Susy Lopes

Bio: Susy Lopes is an academic researcher from University of Coimbra. The author has contributed to research in topics: Conformational isomerism & Infrared spectroscopy. The author has an hindex of 7, co-authored 22 publications receiving 207 citations. Previous affiliations of Susy Lopes include University of Buenos Aires.

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

Abstract: Formic acid (HCOOH, FA) and acetic acid (CH3COOH, 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 f...

Photochemistry and vibrational spectra of matrix-isolated methyl 4-chloro-5-phenylisoxazole-3-carboxylate.

Abstract: Methyl 4-chloro-5-phenylisoxazole-3-carboxylate (MCPIC) has been synthesized, isolated in low temperature argon and xenon matrices, and studied by FTIR spectroscopy. The characterization of the low energy conformers of MCPIC was made by undertaking a systematic investigation of the DFT(B3LYP)/6-311++G(d,p) potential energy surface of the molecule. The theoretical calculations predicted the existence of three low energy conformers. Two of them (I and II) were observed experimentally in the cryogenic matrices. The third one (III) was found to be converted into conformer II during deposition of the matrices, a result that is in agreement with the predicted low III → II energy barrier ( 235 nm) of matrix-isolated MCPIC yielded as final photoproduct the corresponding oxazole (methyl 4-chloro-5-phenyl-1,3-oxazole-2-carboxylate). Identification of the azirine and nitrile-ylide intermediates in the spectra of the irradiated matrices confirmed their mechanistic relevance in the isoxazole → oxazole photoisomerization.

Molecular structure and infrared spectra of dimethyl malonate: A combined quantum chemical and matrix-isolation spectroscopic study

Abstract: The infrared spectra of dimethyl malonate isolated in low-temperature argon and xenon matrices were studied. Theoretical calculations, carried out at the MP4/6-31G**, MP2/6-31++G** and DFT(B3LYP)/6-311++G** levels, predict two different conformers of nearly equal internal energy, both exhibiting the methyl ester moieties in the cis (C–O) configuration. One of the conformers has C2 symmetry with the two ester groups crossed symmetrically with respect to the C–C–C plane. This structure is doubly degenerated by symmetry. The other form (gauche) belongs to the C1 point group. Four identical-by-symmetry minima on the PES correspond to this structure. The energy of this form is predicted (at the MP4 level) to be slightly lower than that of the C2 conformer. The six minima on the PES can be divided into two groups of three (one C2 isomer and two C1 forms in a group). Each structure from one group is related to its counterpart from the other group by the operation of reflection in the C–C–C plane. In each group, the conformers are separated by low energy barriers (less than 2 kJ mol−1), while conformational interconversions between the two groups imply a transition state structure with a vis-a-vis orientation of oxygen atoms and thus are associated with considerably higher energy barriers. The infrared spectra of the matrix isolated compound were found to closely match the spectrum predicted for the C1 conformer. Annealing of the matrices up to 55 K does not lead to significant changes in the spectra, suggesting that the low energy barriers separating the two conformers allow practically all molecules of dimethyl malonate to transform to the more stable gauche conformer, when they are cooled down after landing on the matrix surface. Spectra of the low temperature solid form of the compound (8 K < T < 200 K) also reveal only the presence of the C1 conformer.

Conformational flexibility, UV-induced decarbonylation, and FTIR spectra of 1-phenyl-1,2 propanedione in solid xenon and in the low temperature amorphous phase.

Abstract: 1-Phenyl-1,2-propanedione has been isolated in low-temperature xenon matrixes and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. In good agreement with previous electron diffraction data [Shen, Q.; Hagen, K. J. Phys. Chem. 1993, 97, 985], the calculations predicted the existence of only one stable conformation for the compound, in which the OCCO dihedral angle is 135.6°. On the other hand, the experimental data clearly reveals that, in the as-deposited xenon matrixes (T = 20 K), there is a distribution of molecules with different OCCO dihedral angles around the equilibrium value. This distribution results from the efficient trapping of the conformational distribution existing in the gas phase, prior to deposition, which is determined by the low frequency, large amplitude torsional vibration around the C−C central bond. Upon annealing to higher temperatures (T ∼ 45 K), the initially trapped conformational distribution can be modified in a certain degree, favoring more pol...

Matrix-Isolation FTIR Spectroscopy of Benzil: Probing the Flexibility of the C−C Torsional Coordinate

Abstract: The infrared spectrum and conformational flexibility of benzil, (C6H5CO)2, are studied by matrix-isolation FTIR spectroscopy, supported by DFT calculations. It is shown that the low-frequency (ca. 25 cm-1), large-amplitude torsion around the C−C central bond strongly affects the structural and spectroscopic properties exhibited by the compound. The equilibrium conformational distribution of molecules with different OC−CO dihedral angles, existing at room temperature in the gas phase, and trapped in a low-temperature (T = 9 K) inert matrix can be changed either by in situ irradiation with UV light (λ > 235 nm) or by annealing the matrix to higher temperatures (T ≈ 34 K). In the first case, the increase of the average OC−CO angle results from conformational relaxation in the excited electronic states (S1 and T1), whose lowest-energy conformations correspond, for both S1 and T1 states, to a nearly planar configuration with the OC−CO dihedral angle equal to 180°. In the second case, the decrease of the averag...

Recent Developments in the Synthesis and Reactivity of Isoxazoles: Metal Catalysis and Beyond

Abstract: Isoxazoles are important five-membered aromatic heterocycles in organic chemistry. Recently, many exciting advances in the synthesis and functionalization of isoxazoles have been reported. New transition metal-catalyzed reactions have resulted in the development of attractive and highly efficient synthetic approaches to densely functionalized isoxazoles. Complete control of regioselectivity can be achieved on the basis of a judicious choice of metal catalyst and reaction partners using dipolar cycloaddition and cycloisomerization reactions, while more recent studies have focused on the site-selective functionalization of isoxazoles via CH functionalization. New strategies for the use of isoxazoles as scaffolding templates in asymmetric synthesis have emerged, thus opening new prospects for the synthesis of enantioenriched motifs under the conditions that are orthogonal to other transformations. In this review, recent advances involving the synthesis and reactivity of isoxazoles are summarized. The review covers the period from January 2005 to June 2015.

Single chain folding of synthetic polymers by covalent and non-covalent interactions: Current status and future perspectives

Abstract: The present feature article highlights the preparation of polymeric nanoparticles and initial attempts towards mimicking the structure of natural biomacromolecules by single chain folding of well-defined linear polymers through covalent and non-covalent interactions. Initially, the discussion focuses on the synthesis and characterization of single chain self-folded structures by non-covalent interactions. The second part of the article summarizes the folding of single chain polymers by means of covalent interactions into nanoparticle systems. The current state of the art in the field of single chain folding indicates that covalent-bond-driven nanoparticle preparation is well advanced, while the first encouraging steps towards building reversible single chain folding systems by the use of mutually orthogonal hydrogen-bonding motifs have been made. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Atom Tunneling in Chemistry.

Abstract: Quantum mechanical tunneling of atoms is increasingly found to play an important role in many chemical transformations. Experimentally, atom tunneling can be indirectly detected by temperature-independent rate constants at low temperature or by enhanced kinetic isotope effects. In contrast, the influence of tunneling on the reaction rates can be monitored directly through computational investigations. The tunnel effect, for example, changes reaction paths and branching ratios, enables chemical reactions in an astrochemical environment that would be impossible by thermal transition, and influences biochemical processes.