RelBAC: Relation Based Access Control

TheWeb 2.0, GRID applications and, more recently, semantic desktop applications are bringing the Web to a situation where more and more data and metadata are shared and made available to large user groups. In this context, metadata may be tags or complex graph structures such as file system or web directories, or (lightweight) ontologies. In turn, users can themselves be tagged by certain properties, and can be organized in complex directory structures, very much in the same way as data. Things are further complicated by the highly unpredictable and autonomous dynamics of data, users, permissions and access control rules. In this paper we propose a new access control model and a logic, called RelBAC (for Relation Based Access Control) which allows us to deal with this novel scenario. The key idea, which differentiates RelBAC from the state of the art, e.g., Role Based Access Control (RBAC), is that permissions are modeled as relations between users and data, while access control rules are their instantiations on specific sets of users and objects. As such, access control rules are assigned an arity which allows a fine tuning of which users can access which data, and can evolve independently, according to the desires of the policy manager(s). Furthermore, the formalization of the RelBAC model as an Entity-Relationship (ER) model allows for its direct translation into Description Logics (DL). In turn, this allows us to reason, possibly at run time, about access control policies

Conformers, Vibrational Spectra and Infrared-induced

The results of a combined study of dichloroacetic acid monomer undertaken by matrix-isolated low-temperature infrared spectroscopy in both argon and krypton matrices and ab initio SCFMO calculations are presented. Two s-cis (C-0) conformations of CCI,HCO,H differing by internal rotation about the C-C bond and one s-trans form were found in the matrices. Their spectra are reported within the range 4000-400 cm-' and interpreted. The skewlscis form (H-C-C-0 dihedral angle equal to ca. 144") converts to the most stable syn/scis conformer (H-C-C-0: 0") upon irradiation in the v(0-H) region. The temperature dependences (annealing) of the isolated matrix vibrational spectra of the studied molecule before and after irradiation were found to be different, and a model is proposed to explain the experimental observations. In addition, 3-21G and 6-31G* a6 initio- MO calculations were carried out and the structures and energies of the relevant conformations are reported. The theoretical results agree with the experimental data and provide a good insight into the intramolecular interactions which determine the relative stability of the various conformers. Finally, a normal-mode analysis based on the 6-31G* harmonic force fields is used to review previous assignments of the vibrational spectra of the various conformers of the studied molecule

Conformers, Vibrational Spectra and Infrared-induced

The results of a combined study of dichloroacetic acid monomer undertaken by matrix-isolated low-temperature infrared spectroscopy in both argon and krypton matrices and ab initio SCFMO calculations are presented. Two s-cis (C-0) conformations of CCI,HCO,H differing by internal rotation about the C-C bond and one s-trans form were found in the matrices. Their spectra are reported within the range 4000-400 cm-' and interpreted. The skewlscis form (H-C-C-0 dihedral angle equal to ca. 144") converts to the most stable syn/scis conformer (H-C-C-0: 0") upon irradiation in the v(0-H) region. The temperature dependences (annealing) of the isolated matrix vibrational spectra of the studied molecule before and after irradiation were found to be different, and a model is proposed to explain the experimental observations. In addition, 3-21G and 6-31G* a6 initio- MO calculations were carried out and the structures and energies of the relevant conformations are reported. The theoretical results agree with the experimental data and provide a good insight into the intramolecular interactions which determine the relative stability of the various conformers. Finally, a normal-mode analysis based on the 6-31G* harmonic force fields is used to review previous assignments of the vibrational spectra of the various conformers of the studied molecule

Matrix-Isolation Study and Ab Initio Calculations of the Structure and Spectra of Hydroxyacetone

The structure of hydroxyacetone (HA) isolated in an argon matrix (at 12 K) and in a neat solid phase (at 12−175 K) was characterized by using infrared (IR) spectroscopy. The interpretation of the experimental results was supported by high-level quantum chemical calculations, undertaken by using both ab initio (MP2) and density functional theory methods. A potential-energy surface scan, carried out at the MP2/6-311++G(d,p) level of theory, predicted four nonequivalent minima, Cc, Tt, Tg, and Ct, all of them doubly degenerate by symmetry. The energy barriers for conversion between most of the symmetrically related structures and also between some of the nonequivalent minima (e.g., Tg → Tt and Ct → Tt) are very small and stay below the zero-point vibrational level associated with the isomerization coordinate in the higher-energy form in each pair. Therefore, only Cc and Tt conformers have physical significance, with populations of 99 and 1%, respectively, in gas phase at room temperature. For the matrix-isolated compound, only the most stable Cc conformer was observed. On the other hand, the polarizable continuum model calculations indicated that in water solution, the population of Tt and Ct conformers might be high enough (ca. 6 and 11%, respectively) to enable their experimental detection, thus supporting the conclusions of a previous IR spectroscopy study [Spectrochim. Acta A 2005, 61, 477] in which the presence of more than one HA conformer in aqueous solution was postulated. The signatures of these minor conformers, however, do not appear in the spectra of the neat HA crystal, and the crystal structure was rationalized in terms of centrosymmetric hydrogen-bonded dimers consisting of two Cc-like units. Finally, we calculated 1H, 13C, and 17O NMR chemical shifts at different levels of theory and found them to agree with available experimental data

Color polymorphism in organic crystals

Colour polymorphism is a rare phenomenon that occurs when a compound possesses packing or conformational polymorphs of different colours. Here the authors review representative chemical systems that display colour polymorphism and explore the origins of this property

Conformers, infrared spectrum, UV-induced photochemistry, and near-IR-induced generation of two rare conformers of matrix-isolated phenylglycine

