Browsing by Author "Fausto, Rui"

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5-Methylhydantoin: from isolated molecules in a low-temperature argon matrix to solid state polymorphs characterization
(Amer Chemical Soc, 1155 16th St, Nw, Washington, Dc 20036 USA, 2017-07-20) Nogueira, Bernardo A.; Canotilho, J.; Eusebio, M. E. S.; Henriques, M. S. C.; Paixao, J. A.; Fausto, Rui; ILDIZ, GÜLCE ÖĞRÜÇ; 107326
The molecular structure, vibrational spectra and photochemistry of 5-methylhydantoin (C4H6N2O2; 5-MH) were studied by matrix isolation infrared spectroscopy and theoretical calculations at the DFT(B3LYP)/6-311++G(d,p) theory level. The natural bond orbital (NBO) analysis approach was used to study in detail the electronic structure of the minimum energy structure of 5-MH, namely the specific characteristics of the sigma and pi electronic systems of the molecule and the stabilizing orbital interactions. UV irradiation of 5-MH isolated in argon matrix resulted in its photofragmentation through a single photochemical pathway, yielding isocyanic acid, ethanimine, and carbon monoxide, thus following a pattern already observed before for the parent hydantoin and 1-methylhydantoin molecules. The investigation of the thermal properties of 5-MH was undertaken by differential scanning calorimetry (DSC), polarized light thermal microscopy (PLTM) and Raman spectroscopy. Four different polymorphs of 5-MH were identified. The crystal structure of one of the polymorphs, for which it was possible to grow up suitable crystals, was determined by X-ray diffraction (XRD). Two of the additional polymorphs were characterized by powder XRD, which confirmed the molecules pack in different crystallographic arrangements.
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Auxiliary Differential Diagnosis of Schizophrenia and Phases of Bipolar Disorder Based on the Blood Serum Raman Spectra
(Wiley, 2020) ILDIZ, GÜLCE ÖĞRÜÇ; Bayari, Sevgi; Aksoy, Umut M.; Yorguner, Neşe; Bulut, Hüseyin; Yılmaz, Sultan S.; Halimoğlu, Gökhan; Kabuk, Hayrunnisa Nur; YAVUZ, GİZEM; Fausto, Rui
Schizophrenia (SZ) and bipolar disorder (BP) are severe psychiatric disorders that are characterized by an extensive spectrum of symptoms and affect approximately 2% of the world population. BP exhibits three well-distinct phases, which are classified as manic and depressive episodes and euthymic phase. These disorders are of difficult differential clinical diagnosis due to the similarity of their symptoms. Diagnostic approaches for SZ and BP are based on constructed patient interviews and subjective evaluations of clinical symptoms, and there are still no molecular-based auxiliary diagnostic tools to support the clinical diagnosis. In this study, an analytical model for auxiliary differential diagnosis of SZ and BP, based on the analysis of patients' blood serum Raman spectra, is developed, which is able to account for the different BP phases and can also differentiate SZ and BP patients from healthy individuals. The model is based on a hierarchical sequence of four two-class PLS-DA steps where the Raman spectra are theX-predictor variables. It is concluded that the full 400-3,100 cm(-1)Raman spectroscopic range is a sensitive probe for the disorders, thus working as a general spectroscopic biomarker for the illnesses. The proposed methodology is reliable, fast, cheap, essentially minimal-invasive, and might be implemented easily in the clinical environment.
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Blood Serum-Infrared Spectra-Based Chemometric Models for Auxiliary Diagnosis of Autism Spectrum Disorder
(Elsevier, 2021) ILDIZ, GÜLCE ÖĞRÜÇ; Bayarı, Sevgi; Yorguner, Neşe; Fausto, Rui
This chapter focuses on the use of chemometric models developed from infrared (IR) spectroscopic data of blood serum as an auxiliary tool for autism spectrum disorder (ASD) diagnosis. In the introductory sections the fundamentals of IR spectroscopy are shortly reviewed and brief descriptions of the different chemometric methods used in the case study described in the last section of the chapter are presented. The last section deals with the application of the described strategy to the diagnosis of ASD in children and adolescents. © 2021 Elsevier Inc. All rights reserved.
