Veselko Žagar

17O NQR study of the antiferroelectric phase transition in TlH2PO4

The temperature dependence of the 17O NQR spectra in TlH2PO4 has been measured using a proton‐17 O nuclear quadrupole double resonance technique. The results show that the protons in the short one‐dimensionally linked O1–H1‐‐O1 and O2–H2‐‐O2 hydrogen bonds are moving between two equilibrium sites above Tc and freeze into one of the off‐center sites below Tc. The protons in the asymmetric O3–H3‐‐O4 hydrogen bonds are static and ordered above and below Tc.

Hydrogen bonding in 1,2-diazine–chloranilic acid (2 : 1) studied by a 14N nuclear quadrupole coupling tensor and multi-temperature X-ray diffraction

Protons involved in the H-bond system in 1,2-diazine-chloranilic acid (2 : 1) are assumed to be in jumping motion in the double-minimum potential corresponding to the two extreme electronic states of O-H...N and O-...H-N+. 14N nuclear quadrupole coupling constants were determined by 1H-14N nuclear quadrupole double resonance. Assuming that the observed coupling constants are result of a fast exchange of the two extreme electronic states, the coupling constants for each state were estimated by use of the equilibrium populations of the two extreme states determined from multi-temperature X-ray single-crystal diffraction. It was suggested that not only the population but also the electron distribution of the extreme electronic states itself changes with temperature.

Phase transition and temperature dependent electronic state of an organic ferroelectric, phenazine–chloranilic acid (1:1)

The isotope effect of hydrogen motion in an organic ferroelectric, phenazine (Phz)–chloranilic acid (H2ca and D2ca for normal and deuterated compounds, respectively) co-crystal, was studied by 35Cl nuclear quadrupole resonance (NQR). Besides a ferroelectric transition at Tc = 253 K (303 K), a neutral-to-ionic transition was found below 170 K (200 K) for Phz–H2ca (Phz–D2ca). 1H–14N nuclear quadrupole double resonance measurements were also made in order to study the temperature dependent electronic state of Phz–(H/D)2ca. 14N NQR parameters suggested that donor orbital populations of the two nitrogen atoms in a phenazine molecule become nonequivalent (1.78 and 1.97) in the ferroelectric phase, while they are both equal to 1.89 in the paraelectric phase. In the ionic phase of Phz–D2ca, which was obtained by cooling below 188 K, they became 1.50 and 1.95, suggesting a proton transfer from D2ca to Phz.

1H -14N Nuclear Quadrupole Double Resonance with Multiple Frequency Sweeps

Abstract A new highly sensitive 1H -14N nuclear quadrupole double resonance technique is presented which is based on magnetic field cycling and on the application of multiple frequency sweeps of an r.f. magnetic field. The sensitivity and the resolution o f the new technique are estimated. Some experimental results obtained by the new technique are presented.

Nuclear quadrupole resonance characterization of carbamazepine cocrystals.

Nuclear quadrupole resonance (NQR) is used as a method for the characterization of cocrystals and crystal polymorphs. (14)N NQR spectra of several cocrystals of carbamazepine have been measured together with the (14)N NQR spectra of cocrystal formers. The results show that the (14)N NQR spectrum of a cocrystal and the (14)N NQR spectra of cocrystal formers differ well outside the experimental resolution. It is further described how the NQDR techniques, that have been used to measure the (14)N NQR frequencies, can be used to check the homogeneity of a polycrystalline sample and to monitor the stability of a metastable crystal polymorph.

14N and 35Cl double resonance study of the phase transition in the intercalated bilayer compound C10H21NH3Cl

The pure nuclear quadrupole (NQR) spectra of 14N, 35Cl, and 37Cl have been measured in C10H21NH3Cl as a function of temperature. The chlorine NQR frequency νQ (35Cl)=1215 kHz at 47 °C is determined by the N–H––Cl hydrogen bonds and is the lowest reported so far. The 14N quadrupole coupling data e2qQ/h=760 kHz. η=0 shows that the C–N–H––Cl backbone is practically rigid at this temperature. Above 53 °C the effects of the phase transition from an intercalated to a nonintercalated structure show up in the N–H––Cl backbone as a sharp drop of the NQR frequencies to ∼60% of their rigid lattice values. A model accounting for this effect has been proposed.