The conformational space of α-phenylglycine (PG) have been investigated theoretically at both the DFT/B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels of approximation. Seventeen different minima were found on the investigated potential energy surfaces, which are characterized by different dominant intramolecular interactions: type I conformers are stabilized by hydrogen bonds of the type N-H⋯O=C, type II by a strong O-H⋯N hydrogen bond, type III by weak N-H⋯O-H hydrogen bonds, and type IV by a C=O⋯H-C contact. The calculations indicate also that entropic effects are relevant in determining the equilibrium populations of the conformers of PG in the gas phase, in particular in the case of conformers of type II, where the strong intramolecular O-H⋯N hydrogen bond considerably diminishes entropy by reducing the conformational mobility of the molecule. In consonance with the relative energies of the conformers and barriers for conformational interconversion, only 3 conformers of PG were observed for the compound isolated in cryogenic Ar, Xe, and N2 matrices: the conformational ground state (ICa), and forms ICc and IITa. All other significantly populated conformers existing in the gas phase prior to deposition convert either to conformer ICa or to conformer ICc during matrix deposition. The experimental observation of ICc had never been achieved hitherto. Narrowband near-IR irradiation of the first overtone of νOH vibrational mode of ICa and ICc in nitrogen matrices (at 6910 and 6930 cm-1, respectively) led to selective generation of two additional conformers of high-energy, ITc and ITa, respectively, which were also observed experimentally for the first time. In addition, these experiments also provided the key information for the detailed vibrational characterization of the 3 conformers initially present in the matrices. On the other hand, UV irradiation (λ = 255 nm) of PG isolated in a xenon matrix revealed that PG undergoes facile photofragmentation through two photochemical pathways that are favored for different initial conformations of the reactant: (a) decarboxylation, leading to CO2 plus benzylamine (the dominant photofragmentation channel in PG cis-COOH conformers ICa and ICc) and (b) decarbonylation, with generation of CO plus benzonitrile, H2O and H2 (prevalent in the case of the trans-COOH conformer, IITa).Fil: Borba, Ana. Universidad de Coimbra; PortugalFil: Gomez Zavaglia, Andrea. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Centro de Investigaciones en Criotecnología de Alimentos (i); Argentina. Universidad de Coimbra; PortugalFil: Fausto, Rui. Universidad de Coimbra; Portuga

Molecular structure of S-ethylthioacrylate Combined vibrational spectroscopic and abinitioSCF-MO study

Ab initio 6-31G* SCF-MO calculations have been carried out on S-ethyl thioacrylate [CH2CHC(O)SCH2CH3]. Fully optimized geometries, relative stabilities, dipole moments and harmonic force fields for several conformers of this molecule have been determined and the results compared with those for similar molecules. Together with FTIR and Raman spectroscopic data, the theoretical results demonstrate that S-ethyl thioacrylate exists in two different conformations about the Cα–C bond (the s-cis and s-trans forms, with CC–CO dihedral angles equal to 0° and 180°, respectively); the s-cis conformation being more stable than the s-trans form by ca. 6 kJ mol-1 for the isolated molecule. Comparison of the experimental and theoretical vibrational spectra confirms that, as concluded from our previous study on the analogous trans-S-ethyl thiocrotonate molecule (R. Fausto, P. J. Tonge and P. R. Carey, J. Chem. Soc., FaradayTrans., 1994, 90, 3491), the presence of the s-trans isomer of an α,β-unsaturated thioester can be successfully monitored by the IR band at ca. 1170 cm-1, ascribed to the Cα–C stretching mode of this form. In addition, we were also able to identify some IR bands sensitive to the conformation of the ethyl group that may be used as spectroscopic probes to study conformational equilibria associated with this internal degree of freedom in more complex S-ethyl thioester

An FT-IR spectroscopic study of the crystalline and columnar liquid-crystalline phases of copper (II) carboxylates

The FT-IR spectra of a series of anhydrous copper (II) carboxylates of general formula Cu2[CH3(CH2)nCOO]4 (n = 4–8, 10, 12, 14, 16) show notable changes with the phase transition from the crystalline to the columnar liquid-crystalline phase. These changes indicate that the coordination of the carboxylate groups to the bimetallic centre changes from bridging bidentate to chelating bidentate, which in turn is induced by the conformational disordering in the carbon chains

Automatic Counterfactual Augmentation for Robust Text Classification Based on Word-Group Search

Despite large-scale pre-trained language models have achieved striking results for text classificaion, recent work has raised concerns about the challenge of shortcut learning. In general, a keyword is regarded as a shortcut if it creates a superficial association with the label, resulting in a false prediction. Conversely, shortcut learning can be mitigated if the model relies on robust causal features that help produce sound predictions. To this end, many studies have explored post-hoc interpretable methods to mine shortcuts and causal features for robustness and generalization. However, most existing methods focus only on single word in a sentence and lack consideration of word-group, leading to wrong causal features. To solve this problem, we propose a new Word-Group mining approach, which captures the causal effect of any keyword combination and orders the combinations that most affect the prediction. Our approach bases on effective post-hoc analysis and beam search, which ensures the mining effect and reduces the complexity. Then, we build a counterfactual augmentation method based on the multiple word-groups, and use an adaptive voting mechanism to learn the influence of different augmentated samples on the prediction results, so as to force the model to pay attention to effective causal features. We demonstrate the effectiveness of the proposed method by several tasks on 8 affective review datasets and 4 toxic language datasets, including cross-domain text classificaion, text attack and gender fairness test.Comment: 13 pages, 7 figure