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

Elucidating the Infrared Spectral Properties of Succinic Molecular Acid Crystals: Illustration of the Structure and the Hydrogen Bond Energies of the Crystal and Its Deuterated Analogs

Herein, the infrared spectroscopic properties of molecular succinic acid crystals (SA) and their four isotopic analogs [C₂H₄(COOH)₂, h₆-SA; C₂H₄(COOD)₂, d₂-SA; C₂D₄(COOH)₂, d₄-SA; C₂D₄(COOD)₂, d₆-SA] are reported. The correlation between the structure of succinic acid molecules and their corresponding hydrogen bond energies is elucidated. The effects related to the isotopic dilution as well as the changes in the spectrum recording temperature on the fine structures of the v_O-H and v_O-D bands are interpreted. The infrared spectral anomalies detected in the spectra of isotopically neat succinic nanocrystal acids are confirmed by theoretical calculations using density functional theory (DFT). According to previous spectroscopic studies of succinic acid and those carried out for α,ω-dicarboxylic acids, a decent agreement between the experimental results and the theoretical DFT simulations is obtained. Moreover, the spectra of single crystals of the h₆ and d₄ succinic acid variants prove that the vibrational coupling mechanism between the (COOH)₂ cycles is rigorously convergent to that detected in the spectra of aromatic carboxylic acids, suggesting thereby that the promotion of symmetry-forbidden high stretching IR transitions plays a crucial role. Furthermore, the obtained experimental results reveal that the succinic acid shows a spectral behavior significantly different from that characteristic of hydrogen associations of other acids of homologous series, such as the glutaric, adipic, malonic, and pimelic acid crystals. The results obtained herein shed light on the way to explore the revealed structure of isotopic derivatives of succinic acid crystals and may prove to be useful results for understanding the nature of unconventional interactions as well as the macroscopic energy effects directing the development of hydrogen associations.

Infrared spectra of hydrogen bond network in lamellar perfluorocarboxylic acid monohydrates

The infrared spectra of the long-chain perfluorocarboxylic acid monohydrates differ markedly from those of the anhydrous dimers. Consequently, the structure of the solid perfluorocarboxylic acid monohydrates must differ from any known dimer-containing carboxylic acid crystals. Consideration of the significant features of the infrared spectra of the long-chain perfluorocarboxylic acid monohydrates, supplemented by their Raman spectra, and comparison with the spectra of auxiliary substances have led us to conclude that the rather strong neutral carboxyl-hydroxyl to water bonding can best explain the observations. The infrared spectra indicate the presence of fairly short hydrogen bonds connecting the water molecules to the carbonyl groups. In the construction of the hydrogen bonding pattern of the perfluorocarboxylic acid monohydrates, the oxalic acid dihydrate plays the key role. The striking similarity between the infrared spectra of the oxalic acid dihydrates and the perfluorocarboxylic acid monohydrates in the regions characteristic of water and OH⋯O vibration suggests that the structure of the hydrated carboxyl groups is the same in both crystals. These regions are characterized by the sharp doublet at 3539 cm^-1 and 3464 cm^-1, which is due to the H₂O ν₁ and ν₃ stretching vibrations, respectively, and the broad absorption between 3000 cm^-1 and 1500 cm^-1 with the intense band at 1970 cm^-1, both associated with the vibration of the OH⋯O group. The later peak consists of two band components at near 1980 cm^-1 and 2020 cm^-1. These band components show different behaviour when the temperature, polarization or deuteration is changed. In general, the infrared spectra of long-chain perfluorocarboxylic acids represent the system with very short hydrogen bonds connecting the water molecules to the carboxylates. This hydrogen bond pattern should be very similar to that found in the crystals of α-oxalic acid dihydrate.

Lack of the 'long-distance' dynamical co-operative interactions due to low symmetry of hydrogen-bonded malonic acid aggregates in molecular crystals

The paper explains the relationship between the energy of hydrogen bonds and the distance between associated carboxyl groups of malonic acid (MA) molecules by means of infrared spectroscopic studies of crystals of its four isotopic varieties [CH₂(COOH)₂, h₄-MA; CH₂(COOD)₂, d₂^c-MA; CD₂(COOH)₂, d₂^m-MA; CD₂(COOD)₂, d₄-MA]. The effects associated with impact on the isotopic dilution and changes in the temperature of spectrum registration on the fine structures of the ν_O-H and ν_O-D bands were analyzed. MA molecular crystals are characterized by a tendency to spontaneous H/D isotopic exchange both within centrosymmetric hydrogen bond cycles and methylene groups. The mono- and polycrystalline spectra obtained in the infrared range of isotopically neat and isotopically diluted by deuterons do not indicate the occurrence of anomalous temperature evolution in the course of lowering their registration temperature to 77 K. Theoretical calculations did not give clear confirmation of the nature of the phenomena analyzed.

A unified quantum model susceptible to elucidate the dissimilarity of IR spectral density of dicarboxylic acid crystals: Phthalic and terephthalic acid crystals cases

Over the last decades, several approaches have been developed for elucidating the infrared spectral density of dicarboxylic acid crystals, which has been served as prototype for determining hydrogen bonds dynamics. These approaches differ in how accurately the simulated spectra can superimpose the experimental ones. In this study, we present a superdimer quantum approach susceptible to elucidate the infrared spectral properties of some particular dicarboxylic acid crystals using a newly proposed algorithm, which favors the rule of Davydov coupling in the generation of the spectra. The approach, which is herein effectively applied to terephthalic and phthalic acid dimer crystals, ascribes the non-conventional IR spectral properties of these particular acid crystals to the existence of superdimer structure in their lattices. In this superdimer structure, a strong vibronic coupling mechanism, namely Davydov coupling, takes place between the proton stretching vibrations in the (COOH)₂ cycles. This strong coupling exciton, generated by the resonance arising in the two coupled (COOH)₂ cycles of the aromatic rings of the superdimer, in conjunction with the strong anharmonic coupling between the fast and slow modes of each hydrogen bonds provide a strong support basis for a common explanation of the physical properties of these two different crystalline systems. The numerical simulations, involving the implications of the superdimer model, are systematically correlated with the experimental spectra. A decent agreement between the evaluated spectra and the experimental bandshapes of terephthalic and phthalic dicarboxylic acid crystals was obtained using a set of physically sound parameters as inputs in the theoretical formulation. The superdimer quantum approach thereby underscore the potential of the dynamical cooperative interactions between "Davydov coupling" and "strong anharmonic coupling" mechanisms in the generation of the spectral features of terephthalic and phthalic dicarboxylic acid crystals, suggesting that the congregated effects of these two mechanisms can be considered as the most reliable source of the non-conventional IR spectral properties observed. It is therefore expected that this novel algorithm reduces the discrepancies between the simulated spectra compared to the experimental one and simplify the computation of spectra in more complex hydrogen bonded systems.

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 β-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 β-form, the dimer is formed by a pair of almost identical intermolecular hydrogen bonds and consequently the crystals of β-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.