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

Abstract

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.

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

“…Figure 6 summarizes their experimental methyl torsional barriers. Resembling cresols and methylanisoles (see Section 2.1.2.2 and Figure 5), microwave studies on three isomers of toluic acid have shown that the V 3 potential is so high in the o-isomer (molecule (3) in Figure 6) that no torsional splittings could be observed [156]. The barrier to methyl internal rotation drastically decreases in the meta- [157] and paraisomers [65].…”

Sulfuric acid and aromatic carboxylate clusters H2SO4·ArCOO−: Structures, properties, and their relevance to the initial aerosol nucleation

“…QTAIM is a useful approach to evaluate the nature of hydrogen bond and its strength in addition to the geometrical analysis. [47,65] According to Bader's theory, the critical points (3,-…”

The Borderline between Reactivity and Pre‐reactivity of Binary Mixtures of Gaseous Carboxylic Acids and Alcohols

“…Several molecular complexes involving carboxylic acids [3][4][5][6][7][8][9][10][11][12] have been investigated by rotational spectroscopy to understand the nature of their non-covalent interactions and to have information on their internal dynamics and on their conformational equilibria. Most attention has been paid to the complexes of carboxylic acids,mainly to their dimers [3] and to their adducts with water.…”

“…Most attention has been paid to the complexes of carboxylic acids,mainly to their dimers [3] and to their adducts with water. [4] Plenty of data have been obtained on the dimers,c oncerning proton tunneling, [3a] the Ubbelohde effect, [3b] and conformational equilibria. [3c] HCOOH (FA) is the prototype of the carboxylic acids family and for this reason it is involved in most of the investigations of carboxylic acids with molecules containing other functional groups,s uch as its adducts with H 2 O, [4a] N(CH 3 ) 3 , [5] anhydrides, [6] thes implest aldehyde (CH 2 O), [7] the simplest amide (CH(CO)NH 2 ), [8] ethers (dimethylether), [9] ketones (cyclobutanone), [10] esters (isopropylformate), [11] and azines (pyridine).…”