Spectroscopic study of rigid-rod polymers. II. Protonation effect

The protonation of a heterocyclic rigid-rod polymer poly(p-phenylene benzbisthiazole) and its model compound has been studied by UV-visible and Raman spectroscopy. Because of the two nitrogens on the heterocyclic ring, spectroscopic features of unprotonated, half-protonated, and fully protonated structures have been identified. For the fully protonated molecule, there is also an increase in conjugation between the phenyl ring and the heterocyclic ring.     

Spectroscopic study of rigid-rod polymers. I. Structures of model compounds

Poly(p-phenylene benzbisoxazole) and poly(p-phenylene benzbisthiazole) belong to the class of extended-chain, rigid-rod polymers possessing high modulus, high strength, and good thermal and oxidative resistance. Fibers and films of these polymers are processed from anisotropic solutions in strong acids such as methane sulfonic acid or polyphosphoric acid. The electronic absorption and vibrational spectra of the model compounds have been investigated in order to characterize the structures in the solid state and in nonprotonic solvents. The dramatic intensity differences in the spectra obtained have been interpreted by variations in the dihedral angle between the plane of the phenyl group and the plane of the central heterocyclic ring which affect the resonance configuration between the two rings.     

Spectroscopic signatures of halogens in clathrate hydrate cages. 1. Bromine

We report the first UV-vis spectroscopic study of bromine molecules confined in clathrate hydrate cages. Bromine in its natural hydrate occupies 51262 and 51263 lattice cavities. Bromine also can be encapsulated into the larger 51264 cages of a type II hydrate formed mainly from tetrahydrofuran or dichloromethane and water. The visible spectra of the enclathrated halogen molecule retain the spectral envelope of the gas-phase spectra while shifting to the blue. In contrast, spectra of bromine in liquid water or amorphous ice are broadened and significantly more blue-shifted. The absorption bands shift by about 360 cm-1 for bromine in large 51264 cages of type II clathrate, by about 900 cm-1 for bromine in a combination of 51262 and 51263 cages of pure bromine hydrate, and by more than 1700 cm-1 for bromine in liquid water or amorphous ice. The dramatic shift and broadening in water and ice is due to the strong interaction of the water lone-pair orbitals with the halogen sigma* orbital. In the clathrate hydrates, the oxygen lone-pair orbitals are all involved in the hydrogen-bonded water lattice and are thus unavailable to interact with the halogen guest molecule. The blue shifts observed in the clathrate hydrate cages are related to the spatial constraints on the halogen excited states by the cage walls.     

Spectroscopic signatures of halogens in clathrate hydrate cages. 2. Iodine

UV-vis and Raman spectroscopy were used to study iodine molecules trapped in sII clathrate hydrate structures stabilized by THF, CH(2)Cl(2), or CHCl(3). The spectra show that the environment for iodine inside the water cage is significantly less perturbed than either in aqueous solution or in amorphous water-ice. The resonance Raman progression of I(2) in THF clathrate hydrate can be observed up to v = 6 when excited at 532 nm. The extracted vibrational frequency omega e = 214 +/- 1 cm(-1) is the same as that of the free molecule to within experimental error. At the same time, the UV-vis absorption spectrum of I(2) in the sII hydrate exhibits a relatively large, 1440 cm(-1), blue-shift. This is mainly ascribed to the differential solvation of the I(2) electronic states. We conclude that iodine in sII hydrate resides in a 5(12)6(4) cavity, in which the ground-state I(2) potential is not significantly perturbed by the hydrate lattice. In contrast, in water and in ice, the valence absorption band of I(2) is dramatically broadened and blue-shifted by 3000 cm(-1), and the resonance Raman scattering is effectively quenched. These observations are shown to be consistent with a strong interaction between water molecule and iodine through the lone pair of electrons on water as in the case of bromine in the same media. The results presented here, and the stability of other halogen hydrates, were used to test the predictions of simple models and force-field calculations of the host cage-guest association energy.     

