Daiyu Nakamura

Structural phase transition in various methylammonium hexahalometallates(IV) as studied by the NQR of halogens

Abstract The temperature dependence of halogen NQR frequencies was determined for various methylammonium hexahalometallates(IV): (MA)2MCI6; M=Se, Pd, So, Pt, and Ph, (MA)2MBrb6; M= Se, Pd, Sn, and Pt, and (MA)2PtI6. A single resonance line for each isotope in the chlorine and the bromine complexes and only one v1 line of iodine-127 for the iodine complex were observed at various temperatures studied, indicating that all halogen atoms are crystallographically equivalent in crystals. At liquid nitrogen temperature, both v1 and v2 lines of the iodine complex were observed. X-ray powder patterns taken at room temperature revealed that all of these complexes form crystals isomorphous with each other belonging to the space group R 3 ¯ m . The crystal structure describable as a rhombohedrally distorted K2PtCl6 type is consistent with the results of the present NQR measurements. All the complexes studied show an anomaly in the temperature variation of NQR frequencies below dry-ice temperature. The anomaly was attributed to a structural phase transition from the results of DTA. The transition temperatures determined are concentrated in a rather narrow range of temperature, and no remarkable size effect due to complex anions can be observed. A possible structure of these compounds in the low-temperature phase is deduced, and the mechanism by which the phase transition takes place is discussed.

Motion of methylammonium ions in methylammonium hexachloroplatinate(IV) and hexachlorostannate(IV) as studied by proton magnetic resonance

Abstract The proton spin—lattice relaxation time T 1 at the two frequencies of 20 and 60 MHz and the second moment of proton magnetic resonance were determined for methyl-ammonium hexachloroplatinate(IV), its partially deuterated derivatives, and methylammonium hexachlorostannate(IV) at various temperatures between 4.2 and 400 K. The fully protonated complexes showed narrow PMR signals having the second moment of about 7 G 2 even at 77 K, indicating that the CH 3 and NH 3 groups in methylammonium ions undergo rapid reorientation about the triad axis of the cation. The partially deuterated analogs, (CH 3 ND 3 ) 2 PtCl 6 and (CD 3 NH 3 ) 2 PtCl 6 gave rise to a single T 1 minimum at nearly the same temperature attributable to the reorientation of the cations about their respective trial axes. On the other hand, the fully protonated complexes yielded two T 1 minima, one of which, appearing at a lower temperature, was deep while the other, appearing below the transition point, was shallow. The deep minimum could be assigned to the reorientation of the cation as a whole about its triad axis, and the shallow minimum to the individual reorientations of the CH 3 and NH 3 groups in the cation. The potential barrier for the internal reorientation of the methylammonium ion could be evaluated to be about 8 kJ mol −1 by use of Woessners method for the analysis of T 1 data.

35Cl NQR and 1H NMR studies of [C(NH2)3]AuCl4, [C(NH2)3]2PtCl4, and [C(NH2)3]2PdCl4

Abstract The temperature dependence measurements of 35 Cl NQR frequencies and 1 H NMR spin-lattice relaxation time T 1 were carried out for guanidinium tetrachloro-aurate(III), -platinate(II), and -palladate(II). The gold(III) complex showed four NQR lines at various temperatures between 77 and 344 K, while the platinum-(II) and palladium(II) complexes gave two NQR lines in the temperature ranges 77–169 K and 77–220 K, respectively. An unusual phase transition was located at 363 K for the gold(III) complex. The high-temperature phase was easily supercooled. All the complexes studied yielded a T 1 minimum attributable to the reorientation of the planar cation about its C 3 axis. The motional parameters were evaluated. The Zeeman-quadrupole cross relaxation between protons and chlorine nuclei was observed for the platinum(II) and palladium(II) complexes at various temperatures below room temperature, while it was also detected for the high-temperature phase of the gold(III) complex.

Phase transitions in some hexahalostannates(IV) as revealed by the pure quadrupole resonance of halogens

Abstract The quadrupole resonance of halogens in potassium hexachlorostannate(IV), potassium hexabromostannate(IV), ammonium hexachlorostannate(IV), ammonium( d 4 ) hexachlorostannate(IV), and ammonium hexabromostannate(IV) was observed over a wide range of temperature, and transition points were located. The nature of the phase transitions is discussed from the number of resonance lines, the continuity or discontinuity of resonance frequencies at the transition points, and the temperature coefficient of resonance frequencies.

Nuclear Quadrupole Resonance and Its Application in Inorganic Chemistry

Publisher Summary This chapter discusses pure quadrupole resonance or nuclear quadrupole resonance (NQR) that has been applied to a number of organic as well as inorganic compounds and has yielded valuable information on the electronic structure of chemical bonds. Inorganic compounds having a simple chemical formula are not necessarily simple from the standpoint of nuclear quadrupole spectroscopy. On the other hand, some metal complex compounds are amenable to this method of attack, because to the first approximation one can discuss the electronic state of a single complex ion. The first experimental observation of the NQR absorption of coordination compounds was reported on potassium tetraiodomercurate. The chapter discusses the nature of metal-ligand bonds in metal complexes, use is often made of rather vague terms such as covalent character, ionic character, and the hybrid of these to explain various properties of complex compounds such as, for instance, spectroscopic data, magnetic behavior. However, in order to establish these concepts in a quantitative manner and to express the transition from an ideal covalent bond to a pure ionic bond, the extent of covalent character is defined by resorting to some experimental parameters that are characteristic of metal-ligand bonds and are measurable accurately.

