Accurate Crystal Structure Research Articles

炭酸を含むヒト尿路結石の結晶成分は炭酸カルシウムなのか?

It gives important information in selecting the appropriate treatment for urolithiasis to confirm the component of urinary calculus. Presently component analysis of the urinary calculus is generally performed by infrared spectroscopy which is employed by companies providing laboratory testing services in Japan. The infrared spectroscopy determines the molecular components from the absorption spectra in consequence of atomic vibrations. It has the drawback that an accurate crystal structure cannot be analyzed compared with the X-ray diffraction method which analyzes the crystal constituent based on the diffraction of X-rays on crystal lattice. The components of the urinary calculus including carbonate are carbonate apatite and calcium carbonate such as calcite. Although the latter is reported to be very rare component in human urinary calculus, the results by infrared spectroscopy often show that calcium carbonate is included in calculus. The infrared spectroscopy can confirm the existence of carbonate but cannot determine whether carbonate is originated from carbonate apatite or calcium carbonate. Thus, it is not clear whether calcium carbonate is included in human urinary calculus component in Japan. In this study, we examined human urinary calculus including carbonate by use of X-ray structural analysis in order to elucidate the origin of carbonate in human urinary calculus. We examined 17 human calculi which were reported to contain calcium carbonate by infrared spectroscopy performed in the clinical laboratory. Fifteen calculi were obtained from urinary tract, and two were from gall bladder. The stones were analyzed by X-ray powder method after crushed finely. The reports from the clinical laboratory showed that all urinary culculi consisted of calcium carbonate and calcium phosphate, while the gallstones consisted of calcium carbonate. But the components of all urinary calculi were revealed to be carbonate apatite by X-ray diffraction. The components of gallstones were shown to be calcium carbonate (one calcite and the other aragonite) not only by infrared spectroscopy but by X-ray diffraction. It was shown that component analysis of the calculus could be more accurately performed by adding X-ray diffraction method to infrared spectroscopy. It was shown that calcium carbonate existed in a gallstone. As for the carbonate in human urinary calculi, present study showed that it was not calcium carbonate origin but carbonate apatite origin.

The polymorphism of progesterone: Stabilization of a ‘disappearing’ polymorph by co‐crystallization

Progesterone has been known to be polymorphic for over 70 years, and crystallization conditions for the production of both experimentally characterized polymorphs have been repeatedly reported in the literature up to 1975. Nevertheless, our attempts to produce crystals of the metastable form 2 suitable for single crystal X-ray diffraction failed until the structurally related molecule pregnenolone was introduced as an additive into the crystallization solution. Accurate low temperature crystal structures were obtained for forms 1 and 2, pregnenolone and a newly discovered pregnenolone-progesterone co-crystal, which appeared concomitantly with progesterone forms 1 and 2. Computational work based on the experimental crystal structures and those generated by a search for low energy structures showed that the crystallization of enantiomerically pure progesterone results in a more strained conformation compared with the racemate due to the rotation of the acetyl and 21-methyl groups. The role of impurities or additives in influencing crystallization outcome is discussed.

Refinement of the Crystal Structures of Palladium-rich In-Pd Compounds by X-Ray and Neutron Powder Diffraction

Abstract The ternary indium palladium intermetallics In3Pd5, InPd2, and InPd3 have been synthesized by iodine-catalyzed reactions from the elements. Rietveld refinements on X-ray powder diffraction patterns provide the first accurate crystal structure data for In3Pd5 (Pbam, No. 55, a = 1104.20(2), b = 561.346(8), c = 424.263(6) pm, Rh5Ge3-type) and InPd2 (Pnma, No. 62, a = 561.676(6), b = 421.710(4), c = 822.78(8) pm, Co2Si-type). X-Ray powder diffraction apparently confirms the TiAl3 structure type proposed in the literature for InPd3. However, Rietveld refinement on neutron powder diffraction data reveals an In/Pd distributional disorder. Therefore, we describe the crystal structure of InPd3 in a AuCu-type model instead (P4/mmm, No. 123, a = 287.224(4), c = 380.079(7) pm), with mixed occupancy of one crystallographic site by 50% In and 50% Pd. In contrast to In3Pd5 and InPd2, which can be considered to be line compounds, InPd3 shows a non-negligible homogeneity range with unit cell volumes ranging from 0.126132(5) nm3 for the indium-rich to 0.125474(8) nm3 for the palladium-rich In1+xPd3−x phases. Mean In-Pd distances in these indium palladium intermetallics range from 272.3 pm (In1 in In3Pd5) with coordination number 8 for indium to 281.2 pm for 12-coordinated In in InPd3.

