Publications (126) View all
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Article: NMR study of disorder in BaTiO_ {3} and SrTiO_ {3}
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ABSTRACT: The observation of first order satellites in the NMR spectra of 47Ti and 49Ti in the cubic phases of BaTiO3 and SrTiO3 as well as 18O enriched SrTi18O3 demonstrates the presence of nonzero quadrupole coupling at the Ti sites in all these systems. This is incompatible with the central position of Ti in the oxygen octahedron and requires the presence of off-center Ti sites and dynamic disorder even in the paraelectric cubic phase. The Ti potential surface does not seem to change significantly at the cubic-tetragonal transitions. The pure 8-site “order-disorder” model however does not agree with the experiment. A good agreement with the experimental data is obtained if in addition to the dynamic Ti disorder the “displacive” soft mode induced deformation of the unit cell shape in the dynamic tetragonal nanodomains, existing in the cubic phase, is taken into account. The simultaneous presence of the Ti disorder and the soft mode leads to a special type of phase transition with combined “displacive” and “order-disorder” character.Physical Review B 02/2013; 71(6). · 3.69 Impact Factor -
Article: Tautomerism and possible polymorphism in solid hydroxypyridines and pyridones studied by 14N NQR.
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ABSTRACT: 14N nuclear quadrupole resonance frequencies have been measured in solid 2-pyridone, 3-hydroxypyridine and 4-pyridone by 1H-14N nuclear quadrupole double resonance. Two slightly nonequivalent nitrogen positions are observed in solid 3-hydroxypyridine, whereas only one nitrogen position has been observed in 2-pyridone and 4-pyridone within the experimental resolution. Rather low 14N quadrupole coupling constants in pyridones are the consequence of the delocalization of the nitrogen lone pair electrons in the aromatic rings. Two different compounds have been obtained by crystallization of 4-pyridone from ethanol in a normal and in a dry atmosphere. The compound obtained in the dry atmosphere is identical to the commercial sample. The compound obtained in the normal atmosphere cannot be converted to the commercial polymorph by melting. It is thus not a polymorph of anhydrous 4-pyridone. The temperature coefficient of the 14N quadrupole coupling constant is negative in 3-hydroxypyridine and positive in 2- and 4-pyridone. So in 3-hydroxypyridine molecular librations dominate the temperature variation of the quadrupole coupling constant whereas in in 2- and 4-pyridone the chang.The Journal of Physical Chemistry A 01/2013; · 2.95 Impact Factor -
Article: Phase Transition and Ring-puckering Motion in a Metal-Organic Perovskite [(CH(2))(3)NH(2)][Zn(HCOO)(3)].
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ABSTRACT: Phase transitions in a metal-organic perovskite with an azetidinium cation, which exhibits giant polarizability, were investigated using differential scanning calorimetry (DSC) and 1H 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 T1 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.The Journal of Physical Chemistry A 12/2012; · 2.95 Impact Factor -
Article: Nuclear quadrupole resonance characterization of carbamazepine cocrystals.
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ABSTRACT: 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.Solid State Nuclear Magnetic Resonance 09/2012; · 1.71 Impact Factor -
Article: Electron configuration and hydrogen-bonding pattern in several thymine and uracil analogues studied by 1H-14N NQDR and DFT/QTAIM.
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ABSTRACT: Some thio- and aza-derivatives of natural nucleobases uracil and thymine: 2-thiouracil, 4-thiouracil, 6-methyl-2-thiouracil, 6-azauracil, and 6-aza-2-thiothymine have been studied experimentally in solid state by (1)H-(14)N NMR-NQR double resonance (NQDR) and theoretically by the Density Functional Theory (DFT)/Quantum Theory of Atoms in Molecules (QTAIM). The (14)N resonance frequencies have been measured at 173 and 295 K and assigned to particular nitrogen sites (-N═ and -NH-). The temperature factor has been found negligible. The changes in the molecular skeletons, electric charge distribution, intermolecular interactions pattern, and molecular aggregations caused by oxygen replacement with sulfur and carbon replacement with nitrogen are discussed in detail. Correlations between all the principal components of the (14)N quadrupole coupling tensor have been found helpful in the search for the experimental (14)N NQR frequencies, their assignment to a particular nitrogen positions and estimation of the strength of the inter- and intramolecular interactions. The variation in the NQR parameters have been mainly related to the variation in the population of π-electron orbital. For thiouracil derivatives a general trend is that the stronger the hydrogen bond is, the lower is the asymmetry parameter, while for thymine and 6-aza-2-thiotymine, the opposite relation holds. Differences in correlations of the principal components of the (14)N quadrupole coupling tensor at the amino and iminonitrogen positions in heterocyclic rings are discussed. The effect of C→H and C→N substitution at the amino nitrogen position and C→N substitution at the iminonitrogen position on the quadrupole coupling tensor is analyzed. This study also demonstrates the advantages of combining NQR and DFT/QTAIM to predict an unsolved crystalline structure of 4-thiouracil.The Journal of Physical Chemistry B 07/2012; 116(30):8793-804. · 3.70 Impact Factor
