Deuteron magnetic resonance and hydrogen bond network of ammonium trihydrogen selenite

The DMR spectra of single-crystal ND₄D₃(SeO₃)₂ have been studied. The principal values and the direction cosines of the field-gradient tensor of deuterons located on three nonequivalent O · · · O hydrogen bonds have been determined. The lengths of hydrogen bonds have been calculated from $\text{eQq}\text{h}$ values; the deuterons have been located on hydrogen bonds. The comparison with the DMR data of isomorphous compound RbD₃(SeO₃)₂ was made; the influence of NH · · · O hydrogen bonds on the structural parameters O · · · O hydrogen bonds is discussed.     

Deuteron magnetic resonance study of single crystals of NaDSeO3

A deuteron magnetic resonance study of a single crystal of NaDSeO₃ at room temperature is reported. The observed electric field gradient tensors correspond to two nonequivalent hydrogen bonds. The quadrupole coupling constants ($\text{eQq}\text{h}$) and asymmetry parameters (η) are found to be 172.8 and 0.07 kHz for D(1), and 152.5 and 0.08 kHz for D(2). D(2) are ordered, but D(1) are found to be dynamically disordered. The DMR results are consistent with recent X-ray reinvestigation. No phase transitions have been encountered in the temperature range 336 to 77 K.     

Conduction and tritium diffusion studies in single crystals of potassium hydrogen sulfate

Direct-current electrical conductivity of single crystals of KHSO₄ doped with 5, 11, 30, and 437 ppm of cobalt-II is studied in the temperature range 25 to 165°C. The point defect parameters are calculated by graphical analysis of the conductivity data. Various parameters are obtained: enthalpy for the migration of proton (referred to as the enthalpy for the rotation of HSO₄ ion), H_M = 0.55 ± 0.01 eV; the enthalpy for the formation of the intrinsic defects (referred to as the enthalpy for the breaking of the HSO₄ dimers), H_F = 1.30 ± 0.01 eV; and the enthalpy for the impurity-vacancy association H_a = ?0.14 ± 0.01 eV. The tritium diffusion studies in the pure KHSO₄ single crystals give 0.22 ± 0.01 eV and ?1.09 × 10^?3 eV K^?1 as the activation enthalpy and entropy for the process, respectively. The Nernst-Einstein relation is not obeyed. The mechanisms for the conduction and diffusion preocesses in this crystal are discussed.     

Deuteron magnetic resonance and phase transition studies in deuterated ammonium dichromate

Using deuteron magnetic resonance, it has been shown that deuterated ammonium dichromate has four phase transitions.     

Conformational studies of cyclic pentapeptides by proton magnetic resonance spectroscopy

The proton magnetic resonance spectra of c-(-Gly-L -Ala-Gly-Gly-L -Pro-) (I) and four analogous cyclopentapeptides are presented. At ambient temperature the spectra contain two sets of resonances which correspond to two different molecular conformations of the peptides. The relative concentrations of the two forms depend on the peptide, the solvent, and the temperature. For the two molecular species of peptide I in DMSO solution, the NMR. data imply that the peptide linkage involving the nitrogen of proline is respectively in the cis- and the trans-form, and both conformations contain intramolecular hydrogen bridges. Replacement of L -alanine in I by L -cysteine leaves the molecular conformations essentially unalteed. On the other hand substitution of L -proline by L -proline, or replacement of the two glycines in positions 3 and 4 by two sarcosyl residues gives rise to markedly different types of peptide backbone conformation.     

NMR and Raman spectral studies of ammonium hydrogen selenites single crystals

The chemical composition and peculiarities of the structure of a salt that precipitates from aqueous solution of NH₄HSeO₃ at the 25 °C was studied by NMR and Raman spectroscopy methods using the single crystal samples with different heavy water contents. It was proved that this salt is actually monohydrate of hydrogen selenite, NH₄HSeO₃H₂O but not trihydrate of pyroselenite, (NH₄)₂Se₂O₅3H₂O as was assumed previously based on the data cited in the literature.     