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Conformational Analysis and Photochemistry of 2-Chloro-6-fluorobenzoic Acid Monomers in Solid Xenon
(2019-11) Fausto, Rui; ILDIZ, GÜLCE ÖĞRÜÇ; 107326
In this study, the conformational space of 2-chloro-6-fluorobenzoic acid (CFBA) has been investigated theoretically using density functional theory, with the B3LYP functional and the 6-311++G(d,p) basis set, and experimentally by matrix isolation infrared spectroscopy. The calculations allowed identification of 3 different non-planar conformers of the compound, the lowest energy form (I) exhibiting the carboxylic acid group in the cis arrangement (O=C- O-H dihedral equal to ~0°), and the two higher energy forms (II and III) bearing a trans carboxylic group (O=C-O-H dihedral equal to ~180°) (see Figure). The energies of conformers II and III are larger than that of the most stable conformer I by 17.07 and 17.31 kJ mol-1 , respectively. The considerably large energy difference between the two trans conformers and the cis form implies that only this last species should exist significantly populated in the room temperature gas phase conformational equilibrium. In consonance with the theoretical predictions, only conformer I could be trapped from the room temperature vapor of the compound into low temperature (10 K) solid xenon. The theoretical vibrational characterization of all 3 conformers of CFBA and the assignment of the experimental IR spectrum of conformer I were undertaken.The matrix-isolated monomers of the compound were then irradiated z/7 situ using narrowband UV light with L = 235 nm. The major observed reaction channel was decarboxylation, leading to production of CO2 and l-chloro-3-fluorobenzene. Carbon monoxide was also detected in the infrared spectra of the photolysed solid Xc matrices of the compound, indicated that UV- induced decarbonylation of CFBA takes also place in some extent, though the expected accompanying photoproduct (2-chloro-6-fluorophenol) could not be experimentally sensed.
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Conformational Analysis, Spectroscopy and Photochemistry of Matrix-Isolated 1H- and 2H- Tautomeric Forms of Ethyl Tetrazole-5-Carboxylate
(Elsevier Science, 2021) ILDIZ, GÜLCE ÖĞRÜÇ; Fausto, Rui
The preferred conformations, infrared spectra and photochemistry of matrix-isolated 1H and 2H tautomers of ethyl tetrazole-5-carboxylate (ET5C) were investigated. Experimentally, isolated monomeric forms of the two tautomers were trapped in cryogenic argon matrices (15 K), and studied by infrared spectroscopy. Identification of the conformational species trapped in the matrices was undertaken by combining the experimental spectro-scopic data with results of quantum chemical calculations performed at the DFT(B3LYP)/6-311++G(d,p) level of theory. For each tautomer, two conformers were observed in the matrices, all forms exhibiting the ester group in the anti configuration (C-O-C-C dihedral equal to 180 degrees). In turn, the conformers having a gauche ester group that are predicted by the calculations to be significantly populated in the gas phase equilibrium (room temperature), were found to convert into the anti forms during matrix deposition (conformational cooling). Very interestingly, and contrarily to what is in general found for tetrazoles, the 1H tautomer is the most stable tautomer of ET5C, and strongly predominates in the studied matrices. Narrowband UV irradiation (lambda = 250 nm) was found to lead to tetrazole fragmentation, with release of molecular nitrogen and production of four isomeric photoproducts (diazirine, cyanamide, carbodiimide and nitrile imine), all of them being successfully identified through comparison of the experimental spectroscopic spectra of the irradiated matrices with the DFT(B3LYP)/6-311++G(d, p) predicted infrared spectra of these species. This study also stresses the structural relevance of the N-H//O = C intramolecular interaction between the ring hydrogen atom and the carbonyl group of the substituent in the 1H tautomer of ET5C.
