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In marked contrast to the single, narrow 29Si MAS NMR resonance for pure forsterite (Mg2SiO4), the spectra for synthetic forsterite containing 0.05 to 5% of the Mg2+ replaced with Ni2+, Co2+, or Fe2+ display between 4 and 26 additional, small, paramagnetically shifted peaks that are caused by interactions of the unpaired electron spins on the transition metal cations and the nuclear spins. Analyses of these relative peak areas, their numbers, and comparison of their positions to those in spectra of synthetic monticellites (CaMgSiO4) containing similar levels of transition metals, allows at least partial assignment to the effects of cations in either the M1 octahedral site only or to both M1 and M2 sites. More detailed analyses indicate that in forsterite, Ni2+ occupies only M1, Fe2+ occupies M1 and M2 roughly equally, and Co2+ occupies both M1 and M2 in an approximately 3:1 ratio. These findings for low concentrations agree with expectations from previous studies by other methods (e.g., XRD) of olivines with much higher transition metal cation contents. However, even low concentrations of Mn2+ (e.g., 0.1%), as well as higher Fe2+ contents (e.g., in natural San Carlos olivine) can broaden NMR peaks sufficiently to greatly reduce this kind of information content in spectra.

NMR spin-lattice relaxation efficiency is similar at all carbon and silicon sites in aluminum-doped 4H- and 6H-polytype silicon carbide samples, indicating that the valence band edge (the top of the valence band), where the holes are located in p-doped materials, has similar charge densities at all atomic sites. This is in marked contrast to nitrogen-doped samples of the same polytypes where huge site-specific differences in relaxation efficiency indicate that the conduction band edge (the bottom of the conduction band), where the mobile electrons are located in n-doped materials, has very different charge densities at the different sites. An attempt was made to observe 27Al NMR signals directly, but they are too broad, due to paramagnetic line broadening, to provide useful information about aluminum doping.

Nitrogen doping in silicon carbide converts an insulator to a wide-band gap semiconductor with many potential uses. Most doping studies have involved the 4H and 6H polytypes, which alone among the ...

We report the synthesis and analysis of sol–gel-derived samples of forsterite (Mg2SiO4) and willemite (Zn2SiO4), doped with paramagnetic Cu2+, Ni2+, and Co2+, at a range of dopant concentrations. Electron probe microanalysis and backscattered electron imaging show the presence of major micrometre-scale heterogeneities in the distribution of paramagnetic centres. Despite the inhomogeneities, the 29Si NMR spin–lattice relaxation behaviour is well-behaved and is consistent with the stretched-exponential expression Mz(t) = Mz(∞){1 – a exp[–(t/T′)n]}. The exponent n is 0.5 within the experimental error in some samples. This value is consistent with relaxation by immobile isolated paramagnetic impurities with negligible 29Si spin diffusion from the impurity centres, but careful curve fitting confirms that n is significantly larger than 0.5 in other samples. Relaxation efficiency is highly dependent on the dopant ion and its concentration. Although the purely empirical stretched-exponential function does not provide a unique physical picture, it is noteworthy that it is sufficiently robust to describe spin–lattice relaxation even in highly inhomogeneous systems. Spin–lattice relaxation is a useful probe of paramagnetics-doped solid samples, but NMR does not provide information on homogeneity. Careful sample characterization on the micrometre scale is highly desirable, as a complement to NMR studies.Key words: MAS NMR, spin–lattice relaxation, 29Si, forsterite, willemite, stretched-exponential relaxation, sol–gel, minor-component heterogeneity, backscattered electron analysis.

Simulations of high-resolution 19F-decoupled 27Al and 23Na magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of the aluminofluoride minerals, cryolite, cryolithionite, thomsenolite, weberite, chiolite, prosopite, and ralstonite combined with theoretical modeling have given accurate values of chemical shift (δiso), and quadrupolar interaction parameters ( C q and η), thereby eliminating ambiguities incurred by the complex nuclear interactions. These NMR data have been correlated with local electronic environments in the minerals, which were calculated using Full Potential Linearized Augmented Plane Wave (FP LAPW) modeling based on the structures from X-ray diffraction (XRD) data. This combination of NMR, XRD, and modeling techniques allowed the analysis and optimization of the crystal structures. The electronegativities and distances of neighboring ions, represented here by an environmental parameter χ, are shown to control δiso of both 23Na and 27Al. The calculations using χ, also show that the ions beyond the nearest neighbor play an important role in determining δiso of 27Al and 23Na in these aluminofluoride minerals, and the substitution of OH for F significantly affects the shielding around 27Al in prosopite and ralstonite. There is a positive correlation between the site distortion at the Na and Al sites and the values of C q in these aluminofluoride minerals.