On the application of an experimental multipolar pseudo-atom library for accurate refinement of small-molecule and protein crystal structures

With an increasing number of biomacromolecular crystal structures being measured to ultra-high resolution, it has become possible to extend to large systems experimental charge-density methods that are usually applied to small molecules. A library has been built of average multipole populations describing the electron density of chemical groups in all 20 amino acids found in proteins. The library uses the Hansen & Coppens multipolar pseudo-atom model to derive molecular electron density and electrostatic potential distributions. The library values are obtained from several small peptide or amino acid crystal structures refined against ultra-high-resolution X-ray diffraction data. The library transfer is applied automatically in the MoPro software suite to peptide and protein structures measured at atomic resolution. The transferred multipolar parameters are kept fixed while the positional and thermal parameters are refined. This enables a proper deconvolution of thermal motion and valence-electron-density redistributions, even when the diffraction data do not extend to subatomic resolution. The use of the experimental library multipolar atom model (ELMAM) also has a major impact on crystallographic structure modelling in the case of small-molecule crystals at atomic resolution. Compared to a spherical-atom model, the library transfer results in a more accurate crystal structure, notably in terms of thermal displacement parameters and bond distances involving H atoms. Upon transfer, crystallographic statistics of fit are improved, particularly free R factors, and residual electron-density maps are cleaner.

Organic crystal hydrates: what are the important factors for formation

A database study of 34 770 accurate organic crystal structures reveals that the most important factor determining a higher frequency of hydrates is the sum of the average donor and acceptor counts for the functional groups. Different parameter forms for donor/acceptor properties were investigated for correlation with formation of hydrates, and are discussed. We did not find that the donor/acceptor ratio or the molecular weight significantly affects the hydrate formation. We also examined a wide range of calculated molecular properties and found that increasing polar surface is correlated with increasing hydrate formation. The water environment pattern of donor and acceptor hydrogen bonds in the crystal is influenced by the donor/acceptor ratio of the molecule.

A high-field solid-state 35/37Cl NMR and quantum chemical investigation of the chlorine quadrupolar and chemical shift tensors in amino acid hydrochlorides

A series of six L-amino acid hydrochloride salts has been studied by 35/37Cl solid-state NMR spectroscopy (at 11.75 and 21.1 T) and complementary quantum chemical calculations. Analyses of NMR spectra acquired under static and magic-angle-spinning conditions for the six hydrochloride salts, those of aspartic acid, alanine, cysteine, histidine, methionine and threonine, allowed the extraction of information regarding the chlorine electric field gradient (EFG) and chemical shift tensors, including their relative orientation. Both tensors are found to be highly dependent on the local environment, with chlorine-35 quadrupolar coupling constants (CQ) ranging from -7.1 to 4.41 MHz and chemical shift tensor spans ranging from 60 to 100 ppm; the value of CQ for aspartic acid hydrochloride is the largest in magnitude observed to date for an organic hydrochloride salt. Quantum chemical calculations performed on cluster models of the chloride ion environment demonstrated agreement between experiment and theory, reproducing CQ to within 18%. In addition, the accuracy of the calculated values of the NMR parameters as a function of the quality of the input structure was explored. Selected X-ray structures were determined (L-Asp HCl; L-Thr HCl) or re-determined (L-Cys HCl.H2O) to demonstrate the benefits of having accurate crystal structures for calculations. The self-consistent charge field perturbation model was also employed and was found to improve the accuracy of calculated quadrupolar coupling constants, demonstrating the impact of the neighbouring ions on the EFG tensor of the central chloride ion. Taken together, the present work contributes to an improved understanding of the factors influencing 35/37Cl NMR interaction tensors in organic hydrochlorides.