Open Access
Fourier Transform Infrared Spectroscopy Based Complementary Diagnosis Tool for Autism Spectrum Disorder in Children and Adolescents
(MDPI, 2020) ILDIZ, GÜLCE ÖĞRÜÇ; Bayari, Sevgi; Karadağ, Ahmet; Kaygısız, Ersin; Fausto, Rui
Autism spectrum disorder (ASD) is a neurodevelopmental disorder that begins early in life and continues lifelong with strong personal and societal implications. It affects about 1%-2% of the children population in the world. The absence of auxiliary methods that can complement the clinical evaluation of ASD increases the probability of false identification of the disorder, especially in the case of very young children. In this study, analytical models for auxiliary diagnosis of ASD in children and adolescents, based on the analysis of patients' blood serum ATR-FTIR (Attenuated Total Reflectance-Fourier Transform Infrared) spectra, were developed. The models use chemometrics (either Principal Component Analysis (PCA) or Partial Least Squares Discriminant Analysis (PLS-DA)) methods, with the infrared spectra being the X-predictor variables. The two developed models exhibit excellent classification performance for samples of ASD individuals vs. healthy controls. Interestingly, the simplest, unsupervised PCA-based model results to have a global performance identical to the more demanding, supervised (PLS-DA)-based model. The developed PCA-based model thus appears as the more economical alternative one for use in the clinical environment. Hierarchical clustering analysis performed on the full set of samples was also successful in discriminating the two groups.
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FTIR investigation of the O H Xe interaction in simple carboxylic acids in solid xenon
(2012-01) Nunes, Claudio; Kuş, Nihal; Fausto, Rui; ILDIZ, GÜLCE ÖĞRÜÇ; 107326; 56674
The O–H stretching region of the infrared spectra of a series of carboxylic acids in Xe matrices was investigated as a function of temperature. Upon increasing the temperature, the νO–H band site-components undergo reversible frequency blue-shifts, which are larger for the lowest-frequency components. This unprecedented observation indicates both that different types of O–H···Xe specific interactions occur, depending on different trapping sites, and the prevalence of stronger interactions of this type for molecules trapped in sites corresponding to lower frequency νO–H band site-components. These results are in agreement with previous investigations pointing to an increased stabilization and larger νO–H frequency red-shifts in carboxylic acid/Xe complexes bearing a specific H-bond like O–H···Xe interaction. O–H···Xe interaction energies were obtained theoretically and also estimated from the spectroscopic data. Changes in the interaction energies upon temperature variation were also evaluated.
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FTIR Investigation of the O H Xe Interaction In Simple Carboxylic Acids In Solid Xenon
(2012) Nunes, Cláudio M.; Kuş, Nihal; Fausto, Rui; ILDIZ, GÜLCE ÖĞRÜÇ; 107326; 56674
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FTIR Investigation Of The O-H Center Dot Center Dot Center Dot Xe Interaction In Simple Carboxylic Acids In Solid Xenon
(Amer Inst Physics, Circulation & Fulfillment Div, 2 Huntington Quadrangle, Ste 1 N O 1, Melville, Ny 11747-4501 USA, 2012-08-14) Nunes, Claudio M.; Kuş, Nihal; Fausto, Rui; ILDIZ, GÜLCE ÖĞRÜÇ; TR107326; TR56674
The O-H stretching region of the infrared spectra of a series of carboxylic acids in Xe matrices was investigated as a function of temperature. Upon increasing the temperature, the vO-H band site-components undergo reversible frequency blue-shifts, which are larger for the lowest-frequency components. This unprecedented observation indicates both that different types of O-H center dot center dot center dot Xe specific interactions occur, depending on different trapping sites, and the prevalence of stronger interactions of this type for molecules trapped in sites corresponding to lower frequency vO-H band site-components. These results are in agreement with previous investigations pointing to an increased stabilization and larger vO-H frequency red-shifts in carboxylic acid/Xe complexes bearing a specific H-bond like O-H center dot center dot center dot Xe interaction. O-H center dot center dot center dot Xe interaction energies were obtained theoretically and also estimated from the spectroscopic data. Changes in the interaction energies upon temperature variation were also evaluated. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4740243]
