Molecular Motions and Phase Transitions in Plastic Closocarboranes

AbstractThe temperature dependence of 35C1, 81Br, and 127I NQR frequencies and 1Hspin-lattice relaxation times (T1) for CH3NH3PbX3 (X = Cl, Br, I) was measured through the successive phase transitions in these solids. The isotropic reorientation of the CH3NH3 ions takes place in the higher-temperature phases (tetragonal [I4/mcm] and cubic) of the three salts (Ea= 11 kJ mol -1). T1's in the lowest-temperature phases (orthorhombic) indicate that the cations undergo correlated C3-reorientation in the chloride (Ea = 5.45 kJ mol -1)and in the iodide (Ea = 5.80 kJ mol -1, whereas correlated (Ea = 2.40 kJ mol-1) and uncorrected (Ea = 7.50 kJ mol-1) C3- reorientations are excited in the bromide. It is also revealed that the rotational tunneling of the cations governs T1 at lowtemperature region in the orthorhombic phases of these salts

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NMR Study of Molecular Motions and Phase Transitions in Cyclohexene

The Journal of Chemical Physics ◽

10.1063/1.1671569 ◽

1969 ◽

Vol 50 (8) ◽

pp. 3462-3466 ◽

Cited By ~ 8

Author(s):

Z. M. El Saffar ◽

R. G. Eades ◽

J. P. Llewellyn

Keyword(s):

Phase Transitions ◽

Molecular Motions ◽

Nmr Study

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Studies of Successive Phase Transitions and Molecular Motions in [Mg(H2O)6][SiF6] by 1,2H and 19F NMR

Zeitschrift für Naturforschung A ◽

10.1515/zna-1998-6-728 ◽

1998 ◽

Vol 53 (6-7) ◽

pp. 453-458 ◽

Cited By ~ 7

Author(s):

Junko Kimura ◽

Takeshi Fukase ◽

Motohiro Mizuno ◽

Masahiko Suhara

Keyword(s):

Phase Transitions ◽

Nmr Spectra ◽

Quadrupole Coupling Constant ◽

Coupling Constant ◽

Molecular Motions ◽

Exponential Factor ◽

H Nmr ◽

Quadrupole Coupling ◽

Constant E ◽

Successive Phase

Abstract The successive phase transitions of [Mg(H2O)6][SiF6] were studied by measuring 2H NMR spectra. The quadrupole coupling constant e2Qq/h and asymmetry parameter η changed drastically at each transition temperature. 1,2H and 19F NMR Tl were measured for this compound to study the relation between the molecular motions and the successive phase transitions. The activation energy Ea and the pre-exponential factor τ0 for the reorientation of [SiF6]2- were estimated as 28 kJmol-1 and 6.0 x 10-14 s, and those of the 180° flip of H2O as 33 kJmol-1 and 4.0x 10-14 s. These two motions occur rapidly even in phase V. For the reorientation of [Mg(H2O)6]2+ , Ea = 62 kJmol-1 and τ0 = 1.1 x 10-16 s were obtained from the simulation of 2H NMR spectra. The jump rate of this motion is of the order of 104 -106 s-1 in phase II. These results suggest that the successive phase transitions are closely related to the motion of [Mg(H2O)6]2+ .

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