Molecular Structure of Salicylic Acid and Its Hydrates: A Rotational Spectroscopy Study

We present a study of salicylic acid and its hydrates, with up to four water molecules, done by employing chirped-pulse Fourier transform microwave spectroscopy. We employed the spectral data set of the parent, 13C, and 2H isotopologues to determine the molecular structure and characterize the intra- and intermolecular interactions of salicylic acid and its monohydrate. Complementary theoretical calculations were done to support the analysis of the experimental results. For the monomer, we analyzed structural properties, such as the angular-group-induced bond alternation (AGIBA) effect. In the microsolvates, we analyzed their main structural features dominated by the interaction of water with the carboxylic acid group. This work contributes to seeding information on how water molecules accumulate around this group. Moreover, we discussed the role of cooperative effects further stabilizing the observed inter- and intramolecular hydrogen bond interactions.

Noncovalent Interactions in the Molecular Geometries of 4-Methylthiazole···H2O and 5-Methylthiazole···H2O Revealed by Microwave Spectroscopy

The pure rotational spectra of 4-methylthiazole···H2O and 5-methylthiazole···H2O were recorded by chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. Each complex was generated within the rotationally cold environment of a gas sample undergoing supersonic expansion in the presence of an argon buffer gas. The spectra of five isotopologues of each complex have been measured and analyzed to determine the rotational constants, A0, B0, and C0; centrifugal distortion constants, DJ, DJK, and d1; nuclear quadrupole coupling constants, χaa(N3) and [χbb(N3) - χcc(N3)]; and parameters describing the internal rotation of the CH3 group, V3 and ∠(i,b). The experimentally deduced parameters were obtained using the XIAM and the BELGI-Cs-hyperfine code. For each complex, parameters in the molecular geometry are fitted to experimentally determined moments of inertia. DFT calculations have been performed at the ωB97X-D/aug-cc-pVQZ level in support of the experiments. Each complex contains two hydrogen bonds; a comparatively strong, primary interaction between the N of thiazole and an O-H of H2O, and a weaker, secondary interaction between O and either the hydrogen atom attached to C2 (in 5-methylthiazole···H2O) or the CH3 group attached to C4 (in 4-methylthiazole···H2O). The barrier to internal rotation of the CH3 group, V3, is slightly lower for 4-methylthiazole···H2O (XIAM result is 340.05(56) cm-1) than that for the 4-methylthiazole monomer (357.6 cm-1). This is likely to be a result of internal charge redistribution within the 4-methylthiazole subunit following its coordination by H2O. At the precision of the experiments, V3 of 5-methylthiazole···H2O (XIAM result is 325.16(38) cm-1) is not significantly different from V3 of the 5-methylthiazole monomer (332.0 cm-1).

Water binding to the atmospheric oxidation product methyl vinyl ketone

Methyl vinyl ketone is one of the major oxidation products of isoprene, and therefore, an important precursor of secondary organic aerosol. Understanding its interactions with water is relevant to gain insight into aerosol formation and improve the predictive power of atmospheric chemistry models. The molecular complex formed between methyl vinyl ketone and water has been generated in a supersonic jet and characterized using high-resolution microwave spectroscopy in combination with quantum chemistry calculations. In this study, we show that methyl vinyl ketone interacts with water forming four 1:1 isomers connected by O - H···O and C - H···O hydrogen bond interactions. Water has been found to preferentially bind to the antiperiplanar conformation of methyl vinyl ketone. Evidence of a large amplitude motion arising from the methyl internal rotation has been found in the rotational spectra of the dimer. The threefold methyl internal rotation barrier heights have been further determined and discussed for all the species.

Gas-Phase Hydration of Perillaldehyde Investigated by Microwave Spectroscopy Assisted by Computational Chemistry

The microsolvated complexes of two equatorial conformers of perillaldehyde were experimentally investigated in a supersonic molecular jet coupled to a cavity-based Fourier transform microwave spectrometer, in the 2.3-8 GHz frequency range. The structures of hydrates C₁₀H₁₄O·(H₂O)_n (n = 1,2,3) were first optimized at the MP2/6-311++G(d,p) and B3LYP-D3BJ/def2-TZVP levels of theory. The spectral signatures of four monohydrates and of two dihydrates could then be obtained. Additional rotational constants from the analysis of the spectra of their ¹⁸O isotopologues allowed the calculation of the substitution coordinates of the water oxygen atoms of each hydrate. They were found to be in good agreement with those of the optimized structures. SAPT2 calculations and noncovalent interaction analysis highlight the role of dispersion and quasi-hydrogen bonds in the stabilization of the structures.

Assessing the Conformational Equilibrium of Carboxylic Acid via Quantum Mechanical and Molecular Dynamics Studies on Acetic Acid

Accurate hydrogen placement in molecular modeling is crucial for studying the interactions and dynamics of biomolecular systems. The carboxyl functional group is a prototypical example of a functional group that requires protonation during structure preparation. To our knowledge, when in their neutral form, carboxylic acids are typically protonated in the syn conformation by default in classical molecular modeling packages, with no consideration of alternative conformations, though we are not aware of any careful examination of this topic. Here, we investigate the general belief that carboxylic acids should always be protonated in the syn conformation. We calculate and compare the relative energetic stabilities of syn and anti acetic acid using ab initio quantum mechanical calculations and atomistic molecular dynamics simulations. We focus on the carboxyl torsional potential and configurations of microhydrated acetic acid from molecular dynamics simulations, probing the effects of solvent, force field (GAFF vs GAFF2), and partial charge assignment of acetic acid. We show that while the syn conformation is the preferred state, the anti state may in some cases also be present under normal NPT conditions in solution.