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Hydantoins and mercaptoimidazoles: Vibrational spectroscopy as a probe of structure and reactivity in different environments, from the isolated molecule to polymorphs
(2019) Fausto, Rui; Brás, E.M.; Nogueira, Bernardo A.; ILDIZ, GÜLCE ÖĞRÜÇ
In this chapter, we highlight the power of vibrational spectroscopy as central technique to investigate the structure and reactivity of two relevant families of nitrogen-containing heterocyclic molecules: hydantoins and mercaptoimidazoles. Infrared spectroscopy is used in connection with the matrix isolation technique to investigate the structures of the isolated molecules and their photochemistry, while both infrared and Raman spectroscopies, supplemented by thermodynamics, microscopy, and diffraction techniques, are used to investigate neat condensed phases of the compounds and transitions between these phases. The experimental studies are supported by extensive computational studies, which include several approaches for detailed analysis of the electron density.
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Investigation of Menopause-Induced Changes on Hair by Raman Spectroscopy and Chemometrics
(Pergamon-Elsevier Science Ltd., 2022) Brito, Anna Luiza B.; Brueggen, Carlotta; ILDIZ, GÜLCE ÖĞRÜÇ; Fausto, Rui
The ending of estrogen production in the ovaries after menopause results in a series of important physiologic changes, including hair texture and growth. In this study we demonstrate that Raman spectroscopy can be used successfully as a tool to probe menopause-induced changes on hair, in particular when coupled with suitable chemometrics approaches. The detailed analysis of the average Raman spectra (in particular of the Amide I and vS-S stretching spectral regions) of the hair samples of women pre- and post-menopause allowed to estimate that absence of estrogen in post-menopause women leads to an average reduction of similar to 12% in the thickness of the hair cuticle, compared to that of pre-menopause women, and revealed the strong prevalence of disulphide bonds in the most stable gauche-gauche-gauche conformation in the hair cuticle. From the analysis of the vS-S stretching spectral region it could also be concluded that the amount of alpha-helix keratin is slightly higher for post-menopause than for pre-menopause women. A series of statistical models were developed in order to classify the hair samples. Outperforming the traditional PCA-LDA (principal component analysis - linear discriminant analysis) approach, in the present study a GA-LDA (genetic algorithm - linear discriminant analysis) strategy was used for variable reduction/selection and samples' classification. This strategy allowed to develop of a statistical model (L16), which has exceptional prediction capability (total accuracy of 96.6%, with excellent sensitivity and selectivity) and can be used as an efficient instrument for the hair samples' classification. In addition, a new chemometrics approach is here presented, which allows to overcome the intrinsic limitations of the GA algorithm and that can be used to develop statistical models that use GA as the variable reduction/selection method, but superseding its stochastic nature. Three suitable models for classification of the hair samples according to the menopause status of the women were developed using this novel approach (LV17, BLV20 and PLS7 models), which are based on the Fisher's and Bayers' LDA approaches and the PLS-DA method. The followed new chemometrics approach uses the results of a large set of GA-LDA runs over the full data matrix for the selection of the reduced data matrices. The criterion for the selection of the variables is their statistical significance in terms of number of occurrences as solutions of the whole set of GA-LDA runs. (C) 2022 Elsevier B.V. All rights reserved.
