Rotational isomerization of small carboxylic acids isolated in argon matrices: tunnelling and quantum yields for the photoinduced processes

FTIR spectroscopy and molecular level insight of diluted aqueous solutions of acetic acid

Aqueous solutions of acetic acid (AA) have been intensively explored for decades with a particular attention addressed to the hydrogen bond network generated by COOH group at different concentrations. In majority of studies conducted so far the envelope originated from νCO is decomposed into two bands assigned to differently hydrated monomers: the one presumably to AA···H2O, and another one to AA···(H2O)2. In order to examine if species other than the mentioned monomers produce this spectral signature, we performed computational and FTIR spectroscopic study of AA in aqueous solutions. Dilute solutions of deuterated acetic acid (CD3COOD) in D2O and in C2Cl4 as a reference were prepared (c0 = 0.001, 0.01 and 0.1 mol dm-3) as well as of deuterated sodium acetate (CD3COONa) in D2O. CD3COOD in 0.1 mol dm-3 solution in D2O displays a feature that separated in two signals with maxima at 1706 cm-1 and 1687 cm-1. A combined DFT and molecular dynamics study performed in this work showed the assignation of those spectral bands to be a more complex problem than previously thought, with syn-anti isomerism and hydration contributing to the experimentally observed broad νCO envelope.

On the vibrations of formic acid predicted from first principles

In this article, we review recent first principles, anharmonic studies on the molecular vibrations of gaseous formic acid in its monomer form. Transitions identified as fundamentals for both cis- and trans form reported in these studies are collected and supported by results from high-resolution experiments. Attention is given to the effect of coordinate coupling on the convergence of the computed vibrational states.

CC-stretched formic acid: isomerisation, dimerisation, and carboxylic acid complexation

The cis-trans-isomerism of the propiolic acid monomer (HC[triple bond, length as m-dash]C-COOH) is examined with linear Raman jet spectroscopy, yielding the first environment-free vibrational band centres of a higher-energy cis-rotamer beyond formic acid (HCOOH) in addition to all fundamentals and a large number of hot and combination/overtone bands of the trans-conformer. Two near-isoenergetic trans-fundamentals of different symmetry (CC[double bond, length as m-dash]O bend and OH torsion) prove to be a sensitive benchmarking target, as their energetic order is susceptible to the choice of electronic structure method, basis set size, and inclusion of vibrational anharmonicity. For the infrared- and Raman-active C[double bond, length as m-dash]O stretching fundamentals of the cyclic (C_2h) trans-propiolic acid dimer, resonance couplings are found that in part extend to the C_s-symmetric heterodimer of trans-propiolic and trans-formic acid. Exploratory vibrational perturbation theory (VPT2) calculations show that all perturbing states involve displacements of the OH moieties located on the doubly hydrogen bonded ring. The comparison of the infrared spectra of the propiolic acid dimer and its heterodimer with formic acid to that of several other carboxylic acid dimers from the literature reveals a notable similarity regarding a non-fundamental dimer band around 1800 cm^-1, which in most cases is so far unassigned. VPT2 calculations and a simple harmonic model suggest an assignment to a combination vibration of the symmetric and antisymmetric OH torsion.

Structural Relevance of Intramolecular H-Bonding in Ortho-Hydroxyaryl Schiff Bases: The Case of 3-(5-bromo-2-hydroxybenzylideneamino) Phenol

