acid Monohydrates: A Combined Rotational Spectroscopic and Ab Initio Study

Badran

Jäger

2016

J. Phys. Chem. Lett.

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Using rotational spectroscopy, we have observed two isomers of the monohydrate of oxalic acid, the most abundant dicarboxylic acid in the atmosphere. In the lowest-energy isomer, water hydrogen-bonds to both carboxylic acid groups, and the barrier to decarboxylation decreases. In the second isomer, water bonds to only one carboxylic acid group, and the barrier increases. Though the lower barrier in the former is not unequivocal evidence that water acts as a photocatalyst, the higher barrier in the latter indicates that water acts as an inhibitor in this topology. Oxalic acid is unique among dicarboxylic acids: for the higher homologues calculated, the inhibiting topology of the monohydrate is lowest in energy and most abundant under atmospheric conditions. Consequently, oxalic acid is the only dicarboxylic acid for which single-water catalysis of overtone-induced decarboxylation in the atmosphere is plausible.

“…When the hydroxy group is oriented toward the methoxy group, an intramolecular hydrogen bond occurs, denoted with the label H. This hydrogen bond is stabilizing such that it is present in each of the most populated conformers. The intramolecular hydrogen bond was verified using topographical QTAIM analysis. , As shown in Figure S4a, there is a bond critical point between the hydrogen atom of the hydroxy group and the oxygen atom of the methoxy group, unequivocal evidence of hydrogen bonding. , As for coumaraldehyde, in each of these conformers, the aldehyde substituent is in its trans orientation. The environment of the aldehyde substituent is less isotropic in the presence of the methoxy group than in coumaraldehyde, so the second most-populated conformer has a higher relative energy, 1.1 kJ mol –1 compared to 0.5 kJ mol –1 , and it is the conformer that was used in a previous prediction of the redox potential of coniferaldehyde .…”

Section: Resultsmentioning

confidence: 97%

“…54,56 As shown in Figure S4a, there is a bond critical point between the hydrogen atom of the hydroxy group and the oxygen atom of the methoxy group, unequivocal evidence of hydrogen bonding. 73,74 As for coumaraldehyde, in each of these conformers, the aldehyde substituent is in its trans orientation. The environment of the aldehyde substituent is less isotropic in the presence of the methoxy group than in coumaraldehyde, so the second most-populated conformer has a higher relative energy, 1.1 kJ mol −1 compared to 0.5 kJ mol −1 , and it is the conformer that was used in a previous prediction of the redox potential of coniferaldehyde.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Light Absorption by Cinnamaldehyde Constituents of Biomass Burning Organic Aerosol Modeled Using Time-Dependent Density Functional Theory

Calvert

2023

ACS Earth Space Chem.

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Organic aerosol emitted from biomass burning absorbs visible radiation. However, the impact of this light absorption on the overall climate effects of atmospheric aerosol is not well known, partly due to variability in particle composition and absorptivity. Cinnamaldehydes, which consist of an aromatic ring with an unsaturated aldehyde substituent, are an important class of chromophores in light-absorbing organic aerosol, or brown carbon. Here, light absorption by a homologous series of three cinnamaldehydescoumaraldehyde, coniferaldehyde, and sinapaldehydeis modeled with time-dependent density functional theory (TD-DFT) calculations, in the gas and aqueous phases. Based on a survey of hydration and acid dissociation equilibria, the neutral aldehyde is expected to be the predominant form of each species in the atmospheric aqueous phase. These species have complicated conformational landscapes compared to many other brown carbon constituents, like rigid polycyclic aromatic hydrocarbons. For coumaraldehyde, coniferaldehyde, and sinapaldehyde, a total of 8, 26, and 18 conformers were located, respectively. For each species, most of the total population is accounted for by the four most-populated conformers. The relative contributions of the conformers to the total light absorption of the respective species are dictated more by differences in the relative free energies than by differences in the molar absorption coefficients. As the functionalization increases, the absorption is red-shifted. The peaks predicted in water agree well with experimental spectra of coniferaldehyde and sinapaldehyde. No conformers have vertical transitions in the visible spectral range, so absorption above 380 nm is due to the shoulders of transitions of major conformers at ultraviolet wavelengths. These results demonstrate the importance of exploring potential energy landscapes, determining conformer stability and absorptivity, to predict the light absorption of chromophores in brown carbon.