Theoretical and Experimental Study of the Conformation and Vibrational Frequencies of N‐Acetyl‐L‐alanine and N‐Acetyl‐L‐alaninate

The vibrational frequencies of N‐acetyl‐L‐alanine (NAAL), its potassium salt (NAALK) and its free anionic form (NAAL^?) are calculated using density functional theory (B3LYP) combined with the 6‐311 + + G(d,p) basis set. The experimental Raman spectrum of solid NAALK and the scaling factors for calculated values are discussed as well. The three species are characterized by intramolecular NH…O hydrogen bonds leading to the formation of a five‐membered ring. As indicated by the intramolecular (N)H…O distances and by the ν(NH) frequencies, the strength of the intramolecular hydrogen bond is ordered as follows: NAAL^? < NAALK < NAAL^?. Owing to their difference in the coupling with other vibrational modes, the in‐plane and out‐of‐plane vibrations do not reflect the strength of the hydrogen bond.     

Experimental and Theoretical Study of the Electrochemical Behavior of o‐Benzoquinone and Pyrocatechol

The geometric parameters, vibrational frequencies, and the thermochemical values of o‐benzoquinone (o‐BQ), p‐benzoquinone (p‐BQ), pyrocatechol (PC), and p‐hydroquinone (p‐HQ) were computed using ab initio calculation (HF) and density functional theory (DFT) with the 6‐31G (d) and 6‐31G (d, p) basis sets, respectively. Cyclic voltammetry with a gold electrode of PC solutions in phosphate buffers at pH 7.30 showed that the standard electrode potential of half reaction for o‐BQ and PC is 0.813 V. The standard electrode potential of half reaction for o‐BQ and PC with a p‐BQ, H⁺/p‐hydroquinone (p‐HQ) reference electrode, using the solvation energies and the sum of electronic and thermal free energies of o‐BQ and PC, is consistent with the experimental one.     

Calculation of the Tautomeric Properties of 3‐Formyl‐Tetrinic Acid by the ab initio and DFT Methods

We have carried out a nonempirical quantumchemical calculation with full optimization of the geometry of all theoretically possible tautomeric forms of 3formyltetrinic acid (FTRA) according to the Møller–Plesset secondorder perturbation theory with the use of a 6–31G(d) double zeta basis set. The correlation corrections to the total energy of molecules for optimized geometric configurations were calculated in the 631G(d,p) basis. All the possible tautomeric forms of FTRA and the harmonic vibrational frequencies were also calculated within the density functional theory (DFT) with the use of the Perdew–Burke–Ernzerhof (PBE) functional in a threeexponent basis. It is shown that FTRA in vapors (in the perfect gas approximation) exists in the form of a mixture of three enol forms, among which two exoforms predominate. The frequencies and forms of normal vibrations for each cisenol tautomer in the region of vibrations of ketogroups and double bonds differ widely, which permits identification of the tautomers present in the mixture. The possible mechanisms of enolenol transformations of 3acyltetrinic acids are discussed.     

Spectral‐Luminescence and Generation Properties of 1‐Aza‐2‐Methyl‐6‐Aminopyrene and of Its Protolytic Forms

Spectral manifestations of protolytic forms of 1aza2methyl6aminopyrene in ethanol and aqueous solutions are investigated. The ionization constants of the protolytic forms are determined. The generation properties and photostability of a neutral molecule and a cation in excitation by an excimer XeCl laser are studied.     

Intramolecular Hydrogen Bond in Steroid 5‐Hydroxy‐6‐Ketones and 5‐Hydroxy‐6‐Ketoximes of the Stigmastane Series

The intramolecular hydrogen bond in steroid 5hydroxy6ketones and 5hydroxy6ketoximes of the stigmastane series has been investigated by the method of IR spectroscopy. It is shown that 5hydroxy groups in cisA/Bsteroids are linked by the intramolecular hydrogen bond with polar substituents at C₃ and C₆. These hydrogen bonds are lacking in corresponding transA/Bsteroids. According to the IR spectra, intramolecular associates of polymer type are formed in 5hydroxy6hydroxyminosteroids.     

