First experimental evidence of the third conformer of glycolic acid: combined matrix isolation, FTIR and theoretical study

Experimental and theoretical investigation of hydrogen bonded complexes between glycolic acid and water

Theoretical MP2 and B3LYPD3 calculations, as well as experimental matrix isolation infrared spectroscopy studies, were used to investigate the 1:1 complexes formed between glycolic acid and water. Out of five computationally predicted forms of GA⋯H2O complex the most stable one was detected experimentally in solid argon. This structure is characterized by two intermolecular OH⋯O hydrogen bonds depicting a six-member ring in which water acts both as a proton acceptor and as a proton donor. Two other structures with the alcoholic OH group acting as a proton donor are also tentatively suggested to be present in solid argon.

Sulfur Centered Hydrogen Bonding in Thioglycolic Acid and Its Clusters: A Computational Exploration

The conformational landscape of thioglycolic acid (TGA) was investigated by using the CCSD/cc-pVTZ level of theory. The GGC conformer was identified as the global minimum, followed by the GAC conformer. The calculated rotational constant for the GGC conformer exhibited good agreement with the previously reported experimental results. Subsequently, the study delved into the exploration of sulfur-centered hydrogen bonding in TGA's dimer and trimer clusters, employing the CCSD/cc-pVDZ level of theory. These clusters revealed the participation of both oxygen and sulfur atoms in noncovalent H-bonding, contributing to their stability. The presence of these noncovalent interactions in TGA clusters was elucidated through Atoms in Molecule (AIM), reduced density gradient (RDG), and natural bond order (NBO) analysis, while electrostatic potential (ESP) charge and vibrational mode analysis further supported these findings.

Accurate Quantum Chemical Spectroscopic Characterization of Glycolic Acid: A Route Toward its Astrophysical Detection

The first step to shed light on the abiotic synthesis of biochemical building blocks, and their further evolution toward biological systems, is the detection of the relevant species in astronomical environments, including earthlike planets. To this end, the species of interest need to be accurately characterized from structural, energetic, and spectroscopic viewpoints. This task is particularly challenging when dealing with flexible systems, whose spectroscopic signature is ruled by the interplay of small- and large-amplitude motions (SAMs and LAMs, respectively) and is further tuned by the conformational equilibrium. In such instances, quantum chemical (QC) calculations represent an invaluable tool for assisting the interpretation of laboratory measurements or even observations. In the present work, the role of QC results is illustrated with reference to glycolic acid (CH2OHCOOH), a molecule involved in photosynthesis and plant respiration and a precursor of oxalate in humans, which has been detected in the Murchison meteorite but not yet in the interstellar medium or in planetary atmospheres. In particular, the equilibrium structure of the lowest-energy conformer is derived by employing the so-called semiexperimental approach. Then, accurate yet cost-effective QC calculations relying on composite post-Hartree-Fock schemes and hybrid coupled-cluster/density functional theory approaches are used to predict the structural and ro-vibrational spectroscopic properties of the different conformers within the framework of the second-order vibrational perturbation theory. A purposely tailored discrete variable representation anharmonic approach is used to treat the LAMs related to internal rotations. The computed spectroscopic data, particularly those in the infrared region, complement the available experimental investigations, thus enhancing the possibility of an astronomical detection of this molecule.

Molecular structure, interactions, and antimicrobial properties of curcumin-PLGA Complexes-a DFT study

Density functional calculations are performed to study the molecular structure, interactions, and antimicrobial activity of curcumin-poly lacto glycolic acid (Cur-PLGA) complexes. The calculations are performed on curcumin (Cur), glycolic acid (SSC and AAT conformers), lactic acid (LA), Cur-SSC, Cur-AAT, Cur-LA, and Cur-PLGA complexes using dispersion corrected M06-2X functional with 6-31 + G* basis set. The condensed Fukui functions of Cur are calculated to identify the favorable reactive sites. Inter- and intramolecular H-bond interactions are analyzed in detail through natural bond orbital, Atoms in Molecule, and Reduced density gradient analyses. The interaction energy values indicate that the interaction between Cur and AAT is stronger than the other studied complexes. Further, our calculations show that the PLGA interacted with Cur is having lower LUMO energy and density values. This indicates that the antimicrobial activity is high in this complex.