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Matrix isolation infrared spectra and photochemistry of hydantoin
(2013) Nunes, Claudio M.; Fausto, Rui; ILDIZ, GÜLCE ÖĞRÜÇ; 107326
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Matrix Isolation Infrared Spectra and Photochemistry of Hydantoin
(Amer Inst Physics, 2 Huntington Quadrangle, Ste 1No1, Melville, Ny 11747-4501 USA, 2013-01-31) Nunes, Claudio M.; Fausto, Rui; ILDIZ, GÜLCE ÖĞRÜÇ; 107326
Hydantoin (C3H4N2O2, 2,4-imidazolidinedione) was isolated in argon matrix at 10 K and its infrared spectrum and unimolecular photochemistry were investigated. The molecular structure of the compound was studied both at the DFT(B3LYP) and MP2 levels of approximation with valence triple- and quadruple-zeta basis sets (6-311++G(d,p); cc-pVQZ). It was concluded that the minima in the potential energy surfaces of the molecule correspond to C-1 symmetry structures. However, the energy barrier separating the two-equivalent-by-symmetry minima stays below their zero-point energy, which makes the C-s symmetry structure, which separates the two minima, the experimentally relevant one. The electronic structure of the molecule was studied in detail by performing the Natural Bond Orbital analysis of its electronic configuration within the DFT(B3LYP)/cc-pVQZ space. The infrared spectrum of the matrix isolated compound was fully assigned also with help of the theoretically predicted spectrum. Upon irradiation at lambda = 230 nm, matrix-isolated hydantoin was found to photofragment into isocyanic acid, CO, and methylenimine.
Open Access
Micro-Raman Spectroscopy and X-ray Diffraction Analyses of the Core and Shell Compartments of an Iron-Rich Fulgurite
(MDPI, 2022) KARADAĞ, AHMET; Kaygısız, Ersin; Nikitin, Timur; Öngen, Sinan; ILDIZ, GÜLCE ÖĞRÜÇ; Aysal, Namık; Yılmaz, Ayberk; Fausto, Rui
Fulgurites are naturally occurring structures that are formed when lightning discharges reach the ground. In this investigation, the mineralogical compositions of core and shell compartments of a rare, iron-rich fulgurite from the Mongolian Gobi Desert were investigated by X-ray diffraction and micro-Raman spectroscopy. The interpretation of the Raman data was helped by chemometric analysis, using both multivariate curve resolution (MCR) and principal component analysis (PCA), which allowed for the fast identification of the minerals present in each region of the fulgurite. In the core of the fulgurite, quartz, microcline, albite, hematite, and barite were first identified based on the Raman spectroscopy and chemometrics analyses. In contrast, in the shell compartment of the fulgurite, the detected minerals were quartz, a mixture of the K-feldspars orthoclase and microcline, albite, hematite, and goethite. The Raman spectroscopy results were confirmed by X-ray diffraction analysis of powdered samples of the two fulgurite regions, and are consistent with infrared spectroscopy data, being also in agreement with the petrographic analysis of the fulgurite, including scanning electron microscopy with backscattering electrons (SEM-BSE) and scanning electron microscopy with energy dispersive X-ray (SEM-EDX) data. The observed differences in the mineralogical composition of the core and shell regions of the studied fulgurite can be explained by taking into account the effects of both the diffusion of the melted material to the periphery of the fulgurite following the lightning and the faster cooling at the external shell region, together with the differential properties of the various minerals. The heavier materials diffused slower, leading to the concentration in the core of the fulgurite of the iron and barium containing minerals, hematite, and barite. They first underwent subsequent partial transformation into goethite due to meteoric water within the shell of the fulgurite. The faster cooling of the shell region kinetically trapped orthoclase, while the slower cooling in the core area allowed for the extensive formation of microcline, a lower temperature polymorph of orthoclase, thus justifying the prevalence of microcline in the core and a mixture of the two polymorphs in the shell. The total amount of the K-feldspars decreases only slightly in the shell, while quartz and albite appeared in somewhat larger amounts in this compartment of the fulgurite. On the other hand, at the surface of the fulgurite, barite could not be stabilized due to sulfate lost (in the form of SO2 plus O-2 gaseous products). The conjugation of the performed Raman spectroscopy experiments with the chemometrics analysis (PCA and, in particular, MCR analyses) was shown to allow for the fast identification of the minerals present in the two compartments (shell and core) of the sample. This way, the XRD experiments could be done while knowing in advance the minerals that were present in the samples, strongly facilitating the data analysis, which for compositionally complex samples, such as that studied in the present investigation, would have been very much challenging, if possible.
