Refinement of the alum structures. III. X-ray study of the α alums, K, Rb and NH4Al(SO4)2.12H2O

Behavior of H2O surrounding NH4+ and Al3+ in NH4Al(SO4)2·12H2O by 1H MAS NMR, 14N NMR, and 27Al NMR

In order to understand the thermodynamic properties and structural geometry of NH₄Al(SO₄)₂·12H₂O, we studied the magic angle spinning nuclear magnetic resonance (MAS NMR) and static NMR as a function of temperature.

Neutron diffraction study of alums L The crystal structure of hydroxylammonium aluminium alum L The crystal structure of hydroxylammonium aluminium alum

The crystal structure of the hydroxylammonium aluminium sulfate dodecahydrate alum, (NH3OH)Al(SO4)2 · 12H2O, has been determined from three dimensional neutron diffraction data. The compound crystallizes cubic with a - 12.328(4) Å in the space group Pa3. The (NH3OH) + complex around ½½½, the centre of symmetry, is arranged with equal probability over two orientations related to each other by a centre of symmetry. The elastic diffraction experiment gives a space and time-average picture of two orientations obtained by a rotation of the (NH3OH)+ complex around an axis perpendicular to the N − O bond. The positional and thermal parameters have been refined by full-matrix least-squares analysis to a weighted R-value of 1.9%. Disorder in the (SO4)2− complex has been considered and only 3% disorder has been found.

X-Ray and neutron diffraction study of alums

The crystal structure of the alums, (NH3CH3)Al(SO4)2 · 12H2O and (NH4)Al(SO4)2 · 12H2O, has been determined and refined from X-ray and neutron diffraction data. The compounds crystallize cubic in space group Pa3 (Z = 4) with a 0 = 12.322(3) Å and a 0 = 12.242(1) Å respectively. The positional and thermal parameters of all atoms including hydrogens have been refined by full matrix least-squares analysis resulting in Rw values from the X-ray and neutron data of 0.030 and 0.029 respectively for the methylammonium alum and of 0.024 and 0.014 respectively for the ammonium alum.

The atoms of the cation groups (NH3CH3)+ and (NH4)+ are distributed on 8(c) and 24(d) positions in two orientations of equal probability on and around the [111] direction related to each other by an inversion centre. The disorder in the cation groups is explained by a quantized rotation. Disorder in the sulfate groups has been determined to 4.2% for the methylammonium alum and to about 17% for the ammonium alum. The disordered sulfate tetrahedra are distorted and in a reversed orientation along the threefold axis. The system of the hydrogen bonds is discussed.

Electrogyration effect in the paraelectric phase of ferroelectric alums

The electrogyration effect of four methylammonium alums and of ammonium iron alum in their paraelectric phase has been measured. In all methylammonium alums the electrogyration constant s 123 obeys the CurieWeiss-law s 123 = s′123 (T− T 0)−1 + s″123 with only small s″123-values compared with s′123. In contrast to the strong increase in these crystals, s 123 diminishes in ammonium iron alum when the transition temperature is approached. Within the framework of the phenomenological theory of improper ferroelectrics in both kinds of compounds, the electrogyration effect can be described by a sum of two terms, one depending on polarization Pand the other on an intrinsic order parameter [unk], different from P. From dispersion behavior in methylammonium iron alum a less important influence of the trivalent cation on phase transition is concluded.

Study on the phase transitions by nuclear magnetic resonance of alpha-type RbAl(SO4)2 x 12H2O and beta-type CsAl(SO4)2 x 12H2O single crystals

The thermodynamic properties, spin-lattice relaxation times, T(1), and spin-spin relaxation times, T(2), of the (27)Al, (87)Rb, and (133)Cs nuclei in MAl(SO(4))(2)x12H(2)O (M=Rb and Cs) crystals were investigated, and the two crystals were found to lose H(2)O with increases in temperature. From our results for T(1) and T(2), we conclude that the discontinuities near T(d) in the T(1) curves of the two crystals correspond to structural changes. In both crystals, below T(d) the water molecules surrounding the Al(3+) and M(+) nuclei form distorted octahedra, whereas above T(d) the water molecules around the Al(3+) and M(+) nuclei form regular octahedra and the environment of the Al(3+) and M(+) nuclei has cubic symmetry. Further, the T(1) for the (27)Al and (87)Rb nuclei in RbAl(SO(4))(2)x12H(2)O below T(d) were found to increase with increasing temperature, whereas the T(1) for the (27)Al and (133)Cs nuclei in CsAl(SO(4))(2)x12H(2)O were found to decrease. It is possible that this difference is due to the different characteristics of alpha- and beta-type crystals.