Rheological Properties,Morphology, and Thermal Performance of E‐MA‐GMA/PC Blend

Blends of ethylene–methyl acrylate–glycidyl methacrylate terpolymer (E‐MA‐GMA, a random terpolymer) and polycarbonate (PC) were prepared in a Haake torque rheometer and the rheological properties, phase morphology, and thermal behavior were investigated. The graft reactions of PC terminal hydroxyl groups with the epoxy groups of E‐MA‐GMA and the in situ formation of the E‐MA‐GMA‐g‐PC copolymers at the interface were illustrated by the improved mixing torque and melt viscosity in E‐MA‐GMA/PC blends. Typical variation and significant deformation of the dispersed phase was observed in E‐MA‐GMA/PC blends with different composition, where PC was the matrix. With the E‐MA‐GMA content increasing, a complex co‐continuous phase structure with some dispersed E‐MA‐GMA particles wrapped in the continuous PC phase was present, indicating strengthened interfacial adhesion. When the E‐MA‐GMA content was higher than the PC component, fibrous structure of the dispersed PC phase in the E‐MA‐GMA matrix was caused by shear flow and interfacial interaction. DSC studies showed that the melting point of E‐MA‐GMA shifted to lower temperature with the increase of PC content, indicating that the enhanced interaction and graft structure hindered the process of crystallization and crystal growth.     

An Overview of the High‐Pressure Vibrational Spectra of Clays and Related Minerals

Abstract

An overview of the effect of high external pressures on clays and related minerals is presented. The results show that this is an area that will welcome further investigation in the future, especially in view of the importance of clay minerals in the Earths's mantle and, possibly, even in prebiotic chemistry in the formation of amino acids and the mineralogy of the Martian surface.     

Structural Assignment of Isomeric S‐ and S,N‐1‐Alkoxycarbonylalkylated 6‐Amino‐2‐thiouracil Derivatives by Means of 1H and 13C NMR Spectroscopy

Abstract

The structures of new isomeric 2‐alkoxycarbonylalkylthio‐ and 2‐alkoxy‐ carbonylalkylthio‐1‐alkoxycarbonylalkyl‐6‐aminouracils (1–21) have been established on the basis of the ¹H NMR and ¹³C NMR spectroscopic data. The ¹H NMR and ¹³C NMR spectra of 1–21 have been fully assigned by a combination of two‐dimensional experiments [heteronuclear multiple quantum coherence (HMQC) and heteronuclear multiple bond correlation (HMBC)]. The ¹³C NMR spectra have been shown to be able to differentiate between isomers.     

Calculation of the Parameters of a One‐Coordinate Model of the VGaSAs and VGaSnGa Complexes in Gallium Arsenide

Experimental optical bands of the emitting complexes (a gallium vacancy with a donor at the nearest, from the vacancy, sublattice site) V_GaS_As and V_GaSn_Ga in GaAs are investigated in a wide temperature range. The parameters of a onecoordinate model of these centers are determined. A configurationcoordinate diagram of the V_GaS_As complex is constructed. Calculation of the field dependences of emission rates is carried out in terms of the parameters of the onecoordinate model of the V_GaS_As complex and the data obtained are compared with the results of experimental studies of the emission of electron holes from a deeplying center of the V_GaS_As complex.     

Calculation of Quantum‐Chemical Characteristics of the Ground and Excited Electronic States of Porphin and Its Tetrapyrrole Isomers

Using the CNDO/S method, we have performed quantumchemical calculations of the ground and excited electronic states of porphin molecules (symmetry D _2h ) and a number of porphin isomers: porphycene (C_2h ), hemiporphycene, corphycene (C_2v ), isoporphycene, and three nonsynthesized structural isomers of the porphin skeleton by its bridge groups —(CH) _meso —. The results of the calculations are compared with the corresponding data for the freebase porphin molecule (H₂P). Near the boundary between the occupied and vacant orbitals the isomers form rows of singleelectron levels with similar energy characteristics. For the MOs of these isomers, the H₂P MOs closest in distribution on the comparable atomic centers are given. The weak (Q) and strong (B) transitions in the visible and near UV regions point to the porphyrin type of spectra of all the isomers. The calculation data on the excited electronic states of N, g, and ntype H₂P near and above the B states are given; the presence of such states in other isomers is shown. The calculation does not point to a difference in the position of the first singlet transitions in the first five isomers. The calculation data on the lower singlettriplet transitions are also reported.     

