Hydrogen-Bonding Interactions of Malic Acid with Common Atmospheric Bases.

Malic acid (MA) (C4H6O5) is one of the most important organic constituents of fruits that is widely used in food and beverage industries. It is also detected in the atmospheric aerosol samples collected in different parts of the world. Considering the fact that secondary organic aerosols have adverse impacts on the global atmosphere and climate and a molecular-level understanding of the compositions and formation mechanism of secondary organic aerosols is necessary, we have performed systematic density functional electronic structure calculations to investigate the hydrogen-bonding interactions between MA and several naturally occurring nitrogen-containing atmospheric bases such as ammonia and amines that are derived from ammonia by the substitution of hydrogens by a methyl group. The base molecules were allowed to interact with the carboxylic COOH and the hydroxyl-OH group of the MA separately. While at both sites, MA produces energetically stable binary complexes with bases with large negative values of binding energy, the thermodynamical stability, at an ambient temperature and pressure of 298.15 K and 1 atm, respectively, is favored only for the clusters formed at the COOH site. A much larger red shift of the carboxylic-OH stretch than that of the hydroxyl-OH reinforces the preference of this site for cluster formation. Both the binding electronic energy and binding free energy of MA-ammonia complexes are lower than those of MA-amine complexes, although the amines are derivatives of NH3. The large increase in the Rayleigh activities upon cluster formation indicates that the MA-atmospheric base cluster may interact strongly with solar radiation. The detailed analysis of the structural, energetic, electrical, and spectroscopic properties of the binary complexes formed by MA with atmospheric bases shows that MA could participate in the atmospheric nucleation processes and subsequently contribute effectively to new particle formation in the atmosphere.

Perfluoropropionic Acid-Driven Nucleation of Atmospheric Molecules under Ambient Conditions.

A molecular-level understanding of the compositions and formation mechanism of secondary organic aerosols is important in the context of growing evidence regarding the adverse impacts of aerosols on the atmosphere and human health. The ever-growing emissions of pollutants and particulate matter in the atmosphere are a global concern. A particular class of pollutants, which are being important in this sense, are persistent organic pollutants (POPs) since they represent synthetic organic compounds with a long lifetime in the environment. Among the POPs, the perfluorinated compounds, such as perfluoroalkyl carboxylic acids (CnF2n+1COOH) or PFCAs, draw a lot of attention due to their adverse effect on human health. In the present work, we employ high-level density functional theory to investigate the electrostatic interaction of perfluoropropionic acid (C2F5COOH) or PFPA, a PFCA with n = 2, with well-known atmospheric molecules, namely, HCHO, HCOOH, CH3OH, H2SO4, and CH3SO3H [methanesulfonic acid (MSA)]. A detailed and systematic quantum chemical calculation has been performed to analyze the structural, energetic, electrical, and spectroscopic properties of several binary clusters in the context of atmospheric nucleation process. Our analysis shows that PFPA forms very stable hydrogen-bonded binary clusters with molecules like H2SO4 and MSA, which widely recognized atmospheric nucleation precursors. Scattering intensities of radiation (Rayleigh activities) are found to increase many fold when PFPA forms clusters. Analyses of the cluster-binding electronic energies and the free-energy changes associated with their formation at different temperatures indicate that PFPA could participate in the initial nucleation processes and contribute effectively to the new particle formation in the atmosphere.

Head-to-Tail and Head-to-Head Molecular Chains of Poly(p-Anisidine): Combined Experimental and Theoretical Evaluation.

Poly(p-anisidine) (PPA) is a polyaniline derivative presenting a methoxy (-OCH3) group at the para position of the phenyl ring. Considering the important role of conjugated polymers in novel technological applications, a systematic, combined experimental and theoretical investigation was performed to obtain more insight into the crystallization process of PPA. Conventional oxidative polymerization of p-anisidine monomer was based on a central composite rotational design (CCRD). The effects of the concentration of the monomer, ammonium persulfate (APS), and HCl on the percentage of crystallinity were considered. Several experimental techniques such as X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), multifractal analysis, Nuclear Magnetic Resonance (13C NMR), Fourier-transform Infrared spectroscopy (FTIR), and complex impedance spectroscopy analysis, in addition to Density Functional Theory (DFT), were employed to perform a systematic investigation of PPA. The experimental treatments resulted in different crystal structures with a percentage of crystallinity ranging from (29.2 ± 0.6)% (PPA1HT) to (55.1 ± 0.2)% (PPA16HT-HH). A broad halo in the PPA16HT-HH pattern from 2θ = 10.0-30.0° suggested a reduced crystallinity. Needle and globular-particle morphologies were observed in both samples; the needle morphology might have been related to the crystalline contribution. A multifractal analysis showed that the PPA surface became more complex when the crystallinity was reduced. The proposed molecular structures of PPA were supported by the high-resolution 13C NMR results, allowing us to access the percentage of head-to-tail (HT) and head-to-head (HH) molecular structures. When comparing the calculated and experimental FTIR spectra, the most pronounced changes were observed in ν(C-H), ν(N-H), ν(C-O), and ν(C-N-C) due to the influence of counterions on the polymer backbone as well as the different mechanisms of polymerization. Finally, a significant difference in the electrical conductivity was observed in the range of 1.00 × 10-9 S.cm-1 and 3.90 × 10-14 S.cm-1, respectively, for PPA1HT and PPA16HT-HH.

