Comment on ‘A technique for image encryption using digital signature’

The security of a recently proposed technique for encryption images by Sinha and Singh [A. Sinha, K. Singh, Opt. Commun. 218 (2003) 229], based on the use of digital signatures and error correcting codes, is analyzed. The proposed cryptosystem is shown to have some weakness. In fact, the secret key and the original image can be recovered efficiently by a brute force attack.     

A BEM approach to validate a model for predicting sound propagation over non-flat terrain

A two-dimensional boundary element model for sound propagation in a homogeneous atmosphere above non-flat terrain has been constructed. An infinite impedance plane is taken into account in the Green's function in the underlying integral equation, so that only the non-flat parts of the terrain need to be discretised in the boundary element model. This Green's function is undefined for points below the impedance plane, and therefore valleys and hollows are taken into account by coupling the exterior domain above the ground with one or several interior domains below the ground, as suggested in a recent paper [J. Sound Vibrat. 223 (1999) 355]. The resulting BEM model, which can handle arbitrary combinations of barriers and hollows, has been used for validating a ray model for various difficult configurations, including combinations of valleys and barriers.     

Control of surface properties of self-assembled monolayers by tuning the degree of molecular asymmetry

We have studied self-assembled monolayers (SAMs) of asymmetric dialkyldisulfide derivatives of the form CH₃-(CH₂)_11+m-S-S-(CH₂)₁₁-OH with m = −4, −3, 0, +2 and +4 on gold. Sub-nanoscale changes in the length of the CH₃-terminated alkylchain have been used to selectively protrude one particular end group in the resulting film. The alteration of the chain length in only two methylene units already results in changes of surface properties, which have been detected with local (chemical force microscopy) and macroscopic (contact angle) techniques. In particular, advancing contact angles can be adjusted between 40° and 80°. The adhesion between a hydrophobic tip and these SAMs in water is determined by the chemical nature of the protruding end group. Chemical force microscopy, X-ray photoelectron spectroscopy and infrared reflection absorption spectroscopy have shown that these SAMs are composed of mixed, well-packed CH₃- and OH-alkylthiolate branches. The surface composition ratio is close to 1:1 for all investigated SAMs.     

Preparation and properties of vanadyl(IV) hypophosphite,VO(PO2H2)2·H2O

The preparation of VO(PO₂H₂)₂·H₂O was possible by direct reduction of V₂O₅ with hypophosphorous acid or by interaction of VOSO₄ and H₃PO₂ solutions, and subsequent crystallization. The compound was characterized by IR, electronic and ESR spectroscopy and X-ray powder diffractometry. Magnetic susceptibilities were measured in the temperature range between 100 and 300 K. The thermal decomposition was also investigated in detail, showing a very complex sequence of degradation steps which finally generate a polymeric V(III) phosphite.     

Tables of Fourier, Laplace and Hankel Transforms of (u,n)-Dimensional Generalized Functions

The present note contains the Tables of Fourier, Laplace and Hankel transforms of several dimensional generalized functions. They are, mainly, based on the Laplace transform of retarded, Lorentz-invariant functions and the Fourier transforms of causal distributions.     

INTRAMICELLAR QUENCHING OF 2,3-DIMETHYLNAPHTHALENE FLUORESCENCE BY PEROXIDES and HYDROPEROXIDES

Hydrogen peroxide, t -butyl hydroperoxide, di- t -butyl peroxide and dibenzoyl peroxide efficiently quench the fluorescence of 2,3-dimethylnaphthalene incorporated into sodium dodecyl sulfate and cetyltrimethylammonium chloride micelles. Potassium persulfate only is effective when the probe is incorporated into a cationic micelle. The binding constant of the peroxides and the intramiccllar quenching rate constants have been evaluated for all the systems considered and the data are compared to that obtained in homogeneous solution. It is concluded that, even for processes which are not diffusion controlled, the quenching process is slower in the micelles.     

On the unitary dual of the classical Lie groups, Representations of

In this paper we prove that a unitary representation of whose infinitesimal character satisfies some regularity condition is infinitesimally isomorphic to an module. This is done using similar techniques as those used by the author in earlier work.

     

INCLUSION AND FLUORESCENE QUENCHING OF 2, 3-DIMETHYLNAPHTHALENE IN β-CYCLODEXTRIN CAVITIES

Abstract— The fluorescence quenching of 2,3-dimethylnaphthalene (DMN) incorporated to β-cyclodextrin (β-CD) cavities by different olefins (fumaronitrile, acrylonitrile, acrylamide and 2-hydroxyethylmethacrylate) has been measured as a function of the β-CD concentration. The quenching efficiency decreases when the β-CD concentration increases, but extrapolation of the data to infinite cavities concentration does not indicate complete protection. These results are interpreted in terms of two quenching processes, one of them taking place between 2,3-dimethylnaphthalene associated to a β-CD cavity and free quencher, and the other between the DMN and the quencher molecule, both associated with a different cavity. The rate constants of both quenching processes and the β-CD quencher association constant are obtained from the dependence of the quenching efficiency with β-CD concentration.     

EPR and modelling studies of hydrogen-abstraction reactions relevant to polyolefin cross-linking and grafting chemistry

EPR spectroscopy has been employed to study directly the selectivity of hydrogen-atom abstraction by some alkoxyl radicals from a variety of linear and branched alkanes, as well as linear alkenes, chosen as models for low molecular-weight polyolefin cross-linking systems. In situ thermal and photolytic approaches, as well as spin-trapping, have been employed to provide information relating to an accessible temperature range of 233-453 K. in part to mimic conditions relevant to melt processing of polyolefins. Rate constants (in the range 3 x 10(3)-3.7 x 10(5) dm3 mol-1 s-1 per hydrogen) have been determined for C-H abstraction at room temperature. Radical selectivity is largely governed by enthalpic effects (modelled via bond dissociation energy calculations and kinetic analysis). Direct evidence has been obtained for lack of reactivity, as a result of unfavourable steric interactions, for the secondary and tertiary C-H bonds in 2,4-dimethylpentane and 2,4,6-trimethylheptane, models for polypropylene. This has been rationalized via free-energy calculations using DFT.     

Theoretical study on the inhibition of ribonucleotide reductase by 2'-mercapto-2'-deoxyribonucleoside-5'-diphosphates

Ribonucleotide reductase (RNR) is responsible for the reduction of ribonucleotides into the correspondent 2'-deoxyribonucleotides in the only physiological process that yields the monomers of DNA. The enzyme has thus become an attractive target for chemotherapies that fight proliferation-based diseases, specifically cancer and infections by some viruses and parasites. 2'-Mercapto-2'-deoxyribonucleoside-5'-diphosphates (SHdNDP) are mechanism-based inhibitors of RNR and therefore potential chemotherapeutic agents for those indications. Previous experimental studies established the in vitro and in vivo activity of SHdNDP. In the in vitro studies, it was observed that the activity was dependent on the oxidative status of the medium, with the inactivation of RNR only occurring when molecular oxygen was available. To better understand the mechanism involved in RNR inactivation by SHdNDP, we performed theoretical calculations on the possible reactions between the inhibitors and the RNR active site. As a result, we propose the possible mechanistic pathways for the chemical events that occur in the absence and in the presence of O2. They correspond to a refinement and a complement of those proposed in the literature.