V. G. Krishnan

NQR and X-ray Studies of [N(CH3)4]3M2X9 and (CH3NH3)3M2X9 (M = Sb,Bi; X = Cl,Br)

Abstract 35Cl, 81Br, 121Sb, and 209Bi NQR of the title compounds was observed. According to the results of the temperature dependences of NQR frequencies and the DTA measurements, phase transitions take place in [N(CH3)4]3Bi2Br9 (Ttr=183K), [N(CH3)4]3Bi2Cl9 (Ttr = 155K), and (CH3NH3)3Bi2Cl3 (Ttr = 200 and 249 K). 2D NMR spectra for partially deuterated (CH3ND3)3Bi,Br9 showed that the phase transitions in this compound are related to the motion of the methylammonium cations. Single-crystal X-ray work at room temperature shows that the space group for [N(CH3)4]3Sb2Cl9 is P63/mmc with a = 925.1 pm, c = 2173.4 pm, Z = 2. For (CH3NH3 ) 3Sb2Br9 the space group is P3ml with a = 818.8 pm, c = 992.7 pm, Z = 1; in both cases the cations show dynamical disorder. The Rietveld analysis of the powder X-ray diffraction for (CH3NH3)3Bi2Br9 reveals the space group P3ml with a = 821.0, c - 1000.4 pm, Z = 1 at room temperature; the compound is isomorphous with (CH3NH3 )3Sb2Br9 . The crystal symmetries of the low-temperature phases of (CH3NH3)3Bi2Br9 and [N(CH3)4]3Bi2Br9 were deduced from the results of the NQR spectroscopy

Bromine NQR and Crystal Structures of Tetraanilinium Decabromotricadmate and 4-Methylpyridinium Tribromocadmate

Abstract The 79,81Br NQR spectra of tetraanilinium decabromotricadmate (1) and 4-methylpyridinium tribromocadmate (2 ) were studied as function of temperature and their crystal structures were determined. (C6H5NH3)4Cd3Br10 (1): Space group D1 52h - Pbca , Z = 4, a = 2507.8(7) pm, b = 1985.4(5)pm, c = 763.0(2)pm. Characteristic for the structure are trioctahedral units [Cd3Br10] condensed to planes. Within the units the octahedra are face connected and further condensed to planes via common corners. Two types of hydrogen bonds are observed. The 81Br NQR lines with frequencies (MHz, 298 K) of 62.98, 52.59, 43.39, 41.82, and 40.71 are little temperature dependent with positive and negative coefficients. The wide frequency range of the NQR lines is reflected by the wide range of the intraionic distances, 263 ≤ d(Br-Cd)/pm ≤300. (4-(CH3)C5H4NH)CdBr3 (2): C52h - P21/n, Z = 4, a = 1228.8(5}pm, b= 1168.5(5)pm, c = 758.3(3)pm, β = 95.30(1)°; the CdBr⊖3 ions are condensed to chains. The 81Br NQR spectrum is a triplet with frequencies (MHz, 298 K) of 66.01, 55.39, and 50.75. The temperature dependence is small, with positive and negative temperature coefficients. The distances d(Cd-Br) are 256 pm (Cd-Br(2)), 261 pm (Cd -Br(1)), and 284 pm (Cd-Br(3)) and in the chain [CdBr3]Br(1) and Br(3) are bridging atoms. The relations between Br-NQR and crystal structures are discussed.

Phase transition in para-cyanoanilinium iodide

Abstract 79,81 Br and 127 I NQR spectra of para-cyanoanilinium bromide, 4-CNC 6 H 4 NH ⊕ 3 Br ⊝ , and para-cyanoanilinium iodide, 4-CNC 6 H 4 NH ⊕ 3 I ⊝ , respectively, have been investigated in the temperature range 77 K − 390 K. In para-cyano-anilinium iodide a phase transition has been found to occur at 99.5 K ± 1.0 K. The transition is characterized by a sudden change in the slope ν( 127 I)=f(T), and by a corresponding change in the nuclear quadrupole coupling constant. The asymmetry parameter η, which has a value of zero in the high temperature phase I, shows a small but finite increase below the transition. The nature of the phase transition in para-cyanoanilinium iodide is found to be similar to that taking place in anilinium iodide as well as in para-fluoro- and para-chloroanilinium iodide. In addition, the nuclear quadrupole coupling constant, η, and the transition temperature for para-cyanoanilinium iodide and para-chloroanilinium iodide are remarkably similar. Results for the temperature dependent studies on para-cyanoanilinium bromide are also presented.