Structural anomalies at the ferromagnetic transition and precursor effects in the vicinity of the structural phase transition ofLa0.815Ba0.185MnO3

Rhombohedral ${\mathrm{La}}_{0.815}{\mathrm{Ba}}_{0.185}{\mathrm{MnO}}_{3}$ undergoes a phase transition at ${T}_{c}=251\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ from a paramagnetic insulating state (PMI) toward a ferromagnetic metallic state (FMM) on cooling. At ${T}_{S}=187.1(3)\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ a first-order phase transition occurs toward a structure with monoclinic symmetry. Accurate crystal structures are reported for selected temperatures between ${T}_{S}$ and room temperature. Thermal expansion is found to be different in the PMI and FMM phases, with a larger thermal expansion coefficient in the FMM phase. This feature is related to incoherent distortions of the ${\mathrm{MnO}}_{6}$ octahedral groups, which are smaller in the FMM state than in the PMI state. Precursor effects are observed as anomalous structural behavior within the rhombohedral phase close to ${T}_{S}$. They indicate two independent contributions to the driving force for the structural transition. Shear distortions of ${\mathrm{MnO}}_{6}$ octahedra in the rhombohedral structure are transformed into energetically more stable Jahn-Teller-type distortions in the monoclinic structure. And the larger number of degrees of freedom in monoclinic symmetry allows an energetically more stable coordination of La by oxygen in the monoclinic structure.

Expect the Unexpected or Caveat for Drug Designers: Multiple Structure Determinations Using Aldose Reductase Crystals Treated under Varying Soaking and Co-crystallisation Conditions

In structure-based drug design, accurate crystal structure determination of protein–ligand complexes is of utmost importance in order to elucidate the binding characteristics of a putative lead to a given target. It is the starting point for further design hypotheses to predict novel leads with improved properties. Often, crystal structure determination is regarded as ultimate proof for ligand binding providing detailed insight into the specific binding mode of the ligand to the protein. This widely accepted practise relies on the assumption that the crystal structure of a given protein–ligand complex is unique and independent of the protocol applied to produce the crystals. We present two examples indicating that this assumption is not generally given, even though the composition of the mother liquid for crystallisation was kept unchanged: Multiple crystal structure determinations of aldose reductase complexes obtained under varying crystallisation protocols concerning soaking and crystallisation exposure times were performed resulting in a total of 17 complete data sets and ten refined crystal structures, eight in complex with zopolrestat and two complexed with tolrestat. In the first example, a flip of a peptide bond is observed, obviously depending on the crystallisation protocol with respect to soaking and co-crystallisation conditions. This peptide flip is accompanied by a rupture of an H-bond formed to the bound ligand zopolrestat. The indicated enhanced local mobility of the complex is in agreement with the results of molecular dynamics simulations. As a second example, the aldose reductase–tolrestat complex is studied. Unexpectedly, two structures could be obtained: one with one, and a second with four inhibitor molecules bound to the protein. They are located in and near the binding pocket facilitated by crystal packing effects. Accommodation of the four ligand molecules is accompanied by pronounced shifts concerning two helices interacting with the additional ligands.

Conformational, Concomitant Polymorphs of 4,4‐Diphenyl‐2,5‐cyclohexadienone: Conformation and Lattice Energy Compensation in the Kinetic and Thermodynamic Forms

4,4-Diphenyl-2,5-cyclohexadienone (1) crystallized as four conformational polymorphs and a record number of 19 crystallographically independent molecules have been characterized by low-temperature X-ray diffraction: form A (P2(1), Z'=1), form B (P1, Z'=4), form C (P1, Z'=12), and form D (Pbca, Z'=2). We have now confirmed by variable-temperature powder X-ray diffraction that form A is the thermodynamic polymorph and B is the kinetic form of the enantiotropic system A-D. Differences in the packing of the molecules in these polymorphs result from different acidic C-H donors approaching the C=O acceptor in C-H...O chains and in synthons I-III, depending on the molecular conformation. The strength of the C-HO interaction in a particular structure correlates with the number of symmetry-independent conformations (Z') in that polymorph, that is, a short C-HO interaction leads to a high Z' value. Molecular conformation (Econf) and lattice energy (Ulatt) contributions compensate each other in crystal structures A, B, and D resulting in very similar total energies: Etotal of the stable form A=1.22 kcal mol(-1), the metastable form B=1.49 kcal mol(-1), and form D=1.98 kcal mol(-1). Disappeared polymorph C is postulated as a high-Z', high-energy precursor of kinetic form B. Thermodynamic form A matches with the third lowest energy frame based on the value of Ulatt determined in the crystal structure prediction (Cerius2, COMPASS) by full-body minimization. Re-ranking the calculated frames on consideration of both Econf (Spartan 04) and Ulatt energies gives a perfect match of frame #1 with stable structure A. Diphenylquinone 1 is an experimental benchmark used to validate accurate crystal structure energies of the kinetic and thermodynamic polymorphs separated by <0.3 kcal mol(-1) (approximately 1.3 kJ mol(-1)).