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Molecular Spectroscopy and Molecular Structure - A Collection of Contributions Presented at the 34th European Congress on Molecular Spectroscopy (EUCMOS 2018)
(Elsevier Science BV, PO Box 211, 1000 AE Amsterdam, Netherlands, 2019-11-05) Fausto, Rui; Schmitt, Michael; ILDIZ, GÜLCE ÖĞRÜÇ
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Molecular spectroscopy and molecular structure - Selected communications presented at the 1st International Turkish Congress on Molecular Spectroscopy (TURCMOS 2013) Preface
(Pergamon-Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington, Oxford Ox5 1Gb, England, 2016-01-05) Durig, James R.; Fausto, Rui; Ünsalan, Ozan; Bayarı, Sevgi; Kuş, Nihal; ILDIZ, GÜLCE ÖĞRÜÇ; 106111; 4103; 56674; 107326
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Molecular spectroscopy and molecular structure – Selected communications presented at the 3rd international Turkish congress on molecular spectroscopy (TURCMOS 2017)
(2018-12) Fausto, Rui; Ünsalan, Ozan; ILDIZ, GÜLCE ÖĞRÜÇ; 107326; 106111
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Molecular Structure, Infrared Spectra, Photochemistry, and Thermal Properties of 1-Methylhydantoin
(Amer Chemical Soc, 1155 16Th St, Nw, Washington, Dc 20036 Usa, 2014-08-07) Nogueira, Bernardo A.; Canotilho, Joao; Eusebio, M. Ermelinda S.; Fausto, Rui; ILDIZ, GÜLCE ÖĞRÜÇ; 107326
The structural, vibrational, and photochemical study of 1-methylhydantoin (1-MM, C4H6N2O2) was undertaken by matrix isolation infrared spectroscopy (in argon matrix; 10 K), complemented by quantum chemical calculations performed at the DFT(B3LYP)/6-311++G(d,p) level of approximation. The theoretical calculations yielded the C-s symmetry structure, with planar heavy atom skeleton, as the minimum energy structure on the potential energy surface of the molecule. The electronic structure of this minimum energy structure of 1-MH was then studied in detail by means of the natural bond orbital (NBO) and atoms in molecules (AIM) approaches, allowing for the elucidation of specific characteristics of the molecule's sigma and pi electronic systems. The infrared spectrum of the matrix-isolated 1-MH was fully assigned, also with the help of the theoretically predicted spectrum of the compound, and its UV-induced unimolecular photochemistry (lambda >= 230 nm) was investigated. The compound was found to fragment to CO, isocyanic acid, methylenimine, and N-methyl-methylenimine. Finally, a thermal behavior investigation on 1-MM samples was carried out using infrared spectroscopy (10 K until melting), differential scanning calorimetry and polarized light thermal microscopy. A new polymorph of 1-MH was identified. The IR spectra of the different observed phases were recorded and interpreted.