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.

Conformational isomerizations triggered by vibrational excitation of second stretching overtones

Vibrational excitation using frequency-tunable IR laser light has been developed as a powerful tool for selective manipulation of molecular conformations. In this methodology, vibrational excitation has been typically applied to the first stretching overtones (∼80 kJ mol^-1) but also to the fundamental modes (∼40 kJ mol^-1). Here, we demonstrate that selective conformational isomerizations are also achieved using excitation to second stretching overtones (∼120 kJ mol^-1). The extremely weak absorptions of the second stretching overtones of molecules isolated in low-temperature matrices were measured for the first time; here using three prototype molecules: hydroxyacetone (HA), glycolic acid (GAc) and glycolamide (GAm). Benchmarking of computed anharmonic IR spectra showed that the B3LYP/SNSD method provides the best agreement with experimental frequencies of the ν(OH), 2ν(OH) and 3ν(OH) modes for the studied molecules in argon matrices. Selective irradiation at the 3ν(OH) frequencies (9850-10 500 cm^-1) of HA, GAc and GAm monomers in argon matrices at 15 K successfully triggers their conformational isomerization. These results open the door to extend control over conformations separated by higher barriers and to induce other transformations not energetically accessible by excitation to the fundamental or first stretching overtone modes.

Complexes of Glycolic Acid with Nitrogen Isolated in Argon Matrices. I. Structures and Thermal Effects

Molecular complexes between glycolic acid and nitrogen were studied in a low-temperature argon matrix with FTIR spectroscopy, and supported by MP2 and BLYPD3 calculations. The calculations indicate 11 and 10 stable complex structures at the MP2 and BLYPD3 levels of theories, respectively. However, only one hydrogen-bonded complex structure involving the most stable SSC conformer of glycolic acid was found experimentally, where the nitrogen molecule is bound with the carboxylic OH group of the SSC conformer. The complex shows a rich site structure variation upon deposition of the matrix in different temperatures and upon annealing experiments, which provide interesting prospects for site-selective chemistry.

An experimental and computational IR and hybrid DFT-D3 study of the conformations ofl-lactic and acrylic acid: new insight into the dehydration mechanism of lactic acid to acrylic acid

Effect of the intra-molecular H-bond ofl-lactic acid on its dehydration mechanism: IR and DFT-D3 study,

Studies of intramolecular H-bond interactions and solvent effects in the conformers of glycolic acid — A quantum chemical study

In this work, density functional theory is applied to understand the conformational stability and solvent effects on glycolic acid conformers in different solvents. In addition, the role of intramolecular hydrogen bond (H-bond) interactions in the stability of conformers are investigated. The molecular geometries of selected conformers are optimized using B3LYP and PBE0 functionals with 6-311[Formula: see text]G(d,p) basis set. The effects of solvent on the geometrical parameters, relative stability, dipole moment, chemical hardness, chemical potential, etc. are studied for the conformers of glycolic acid. Our calculations show that the order of stability of the SSC and AAT conformers does not change in liquid phase. However, the energy of SSC and AAT conformers is very close to each other in water media. In water media, strong intramolecular H-bond interaction is present in AAT conformer which causes the energy of AAT conformer to be very close to that of SSC conformer. This may be due to the influence of water media.

Raman spectroscopic evidence of low temperature stability of D,L-glycolic and L-(+)-lactic acid crystals

Raman and infrared spectra of polycrystalline D,L-glycolic and L-(+) lactic acid are presented and assigned both by an ab initio calculation of normal modes of free conformers and by self-consistent-charge density-functional-theory computational program DFTB+. Temperature dependent Raman spectra from 295 K to 10 K reveal great stability of crystal lattices, since no soft modes and no band splittings that could be attributed to changes of the number of molecules per unit cell were observed. A semiempirical calculation with GULP program was used to estimate the strength of hydrogen bonds in crystals: in glycolic acid they have energies of -0.337 eV/mol, -0.329 eV/mol, -0.262 eV/mol and -0.242 eV/mol, while in lactic acid two hydrogen bonds have energies of -0.283 eV/mol and -0.202 eV/mol.