Mineralogy and crystal structure determination of Mg-fillowite

Accurate crystal structure determination of complex phosphate Mg-fillowite ¾ Na 2 Ca(Mn 4 Mg 2 Fe) 7 [PO 4 ] 6 has been finished. Mg-fillowite occurs in muscovite-pegmatite in the Altay area, Xinjiang, China. It is of the trigonal lattice, with space group R-3 , unit cell a =1.5143(3) nm, c =4.3191(2) nm, V =8.5736 nm 3 , Z = 18. The R-factor of the determination accuracy is R ( I > 2 σ ( I )) = 0.0776. The cationic polyhedra consist of two kinds of structural units: compound col-umn and screw column. In three-dimensional space, they are linked with each other through corner sharing or edge sharing to build the entire structure. There are 45 atoms in an asymmetric unit and 720 atoms in a unit cell. In terms of types and connection manners of the coordination polyhedra, the degree of complicity is rarely seen in all mineral crystal structures. Study of such a structure is significant for the classification of phosphate and the building rule of coordination polyhedra in a complex crystal structure.

Correlation between electronic structure and thermodynamic stability of Ti xZr 1− xMn 2 hydrides

DV–X α cluster calculations based upon accurate crystal structures revealed that each hydrogen atom was strongly bonded to Mn atoms rather than to Zr or Ti atoms in Ti x Zr 1− x Mn 2 ( x = 0, 0.1, 0.2 and 0.3) hydrides and that metal–metal bonds were significantly weakened on hydriding. Assuming that more stable hydride would form as the Mn Mn bond became weaker and both the Zr(Ti) Zr(Ti) bond and the Zr(Ti) Mn bond became stronger, we tried to correlate the thermodynamic stability of the Ti x Zr 1− x Mn 2 hydrides with the ratio, R, of the sum of Zr(Ti) Mn and Zr(Ti) Zr(Ti) bond order differences to the Mn Mn bond order difference, where the differences were taken between the alloys and the corresponding hydrides. It was found that experimental enthalpy changes on hydriding linearly increased (absolute values decreased) with R. Approximate values for R, which were calculated with lattice expansion on hydriding neglected, also correlated with enthalpies, although correlation was not so good. A rough survey of multi-component alloys would be possible without hydriding experiments only by using the approximate values for the R parameter.

Performance of a new furnace for high-resolution synchrotron powder diffraction up to 1900 K: application to determine electron density distribution of the cubic CaTiO3perovskite at 1674 K

Accurate crystal structure analysis at high temperatures is an important challenge in science and technology. A new electric furnace for the measurement of high-resolution (δd/d= 0.03%) synchrotron radiation powder diffraction profiles from materials at high temperatures (up to 1900 K in air) has been designed and fabricated. This furnace consists of a ceramic refractory with MoSi2heaters, an aluminium body cooled by flowing water, and a sample stage with a spinner and a controller for sample-height adjustment.In situsynchrotron powder diffraction measurement for a calcium titanate perovskite specimen at 1674 K has been performed using the furnace at beamline 3A of the Photon Factory. The electron density distribution of the cubic perovskite at 1674 K was successfully obtained using a combination of Rietveld refinement, the maximum-entropy method (MEM) and MEM-based pattern-fitting techniques. The Ti atoms exhibit covalent bonding with the O atoms in the cubic CaTiO3perovskite at this temperature, while the Ca atoms are ionic. These results indicate that the new furnace yields high-quality data for accurate crystal structure analysis.