Satellite transitions in natural abundance solid-state (33)S MAS NMR of alums--sign change with zero-crossing of C(Q) in a variable temperature study

Experiences obtained from recent improvements in the performance of solid-state (14)N MAS NMR spectroscopy have been used in a natural abundance (33)S MAS NMR investigation of the satellite transitions for this interesting spin I=3/2 isotope. This study reports the first observation of manifolds of spinning sidebands for these transitions in (33)S MAS NMR as observed for the two alums XAl(SO(4))(2) x 12H(2)O with X=NH(4) and K. For the NH(4)-alum a variable temperature (33)S MAS NMR study, employing the satellite transitions, shows that the (33)S quadrupole coupling constant (C(Q)) exhibits a linear temperature dependence (in the range -35 degrees C to 70 degrees C) with a temperature gradient of 3.1 kHz/ degrees C and undergoes a sign change with zero-crossing for C(Q) at 4 degrees C (277 K). For the isostructural K-alum a quite similar increase in the magnitude of C(Q) with increasing temperature is observed, and with a temperature gradient of 2.3 kHz/ degrees C. Finally, for optimization purposes, a study on the effect of the applied pulse widths at constant rf field strength on the intensity and variation in second-order quadrupolar lineshape for the central (1/2<-->-1/2) transition of the K-alum has been performed.

Effects of T2-relaxation in MAS NMR spectra of the satellite transitions for quadrupolar nuclei: a 27Al MAS and single-crystal NMR study of alum KAl(SO4)2.12H2O

Asymmetries in the manifold of spinning sidebands (ssbs) from the satellite transitions have been observed in variable-temperature 27Al MAS NMR spectra of alum (KAl(SO4)2.12H2O), recorded in the temperature range from -76 to 92 degrees C. The asymmetries decrease with increasing temperature and reflect the fact that the ssbs exhibit systematically different linewidths for different spectral regions of the manifold. From spin-echo 27Al NMR experiments on a single-crystal of alum, it is demonstrated that these variations in linewidth originate from differences in transverse (T2) relaxation times for the two inner (m=1/2<-->m=3/2 and m=-1/2<-->m=-3/2) and correspondingly for the two outer (m=3/2<-->m=5/2 and m=-3/2<-->m=-5/2) satellite transitions. T2 relaxation times in the range 0.5-3.5 ms are observed for the individual satellite transitions at -50 degrees C and 7.05 T, whereas the corresponding T1 relaxation times, determined from similar saturation-recovery 27Al NMR experiments, are almost constant (T1=0.07-0.10 s) for the individual satellite transitions. The variation in T2 values for the individual 27Al satellite transitions for alum is justified by a simple theoretical approach which considers the cross-correlation of the local fluctuating fields from the quadrupolar coupling and the heteronuclear (27Al-1H) dipolar interaction on the T2 relaxation times for the individual transitions. This approach and the observed differences in T2 values indicate that a single random motional process modulates both the quadrupolar and heteronuclear dipolar interactions for 27Al in alum at low temperatures.

Solid-state 33S MAS NMR of inorganic sulfates

Solid-state (33)S MAS NMR spectra of a variety of inorganic sulfates have been obtained at magnetic field strengths of 4.7, 14.1, 17.6, and 18.8 T. Some of the difficulties associated with obtaining natural abundance (33)S NMR spectra have been overcome by using a high magnetic field strength and magic angle spinning (MAS). Multiple factors were considered when analyzing the spectral linewidths, including magnetic field inhomogeneity, dipolar coupling, chemical shift anisotropy, chemical shift dispersion, and quadrupolar coupling. In most of these sulfate samples, quadrupolar coupling was the dominant line broadening mechanism. Nuclear electric quadrupolar coupling constants (C(q)) as large as 2.05 MHz were calculated using spectral simulation software. Spectral information from these new data are compared with X-ray measurements and GAUSSIAN 98W calculations. A general correlation was observed between the magnitude of the C(q) and the increasing difference between S-O bond distances within the sulfate groups. Solid-state (33)S spin-lattice (T(1)) relaxation times were measured and show a significant reduction in T(1) for the hydrated sulfates. This is most likely the result of the modulation of the time-dependent electric field gradient at the nuclear site by motion of water molecules. This information will be useful in future efforts to use (33)S NMR in the compositional and structural analysis of sulfur containing materials.