Spectral‐Luminescence Characteristics of Polyphenyls and Polyacenes in the Gas Phase in Electron‐Beam Excitation

The spectra of electronenergy loss, excitation functions, and fluorescence spectra in excitation of the vapor of polyphenyls and polyacenes by electron beams of different energies are determined. The influence of successive complication of the molecules under study on these spectralluminescence characteristics is tracked. Unlike the optical absorption spectra, in the spectra of electronenergy loss of all the substances studied one observes a band which is related to the singlettriplet transition S ₀–T ₁. The transitions up to S ₀–S ₅ are recorded in excitation of the molecules by highenergy electrons, including the region of vacuum ultraviolet. From the functions of fluorescence excitation the authors have determined the excitation thresholds that correlate with the energies of the S ₁ levels, except for pyrene in which the S ₀–S ₁ transition is forbidden and does not show up not only in photon excitation but also in electronbeam excitation, although the intercombination forbiddenness in the latter case is removed and the S ₀–T ₁ band is observed.     

Characterization of Mango Juice by High‐Resolution NMR,Hyphenated NMR,and Diffusion‐Ordered Spectroscopy

Abstract

The application of nuclear magnetic resonance (NMR) spectroscopy, hyphenated NMR, and diffusion‐ordered spectroscopy (DOSY) to the characterization of mango juice, as an example of a complex food mixture, is described. The compositional changes taking place as a function of ripening were followed, and selected metabolites were quantified by integration of the corresponding NMR peaks. In this way, an overall view of the metabolite changes is obtained, enabling the study of the biochemical mechanisms involved in the ripening process. More than 50 compounds were identified by 1D‐ and 2D‐NMR, but many ambiguous assignments remain due to spectral overlap or insufficient coupling information. The use of Liquid Chromatography (LC‐NMR) and LC‐NMR/Mass Spectrometry (MS) enables a fuller characterization of the soluble pectin fraction to be made; its dependence on ripening stage is discussed. Finally, DOSY adds information on the M_r of many metabolites, including the pectin fractions of ripe and unripe mango juices, and enables further peak assignments to be made.     

Infrared Spectroscopic Study of N,N‐Dimethylthioformamide Complexes of the Hofmann Type

Four new Hofmanntype complexes, M(DMTF)₂Ni(CN)₄, (where DMTF is dimethylthioformamide, M=Mn, Cd, Co, or Ni) were synthesized and their structure was determined by an elemental analysis and infrared spectroscopy. The IR spectra of DMTF and its nickel tetracyanine complexes with Mn(II), Cd(II), Co(II), and Ni(II) have been investigated within the range 4000–400 cm^–1. The frequency shifts in the metal complexes agree with the assignment of the CS and CN frequencies. The complexes consist of infinite planar polymer layers of |MNi(CN)₄|. Ndimethylthioformamide is coordinated to this layer from above and below; it is a monodentate ligand and is Sbonded through the metal atom in these complexes.     

Kinetics of the Processes of the Change in the Concentration of Photosensitive Paramagnetic Centers on Excitation and De‐Excitation of ZnCdS Single Crystals

The EPR technique is used to study the behavior of photosensitive paramagnetic centers (PPC) in zinc sulfidebased single crystals under different conditions of excitation by UV radiation.     

Thermal and Tribological Properties of Fullerene‐Containing Composite Systems. Part 3. Features of the Mechanism of Thermal Degradation of Poly‐(N‐Vinyl‐Pyrrolidone) and Its Compositions with Fullerene C60

By mass‐spectrometric thermal analysis (MTA) the thermochemical features of poly(N‐vinyl pyrrolidone) (PVP) and its compositions with fullerene C₆₀ were studied. The mechanism of PVP thermal degradation was investigated; in particular the nature of the low‐temperature degradation (between 75 and 300°C) accompanied by output of pyrrolidone was explained as well as the influence of fullerene C₆₀ on this mechanism. It was shown that during thermal degradation of copolymer PVP‐C₆₀, there is a disappearance of the low‐temperature peaks of the output of pyrrolidone that is interpreted as an increase of the thermal stability of N‐vinyl‐pyrrolidone fragments in this product in comparison with their thermal stability in pure PVP.     