A new Schiff base compound, 3-(5-bromo-2-hydroxybenzylideneamino)phenol (abbreviated as BHAP) was synthesized and characterized by ¹H- and ¹³C- nuclear magnetic resonance and infrared spectroscopies. DFT/B3LYP/6-311++G(d,p) calculations were undertaken in order to explore the conformational space of both the E- and Z- geometrical isomers of the enol-imine and keto-amine tautomers of the compound. Optimized geometries and relative energies were obtained, and it was shown that the most stable species is the E-enol-imine form, which may exist in four low-energy intramolecularly hydrogen-bonded forms (I, II, V, and VI) that are almost isoenergetic. These conformers were concluded to exist in the gas phase equilibrium with nearly equal populations. On the other hand, the infrared spectra of the compound isolated in a cryogenic argon matrix (10 K) are compatible with the presence in the matrix of only two of these conformers (conformers II and V), while conformers I and VI convert to these ones by quantum mechanical tunneling through the barrier associated with the rotation of the OH phenolic group around the C–O bond. The matrix isolation infrared spectrum was then assigned and interpreted with help of the DFT(B3LYP)/6-311++G(d,p) calculated infrared spectra for conformers II and V. In addition, natural bond orbital (NBO) analysis was performed on the most stable conformer of the experimentally relevant isomeric form (E-enol-imino conformer V) to shed light on details of its electronic structure. This investigation stresses the fundamental structural relevance of the O–H···N intramolecular H-bond in o-hydroxyaryl Schiff base compounds.

Vibrationally Induced Conformational Isomerization and Tunneling in Pyrrole-2-Carboxylic Acid

The conformational behavior of carboxylic acids has attracted considerable attention, as it can be used as a gateway for the study of more complex phenomena. Here, we present an experimental and computational study of pyrrole-2-carboxylic acid (PCA) conformational space and the vibrational characterization of the compound by infrared spectroscopy. The possibility of promoting conformational transformations using selective vibrational excitation of the 2ν(OH) and 2ν(NH) stretching overtones is explored. Two conformers, exhibiting the cis configuration of the COOH group (O═C-O-H dihedral angle near 0°) and differing by the orientation of the carboxylic group with respect to the pyrrole ring (i.e., showing either a cis or a trans NCC═O arrangement), were found to coexist initially for the compound isolated in a cryogenic nitrogen matrix, in an 86:14 ratio, and were characterized by infrared spectroscopy. A third conformer, with the COOH group in the trans configuration, was produced, in situ, by narrowband near-infrared (NIR) excitation of the most stable PCA form (with a cis NCC═O moiety). The photogenerated PCA conformer was found to decay back to the most stable PCA form, by H-atom quantum mechanical tunneling, with a characteristic half-life time of ∼10 min in the nitrogen matrix at 10 K. Tunneling rates were theoretically estimated and compared for the observed isomerization of pyrrole-2-carboxylic acid and for the structurally similar furan-2-carboxylic acid. This comparison showcases the effect of small modifications in the potential energy surface and the implications of quantum tunneling for the stability of short-living species.

Effect of intramolecular hydrogen-bond formation on the molecular conformation of amino acids

The molecular conformation of the carboxyl group can be crucial for its chemical properties and intermolecular interactions, especially in complex molecular environments such as polypeptides. Here, we study the conformational behaviour of the model amino acid N-acetylproline in solution at room temperature with two-dimensional infrared spectroscopy. We find that the carboxyl group of N-acetylproline adopts two distinct conformations, syn- and anti-. In the syn-conformer the O-H group is oriented at  ~60^∘ with respect to the C=O and in the anti-conformer the O-H is anti-parallel to the C=O. In hydrogen-bond accepting solvents such as dimethyl sulfoxide or water, we observe that, similar to simple carboxylic acids, around 20% of the -COOH groups adopt an anti-conformation. However, when N-acetylproline is dissolved in a weakly hydrogen-bond accepting solvent (acetonitrile), we observe the formation of a strong intramolecular hydrogen bond between the carboxyl group in the anti-conformation and the amide group, which stabilizes the anti-conformer, increasing its relative abundance to ~60%.