Geometric and electronic structure of polymeric C 70-fullerides: the case of [formula omitted][C 703−

The reduction of C 70 with strontium, dissolved in liquid ammonia, in a molar ratio of 2:3 and crystallisation under solvothermal pressure at room-temperature results in the formation of crystalline [Sr(NH 3) 8] 3(C 70) 2·19NH 3. The crystal structure ( P 1 ̄ , a=1594.7(1) pm, b=1656.3(1) pm, c=2409.5(2) pm, α=79.664(1)°, β=75.207(1)°, γ=80.460(1)°, Z=2, R 1=0.082, 14301 reflections) is built up of polymeric chains 1 ∞ [C 70 3−] arranged in a distorted hexagonal close packing of rods parallel [ 1 ̄ 2 1 ̄ ] intersected by layers of [Sr(NH 3) 8]-polyhedra extending parallel (111). Quantum chemical results on the model systems [C 70(CH 3) 2] n− ( n=2, 3, 4) are discussed in comparison to the accurate crystal structure data in order to analyse the total charge, charge distribution and the bonding character due to the structural response of the C 70 fullerene units upon reduction.

Global Rietveld Refinement.

Global optimisation methods of structure determination from powder diffraction data have risen to prominence in a relatively short space of time and they now constitute a key approach in the examination of polycrystalline molecular organic materials. A correctly formulated global optimisation approach may be regarded as a “global Rietveld refinement” that is capable of delivering accurate crystal structures from high-quality powder diffraction data. This paper focuses on how accuracy at all stages of a powder diffraction experiment impacts upon the overall structure solution process and particular attention is paid to assessing the degree of accuracy with which structures are returned from the global optimisation process.

Magnetic Properties of Single Crystalline Mn4Si7.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Preparation and precise structural determination of a second Ga84 cluster compound. A first hint for cluster doping and its fundamental consequences in the field of chemistry and physics of nanoscaled metalloid cluster material.

The Ga(84)R(20)(4-) [R = N(SiMe(3))(2)] species, which represents the largest metalloid cluster entity structurally characterized so far, has been electronically and topologically modified: Via changing the redox potential of the reaction solution, crystals different from those containing the Ga(84)R(20)(4-) anion can be isolated, featuring similar Ga(84)R(20)(3-) entities. An accurate crystal structure analysis via synchrotron radiation is presented, which might be the first step toward an understanding of the metallic conductivity and superconductivity of the Ga(84)R(20)(4-) cluster compound, physical properties which are singular in the field of metalloid clusters so far.

The Cofactor of Tetrachloroethene Reductive Dehalogenase of Dehalospirillum multivorans Is Norpseudo‐B12, a New Type of a Natural Corrinoid

AbstractThe corrinoid cofactor of the tetrachloroethene reductive dehalogenase of Dehalospirillum multivorans was isolated in its Coβ‐cyano form. This cofactor represents the main corrinoid found in D. multivorans cells. Analysis of the isolated cyano‐corrinoid by a combination of HPLC and UV/VIS‐absorbance spectroscopy revealed it to be nonidentical to a variety of known natural B12 derivatives. From high‐resolution mass‐spectrometric analysis, the molecular formula of the corrinoid isolated from D. multivorans could be deduced as C58H81CoN17O14P. The sample of the novel corrinoid from D. multivorans was further analyzed by UV/VIS, CD, and one‐ and two‐dimensional 1H‐, 13C‐, and 15N‐NMR spectroscopy, which indicated its structure to be closely related to that of pseudovitamin B12 (Coβ‐cyano‐7″‐adeninylcobamide). By the same means, the corrinoid could be shown to differ from pseudovitamin B12 only by the lack of the methyl group attached to carbon 176, and, therefore, it was named norpseudovitamin B12 (or, more precisely, 176‐norpseudovitamin B12). Norpseudovitamin B12 represents the first example of a ‘complete’ B12‐cofactor that lacks one of the methyl groups of the cobamide moiety, indicating that the B12‐biosynthetic pathway in D. multivorans differs from that of other organisms. X‐Ray crystal‐structures were determined for norpseudovitamin B12 from D. multivorans and the analogues pseudovitamin B12 and factor A (Coβ‐cyano‐7″‐[2‐methyl]adeninylcobamide). These first accurate crystal structures of complete corrinoids with an adeninyl pseudonucleotide confirmed the expected coordination properties around Co and corroborated the close conformational similarity of the nucleotide moieties of norpseudovitamin B12 and its two homologues.