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.

Near-infrared laser-induced generation of three rare conformers of glycolic acid

Structural transformations were induced in conformers of glycolic acid by selective excitation with monochromatic tunable near-infrared laser light. For the compound isolated in Ar matrixes, near-IR excitation led to generation of two higher-energy conformers (GAC; AAT) differing from the most stable SSC form by 180° rotation around the C-C bond. A detailed investigation of this transformation revealed that one conformer (GAC) is produced directly from the near-IR-excited most stable conformer. The other higher-energy conformer (AAT) was effectively generated only upon excitation of the primary photoproduct (GAC) with another near-IR photon. Once these higher-energy conformers of glycolic acid were generated in an Ar matrix, they could be subsequently transformed into one another upon selective near-IR excitations. Interestingly, no repopulation of the initial most stable SSC conformer occurred upon near-IR excitation of the higher-energy forms of the compound isolated in solid Ar. A dramatically different picture of near-IR-induced conformational transformations was observed for glycolic acid isolated in N2 matrixes. In this case, upon near-IR excitation, the most stable SSC form converted solely into a new conformer (SST), where the acid OH group is rotated by 180°. This conformational transformation was found to be photoreversible. Moreover, SST conformer, photoproduced in the N2 matrix, spontaneously converted to the most stable SSC form of glycolic acid, when the matrix was kept at cryogenic temperature and in the dark.

Oxidative cleavage of hydrobenzoin by ACC/silica gel under ultrasound irradiation

A mild and efficient method has been developed using ultrasound irradiation for oxidative cleavage of hydrobenzoin with ACC/silica gel in dichloromethane. A high yield of 72-99% has been obtained for the oxidation of a series of hydrobenzoins into aromatic aldehydes at room temperature.

Cα−H Bond-Stretching Frequency in Alcohols as a Probe of Hydrogen-Bonding Strength: A Combined Vibrational Spectroscopic and Theoretical Study of n-[1-D]Propanol

The sensitivity of the nu(C)()alpha(-)(H/D) vibrational stretching frequency to hydrogen bonding in alcohols is examined by infrared and Raman spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. The model compound studied is (R,S)-n-[1-D]propanol. It is shown that the nu(C)()alpha(-)(H/D) mode can be successfully correlated with the hydrogen-bond strength in a given solvent, provided the O-H group involved in the hydrogen bond is not acting simultaneously as a hydrogen-bond donor and acceptor. In addition, a detailed analysis of the spectroscopic features observed in both the nu(O)(-)(H) and nu(C)()alpha(-)(H/D) spectral regions of the spectra of n-propanol and (R,S)-n-[1-D]propanol, in a series of different experimental conditions, which include the matrix-isolated compound (in argon matrix), pure liquid and low-temperature glassy states, and solution in different solvents, is undertaken. This permits the contribution of the different conformers of the studied compounds to be assigned to the bands observed in the nu(O)(-)(H) and nu(C)(-)(H) spectral regions.

Clusters of glycolic acid with three to six water molecules

Semiempirical, ab initio, and density functional theory calculations are used to locate many low-energy minima on the potential energy surfaces of the CH2OHCOOH-(H2O)n complexes with n = 3,4,5,6. In the clusters with three, four, and five water molecules, the lowest-energy structure consists of a (H2O)n complex, not necessarily of lowest energy, hydrogen bonded to the carboxylic group of the glycolic acid. The lowest-energy structure for n = 6 is similar except that the water hexamer is hydrogen bonded to both the carboxylic and alpha-hydroxyl groups of the acid. In all the lowest-energy clusters, the intramolecular hydrogen bond remains intact in the glycolic acid.