79,81Br NQR nuclear quadrupole resonance study of the hexabromocadmate (II)-ion, [CdBr6]4−

Abstract The 79,81 Br NQR frequencies of coordination compounds A 4 CdBr 6 , A = NH 4 , ND 4 , and Rb, and of M 2 CdBr 6 , M = Mg(H 2 O) 6 , were studied as a function of temperature. ν( 79 Br) = f(T) of the ammonium compound passes a maximum which is considerably influenced in temperature and frequency by the exchange H → D. A positive temperature coefficient is observed for ν( 81 Br) in (Mg(H 2 O) 6 ) 2 CdBr 6 throughout the whole range of T studied. This behaviour is discussed as arising from the dynamic of the ions NH 4 and Mg(H 2 O) 6 , respectively, and the influence of the hydrogen bonds NH…Br and OH…Br on the electric field gradient at the bromine sites.

Tetraguanidium Hexabromocadmate, [C(NH2)3]4[CdBr6]. Crystal Structure and Bromine Nuclear Quadrupole Resonance

Abstract The 79, 81Br NQR spectra of tetraguanidinium hexabromocadmate, [C(NH2)3]4[CdBr6] have been studied as a function of temperature from 77 K to 390 K and the crystal structure of the compound was determined at room temperature. The title compound crystallizes monoclinic, P21/c, with four formula units in the unit cell, a = 839.2(3) pm, b = 1895.8(6) pm, c= 1527.4(5) pm, β= 108.14(1)°. The anion [CdBr6]4⊖ is an isolated octahedron, with bond lengths 275≤d(Cd-Br)/pm ≤ 281, and bond angles 88 ≤(Br-Cd-Br)/° ≤95, slightly distorted by hydrogen bonds N -H ··· Br. The 81Br NQR sextet, in dependence from temperature, shows positive and negative temperature coefficients. At 77 K the 81Br NQR frequencies have been found between 42.42 MHz and 31.99 MHz; the 79Br NQR at the frequencies expected from the nuclear quadrupole moment ratio Q(79Br)/Q(81Br). Relations between the 81Br NQR spectrum, the crystal structure, and the hydrogen bonds are discussed.

Structure and Bonding of Tribromocadmates, ACdBr3, A = NH4, Rb, Cs, CH3NH3, (CH3)2NH2, (CH3)4N], [H2NNH3, and (H2N)3C. An X-ray Diffraction and 79-81Br NQR Study

Abstract The 79-81Br NQR spectra of tribromocadmates with the cations K⊕, NH4⊕, Rb⊕, Cs⊕, CH3NH3⊕, (CH3)2NH2⊕, (CH3)4N⊕, H2NNH3⊕, and C(NH2)3⊕ were studied as functions of temperature from 77 K on up to T>300 K. CsCdBr3 shows a singlet 81Br NQR spectrum over the whole temperature range studied. [CH3NH3]CdBr3, with one 81Br NQR line spectrum at room temperature, experiences a phase transition at 167 K; below this temperature an 18-line spectrum is observed. In [(CH3)4N]CdBr3 (phase II), at 290 K, a singlet 81Br NQR is present as is in the high temperature phase III (TII.1 , = 390 K); the low temperature phase III (TII,m, = 160 K has a triplet 81Br NQR spectrum. KCdBr3 shows an 81Br NQR doublet spectrum, as do RbCdBr3, [H2NNH3]CdBr3, and [C(NH2)3]CdBr3. 81Br NQR triplets are observed for [(CH3)2NH2]CdBr3 and NH4CdBr3. Several crystal structures were determined (at room temperature). [(CH3)4N]CdBr3: P63/m, Z = 2, a - 940 pm, c = 700 pm, disordered cation, single chain Perovskite with face connected [CdBr6]- octahedra (nearly CsNiCl3-type). [(CH3)2NH2]CdBr3: P21/c, Z = 4, a = 898 pm, 6 = 1377 pm, c = 698 pm, ß = 91.2°, face connected [CdBr3-octahedra single chain Perovskite. NH4CdBr3: Pnma, Z = 4, a = 950 pm, b = 417 pm, c= 1557 pm, with a double chain of condensed [CdBr6]-octahedra, NH4CdCl3-type. [N2H5]CdBr3: P2,/c, Z = 4, a = 395 pm, 6 = 1749 pm,c = 997 pm,ß = 94.2°, double chain polyanion similar to NH4CdBr3. [C(NH2)3]CdBr3: C2/c, Z = 4, a = 778 pm, 6 = 1598 pm, c = 746 pm, ß = 110.2°, a single chain Perovskite with a chain of condensed trigonal bipyramids [CdBr5]. Three types of anion chains of CdBr3 have been observed: Single octahedral chains, face connected; double octahedral chains, edge connected; a trigonal-bipyramidal chain, edge connected. The relation between the crystal structure and the Br NQR is discussed

