Juan Ramón Avilés Moreno

Conformational flexibility in terpenes: vibrational circular dichroism (VCD), infrared and Raman study of S-(-)-perillaldehyde.

S-(-)-Perillaldehyde (4-isopropenylcyclohex-1-ene-1-carbaldehyde) is a secondary metabolite and an atmospheric pollutant obtained from the oxidation of other terpenes, as limonene and alpha-pinene, among others. For the first time, a study of the molecular structure and the infrared and Raman spectra of this chemical is presented. Theoretical calculations reveal the existence of two conformers depending on the position of the isopropenyl group (axial and equatorial) and twelve rotamers (six equatorials and six axials), being three of these equatorial rotamers the most stable (ca. 93%). A complete and reliable assignment of the IR and Raman spectra of the title compound is made, taking into account these three last rotamers, and the results are used to carry out a detailed interpretation, for the first time to our knowledge, of the VCD spectrum of this compound in the 1500-900 cm (-1) region.

Conformational relaxation of S-(+)-carvone and R-(+)-limonene studied by microwave Fourier transform spectroscopy and quantum chemical calculations

S-(+)-carvone (C10H14O, 5-isopropenyl-2-methylcyclohex-2-en-1-one) and R-(+)-limonene (C10H16, 4-isopropenyl-1-methylcyclohexene) have been characterized in the gas phase using a Fourier transform microwave spectrometer coupled to a supersonic molecular beam. Two conformers—with the isopropenyl group in the equatorial position—have been detected for each compound and described by a set of molecular parameters including the principal rotational constants and the quartic centrifugal distortion parameters. Quantum chemical calculations indicate that a third conformer might not be observed due to relaxation processes in the jet. The gas phase results are compared with the liquid phase IR-Raman-VCD spectra.

The chiral structure of 1H-indazoles in the solid state: a crystallographic, vibrational circular dichroism and computational study

A survey of the Cambridge Crystallographic Database reveals that there are 17 1H-indazoles forming dimers (7), trimers (4) and catemers or chains (6). Amongst the catemers there is one centrosymmetric compound and five non-centrosymmetric ones that crystallize in chiral helices, either M or P, including indazole itself, whose structure has been determined anew. An explanation will be provided for how an achiral molecule crystallizes in a chiral space group. The second part of the paper is devoted to the use of Vibrational Circular Dichroism (VCD) for determining the absolute configuration of a crystal of indazole. This previously needs the complete assignment of its IR and Raman spectra in solution and in the solid-state based on B3LYP/6-31G(d) calculations of indazole dimers and trimers, which are already chiral.

Study of the Photoinduced Supramolecular Chirality in Columnar Liquid Crystals by Infrared and VCD Spectroscopies

IR and VCD spectroscopies are employed to clarify the molecular origins of supramolecular chirality in azobenzene-containing columnar liquid crystals. The different columnar mesomorphic assemblies, Colr and Colh, of an achiral and a chiral propeller-like hydrogen-bonded complex, respectively, are used for this study. The mesomorphic behavior of the achiral complex is studied here for the first time, and the structural parameters of its Colr mesophase are determined by X-ray diffraction. Both complexes bear azobenzene units and this makes it possible to implement photoresponsive columnar architectures, the chirality of which can be induced and modulated by irradiation with 488 nm circularly polarized light (CPL). Thin films of the respective rectangular and hexagonal columnar phases of these complexes have been processed in order to study the IR absorption spectra and the vibrational circular dichroism responses upon irradiation with CPL. These studies allow confirming that the outer part of the columns, consisting on azobenzene groups, is mainly involved in the photoinduced supramolecular chirality, rather than the inner part of the columns where a melamine core is located. This supports a structural model based on the helical disposition of the azobenzene groups along the stacked propeller-like complexes.

Chiral terpenes in different phases: R -(−)-camphorquinone studied by IR–Raman–VCD spectroscopies and theoretical calculations

R-(−)-camphorquinone is a bicyclical terpenoid with many usages and application in different fields. Different experimental and theoretical works reveal that there is only one stable conformer of this chiral chemical species in agreement to the sterical restriction that the bicycle introduces. In the current work, from a complete assignment of the vibrational IR and Raman spectra, we are able to explain the VCD spectrum of the title compound. The recorded spectra of R-(−)-camphorquinone in different phases have been analyzed and compared: the first one in CCl4 solution, the second one using Nujol and Fluorolube suspensions and the third one using thin films. Finally, to study the coupling between the two C=O stretching normal modes, a NBO analysis is performed. The present work reveals that IR, Raman and VCD, combined with quantum chemical calculations, are helpful complementary techniques to characterize chiral systems, as terpenes, in different phases.

Rotational strength sign and normal modes description: A theoretical and experimental comparative study in bicyclic terpenes.

A comparative study of the Vibrational circular dichroism (VCD) patterns in three bicyclic terpenes with related molecular structures, (+)-(R)-α-pinene, (-)-(S)-β-pinene, and (-)-(S)-verbenone, is carried out. The study is done in the light of normal mode descriptions from a previous vibrational analysis. Although normal modes corresponding to identical molecular moieties shown the same description, rotational strength does not follow the Cahn-Ingold-Prelog descriptor-related sign in some cases, i.e., they do not present opposite signs depending on stereochemical description of the molecule.

Hydrogen bonding network in a chiral alcohol: (1R,2S,5R)-(−)-menthol. Conformational preference studied by IR–Raman–VCD spectroscopies and quantum chemical calculations

A study of the molecular structure of (1R,2S,5R)-(−)-menthol and the hydrogen bond networks formed by this species in solution is carried out. Molecular structures of monomers and H-bonded dimers and trimers of the title compound are optimized using quantum chemical calculations in the isolated molecule approach. In addition, IR, Raman and VCD techniques are used to study CCl4 solutions and thin films of the target compound. Their corresponding vibrational spectra are then analysed, both theoretically (HF and DFT) and experimentally, to characterize the different monomers (rotamers) and H-bonded oligomer species in menthol solutions as a function of the concentration.