Proton and boron-11 magnetic resonance studies of some potassium tetraarylborates

Proton and boron-11 magnetic resonance spectra for several potassium para-substituted tetraarylborate compounds [KB(C₆H₄-pX)₄, where X is H, OCH₃, CH₃, Br, Cl, F, CF₃] have been obtained. The chemical shift between the centers of the AA′ and XX′ multiplets for the ring proton multiplets, relative to a reference chemical shift of 0·39 ppm for potassium tetraphenylborate, correlated with the corresponding Hammett σ values for the para-substituent. Additionally, the boron-11 chemical shifts gave a good correlation with corresponding σ values for the substituents. Electronegativities of para-substituted phenyl rings were calculated and found to be approximately 2·70 for all compounds studied. It was shown that electronic substituent effects do not greatly influence the electron density surrounding the central boron atom in the tetraarylborate ions.     

Organic-organic epitaxy of incommensurate systems: quaterthiophene on potassium hydrogen phthalate single crystals

Hot-wall epitaxy and molecular-beam epitaxy have been employed for growing quaterthiophene thin films on the (010) cleavage face of potassium hydrogen phthalate, and the results are compared in terms of film properties and growth mode. Even if there is no geometrical match between substrate and overlayer lattices, these films are epitaxially oriented. To investigate the physical rationale for this strong orientation effect, optical microscopy, atomic force microscopy, and X-ray diffraction are employed. A clear correlation between the morphology of the thin films and the crystallographic orientation is found. The results are also validated by surface potential calculations, which demonstrate the primary role played by the corrugation of the substrate surface.     

Deuteron activation analysis of ultra-trace carbon in boron-doped Si single crystals

The charged particle activation analysis of ultra-trace carbon in boron-doped silicon with the¹²C(d,n)¹³N reaction has been developed. In order to apply¹³N substoichiometric separation to determine carbon in silicon, we studied the rapid dissolution of silicon using nitric acid as the¹³N carrier. Its amounts were as small and definite as possible and the nitrogen oxide gas produced during the dissolution was collected. Silicon was dissolved for 6 min in a mixture of hydrofluoric acid, acetic acid and phosphoric acid, which contained potassium nitrate as the¹³N carrier and nitrogen oxide was collected in the sodium hydroxide solution. In order to combine the dissolution method with¹³N substoichiometric separation, the conditions for steam distillation as pre-separation were also refined in relation to increases in the amount of carrier. Nitrogen-13 was separated substoichiometrically after silicon dissolution and steam distillation. This analytical procedure was used to determine carbon in boron-doped CZ–Si. Carbon at 0.7–12.7·10¹⁵ atoms/cm³ was determined with good reproducibility. It took less than 30 min to start the radioactivity measurement after the end of iradiation. The detection limit was 2·10¹³ atoms/cm³ (0.2 ppb).     

Hydrogen bonding in potassium hydrogen carbonate studied by compton scattering on single crystals

Compton spectra excited by ²⁴¹Am radiation have been measured on thin single-crystal slices with the scattering vector oriented parallel and perpendicular to the pair of hydrogen bonds in the (HCO₃) dimers as well as to the plane of the dimers. the reciprocal form factors extracted from the spectra are strongly anisotropic with two extra zero passages in the direction of the hydrogen bonds. the results are in very satisfactory agreement with theoretical data that are calculated for the dimer with and without additional point charges simulating the neighboring ions in the crystal. The calculations have been done with Gaussian basis sets of double-zeta quality within the Hartree–Fock approximation. The theoretical reciprocal form factors of the monomer HCO and the dimer show only small differences in the directions perpendicular to the hydrogen bonds, whereas the differences in the hydrogen bond direction are remarkable and account for the experimentally observed features. In comparison to this effect the influence of the neighboring ions by their charges is much smaller, which is even true when taking into account the influence of the neighboring dimers in the crystalline stack in more detail by symmetrical orthogonalization of the combined wave functions. The same orthogonalization procedure applied to a pair of monomers in the spatial arrangement of the dimer yields a reciprocal form factor that in the experimentally reliable range above 1.8 Å accounts for most of the dimerization effect in the direction of the hydrogen bonds. Thus also for this prototype of paired hydrogen bonds (cf., carboxylic acids, DNA, and RNA), it confirms our earlier experimental finding on liquid water now unambiguously, namely that, in the same way as the cohesion in ionic crystals, hydrogen bond formation in bulk matter is distinctly dominated by electrostatic attraction, which is compensated by repulsion owing to the Pauli principle.     