P. M. R. Spectroscopic Analysis of Bacterial Polysaccharides Containing Pyruvic Acid

Abstract

The characteristic p.m.r. signal at τ 8.4–8.6 given by the CH₃-C group of a pyruvic acid ketal (1-carboxyethylidene derivative)may be used to demonstrate the presence of such a substituent in polysaccharides. Similar signals are given when the pyruvic acid ketal is methylated or reduced. The method is convenient for monitoring purification procedures and chemical transformations of polysaccharides such as are involved in structural studies. In certain cases the molar ratio of pyruvic acid may be estimated.     

An 1H-NMR. Spectroscopic Study of Alloxazines and Isoalloxazines

¹H-NMR.-spectra of a series of alloxazines and isoalloxazines and certain cationic derivatives were investigated (Tab. 2 and 5). Unequivocal assignment of all resonance signals was achieved in some compounds by selective deuteriation also by double resonance technique. The coupling constants were verified by computer simulation. Considerable enhancement of the signals due to H-C(9) and H-C(6) is found on decoupling of H₃C-C(7), H₃C-C(8) and H₃C-N(10), resp. These results are compared with those obtained with FAD. The methyl resonance signal of the H₃C-C(7) compounds give doublets due to coupling with H-C(6). The difference in chemical shifts observed upon successive formal introduction of methyl groups into the benzene nucleus of (iso)alloxazines indicates that the molecule becomes less planar thereby. The pyrimidine ring of (iso)alloxazines does not contribute to the ring current except by indirect effects via the carbonyl groups. The experimental data are compared with published MO calculations and discussed.     

11,16 Oxetane lactones. Spectroscopic evidences and conformational analysis

Sesquiterpene lactones constitute a wide group of compounds with several biological activities, including allelopathic. The naturally occurring sesquiterpene lactones dehydrocostuslactone and cynaropicrin have been modified in three different ways: preparation of 11,13-oxetane lactones, addition of a second Michael acceptor and reduction of the α-methylene-γ-lactone in order to future structure-activity relationship (SAR) studies. We have obtained all oxetane lactones stereoisomers at C-11 and C-16 positions. This fact has allowed us to establish some correlations between experimental data, derived by NMR and X-ray analysis, and the configuration at C-11 and C-16, which could be a useful tool to establish the stereochemistry as well as to confirm the presence of an oxetane ring on similar compounds. Comparative conformational analyses as a key aspect in the biological behaviour of those compounds in future structure-activity relationship (SAR) studies are presented.     

Cyclobutene-1,2-diones. A Theoretical and Spectroscopic Study

The properties of substituted cyclobutene-1,2-diones 1 are examined by the use of (17)O NMR spectroscopy and theoretical calculations and compared to those of cyclopropenones 2 and other models. Cyclobutene-1,2-diones have less negative charge per oxygen compared to cyclopropenones, and electron donation by substituents enhances the negative charge on oxygen. Calculated (17)O chemical shifts reproduce the measured trends. The dianions of squaric and deltic acids are highly stabilized by negative charge delocalization to the oxygens.     

The Fundamental Basis for Cyclopolymerization. III. A Spectroscopic Study of Dimethacrylamides

The purpose of this investigation was to prepare suitable solid 1,6-dienes and the corresponding monoenes and to compare their UV and NMR spectra. A comparison of their polymerization characteristics will be reported in the succeeding paper of this series. The compounds chosen for this study were dimethacrylamide, N-isobutyrylmethacrylamide, and their N-methyl and N-phenyl derivatives. Since the dimethacryl derivatives are solids at room temperature, they are suitable monomers for solid-state polymerization studies, the results of which could lead to significant conclusions regarding the fundamental basis for cyclopolymerization. A previously postulated electronic interaction between the neighboring double bonds of the dienes should be detectable when their UV spectra are compared with their monoene counterparts. The UV spectra of the N-isobutyrylmethacrylamides and dimethacrylamides were compared with the spectra of the related di-isobutyramides, methacrylamides, and isobutyramides. The spectra of the diacylamides were much more complicated than predicted, and it seems that interaction of the carbonyls through the nitrogen is the major factor affecting the observed spectral transitions. This is different from the corresponding anhydrides in which the carbonyl groups appear to behave as isolated chromophores. The NMR spectra of the compounds were determined since a comparison of chemical shifts and variations of chemical     

Spectroscopic study of the interactions of alkali fluorides with D-xylose.