Magnetic properties of single crystalline Mn 4Si 7

A single crystal of MnSi 2− x was obtained by a modified Czochralski pulling technique in a cold copper crucible. The quality and the nature of the sample were checked by an accurate crystal structure determination, which revealed the sample to be Mn 4Si 7. Resistivity and magnetic measurements were performed on this sample in the temperature range between 2 K and room temperature and in magnetic fields up to 7.5 T. Mn 4Si 7 shows a metallic behaviour and the good crystal quality was revealed by the high residual resistance ratio of 360. For the magnetic susceptibility we observed a Curie–Weiss law above about 40 K with a low effective moment of p eff=0.365 μ B/Mn. Below this temperature, moments order in an anisotropic helical state, and in fields above 1 T, they align with a saturation moment of p sat=0.012 μ B/Mn. Mn 4Si 7 is a weak itinerant magnetic system that could be a good candidate for the observation of the critical quantum fluctuations expected for marginal Fermi liquids.

Crystal chemistry and binding of NO2, SCN and SeCN to Co in cobalamins.

Results of the accurate crystal structure determination of NO(2)Cbl.2LiCl (1), NO(2)Cbl.NaCl (2), NCSCbl (3) and NCSeCbl (4), based on synchrotron diffraction data collected at 100 K, are described. The nitro group in (1) was found to be disordered with two orientations that differ by a rotation of approximately 60 degrees about the Co-NO(2) bond, whereas in (2) the nitro group has only one orientation. The first X-ray structural determination of a cobalamin with a Co-Se bond is reported. Comparison of the axial distances indicates that SeCN has a bond length of 2.384 (3) A and that the trans influence on the Co-N bond is only slightly greater than that of SCN. The crystals of the thiocyanate cobalamin contain both the S- and N-bonded coordination isomers in a 3:2 ratio. The structural features of the Co-S bond in cobalamins are discussed. The crystal chemistry of cobalamins is discussed in terms of packing of roughly spherical molecules. The unit-cell parameters can be used to group the cobalamins' crystal structures in different arrays intermediate between distorted hexagonal close packing and primitive hexagonal arrangements. The structural features of cobalamins, and of cobaloximes that have the same axial fragment as the cobalamins, are reviewed and discussed in terms of the cis influence of the equatorial ligand.

Structure and dynamics of ND3BF3 in the solid and gas phases: a combined NMR, neutron diffraction, and Ab initio study.

The decrease in D-->A bond lengths, previously reported for some Lewis acid/base complexes, in going from the gas to the solid phases is investigated by obtaining an accurate crystal structure of solid ND(3)BF(3) by powder neutron diffraction. The B-N internuclear distance is 1.554(3) A, 0.118 A shorter than the most recent gas-phase microwave value and 0.121 A shorter than the single molecule geometry optimized (1.672 A, CISD/6-311++G(d,p)) bond length. The crystal structure also shows N-D.F-B hydrogen bonds. The effects of this change in structure and of intermolecular hydrogen-bonding on nuclear magnetic shielding (i.e., chemical shifts) and the nuclear quadrupolar coupling constants (QCC) are investigated by ab initio molecular orbital and density functional theory calculations. These calculations show that the nitrogen ((15)N and (14)N) and boron ((11)B and (10)B) chemical shifts should be rather insensitive to changes in r(BN) and that the concomitant changes in molecular structure. Calculations on hydrogen-bonded clusters, based on the crystal structure, indicate that H-bonding should also have very little effect on the chemical shifts. On the other hand, the (11)B and (14)N QCCs show large changes because of both effects. An analysis of the (10)B[(19)F] line shape in solid ND(3)(10)BF(3) yields a (11)B QCC of +/-0.130 MHz. This is reasonably close an earlier value of +/-0.080 MHz and the value of +/-0.050 MHz calculated for a [NH(3)BF(3)](4) cluster. The gas-phase value is 1.20 MHz. Temperature-dependent deuterium T(1) measurements yield an activation energy for rotation of the ND(3) group in solid ND(3)BF(3) of 9.5 +/- 0.1 kJ/mol. Simulations of the temperature-dependent T(1) anisotropy gave an E(a) of 9.5 +/- 0.2 kJ/mol and a preexponential factor, A, of 3.0 +/- 0.1 x 10(12) s(-)(1). Our calculated value for a [NH(3)BF(3)](4) cluster is 16.4 kJ/mol. Both are much higher than the previous value of 3.9 kJ/mol, from solid-state proton T(1) measurements.