Novel ionic plastic phase of [(CH3)4N]SCN obtainable above 455 K studied by proton magnetic resonance, electrical conductivity and thermal measurements

A new high-temperature solid phase of tetramethylammonium thiocyanate has been found above the phase transition at 455 K, where a large entropy change (15 J K–1 mol–1) was observed. X-ray powder patterns recorded at ca. 470 K showed that this phase forms a CsCl-type cubic lattice with aO= 600 pm, indicating that the SCN– anions take highly disordered orientations. The presence of rapid three-dimensional self-diffusion, as well as the overall rotation of the cations in this phase, was revealed by the measurements of 1H NMR spin–lattice relaxation times, T1 and T1ρ, the second moment of NMR absorptions and the electrical conductivity. The large ionic-diffusion constant (of the order of 10–13 m2 s–1) obtained around 490 K is roughly the same as those reported for ‘ionic plastic’ crystals. An activation energy of ca. 110 kJ mol–1 was determined for cationic self-diffusion. These results imply that this new phase is the first example of an ionic plastic crystal containing tetramethylammonium cations.

Ionic dynamics in the rotator phase of n-alkylammonium chlorides (C6–C10), studied by 1H nuclear magnetic resonance, electrical conductivity and thermal measurements

1 H NMR, electrical conductivity and thermal measurements have been performed on the rotator phase of n-alkylammonium chlorides CnH2n+1 NH3Cl (n= 6–10). Rapid two-dimensional self-diffusion and uniaxial rotation of the long rod-like cations were observed in all salts studied by measuring 1H NMR spin–lattice relaxation times T1 and T1ρ, indicating the highly disordered structure of the rotator phase. Marked ionic self-diffusion was detected in this phase by measuring a.c. electrical conductivity which increased to ca. 10–2 S m–1 near the respective melting temperatures. Analysing the NMR relaxation and conductivity data, it was found that the anions also diffuse in this phase to a similar extent to the cations. From the characteristic dynamic behaviour in this phase, together with the small melting entropies (<20 J K–1 mol–1) reported, the rotator phase can be considered to be a low-dimensional plastic crystal or an intermediate state between the plastic crystal and smectic liquid crystal.

Phase transitions in rhombohedral (CH3NH3)2MCl6 crystals as revealed by the NQR of chlorine

Abstract In order to obtain some knowledge on the dynamical properties of (CH3NH3) 2 PtCl 6 and (CH 3 NH 3 ) 2 SnCl 6 , the temperature dependence of chlorine NQR and PMR was observed over a wide range of temperature. Both complexes gave rise to a single chlorine resonance line (26.256 and 15.815 MHz, respectively, at 300 K) at various temperatures investigated, indicating that all chlorine atoms in crystals are crystal lographically equivalent. Although chlorine frequencies v ( T ) decreased continuously with increasing temperature, dv ( T )/ dT varied drastically at 125 K for the former compound and at 156 K for the latter, indicating the occurrence of phase transitions, which were confirmed by the measurement of DTA. However, no changes were observed in the second moment of PMR absorption curves above and below the transition points. The analysis of the results indicated that both -CH 3 and −NH 3 groups rotate about their C 3 axes even in the low-temperature phase.

Vibrational spectra of water molecules in some transition metal dichloride dihydrates

Abstract The i.r. spectra of MCl2.2H2O (M = Co, Fe, Mn) and partially and almost completely deuterated derivatives thereof were recorded at liquid nitrogen and room temperature in a low wavenumber region covering 700-200 cm−1. Librational bands were assigned unambiguously to rocking and wagging modes by taking advantage of a theoretical conclusion that the isotopic frequency ratio, v H 2 O v HDO (but not v H 2 O v D 2 O , is considerably different between the rocking mode and the wagging mode. The force constant of rocking is one order of magnitude greater than that of wagging. The result is explained by means of a simple point-charge model.

Cationic Dynamics and Phase Transitions in (CH3NH3)3Bi2Br9 as Studied by Proton Magnetic Resonance

Molecular motions and phase transitions in tris(methylammonium) nonabromodibismuthate (III), (CH 3 NH 3 ) 3 Bi 2 Br 9 , were investigated by measuring the temperature dependences of static and rotating frame 1 H spin-lattice relaxation times ( T 1 ans T 1ρ ) and the second moment ( M 2 ) of 1 H NMR absorptions. Near 80 K in the lowest-temperature phase IV, the orientation of the methylammonium cation is almost fixed but the cation itself is rapidly rotating about the C-N bond axis. On heating, gradual disordering of the cationic orientations takes place in phases IV and III for two thirds of crystallographically nonequivalent cations and in phase II for the remaining cations. Finally, all the cations become to be dynamically disordered and undergo rapid overall reorientations in the room-temperature phase I. The IV-III transition at 108 K may be connected to dynamics of the inorganic (Bi 2 Br 9 3- ) n subsystem rather than the ionic motions. An anomalous behavior of 1 H T 1ρ , attributable to cross relaxa...