Thermally and vibrationally induced conformational isomerizations, infrared spectra, and photochemistry of gallic acid in low-temperature matrices

Infrared Spectra and Phototransformations of meta-Fluorophenol Isolated in Argon and Nitrogen Matrices

Monomers of meta-fluorophenol (mFP) were trapped from the gas phase into cryogenic argon and nitrogen matrices. The estimated relative energies of the two conformers are very close, and in the gas phase they have nearly equal populations. Due to the similarity of their structures (they only differ in the orientation of the OH group), the two conformers have also similar predicted vibrational signatures, which makes the vibrational characterization of the individual rotamers challenging. In the present work, it has been established that in an argon matrix only the most stable trans conformer of mFP exists (the OH group pointing away from the fluorine atom). On the other hand, the IR spectrum of mFP in a nitrogen matrix testifies to the simultaneous presence in this matrix of both the trans conformer and of the higher-energy cis conformer (the OH group pointing toward the fluorine atom), which is stabilized by interaction with the matrix gas host. We found that the exposition of the cryogenic N₂ matrix to the Globar source of the infrared spectrometer affects the conformational populations. By collecting experimental spectra, either in the full mid-infrared range or only in the range below 2200 cm^-1, we were able to reliably distinguish two sets of experimental bands originating from individual conformers. A comparison of the two sets of experimental bands with computed infrared spectra of the conformers allowed, for the first time, the unequivocal vibrational identification of each of them. The joint implementation of computational vibrational spectroscopy and matrix-isolation infrared spectroscopy proved to be a very accurate method of structural analysis. Some mechanistic insights into conformational isomerism (the quantum tunneling of hydrogen atom and vibrationally-induced conformational transformations) have been addressed. Finally, we also subjected matrix-isolated mFP to irradiations with UV light, and the phototransformations observed in these experiments are also described.

IR-induced and tunneling reactions in cryogenic matrices: the (incomplete) story of a successful endeavor

In this article, IR-induced and tunneling-driven reactions observed in cryogenic matrices are described in a historical perspective, the entangling of the two types of processes being highlighted. The story of this still ongoing fascinating scientific endeavor is presented here following closely our own involvement in the field for more than 30 years, and thus focuses mostly on our work. It is, because of this reason, also an incomplete story. Nevertheless, it considers a large range of examples, from very selective IR-induced conformational isomerizations to IR-induced bond-breaking/bond-forming reactions and successful observations of rare heavy atom tunneling processes. As a whole, this article provides a rather general overview of the major progress achieved in the field.

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.

A general synthesis of abundant metal nanoparticles functionalized mesoporous graphitized carbon

A general coordination–polymerization strategy combining flexible coordination mode with stable polymer network is reported to construct metal–gallic acid resin and mesoporous graphitized carbon materials with abundant metal nanoparticles.