Defect induced forbidden X-ray reflections in RbH2PO4.

Resonant X-ray diffraction was used to study the proton jumps in hydrogen-bonded rubidium dihydrogen phosphate (RDP) crystals. In the paraelectric RDP phase, hydrogen is delocalized between two crystallographically equivalent positions. At lower temperatures, this symmetry can be broken, which defines the processes that lead to the para- to ferroelectric phase transition. We have measured the energy spectra of the forbidden reflections 006 and 550 at incident radiation energies close to the Rb K-edge in a wide temperature range, down to the temperature of the ferroelectric phase transition. In the paraelectric phase we observed a growth of integrated intensity for both forbidden reflections with temperature. This behavior is opposite to conventional non-resonant Bragg reflections, where the intensity decreases in accordance with the Debye-Waller factor. The developed theoretical model explains this effect with the thermal motion induced (TMI) scattering mechanism and also confirms the adiabatic approximation stating that electrons instantly follow the nuclei movements. In the 550 energy spectra, we have observed an additional contribution to the resonant structure factor which could be associated with the presence of transient Slater-type proton configurations (PC) in the half-filled hydrogen position.

A multinuclear NMR and quantum chemical study of solid trimethylammonium chloride

The solid salt, trimethylammonium chloride (TMAC), is investigated by a combination of NMR spectroscopic techniques and quantum chemical calculations. Chemical shift and nuclear quadrupolar interaction parameters have been measured for 35Cl, 1H/2H, and 15N/14N. These parameters have also been calculated as a function of the hydrogen position in the N···H···Cl fragment. Overall, the measured parameters are consistent with a structure in which the hydrogen is completely transferred to the nitrogen (i.e., N–H···Cl). The high hydrogen chemical shift (10.9 ppm by 2H CP/MAS) and relatively small deuterium quadrupolar coupling constant (127 kHz) indicate a moderately strong N–H···Cl hydrogen bond. A pronounced deuterium isotope effect on the 35Cl quadrupolar coupling constant is observed.

Development of the trappe force field for ammonia

The transferable potentials for phase equilibria (TraPPE) force field is extended through the development of a non-polarizable five-site ammonia model. In this model, the electrostatic interactions are represented by three positive partial charges placed at the hydrogen position and a compensating partial charge placed on an M site that is located on the C3 molecular axis and displaced from the nitrogen atom toward the hydrogen atoms. The repulsive and dispersive interactions are represented by placing a single Lennard–Jones site at the position of the nitrogen atom. Starting from the five-site model by Impey and Klein (Chem. Phys. Lett. 1984, 104, 579), this work optimizes the Lennard–Jones parameters and the magnitude of the partial charges for three values of the M site displacement. This parameterization is done by fitting to the vapor–liquid coexistence curve of neat ammonia. The accuracy of the three resulting models (differing in the displacement of the M site) is assessed through computation of the binary vapor–liquid equilibria with methane, the structure and the dielectric constant of liquid ammonia. The five-site model with an intermediate displacement of 0.08 Å for the M site yields a much better value for the dielectric constant, whereas differences in the other properties are quite small.

Determination of Site-Specific (Deuterium/Hydrogen) Ratios in Vanillin by 2H-NuclearMagnetic Resonance Spectrometry: Collaborative Study

The results of collaborative study are reported for a method that determines the site-specific isotope ratios of deuterium/hydrogen (D/H)i in vanillin by deuterium-nuclear magnetic resonance (2H-NMR) spectrometry. This method allows characterization of all the main commercial sources of commercial vanillin and detection of undeclared mixtures. It is based on the fact that the amounts of deuterium at various positions in the vanillin molecule are significantly different from one source to another. Vanillin is dissolved in acetonitrile and analyzed with a high-field NMR spectrometer fitted with a deuterium probe and a fluorine lock. The proportions of isotopomers monodeuterated at each hydrogen position of the molecule are recorded, and the corresponding (D/H) ratios are determined by using a calibrated reference. Nine laboratories analyzed 5 materials supplied as blind duplicates (1 natural vanillin from vanilla beans, 2 synthetic vanillins from guaiacol, 1 semisynthetic vanillin from lignin, and a mixture of natural and synthetic vanillins). The precision of the method for measuring site-specific ratios was as follows: for (D/H)1 the within-laboratory standard deviation (sr) values ranged from 2.2 to 5.8 ppm, and the among-laboratories standard deviation (sR) values ranged from 3.6 to 5.1 ppm; for (D/H)3 the sr values ranged from 1.7 to 3.2 ppm, and the sR values ranged from 2.4 to 3.7 ppm; for (D/H)4 the sr values ranged from 2.3 to 6.2 ppm, and the sR values ranged from 2.4 to 6.4 ppm; for (D/H)5 the sr values ranged from 0.8 to 2.7 ppm, and the sR values ranged from 0.9 to 2.3 ppm. It was shown that these values allow a satisfactory discrimination between vanillin sources. Therefore, the Study Director recommends the method for adoption as a First Action Official Method by AOAC INTERNATIONAL.

Temperature Dependence of 2H Nuclear Quadrupole Interaction in Very Short Hydrogen Bonds in Some Organic Acidic Salt Crystals

The 2H nuclear quadrupole interaction parameters, e2Qq/h and η, are closely related to the shape of the potential energy surface at hydrogen bonds and depend sensitively on their geometry. We measured the temperature dependence of the 2H NMR spectra of the crystalline acidic salts KDCO3, KD cetylenedicarboxylate, RbD acetylenedicarboxylate, and KD maleate, which contain very short O-D···O type hydrogen bonds. The temperature coefficient decreases with increase in the O···O distance in the hydrogen bond. Ab inito molecular orbital calculations of the electric field gradient tensor based on the temperature dependent structure of each crystal indicate that thermal expansion of the hydrogen bond geometry is not responsible for this tendency. Ab initio calculations also predict that a fictitious off center shift of the hydrogen position in a symmetric hydrogen bond causes very high e2 Qq/h values. This suggests that low energy vibrational excitation may be responsible for large positive d (e2 Qq/h)/dT values in symmetric hydrogen bonds.