Positional and Conformational Isomerism in Hydroxybenzoic Acid: A Core-Level Study and Comparison with Phenol and Benzoic Acid

Abstract

Three positional isomers of hydroxybenzoic acid, as well as phenol and benzoic acid, were studied using core-level photoemission and X-ray absorption spectroscopies, supported by quantum chemical calculations. While 2-hydroxybenzoic (salicylic) acid exists as a single conformer with an internal hydrogen bond, 3and 4-hydroxybenzoic acids are mixtures of multiple conformers. The effects due to isomerism are clearly seen in the C 1s and O 1s photoelectron spectra, whereas the conformational effects on the binding energies are less pronounced. The O 1s photoelectron spectrum of salicylic acid is significantly different from that of the other two isomers, providing a signature of the hydrogen bond. In contrast, the oxygen K edge X-ray absorption spectra of the three hydroxybenzoic acids show only minor differences. The salicylic acid absorption spectrum at the carbon K edge shows a more resolved vibrational structure than the spectra of the other molecules, which can be explained in part by the existence of a single conformer. Our theoretical study of vibrational excitations in the lowest C 1s absorption bands of salicylic and 4-hydroxybenzoic acids indicates that the observed structure can be assigned to 0−0 lines of various electronic transitions since most of the totally symmetric vibrational modes with sufficiently large frequencies to be resolved are predicted to be inactive. Significant sensitivity of the C 1s excitations in 3-hydroxybenzoic acid to rotational conformerism was predicted but not observed due to spectral crowding.

Dehydrogenation of Ammonia Borane Impacts Valence and Core Electrons: A Photoemission Spectroscopic Study

“…The present calculated VIE of AB at 10.57 eV agrees well with the measurements which are between the two experimental measurements of 10.58 eV and 10.00 eV . In addition, our recent joint XPS experimental and theory study of hydroxybenzoic acids revealed that the vibrational impact is small for fast ionization processes so that in the present study, only the first-order VIEs are considered. In addition, the fact that only the first IE available without any higher valence IEs of the boron–nitrogen hydrides also makes such high level calculations less useful.…”

“…To further study the electronic structure and their behavior of these compounds may require information of other properties such as excitations of both valence and core electrons. As such, recent synchrotron sourced X-ray techniques including near-edge X-ray absorption fine structure (NEXAFS) enables an unambiguous identification of fingerprints for elements in molecules and core excitation spectra . It will be interesting to use new measured NEXAFS of the boron–nitrogen hydrides in the near future to benchmark the existing extended second-order algebraic–diagrammatic construction approximation scheme for polarization propagator ADC(2)­x , against the recently developed coupled-cluster (CC) theory obtained from a new time-dependent formalism based (EOM-CCSDT) and quadruples (EOM-CCSDTQ) , for core excitation spectra.…”

Future of computational molecular spectroscopy—from supporting interpretation to leading the innovation

“…Recent joint CMS and XPS studies reveal that intramolecular hydrogen bonding (OÁ Á ÁH) deeply impacts to core electrons (O1s) of molecules. 63 Combining the measured O1s XPS (for hydrogen acceptor) and 1 H-NMR (for hydrogen donor) with quantum mechanics, intramolecular hydrogen bonding (OÁ Á ÁH) of molecules can be studied locally. 64 In addition, XPS provides core electron ionization of a molecule which localised on specific atoms.…”

A photoelectron spectroscopic investigation of aspirin, paracetamol and ibuprofen in the gas phase

“…The rotamer aspirin3 contains an internal hydrogen bond between the carboxylic acid hydrogen and the ether-like oxygen bonded to the acetyl group. Often internal hydrogen bonds are found in the lowest energy conformer [35][36][37][38] of a molecule, but in this case the two lowest energy forms do not form an internal hydrogen bond. As noted above, this has been attributed 7 to the stabilization of the lower energy forms by the Bu ¨rgi-Dunitz trajectory.…”