High-resolution proton nuclear magnetic resonance studies of urine from asphyxiated newborn infants

High-resolution proton nuclear magnetic resonance spectroscopy was used to study human urine obtained from 10 normal babies and twenty babies with various degrees of neonatal asphyxia, respiratory distress syndrome (RDS), and meconium aspiration syndrome (MAS). All sick babies showed different degrees of oxygen deficiency, indicated by an obvious increase of the lactate signal level in the urine spectra. Changes in the concentration of other urinary metabolites produced from the citric acid cycle were also observed. In extremely serious cases, the signals of some of the major components, including citrate, α-ketoglutarate, and succinate, simply disappeared. The spectra of urine, serum, and CSF of an infant suffering from SIDS showed common characteristics of the metabolites.     

Free radical conformations and conversions in X-irradiated single crystals of L-cysteic acid by electron magnetic resonance and density functional theory studies

Single crystals of L-cysteic acid monohydrate were X-irradiated and studied at 295 K using EPR, ENDOR, and EIE techniques. Three spectroscopically different radicals were observed. These were a deamination radical reduction product (R1), and two oxidation products formed by hydrogen abstraction (radicals R2, R3). R2 and R3 were shown to exhibit the same chemical structure while exhibiting very different geometrical conformations. Cluster DFT calculations at the 6-31G(d,p) level of theory supported the experimental observations for radicals R1 and R2. It was not possible to simulate the R3 radical in any attempted cluster; hence, for this purpose a single molecule approach was used. The precursor radicals for R1, R2, and R3, identified in the low-temperature work on L-cysteic acid monohydrate by Box and Budzinski, were also investigated using DFT calculations. The experimentally determined EPR parameters for the low-temperature decarboxylated cation could only be reproduced correctly within the cluster when the carboxyl group remained in the proximity of the radical. Only one of the two observed low-temperature carboxyl anions (stable at 4 and 48 K) could be successfully simulated by the DFT calculations. Evidence is presented in support of the conclusions that the carboxyl reduction product already is protonated at 4 K and that the irreversible conversion between the two reduction products is brought forward by an umbrella-type inversion of the carboxyl group.     

Conformational studies of cyclo-gly-pro(S) by X-ray diffraction and proton magnetic resonance

The X-ray crystal structure and the PMR spectrum in ²H₂O and DMSO-d₆ of cyclo-glycyl-4-thiaprolyl has been determined. In the crystal, the diketopiperazine ring of the molecule adopts a boat conformation and the thiazolidine ring an envelope conformation, very similar to the analogous compound cyclo-glycyl-prolyl. Comparison of proton-proton dihedral angles derived from the crystal structure and from vicinal coupling constants in solution indicates that the conformation of cyclo-glycyl-4-thiaprolyl is nearly the same in crystal and in solution in the limits of applied mehods.     

The proton magnetic resonance of heterocyclic diketones

A full analysis of the 60 MHz ¹H NMR spectra of heterocyclic analogues of 1,3-indandione [N-(2-bromoethyl)-phthalimide, phthalic anhydride and thiophthalic anhydride] and of 1,2-indandione (N-methylisatin, coumarandione and 2,3-dihydrothionapthene-2,3-dione) has been carried out. The protons of the symmetric compounds resonate at lower fields than those of the asymmetric ones and the proton ortho to the CO function is more deshielded than the proton ortho to the heteroatom. Coupled Hartree-Fock perturbation theory has been applied to estimate the ring current contributions to the proton chemical shifts and to the magnetic susceptibilities. MO calculations based on the finite perturbation theory at the CNINDO levels provide information on the relative importance of σ and π pathways for the various coupling constants.     