Conformations and intermolecular interactions pattern in solid chloroxylenol and triclosan (API of anti-infective agents and drugs). A 35Cl NQR, 1H-14 N NQDR, X-ray and DFT/QTAIM study

Two antibacterial and antifungal agents, chloroxylenol (4‐chloro‐3,5‐dimethyl‐phenol) and triclosan (5‐chloro‐2‐(2’,4’‐dichlorophenoxy)‐phenol), were studied experimentally in solid state with an X‐ray, 35Cl‐nuclear quadrupole resonance (NQR) and 17O‐nuclear quadrupole double resonance (NQDR) spectroscopies and, theoretically, with the density functional theory/quantum theory of atoms in molecules (DFT/QTAIM). The crystallographic structure of triclosan, which crystallises in space group P31 with one molecule in the asymmetric unit [a = 12.64100(10), b = 12.64100(10), c = 6.71630(10) Å], was solved with an X‐ray and refined to a final R‐factor of 2.81% at room temperature. The NQR frequencies of 35Cl and 17O were detected with the help of the density functional theory (DFT) assigned to particular chlorine and oxygen sites in the molecules of both compounds.

Phase Transition and Ring-Puckering Motion in a Metal–Organic Perovskite [(CH2)3NH2][Zn(HCOO)3]

Phase transitions in a metal-organic perovskite with an azetidinium cation, which exhibits giant polarizability, were investigated using differential scanning calorimetry (DSC) and (1)H nuclear magnetic resonance (NMR) measurements. The DSC results indicated successive phase transitions at 254 and 299 K. The temperature dependence of the spin-lattice relaxation time T(1) determined by NMR indicated that the activation energy for cation ring-puckering motion was 25 kJ mol(-1) in phase I (T > 299 K). The potential energy at the transition state of puckering is expected to decrease when the potential for the motion becomes asymmetric with decreasing temperature in phases II and III. A possible mechanism for the onset of an extraordinarily large dielectric anomaly is discussed.

14N NQR, 1H NMR and DFT/QTAIM study of hydrogen bonding and polymorphism in selected solid 1,3,4-thiadiazole derivatives

The 1,3,4-thiadiazole derivatives (2-amino-1,3,4-thiadiazole, acetazolamide, sulfamethizole) have been studied experimentally in the solid state by (1)H-(14)N NQDR spectroscopy and theoretically by Density Functional Theory (DFT). The specific pattern of the intra and intermolecular interactions in 1,3,4-thiadiazole derivatives is described within the QTAIM (Quantum Theory of Atoms in Molecules)/DFT formalism. The results obtained in this work suggest that considerable differences in the NQR parameters permit differentiation even between specific pure association polymorphic forms and indicate that the stronger hydrogen bonds are accompanied by the larger η and smaller ν(-) and e(2)Qq/h values. The degree of π-electron delocalization within the 1,3,4-thiadiazole ring and hydrogen bonds is a result of the interplay between the substituents and can be easily observed as a change in NQR parameters at N atoms. In the absence of X-ray data NQR parameters can clarify the details of crystallographic structure revealing information on intermolecular interactions.

Supramolecular synthon pattern in solid clioquinol and cloxiquine (APIs of antibacterial, antifungal, antiaging and antituberculosis drugs) studied by 35Cl NQR, 1H-17O and 1H-14N NQDR and DFT/QTAIM

AbstractThe quinolinol derivatives clioquinol (5-chloro-7-iodo-8-quinolinol, Quinoform) and cloxiquine (5-chloro-8-quinolinol) were studied experimentally in the solid state via 35Cl NQR, 1H-17O and 1H-14N NQDR spectroscopies, and theoretically by density functional theory (DFT). The supramolecular synthon pattern of O–H···N hydrogen bonds linking dimers and π–π stacking interactions were described within the QTAIM (quantum theory of atoms in molecules) /DFT (density functional theory) formalism. Both proton donor and acceptor sites in O–H···N bonds were characterized using 1H-17O and 1H-14N NQDR spectroscopies and QTAIM. The possibility of the existence of O–H···H–O dihydrogen bonds was excluded. The weak intermolecular interactions in the crystals of clioquinol and cloxiquine were detected and examined. The results obtained in this work suggest that considerable differences in the NQR parameters for the planar and twisted supramolecular synthons permit differentiation between specific polymorphic forms, and indicate that the more planar supramolecular synthons are accompanied by a greater number of weaker hydrogen bonds linking them and stronger π···π stacking interactions. FigureThe 1H-14N solid effect double resonance spectra and the relief map of the Laplacian of electron density in the OH...N plane for clioquinol and cloxiquine