Pyridine–acetaldehyde, a molecular balance to explore the n→π*interaction

Weak n→π*and C–H⋯O interactions determine the structure of pyridine–acetaldehyde adduct. The n→π*distance oscillates with the methyl group internal rotation which acts as a sort of molecular balance to explore the n→π*interaction energy.

Conformational preference determined by inequivalent n-pairs: rotational studies on acetophenone and its monohydrate

Acetophenone and its complex with water have been investigated by using pulsed jet Fourier transform microwave spectroscopy complemented with quantum chemical calculations.

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

By mixing primary and secondary alcohols with carboxylic acids just before the supersonic expansion within pulsed Fourier transform microwave experiments, only the rotational spectrum of the ester was observed. However, when formic acid was mixed with tertiary alcohols, adducts were formed and their rotational spectra could be easily measured. Quantum mechanical calculations were performed to interpret the experimental evidence.

A butterfly motion of formic acid and cyclobutanone in the 1 : 1 hydrogen bonded molecular cluster

Large tunnelling splittings reveal the relative motions and the inversion pathway in the formic acid–cyclobutanone cluster.

Hydration of the simplest α-keto acid: a rotational spectroscopic and ab initio study of the pyruvic acid–water complex

Non-covalent interactions analysis of hydrogen bonding in the pyruvic acid water complex.

Rotational spectroscopy of the methyl glycidate–water complex: conformation and water and methyl rotor tunnelling motions

Rotational transitions of methyl glycidate–water exhibit relatively large water tunnelling splittings, a surprise considering that water is quite tightly bound.

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

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.

Acetic acid-water complex: The first observation of structures containing the higher-energy acetic acid conformer

Non-covalent interaction of acetic acid (AA) and water is studied experimentally by IR spectroscopy in a nitrogen matrix and theoretically at the MP2 and coupled-cluster with single and double and perturbative triple excitations [CCSD(T)]/6-311++G(2d,2p) levels of theory. This work is focused on the first preparation and characterization of complexes of higher-energy (cis) conformer of AA with water. The calculations show three 1:1 structures for the trans-AA⋯H2O complexes and three 1:1 structures for the cis-AA⋯H2O complexes. Two trans-AA⋯H2O and two cis-AA⋯H2O complexes are found and structurally assigned in the experiments. The two cis-AA⋯ ⋅ H2O complexes are obtained by annealing of a matrix containing water and cis-AA molecules prepared by selective vibrational excitation of the ground-state trans form. The less stable trans-AA⋯H2O complex is obtained by vibrational excitation of the less stable cis-AA⋯H2O complex. In addition, the 1:2 complexes of trans-AA and cis-AA with water molecules are studied computationally and the most stable forms of the 1:2 complexes are experimentally identified.

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

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.

Hydrated forms of fluoroacetic acid: a rotational study

Two conformers have been observed in the rotational spectrum of the 1 : 1 adduct of fluoroacetic acid and water.

The benzoic acid-water complex: a potential atmospheric nucleation precursor studied using microwave spectroscopy and ab initio calculations

The pure rotational, high-resolution spectrum of the benzoic acid-water complex was measured in the range of 4-14 GHz, using a cavity-based molecular beam Fourier-transform microwave spectrometer. In all, 40 a-type transitions and 2 b-type transitions were measured for benzoic acid-water, and 12 a-type transitions were measured for benzoic acid-D2O. The equilibrium geometry of benzoic acid-water was determined with ab initio calculations, at the B3LYP, M06-2X, and MP2 levels of theory, with the 6-311++G(2df,2pd) basis set. The experimental rotational spectrum is most consistent with the B3LYP-predicted geometry. Narrow splittings were observed in the b-type transitions, and possible tunnelling motions were investigated using the B3LYP/6-311++G(d,p) level of theory. Rotation of the water moiety about the lone electron pair hydrogen-bonded to benzoic acid, across a barrier of 7.0 kJ mol(-1), is the most likely cause for the splitting. Wagging of the unbound hydrogen atom of water is barrier-less, and this large amplitude motion results in the absence of c-type transitions. The interaction and spectroscopic dissociation energies calculated using B3LYP and MP2 are in good agreement, but those calculated using M06-2X indicate excess stabilization, possibly due to dispersive interactions being over-estimated. The equilibrium constant of hydration was calculated by statistical thermodynamics, using ab initio results and the experimental rotational constants. This allowed us to estimate the changes in percentage of hydrated benzoic acid with variations in the altitude, region, and season. Using monitoring data from Calgary, Alberta, and the MP2-predicted dissociation energy, a yearly average of 1% of benzoic acid is expected to be present in the form of benzoic acid-water. However, this percentage depends sensitively on the dissociation energy. For example, when using the M06-2X-predicted dissociation energy, we find it increases to 18%.