Peptide Side-COOH Groups Have Two Distinct Conformations under Biorelevant Conditions

The carboxyl (COOH) side chain groups of amino acids, such as aspartic acid, play an important role in biochemical processes, including enzymatic proton transport. In many theoretical studies, it was found that the (bio)chemical reactivity of the carboxyl group strongly depends on the conformation of this group. Interestingly, up to now there has been no experimental investigation of the geometry and the stability of different COOH conformers under biorelevant conditions. Here, we investigate the conformational isomerism of the side chain COOH group of N-acetyl aspartic acid amide using polarization-resolved two-dimensional infrared spectroscopy. We find that the carboxyl group shows two distinct near-planar conformers (syn and anti) when dissolved in water at room temperature. Both conformers are significantly populated in aqueous solution (75 ± 10% and 25 ± 10% for syn and anti, respectively). Molecular dynamics simulations show that the anti conformer interacts more strongly with water molecules than the syn conformer, explaining why this conformer is significantly present in aqueous solution.

Observation of Distinct Carboxylic Acid Conformers in Aqueous Solution

We investigate the molecular geometry of the carboxyl group of formic acid in acetonitrile and aqueous solutions at room temperature with two-dimensional infrared spectroscopy (2D-IR). We found that the carboxyl group adopts two distinct configurations: a configuration in which the carbonyl group is oriented antiparallel to the hydroxyl (anti-conformer), and a configuration in which the carbonyl group is oriented at an angle of ∼60° with respect to the hydroxyl (syn-conformer). These results constitute the first experimental evidence that carboxyl groups exist as two distinct and long-living conformational isomers in aqueous solution at room temperature.

S–H rotamerizationviatunneling in a thiol form of thioacetamide

Rotamerization of the S–H groupviahydrogen tunneling is reported for the first time.

Nunes C, Tarczay G, Fausto R. Selective conformational control by excitation of NH imino vibrational antennas

We provide experimental evidence for the occurrence of selective and reversible conformational control over the SH group by vibrational excitation of remote NH groups. Using an imino group that acts as a molecular antenna has no precedents.

Intricate Conformational Tunneling in Carbonic Acid Monomethyl Ester

Disentangling internal and external effects is a key requirement for understanding conformational tunneling processes. Here we report the s- trans/ s- cis tunneling rotamerization of carbonic acid monomethyl ester (1) under matrix isolation conditions and make comparisons to its parent carbonic acid (3). The observed tunneling rate of 1 is temperature-independent in the 3-20 K range and accelerates when using argon instead of neon as the matrix material. The methyl group increases the effective half life (τ_eff) of the energetically disfavored s- trans-conformer from 3-5 h for 3 to 11-13 h for 1. Methyl group deuteration slows the rotamerization further (τ_eff ≈ 35 h). CCSD(T)/cc-pVQZ//MP2/aug-cc-pVTZ computations of the tunneling probability suggest that the rate should be almost unaffected by methyl substitution or its deuteration. Thus the observed relative rates are puzzling, and they disagree with previous explanations involving fast vibrational relaxation after the tunneling event facilitated by the alkyl rotor.

Experimental Evidence of Long-Range Intramolecular Vibrational Energy Redistribution through Eight Covalent Bonds: NIR Irradiation Induced Conformational Transformation of E-Glutaconic Acid

Long-range intramolecular vibrational energy redistribution (IVR) driven conformational changes were investigated in a matrix-isolated open-chain, asymmetrical dicarboxylic acid, E-glutaconic acid. Although the analysis was challenging due to the presence of multiple backbone conformers and short lifetimes of the prepared higher energy cis conformers, it was shown that the selective excitation of the O-H stretching overtone of one of the carboxylic groups can induce the conformational change (trans to cis) of the other carboxylic group, located at the other end of the E-glutaconic acid molecule. This is a direct proof that the IVR process can act through eight covalent bonds in a flexible molecule before the excess energy completely dissipates into the matrix. The lifetime of the prepared higher energy conformers (averaged over the different backbones) was measured to be 12 s.

Double deuterated acetylacetone in neon matrices: infrared spectroscopy, photoreactivity and the tunneling process

Open enol conformers of double deuterated acetylacetone are produced by UV and IR irradiation, allowing the analysis of the tunnelling effect between them.