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Molecular Structure, Spectroscopy and Photochemistry of Alprazolam
(Elsevier, 2022) ILDIZ, GÜLCE ÖĞRÜÇ; Tabanez, Andreia M.; Nunes, Antonio; Roque, Jose P. L.; Justino, Licinia L. G.; Ramos, M. Luisa; Fausto, Rui
In this article, a comprehensive study of the molecular structure, spectroscopy, and photochemistry of alprazolam (Xanax (R); 8-chloro-1-methyl-6-phenyl-4H-[1,2,4]triazolo[4,3-a][1,4]benzodiazepine) is reported. The structure of the isolated molecule of the compound was investigated using density functional theory (DFT), revealing that the molecule exists in a single conformer, which is associated with 12 equivalent-by symmetry minima. The molecules of the compound were trapped from the gas phase into a low temperature (10 K) argon matrix, and the infrared (IR) spectrum of the matrix-isolated monomers was obtained and assigned. The matrix-isolated molecules were then subjected to in situ ultraviolet (UV) irradiation . It was concluded that alprazolam is photostable under these experimental conditions, contrarily to what is known to happen for the compound in solution or in solid state in the presence of excipients used in the pharmaceutical formulations. An explanation for the photostability of the matrix-isolated compound is provided, based on rapid recombinations of the biradical formed from the UV-induced diazepine ring cleavage or the chlorine atom and the complementary radical resulting from the scissoring of the C-Cl bond, which are favored by the cage confinement of the matrix-isolated molecules. The major fragmentation channels of the alprazolam molecule upon electron bombardment (70 eV) were determined by analysis of its electron ionization mass spectrum, which reveals that the major primary fragmentation processes lead to formation of cyanobenzene, N-2 , HCl (Cl-2), and benzene. The compound was also investigated in solution by multinuclear (H-1, C-13 and N-15) nuclear magnetic resonance (NMR) and ultraviolet (UV) spectroscopies, and in crystalline phase (P-1 polymorph) through IR and Raman spectroscopies. In addition, the structure of the crystal, previously reported in the literature [M. R. Caira, B. Easter, S. Honiball, A. Horne and L. R. Nassimbeni, Structure and Thermal Stability of Alprazolam and Selected Solvates, J. Pharm. Sci., 1995 , 84 , 1379-1384], was revisited in order to evaluate the relative importance of the different types of intermolecular interactions , using Hirshfeld surface analysis, the CE-B3LYP energy decomposition model, and the harmonic oscillator model of aromaticity (HOMA) index. Finally, the enthalpy of sublimation of the crystal was estimated from the CE-B3LYP calculated lattice energy.
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Near-infrared in situ generation of the higher-energy trans conformer of tribromoacetic acid: Observation of a large-scale matrix-site changing mediated by conformational conversion
(Amer Inst Physics, 1305 Walt Whitman Rd, Ste 300, Melville, Ny 11747-4501 USA, 2018-01-28) Apostolo, Rui F. G.; Bazso, Gabor; Tarczay, Gyorgy; Fausto, Rui; ILDIZ, GÜLCE ÖĞRÜÇ; 107326
The first observation of the higher-energy conformer of tribromoacetic acid (trans-TBAA) is reported. The conformer was produced in cryogenic matrices (Ar, Kr, and N-2) by in situ selective narrowband near-infrared excitation of the lower-energy cis-TBAA conformer and characterized both structurally and vibrationally. The novel trans-TBAA conformer is shown to spontaneously decay to the most stable cis-TBAA form in all studied matrix media, by tunneling, and the measured decay rates in the different matrices were compared with those of the trans conformers of other carboxylic acids in similar experimental conditions. In the N-2 matrix, where trans-TBAA establishes a specific stabilizing intermolecular interaction with the host N-2 molecules via its OH group and is about 11 times more stable than in rare gas matrices, the effect of changing the irradiation wavenumber within the 2 nu OH absorption profile was investigated in detail. An interesting phenomenon of matrix-site changing mediated by conformational conversion was observed in the N-2 matrix: vibrational excitation of cis-TBAA in the 2 nu OH wavenumber range predominantly converts the molecules located in a specific "matrix site" into trans-TBAA; then, relaxation (by tunneling) of the produced higher-energy conformer back to the cis form populates almost exclusively another "matrix site." The experimental studies received support from quantum chemistry calculations, which allowed a detailed characterization of the relevant regions of the potential energy surface of the molecule and the detailed assignment of the infrared spectra of the two conformers in the various matrices. Published by AIP Publishing.