Vibrational Spectra of Atmospherically Relevant Hydrogen-Bonded MSA···(H2SO4)n (n = 1-3) Clusters.

Methanesulfonic acid (CH3SO3H), also known as MSA, has been found to be capable of forming a strong hydrogen-bonded interaction with sulfuric acid (H2SO4) under ambient conditions. The energetic stability of the MSA···H2SO4 clusters increases with decreasing temperature at higher altitudes in the troposphere, which is relevant in the context of atmospheric aerosol formation. We have performed, in the present work, a detailed and systematic quantum-chemical calculation with high-level density functional theory to characterize the hydrogen bond formation in the binary MSA···H2SO4, ternary MSA···(H2SO4)2, and quaternary MSA···(H2SO4)3 clusters. The five different conformations of MSA···(H2SO4)2 and six conformations of MSA···(H2SO4)3, considered in the present work for the spectroscopic analysis, have been taken from our previous work [J. Phys. Chem. A. 2020, 124, 11072-11085]. The hydrogen bonds were analyzed on the basis of infrared vibrational frequencies of different O-H stretching modes and quantum theory of atoms in molecules (QTAIM). A strong positive correlation has been observed between the red shift of the OH groups in MSA and H2SO4 and the corresponding O-H elongation as a result of hydrogen bond formation. Topological analysis employing QTAIM shows that most of the charge density and the Laplacian values at bond critical points (BCPs) of the hydrogen bonds of the MSA···(H2SO4)n (n = 1-3) complexes fall within the standard hydrogen-bond criteria. However, those outside these criteria fall in the category of a very strong hydrogen bond with a hydrogen bond length as low as 1.41 Å and an O-H bond elongation as high as 0.096 Å. In general, the charge densities of the BCPs located on hydrogen bonds increase as the hydrogen-bond lengths decrease. Proportionately, a larger number of hydrogen bonds in ternary MSA···(H2SO4)2 demonstrate a partial covalent character when compared with the quaternary clusters.

Atmospherically Relevant Hydrogen-Bonded Interactions between Methanesulfonic Acid and H2SO4 Clusters: A Computational Study.

A detailed and systematic quantum-chemical calculation has been performed with high-level density functional theory (DFT) to analyze the electrostatic interaction of methanesulfonic acid (CH3SO3H), also known as MSA, with pre-formed clusters of sulfuric acid (H2SO4) molecules in ambient conditions. Both MSA and H2SO4 are considered as atmospheric molecules that might play active roles in aerosol formation. The interactions between MSA and H2SO4 clusters lead to the formation of MSA···(H2SO4)n (n = 2, 3) complexes stabilized by the formation of different types of intermolecular hydrogen bond networks. Analyses of cluster binding energies and free energy changes associated with their formation indicate that MSA could bring additional stability into the atmospheric molecular clusters responsible for aerosol formation. Variations of Gibbs free energy with temperature and pressure have been analyzed. The lower temperatures and pressures at the higher altitudes of the troposphere are found to play in favor of higher stability of the MSA···(H2SO4)n clusters. Effects of hydrogen bond formation on dipole moment, mean polarizability, and anisotropy of polarizability of the clusters have been analyzed. Rayleigh scattering intensities are found to increase many-fold when light interacts with the MSA···(H2SO4)n clusters.

Effect of hydrogen bond formation on the NMR properties of glycine-HCN complexes.

The influence of the hydrogen bond formation on the nuclear magnetic resonance parameters has been investigated for the binary (1:1) and ternary (1:2) glycine-HCN complexes in the gas phase using high-level density functional theory with the B3LYP/6-31++G(2d,2p)//B3LYP/6-31++G(d,p) model of quantum chemistry. The calculated isotropic/anisotropic shielding parameters of the isolated glycine and HCN molecules are reported and compared with other theoretical results and experimental measurements. Six different conformations of hydrogen-bonded clusters have considered for both 1:1 and 1:2 glycine-HCN complexes. The isotropic and anisotropic chemical shifts for all the constituent atoms of the complexes have been calculated. The spin-spin coupling constants and the Fermi contact terms have also been analyzed in the context of hydrogen-bond formation.

Effects of microhydration on the electronic properties of ortho-aminobenzoic acid.

High-level density functional electronic structure calculations have been performed to analyze the effect of microsolvation with water on the electronic properties of ortho-aminobenzoic acid (o-Abz). The hydrogen-bonded interaction of the o-Abz molecule with one to three water molecules, o-Abz···(H2O)n (n = 1­3), has been considered in two different situations, once the solvent water molecules are placed close to the carboxyl (−COOH) group of o-Abz producing the o-Abz···[H2O]nCOOH complexes and when the water molecules are placed close to the amino (−NH2) group producing the o-Abz···[H2O]nNH2 clusters. Variation of the vibrational spectra and energetics upon hydrogen-bond formation are analyzed and compared with available experimental data. The effect of cooperativity is also analyzed. Overall, the hydrogen-bonded o-Abz···[H2O]nCOOH clusters are found to be more stable than the o-Abz···[H2O]nNH2 clusters.