Carbon-13 and proton magnetic resonance studies of some carbonyl-bis-(amino acid esters)

A number of carbonyl-bis-(amino acid esters) have been examined by proton and carbon-13 nuclear magnetic resonance techniques. All but one of the compounds were synthesized with two chiral centers of like-configuration. In one series, the diastereotopic nonequivalence of isopropyl methyl groups attached to the asymmetric centers is apparent in both the proton and the carbon spectra, and the relative magnitude of the observed nonequivalence increases slightly with increasing ‘bulk’ of the neighboring ester groups. Carbon-13 chemical shifts are reported, and a linear correlation of Taft σ* inductive constants with ester carbonyl carbon chemical shifts and with amide proton chemical shifts (for a series in which only variation of the ester substituent occurs) is presented. In addition, the effect in terms of chemical shift differences of keeping the same ester group at the asymmetric centers while varying the other substituent group, is examined.     

Nuclear magnetic resonance proton dipolar order relaxation in thermotropic liquid crystals: a quantum theoretical approach

By means of the Jeener-Broekaert nuclear magnetic resonance pulse sequence, the proton spin system of a liquid crystal can be prepared in quasiequilibrium states of high dipolar order, which relax to thermal equilibrium with the molecular environment with a characteristic time (T1D). Previous studies of the Larmor frequency and temperature dependence of T1D in thermotropic liquid crystals, that included field cycling and conventional high-field experiments, showed that the slow hydrodynamic modes dominate the behavior of T1D, even at high Larmor frequencies. This noticeable predominance of the cooperative fluctuations (known as order fluctuations of the director, OFD) could not be explained by standard models based on the spin-lattice relaxation theory in the limit of high temperature (weak order). This fact points out the necessity of investigating the role of the quantum terms neglected in the usual high temperature theory of dipolar order relaxation. In this work, we present a generalization of the proton dipolar order relaxation theory for highly correlated systems, which considers all the spins belonging to correlated domains as an open quantum system interacting with quantum bath. As starting point, we deduce a formulation of the Markovian master equation of relaxation for the statistical spin operator, valid for all temperatures, which is suitable for introducing a dipolar spin temperature in the quantum regime, without further assumptions about the form of the spin-lattice Hamiltonian. In order to reflect the slow dynamics occurring in correlated systems, we lift the usual short-correlation-time assumption by including the average over the motion of the dipolar Hamiltonian together with the Zeeman Hamiltonian into the time evolution operator. In this way, we calculate the time dependence of the spin operators in the interaction picture in a closed form, valid for high magnetic fields, bringing into play the spin-spin interactions within the microscopic time scale. Then, by adopting the spin-temperature density operator to represent the collective state of the spin system, and removing the traditional hypothesis of high temperature, we deduce an expression for the first order quantum contribution to T1D (-1), in terms of spectral densities, with coefficients in form of spin traces. The properties that distinguish our result from the high-temperature T1D (-1) are as follows. (a) It is exclusively associated to cooperative fluctuations. (b) Because of its quantum character, it relies on both considering the lattice degrees of freedom quantum mechanically and including the spin-spin interactions in the microscopic time scale. With regard to the average dipolar Hamiltonian, only the nonsecular part plays a relevant role. (c) Associated with the structure of the spin operator involved in the quantum contribution, a term arises which is proportional to the number of spins in the correlated molecular domains, showing that the quantum contribution may be of macroscopic size in highly correlated systems. When applied to nematic liquid crystals, the new term exhibits the typical nu(-1/2) Larmor frequency dependence through the spectral density of the OFD, in consistence with the experimental results.