Isolated versus Condensed Anion Structure VI: X-ray Structure Analysis and 81Br NQR of Guanidinium Pentabromodicadmate(II), [C(NH2)3]Cd2 Br5 , tris-Hydrazinium Pentabromocadmate(II), [H2NNH3]3 CdBr5 , and bis-Hydrazinium Tetrabromocadmate(II)-Tetra Hydrate, [H2NNH3]2CdBr4-4H20

The structure of the condensed bromocadmate anions in [C(NH2)3]Cd2Br5 (1) and [H2NNH3]3CdBr5 (2) were studied at room temperature by X-ray diffraction. (1) crystallizes with double-chains bridged by Br atoms (orthorhombic, Pmmn, Z= 2, a = 1394.0(5), b = 394.5(1), c = 1086.9(5) pm). This chain structure was not described previously. (1) shows three 81Br NQR lines at temperatures between 77 and 323 K. Structural phase transitions take place at 283 K and at 535 K. (2) crystallizes with Br bridged zigzag-chains (monoclinic, P21, Z=2,a = 943.1(1), b = 778.8(2), c = 942.0(2) pm, β = 102.10(2)°) and shows a first-order phase transition around 304 K with a large thermal hysteresis. Below the transition point five 81Br NQR lines are observed at temperatures between 122 and 304 K, and above the transition point four 81 Br NQR lines at tem-peratures between 288 and 353 K. Two 81Br NQR lines are observed in [H2NNH3]2CdBr4 -4H20 (3) at temperatures between 77 and around 320 K with positive temperature coefficients

Isolated versus Condensed Anion Structure: the Influence of the Cation Size and Hydrogen Bond on Structure and Phase Transition in MX42- Complex Salts. 2,2-Dimethyl-1,3-propanediammonium Tetrabromocadmate(II) Monohydrate, Dimethylammonium Tetrabromozincate(II), and Dimethylammonium Tetrabromocadmate(II)

Abstract In the course of work on the anion condensation of Cd and Zn halides, the crystal structures and 79,81 Br NQR spectra of 2,2-dimethyl-1,3-propanediammonium tetrabromocadmate(II) monohydrate (1) and dimethylammonium tetrabromozincate(II) (2), and 79-81 Br NQR spectra of dimethyl-ammonium tetrabromocadmate(II) (3) have been studied. An isolated [CdBr4]2- ion of the salt [H3NCH2C(CH3)2CH2NH3 ]CdBr4 • H2O (1) is formed, with polar orthorhombic space group Pbc21, Z = 4, a = 645.5(3), b = 1628.3(8), c -1389.5(6) pm. [(CH3)2 NH2 ]2ZnBr4 (2) has also an isolated anion lattice, with monoclinic space group, P21 /c, Z = 4, a = 870.6(3), b = 1195.6(4), c = 1628.9(5) pm, β = 121.84(1)°. The 81 Br NQR spectrum of (1) is a quartet (ν in MHz, T = 77 K/ 303 K): ν1 = 68.403/64.278; ν2 = 65.296/62.663; ν3 = 56.525/54.660; V4 = 43.211/45.422. The 81 Br NQR spectrum of (2) is a quartet too (ν in MHz, T= 77 K/300 K): ν1 = 66.848/64.784; ν2= 63.152/ 61.883; ν3 = 57.498/55.890; ν4 = 49.416/50.224. The 81 Br NQR spectrum of [(CH3)2NH2 ]2CdBr4 (3) is also a quartet (ν in MHz, T =77 K/310 K): ν1 = 68.606/67.272; ν2 = 65.806/63.296; ν3 = 58.096/ 57.369; ν4 = 55.757/53.393. The change of NQR frequencies with temperature is smooth for all three compounds. One of the 81 Br NQR lines in both (1) and (2) increases slightly in frequency with increasing T. In the compound (1), the H2O coordinates to NH3+ groups and to bromines forming a distorted tetrahedron (Br...)2 ...H2O... (... HN)2 , the six angles lying between 69° and 131°. The relations between the bromine NQR and the crystal structures are discussed for the title compounds.