How far the vibrational exciton interactions are responsible for the generation of the infrared spectra of oxindole crystals?

Oxindole (indolin-2-one, Ox) is a unique and a crucial molecular system in spectroscopic studies. Indole is the core structure of many substances found in the human body (tryptophan, serotonin) and the indole alkaloids have highly differentiated pharmacological properties such as analgesic, anti-fever and anti-inflammatory. The Ox's structural results given in the Cambridge Structural Database revealed the existence of only one crystalline form of Ox, referred to the α-form. However, we have experimentally noticed the existence of two polymorphic forms during the crystallization of Ox. Furthermore, the significant spectral differences that we have observed in the solid state infrared spectra of these two forms additionally confirm the existence of the polymorphism phenomenon. Of the four polymorphic forms of Ox, two of them - α - and ß-forms - were of particular interest. In the crystalline lattices of both polymorphs, we observed a similar pattern of molecular arrangements giving rise to the supramolecular synthon according to the terminology of Etter. Moreover, hydrogen bonds in the dimer of the α-form are found to be non-equivalent (non-centrosymmetric dimers), having a length of 2797 Å and 2979 Å, respectively. Comparatively, in the most densely packed crystalline structure of Ox, the ß-form, the dimer is formed by a pair of almost identical intermolecular hydrogen bonds and consequently the crystals of ß-form exhibited spectral properties typical to centrosymmetric hydrogen bond dimers. In addition, the spectroscopic studies that we have conducted to polymorphic forms of Ox, isotopically diluted with deuterium, show the dramatic influence of isotopic substitution in the hydrogen bridge on the infrared spectra of hydrogen bonding. Thus, the main goal of this work is the proposition of a theoretical approach that can describe the main features of the crystalline infrared spectra of the Ox polymorphs. The proposed approach is based on the phenomenon of the exciton coupling results directly from intermolecular interactions in the vibrationally excited state which leads to the delocalization of the excitation over the molecules in the lattice and to the Davydov splitting effect in the crystalline spectra.

Elucidating the theoretical X-ray photoabsorption spectrum of Argon near the KM edge: Transition probabilities calculations.

We report new calculations of the transition probabilities for double electron photo-excitations near the KM edge in atomic Argon in the energy range 3220-3238 eV. The calculations were performed to study the X-ray photoabsorption spectrum of Argon near the KM edge using Multi-configurational Dirac-Hartee Fock method. Configuration interaction calculations showed that excitations to the doubly excited state [1s3p]3d2 is almost small compared to [1s3p]4p2 excitations, thus [1s3p]4p2 is the dominant transition in this energy range. The transition probabilities were convoluted into Breit-Wigner line shapes and compared to previous measurements of the X-ray photoabsorption spectrum of Argon near the KM edge, the comparison showed good agreement with experiment. The results of the calculations presentded herein therefore shed light on understanding of near KM edge features, and thereby the preferred excitation channels of the atom.

Towards accurate infrared spectral density of weak H-bonds in absence of relaxation mechanisms.

Following the previous theoretical developments to completely reproduce the IR spectra of weak hydrogen bond complexes within the framework of the linear response theory (LRT), the quantum theory of the high stretching mode spectral density (SD) of weak H-bonds is reconsidered. Within the LRT theory, the SD is the one sided Fourier transform of the autocorrelation function (ACF) of the high stretching mode dipole moment operator. In order to provide more accurate theoretical bandshapes, we have explored the equivalence between the SDs given in previous studies with respect to a new quantum one, and revealed that in place of the basic equations used in the precedent works for which the SD IOld(ω)=2Re∫0∞GOld(t)e-iωtdt where the ACF GOld(t) = ⟨µ(0)µ(t)+⟩ = tr {ρ {µ(0)} {µ(t)}+}, one can use a new expression for the SD, given by INew(ω)=2ωRe∫0∞GNew(t)e-iωtdt where GNew(t)=µ(0)µ(t)+=1ßtrρB∫0ßµ(0)µ(t+iλℏ)+dλ. Here ρB is the Boltzmann density operator, µ(0) the dipole moment operator at initial time and µ(t) the dipole moment operator at time t in the Heisenberg picture, ℏ is the Planck constant, ß is the inverse of the Boltzmann factor kBT where T is the absolute temperature and kB the Boltzmann constant. Using this formalism, we demonstrated that the new quantum approach gives the same final SD as used by previous models, and reduces to the Franck-Condon progression appearing in the Maréchal and Witkowski's pioneering approach when the relaxation mechanisms are ignored. Results of this approach shed light on the equivalence between the quantum and classical IR SD approaches for weak H-bonds in absence of medium surroundings effect, which has been a subject of debate for decades.