Local structure and tunneling kinetics of ammonium aluminum alum by infrared hole-burning

The title compound, crystallized with a few percent of deuterium, contains some NH(3)D(+) and HOD. At low temperatures, five N-D stretch bands are observed. These belong to two ammonium sites with apparent C(s) symmetry in the low-temperature phase. The N-D bands can be hole-burned with an infrared laser. Burning bands belonging to the A-sites transforms some of them to the higher-energy B-sites. The A- and B-sites probably differ from each other by the arrangement of water and sulfate about the ammonium.

Tetraaza analogues of lithium and sodium alums: synthesis and X-ray structures of the single strand polymer [Li(THF)2(Al[SO2(NtBu)2]2)] infinity and the contact ion pairs [Na(15-crown-5)][Al(SO2(NtBu)2)2] and ([Na(15-crown-5)][O2S(mu-NBn)2Al(mu-NBnSO2NBn)])2

The reaction of an alkali metal aluminohydride MAlH4 (M = Li, Na) with N,N'-bis-(tert-butyl)sulfamide or N,N'-bis-(benzyl)sulfamide in THF produces the complex ions (Al[SO2(NR)2]2)- (R = tBu, Bn). The X-ray structures of [Li(THF)2(Al[SO2(NtBu)2]2)] infinity (1), [Na(15-crown-5)][Al(SO2(NtBu)2)2], (2) and ([Na(15-crown-5)][O2S(mu-NBn)2Al(mu-NBnSO2NBn)])2 (3.3THF) are reported. The two diazasulfate ligands [SO2(NtBu)2]2- are N,N' chelated to Al3+ in both 1 and 2. In the lithium derivative 1 the spirocyclic (Al[SO2(NtBu)2]2)- anions are bridged by the bis-solvated cations Li(THF)2+ to give a polymeric strand. In the sodium salt 2 the complex anion is O,O' chelated to Na+, which is further encapsulated by a 15-crown-5 ligand to give a monomeric ion-pair complex. By contrast, the benzyl derivative 3 forms a dimer in which the terminal [SO2(NBn)2]2- ligands are (N,N'),(O,O') bis-chelated to Al3+ and Na+, respectively, and the bridging ligands adopt a novel N,O-chelate, N'-monodentate bonding mode. The central core of 3 consists of two four-membered AlOSN rings bridged by two NtBu groups. Crystal data: 1, orthorhombic, Pna2(1), a = 20.159(5) degrees, b = 10.354(3) degrees, c = 15.833(4) degrees, alpha = beta = gamma = 90 degrees, V = 3304.7(15) A3, Z = 4; 2, monoclinic, P2(1)/n, a = 16.031(2) A, b = 9.907(2) A, c = 23.963(4) A, beta = 103.326(2) degrees, Z = 4; 3, triclinic, P1, a = 12.7237(11) A, b = 14.0108(13) A, c = 16.2050(14) A, alpha = 110.351(2) degrees, beta = 111.538(2) degrees, gamma = 97.350(2) degrees, Z = 1.

Single-Crystal Raman Spectroscopy of the Rubidium Alums RbM(III)(SO(4))(2).12H(2)O (M(III) = Al, Ga, In, Ti, V, Cr, Fe) between 275 and 1200 cm(-)(1): Correlation between the Electronic Structure of the Tervalent Cation and Structural Abnormalities

Low-temperature single-crystal Raman spectra for RbM(III)(SO(4))(2).12H(2)O (M(III) = Al, Ga, In, Ti, V, Cr, Fe) and RbM(III)(SO(4))(2).12D(2)O (M(III) = Al, V) have been collected and assigned in the range 275-1200 cm(-)(1). These results permit classification of the Ti and V rubidium sulfate alums to the beta modification, whereas the remaining tervalent cations give the expected alpha modification. The dimorphism of the rubidium sulfate alums is explained in terms of the electronic structure of the tervalent cation, where the observation of the beta modification is associated with unequal occupancy of the t(2g) (O(h)()) orbitals. For the rubidium vanadium alums the (3)E(g) <-- (3)A(g) electronic Raman (eR) transition permits quantification of the trigonal field splitting of the t(2g) (O(h)()) orbitals (ca. 1940 cm(-)(1)). The profile of the eR band is sensitive both to changes in temperature and to deuteration. Analysis of the eR band profile suggests a reduced spin-orbit splitting of the (3)E(g) manifold, this being ascribed to excited state Jahn-Teller (J-T) effects. The similarity of the Raman spectra of the cesium and rubidium titanium sulfate alums suggest that they exhibit closely related structural chemistry, with both subject to phase transitions below 80 K. The observation that modes of E(g) symmetry are coupled to the structural change is consistent with the interpretation that the trigonal field leaves an orbital doublet ground term for titanium(III), leading to a cooperative J-T effect.