Structural Phase Transitions and Ionic Motions in Pyridinium Hexachlorotellurate(IV),Hexachlorostannate(IV), and Hexabromostannate(IV) Crystals as Studied by ¹H NMR

Abstract

in P y rid in iu m H e x a c h lo r o te llu r a te ( I V ) , H e x a c h lo r o s ta n n a te ( I V ) , a n d H e x a b r o m o s ta n n a te ( I V ) C r y s ta ls a s S tu d ie d b y N M R Yutaka Tai, Tetsuo Asaji, Ryuichi Ikeda*, and Daiyu Nakamura Department of Chemistry, Faculty of Science, Nagoya University, Chikusa, Nagoya, Japan Z. Naturforsch. 44 a, 300-306 (1989); received January 21, 1989The 'H NMR second moment M2 and the spin-lattice relaxation time T, are determined for pyridinium hexachlorotellurate(IV), hexachlorostannate(IV), and hexabromostannate(IV) at various temperatures above ca. 140 K. The phase transition temperatures already reported from halogen NQR experiments are determined as 272, 331, and 285 K, respectively, by differential thermal analysis (DTA). The DTA as well as differential scanning calorimetry measurements show that the above phase transitions are of second-order. For pyridinium hexachlorotellurate(IV) and hexabromostannate(I V), a sharp 'H T, dip was observed at the transition temperature. This is interpreted in terms of a phenomenon related to the critical fluctuation of an order parameter. From the measurements of 'H M2, 60° two-site jumps (60c flips) around the pseudo C6 axis of the cation are suggested to occur in the high temperature phases of the complexes. Modulation of X...'H (X = CI, Br) magnetic dipolar interactions due to the reorientational motion of the complex anions is considered as a possible relaxation mechanism in the high temperature phases. S t r u c t u r a l P h a s e T ra n s itio n s a n d Io n ic M o tio n s Introduction Recently, in the crystals of pyridinium hexachlorometallates(IV), (C5H6N)2MC16 (M = Sn, Te, Pb, Pt) [1] and hexabromostannate(IV), (C5H6N)2SnBr6 [2] (hereafter C5H6N + is abbreviated to pyH+) a struc tural phase transition was found from the temper ature dependence of halogen NQR frequencies. The 'H NMR spin-lattice relaxation time, T{, determined for (pyH)2TeCl6 as a function of temperature shows a sharp dip at the transition temperature, Tc ( = 273 K) [3]. This has been explained as relaxation modu lated by the anion reorientation through dipolar re laxation of the second kind [4], A previous powder X-ray diffraction study [1] has shown that the crystals of (pyH)2MCl6 (M = Sn, Te) in their high temperature phases are isomorphous. The accurate X-ray analysis of these complexes has re vealed that the tin complex forms triclinic crystals with the space group PI (Z = 1) in the low tempera ture phase [1], while the high temperature phase of the tellurium complex is monoclinic, belonging to the space group B2/m with Z = 2. Accurate structural data have not been reported as yet for the high tem * Institute for Molecular Science, Myodaiji, Okazaki 444, Japan. Reprint requests to Prof. Dr. D. Nakamura. Nagoya Univer sity, Faculty of Science. Nagoya 464-01, Japan. perature phase of the former complex and the low temperature phase of the latter. From a structural analysis of the high temperature phase of (pyH)2TeCl6, the most probable orienta...

Calorimetric and single crystal X-ray study of the phase transition of (PyH)2PdCl4

“…Displacement ellipsoids are drawn at the 50% probability level. Table 2 Fractional atomic coordinates and displacement parameters (Å 2 ) for the low-temperature phase of (C 5 H 6 N) 2 (15) to explain the temperature dependence of M 2 by assuming a three-wells potential model with minima at G308 rotated positions instead of G608 rotated positions. The M 2 reduction factor, R M 2 Z hM motion 2 i p =hM rigid 2 i p , was observed to be about 0.7 at T c Z241 K as shown in Fig.…”

“…The potential well for the pyridinium reorientation in the low-symmetric crystal phase, has been discussed by the model of reorientation between inequivalent potential wells [12,[14][15][16][17][18]. Kozak et al showed by NMR study that the energy difference, which characterizes the inequivalence of the wells, is a function of temperature in pyridinium hexafluorophosphate [16].…”

Chlorine Nuclear Spin-Spin Relaxation Mechanism in Mg(H2O)6SnCl6 as Studied by NQR and NMR Techniques

“…Recently, we have studied the dynamics of molecular ions in pyridinium tetrachloroaurate(III) and pyridinium hexachlorometallates(IV) by chlorine NQR and proton NMR experiments [4][5][6][7], In these complexes, an unusual relaxation mechanism was found on measuring the 35 C1 NQR spin-lattice relaxation time T 1Q and the NMR spin-lattice relaxation time Ti. That is, it was shown that the relaxation mechanisms of the chlorine and hydrogen nuclei are fluctuations of the electric field gradient (EFG) due to cationic motions and fluctuations of the local magnetic field due to anionic motions, respectively.…”

Structural Phase Transitions and Cationic Motions in Pvridinium Dichloroiodate(I) as Studied by 1H NMR, Differential Thermal Analysis, and Powder X-Ray Diffraction

“…Reorientation of ions in crystals has extensively been studied by *H NMR and NQR experiments for the pyridinium cation (pyH + ) in (pyH)X (X = C1, Br, I) [1][2][3], (pyH)AuX4 (X = C1, Br) [4,5], and (pyH)2MX6 (M =Te, Sn; X = C1, Br) [6][7][8]. In these crystals, except for (pyH)AuX4, structural phase tran sitions have been detected and discussed [1][2][3][6][7][8],…”

“…In these crystals, except for (pyH)AuX4, structural phase tran sitions have been detected and discussed [1][2][3][6][7][8],…”