Articles

Espectroscopia Rotacional

Abstract

“Rotational spectroscopy” is the technique that measures the interaction of electromagnetic radiation with the rotational movements of a molecular system. The observation of transitions between rotational quantum levels requires that the system under study has rotational freedom – typically, molecules, ions or radicals in the gas phase. This limitation does not allow the widespread application of rotational spectroscopy as an analytical technique, as in the case of other spectroscopic techniques that admit samples in condensed phase (i.e., liquids or solids). However, it is a historically important technique in the rigorous determination of molecular geometries and also has a very relevant contribution in astrochemistry: it is through rotational spectroscopy that several molecules (and molecular ions) are identified in space, using large radio telescopes such as ALMA (Atacama Large Millimeter Array).

Paulo Ribeiro-Claro.

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Espectroscopia.

Abstract

Spectroscopy is an experimental technique that is based on the use of light to study the composition, structure and properties of matter. The root of the word, from the Latin spectrum (image, appearance), refers to something like “observation of the hidden image”. Although it historically originates from the observation of visible light dispersed by a prism, the current concept encompasses any description of the radiation-matter interaction as a function of radiation energy.

Paulo Ribeiro-Claro.

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Conformational and vibrational reassessment of solid paracetamol.

Abstract

This work provides an answer to the urge for a more detailed and accurate knowledge of the vibrational spectrum of the widely used analgesic/antipyretic drug commonly known as paracetamol. A comprehensive spectroscopic analysis – including infrared, Raman, and inelastic neutron scattering (INS) – is combined with a computational approach which takes account for the effects of intermolecular interactions in the solid state. This allows a full reassessment of the vibrational assignments for Paracetamol, thus preventing the propagation of incorrect data analysis and misassignments already found in the literature. In particular, the vibrational modes involving the hydrogen -bonded N-H and O-H groups are correctly reallocated to bands shifted by up to 300 cm(-1) relatively to previous assignments.

Amado, A.M.; Azevedo, C.; Ribeiro-Claro, P.J.A.

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Inelastic neutron scattering study of reline: shedding light on the hydrogen bonding network of deep eutectic solvents

Abstract

The solids choline chloride and urea, mixed in a 1 : 2 molar proportion, form the iconic deep eutectic solvent “Reline”. A combination of computational and vibrational spectroscopy tools, including inelastic neutron scattering (INS), have been used to probe intermolecular interactions in the eutectic mixture. Reline’s experimental spectra were estimated using discrete and periodic ab initio calculations of a molecular aggregate with two choline chloride and four urea units. This is the minimum size required to achieve satisfactory agreement with experiment, as smaller clusters cannot represent all of reline’s significant intermolecular interactions. The INS spectrum of reline, compared with that of pure choline chloride, reveals a displacement of chloride anions away from their preferred positions on top of choline’s methyl groups, whose torsional movement becomes less hindered in the mixture. Urea, which adopts a planar (sp(2)) shape in the crystal, becomes non-planar (sp(3)) in reline, a feature herein discussed for the first time. In reline, urea molecules form a wide range of hydrogen bonds, from soft contacts to stronger associations, the latter being responsible for the deviation from ideality. The chloride’s interactions with choline are largely conserved at the hydroxyl end while becoming weaker at the cationic headgroup. The interplay of soft and strong interactions confers flexibility to the newly formed hydrogen-bond network and allows the ensemble to remain liquid at room temperature.

Araujo, C.F.; Coutinho, J.A.P.; Nolasco, M.M.; Parker, S.F.; Ribeiro-Claro, P.J.A.; Rudic, S.; Soares, B.I.G.; Vaz, P.D.

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Studies on polymorph conversion in a new cyclodextrin inclusion compound.

Abstract

A novel beta-cyclodextrin (beta CD) inclusion compound was prepared using 4-phenylpyridine-N-oxide (PPNO) as the organic guest. The inclusion compound, beta CD.PPNO, was characterised both in solution and in the solid state using numerous techniques. H-1 NMR in aqueous solution allowed the determination of a 1 : 1 stoichiometry and an association constant of 164 +/- 21 M-1. Powder and single-crystal X-ray diffraction studies showed the formation of two distinct crystal phases, appearing at different timings. Over time, one of the crystal phases converts spontaneously into the other. This work is the first to monitor the conversion of different polymorphs of cyclodextrin inclusion compounds in real time.

José A. Fernandes, Ana I. Ramos, Paulo Ribeiro-Claro, Filipe A. Almeida Paza and Susana S. Braga.

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