Application of NMR Spectroscopy and Mass Spectrometry to the Structural Elucidation of Modified Flavan‐3‐ols and Their Coupling Reaction Products*

NMR and MS techniques were used for the unambiguous structural elucidation of synthesized modified monomeric and dimeric flavan‐3‐ols presenting different substituents on the A‐ring C8 position. The full characterization of the synthesized compounds was achieved by concerted use of NMR and ESI‐MS techniques. Assignments of proton and carbon atoms was achieved through analysis of the 1D ¹H and ¹³C NMR spectra combined with homo‐ and heteronuclear 2D NMR experiments. In each case, HMBC correlation between proton H2 and carbon C8a was observed allowing assignment of this carbon, which represents the key for attribution of the A‐ring carbon atoms. The synthesis and structural characterization of activated monomeric and dimeric flavanols were also achieved and used as precursors for preparation of natural and modified dimeric procyanidin derivatives. The preparation of various dimeric species involving modified flavanols was explored through different coupling reactions. The structures of the compounds formed were characterized on the basis of their MS and NMR spectral analysis. Dimeric species were characterized through proton–proton and proton–carbon correlations, which distinguished between the different flavanol moieties and established their sequences.     

Photoluminescence of the n+‐n‐i Submicron Structures of Gallium Arsenide

The special features of the photoluminescence of the n ⁺ni submicron structures of gallium arsenide are investigated. It is found that the character of the dependence of the fluorescence intensity on the thickness of the structure is determined by the relationship between three characteristic lengths: diffusional length, thickness of the n ⁺ layer, and depth of absorption of exciting radiation, whereas the decrease in the mobility of charge carriers in epitaxial layers leads to an increase in the fluorescence intensity of the structure, with exciting radiation being strongly absorbed.     

Helmholtz, Riemann, and the Sirens: Sound, Color, and the “Problem of Space”

Emerging from music and the visual arts, questions about hearing and seeing deeply affected Hermann Helmholtz’s and Bernhard Riemann’s contributions to what became called the “problem of space [Raumproblem],” which in turn influenced Albert Einstein’s approach to general relativity. Helmholtz’s physiological investigations measured the time dependence of nerve conduction and mapped the three-dimensional manifold of color sensation. His concurrent studies on hearing illuminated musical evidence through experiments with mechanical sirens that connect audible with visible phenomena, especially how the concept of frequency unifies motion, velocity, and pitch. Riemann’s critique of Helmholtz’s work on hearing led Helmholtz to respond and study Riemann’s then-unpublished lecture on the foundations of geometry. During 1862–1870, Helmholtz applied his findings on the manifolds of hearing and seeing to the Raumproblem by supporting the quadratic distance relation Riemann had assumed as his fundamental hypothesis about geometrical space. Helmholtz also drew a “close analogy … in all essential relations between the musical scale and space.” These intersecting studies of hearing and seeing thus led to reconsideration and generalization of the very concept of “space,” which Einstein shaped into the general manifold of relativistic space-time.     

Analysis of the common characteristics of the Hartmann,Ronchi, and Shack–Hartmann tests

An overview of the settings of the planes for the filters and observed patterns in the Hartmann and Ronchi tests is presented. Also a new set of filters for both test were developed. In a similar way, it is easy to extend this analysis to the Shack–Hartmann test, and to propose a new Null Shack–Hartmann filter.     

Influence of Thermal Treatment on the Thermal Behavior of Poly‐L‐lactide

The influences of thermal treatment on cold crystallization and the thermal behavior of poly‐L‐lactide (PLLA) were investigated by DSC and polarizing microscopy. Both the cooling and heating rates had effects on cold crystallization. Double peaks were observed for the samples on subsequently heating at 10°C min^?1 after cooling between 5 and 20°C min^?1. The degrees of crystallinity dramatically increased with decreasing cooling rate, and the size of PLLA spherulites increased with a decrease in the cooling rate. Double cold crystallization peaks were also observed during heating traces at higher rates for this material after fast cooling (20°C min^?1) from the melt. The competition between the crystallization from the nuclei formed during cooling, and that from spontaneous nucleation might be responsible for the appearance of double peaks.