The interactions of alkali fluorides with D-xylose have been studied by X-ray diffraction (XRD), infrared spectroscopy (IR), nuclear magnetic resonance (NMR, 1H and 13C) and atomic absorption spectrophotometry. KF and CsF form complexes with D-xylose in a 1:1 molar ratio. These complexes can be obtained by solid state milling the reactants in an agate mortar or from methanolic solutions of the sugar and the salt. LiF and NaF do not form complex with D-xylose. IR and XRD prove the identical nature of the complexes obtained by milling and from solution. IR spectra indicate strong perturbation of the OH stretching vibrations with considerable shifts to lower frequencies, which must be caused by strong hydrogen bond formation to the fluorine anion. The perturbations of C-O bond are weak, indicating that cation binding to the oxygen atoms is not the main interaction responsible for the complex formation. 1H NMR spectra of the D-xylose-KF complex dissolved in deuterium oxide is equal to that of pure D-xylose, indicating the destruction of the complex in solution. The complex is stable in DMSO, and 13C spectra of the complex in DMSO-d6 and in solid state (CPMAS) spectra are in accordance with the observed interactions in the IR spectra. As far as we know, this is the first report of a sugar-halide salt complex in which the anion instead of the cation provides the binding forces.     

Spectroscopic studies of azabicyclospirohydantoins

The IR spectra of 3-alkyl-3-azabicyclo(3.2.1)octane-8-spiro-, 3-alkyl-3-azabicyclo(3.3.1) nonane-9-spiro-, and 8-alkyl-8-azabicyclo(4.3.1)decane-10-spiro-5'-hydantoins have been studied in the solid state and in solution and the existence of different types of hydrogen bonding deduced from the spectra.The presence of intermolecular N_3'-H?N bonds in the solid state depends mainly on the steric hindrance of the polymethylene chain and on the bulk and shape of the piperidine _N-substituent. The possibility of strong N_3'-H?N bond formation is smaller than for isomeric nortropane- and granatanine-spirohydantoins.Results are confirmed in some cases by Raman and X-ray measurements.     

Spectroscopic observation of solvent interaction with selected retinal Schiff bases.

Iodide salts of several N-retinylidenedialkylamines were prepared and their UV-VIS spectra recorded. Their lambda max values increased as the number of hydrogen atoms on the carbons alpha to nitrogen increased. In separate experiments, iodide salts of N-retinylidene-n-butylammonium (1) and N-retinylidene-n-butylmethylammonium (2) were prepared, and their excitation energies (delta E) were measured in selected solvents of varying dielectric constant (epsilon). Data from each compound gave a straight line which converged at epsilon = 0. On the other hand, when delta E values of the iodide and bromide of 2 were plotted vs solvent epsilon, parallel rather than convergent lines were obtained. When lambda max values of 1 and 2 were measured in a greater number of solvents, the solvents fell into four main groups. The first group, regular solvents, are rigid, have fixed dipoles, neither donate nor accept H-bonds, and the delta E of 1 and 2 decreases linearly as solvent epsilon increases. This line for 2 is taken as a standard against which other solvents are judged. A second class of solvents that are good H-bond donors like CHCl3, whose dipole moment coincides with the C-H bond axis, is located in an area below the standard line. A third group, nucleophilic solvents like tetrahydrofuran, whose dipole moment is coincident with a strongly nucleophilic oxygen atom are good H-bond acceptors and are found above the standard line. Solvents with unpredictable spectroscopic behavior are classed as anomalous.     

Spectroscopic analysis of charge transfer complexes between morpholine and iodine.

It has been demonstrated spectroscopically that many nitrogen-containing heterocyclic compounds can form charge transfer complexes with iodine. The complexes of morpholine with iodine were shown to be of the n-sigma type with a 1:1 stoichiometry. A strong donor-acceptor interaction was found (Kc = 1261 +/- 12 mol-1 at 20 degrees C in CCl4), considerably higher than those of complexes of aromatic compounds with iodine. The high value of the formation constant for this complex indicated that morpholine could serve as a starting point for the synthesis of novel anti-thyroid drugs.