The mechanism of tautomerisation and geometric isomerisation in thioformic acid and its water complexes: exploring chemical pathways for water migration

A systematic and automated search for chemical pathways of isomerisation between geometric and tautomeric forms of gas-phase thioformic acid (TFA) and its water complexes is performed using a global reaction route mapping (GRRM) method, and an uncovered pathway for cis-trans isomerisation in the thiol form of TFA has been explored through computations performed at CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311++G(2d,2p) level of the coupled cluster and density functional theories. To explore the routes for water migration, a detailed analysis of complexes of TFA with a single-water molecule is presented. Notably, during the isomerisation process in TFA, a positive catalytic effect of water was observed that can arise either by the stabilization of the reactant and/or of the transition state through extensive hydrogen bonding. Interesting behavior of isomeric forms of TFA along the pathways analysed is revealed through the Gibbs free-energy change and its temperature-dependence. The cis form of TFA(thiol) in the complexes of TFA with a single-water molecule is found to be thermodynamically equally feasible as the trans form which though is known to be the most dominating among the isomeric forms of TFA. Besides these, various complexes of TFA with two-water molecules have also been explored to study the hydrogen-bonding interaction through natural bond-orbital (NBO) analysis. The complexes of TFA with two-water molecules have also been characterized using spectral features including vibrational frequency analysis, and the effect of complexation has been observed by noting frequency shift.

Conformational switching in pyruvic acid isolated in Ar and N₂ matrixes: spectroscopic analysis, anharmonic simulation, and tunneling

Monomers of pyruvic acid (PA) isolated in cryogenic argon and nitrogen matrixes were characterized by mid- and near-infrared spectroscopy. Interpretation of the experiments was aided by fully anharmonic calculations of the fundamental modes, overtones, and combinations up to two quanta, including their infrared intensities. The initially dominating PA conformer (Tc) has a cis CCOH arrangement and is stabilized by a strong intramolecular H-bond. Selective near-infrared excitation of Tc at the first OH overtone (6630 cm(-1) in Ar, 6643 cm(-1) in N2) induced a large scale conformational conversion to the higher-energy conformer (Tt) with trans CCOH arrangement. Tt was then converted back to Tc by selective NIR irradiation at the first Tt OH overtone (6940 cm(-1) in Ar, 6894 cm(-1) in N2). In N2 matrix, the Tt form was stabilized due to interaction between the OH group and the matrix molecules. This stabilization manifested itself in the absence of Tt → Tc relaxation and in a considerable change of the vibrational Tt signature upon going from argon to nitrogen matrix. In argon, the Tt form spontaneously decayed back to Tc in the dark (characteristic lifetime +16 h). In the presence of broad-band near-infrared light, the Tt → Tc relaxation speed considerably increased. The decay mechanisms are discussed.

Formic and acetic acids in a nitrogen matrix: Enhanced stability of the higher-energy conformer

Formic acid (HCOOH, FA) and acetic acid (CH(3)COOH, AA) are studied in a nitrogen matrix. The infrared (IR) spectra of cis and trans conformers of these carboxylic acids (and also of the HCOOD isotopologue of FA) are reported and analyzed. The higher-energy cis conformer of these molecules is produced by narrowband near-IR excitation of the more stable trans conformer, and the cis-to-trans tunneling decay is evaluated spectroscopically. The tunneling process in both molecules is found to be substantially slower in a nitrogen matrix than in rare-gas matrices, the cis-form decay constants being approximately 55 and 600 times smaller in a nitrogen matrix than in an argon matrix, for FA and AA respectively. The stabilization of the higher-energy cis conformer is discussed in terms of specific interactions with nitrogen molecule binding with the OH group of the carboxylic acid. This model is in agreement with the observed differences in the IR spectra in nitrogen and argon matrices, in particular, the relative frequencies of the νOH and τCOH modes and the relative intensities of the νOH and νC=O bands.