35Cl (spin 3/2) in sodium chlorate has a strong quadrupole coupling, similar in size to its interaction with a magnetic field of 11.7 T (500 MHz for protons). This means that it obeys neither the standard nuclear quadrupole resonance (NQR) nor the nuclear magnetic resonance (NMR) rules. Recently, we have published a theoretical approach that allows us to calculate the transition frequencies of such a system without approximations. This exact solution is applicable to any spin, with any relative ratio of Zeeman to quadrupole coupling, so we can map out how NQR becomes NMR as the magnetic field is increased. In the general situation, the selection rules restricting direct observation to only single quantum transitions break down and nominal multiple-quantum transitions become directly observable. The transition frequencies depend on the Zeeman interaction, the quadrupole coupling and the orientation of the crystal in the magnetic field. Sodium chlorate is an ideal sample, since we can use the 23Na NMR spect...

The substitution and decomposition reactions of the difluorobis(pyridine)boron(III) cation, and of several substituted pyridine and other difluoroboron cations (D)2BF2+, are explored. Donor molecule displacement is greatly accelerated by the presence of electron-withdrawing meta and para pyridine substituents in (substituted pyridine)2BF2+ cations, and by electron-donating substituents on pyridine when the substituted pyridine is the attacking species. Increased steric hindrance of D in D2BF2+, and of the attacking base D′, slows or prevents displacement of D by D′. (py)2BF2+·PF6− undergoes slow decomposition in acetone, giving initially py·BF3 and finally py·H+·BF4− as the major fluoroboron species. A second decomposition pathway, giving rise to BF3OH− and several additional fluorine-containing products, is favoured by the presence of strong bases such as amidines and by the presence of NH and OH bonds. Increased susceptibility to ligand displacement facilitates the synthesis of new fluoroboron cations, but also increases the susceptibility to decomposition.

Factors affecting the ability of potentially chelating amine ligands to form chelated fluoroboron cations are explored by 19F and 11B NMR spectroscopy and fast atom bombardment mass spectrometry (FAB-MS). Five-membered chelate rings form much more readily than six-membered. Some potentially chelating ligands give rise to additional fluoroboron species by various redistribution and decomposition reactions.Key words: chelated fluoroboron cations, 19F NMR, 11B NMR; FAB-MS, N,N,N',N'-tetraethylethylenediamine.

Alumina was synthesized by a sol–gel method using modifying agents such as 2,4-pentanedione, acetophenone, benzophenone, and dibenzoyl methane. The alumina supports were characterized employing N 2 adsorption studies and 27 Al solid-state NMR. The ZnCl 2 /Al 2 O 3 catalysts were made by impregnation of methanolic solution containing calculated amounts of ZnCl 2 on modified alumina supports. The influence of the loading of ZnCl 2 and also the effect of additive used during the sol–gel synthesis of alumina, were probed using Friedel–Crafts alkylation reaction. The alkylation activities indicated the predominance of pore volume in determining the catalytic activities of the catalysts, rather than pore size distribution.

Possible methods of formation of chelated fluoroboron cations (DD)BF 2 + and (DD)BFX + , where DD is 2,2′-bipyridine (bpy), 1,10-phenanthroline (1,10-phen), 2,2′,6′,2″-terpyridine (terpy) and 1,8-bis(dimethylamino)naphthalene (DMAN, ‘proton sponge’), have been investigated, primarily by 19 F and 11 B NMR and fast atom bombardment mass spectrometry. The most successful method involves displacement of weakly basic isoxazole (ISOX) and heavy halide ion from ISOX·BF 2 X and ISOX·BFX 2 (X=Cl, Br). Displacement of pyridine from py·BF 2 Cl and py·BFCl 2 is less effective because pyridine, more basic than isoxazole, competes with the chelating donor in forming fluoroboron cations when displaced from its adducts. 1,10-Phen, terpy and DMAN form (DD)BF 2 + ·BF 4 − when reacted with Et 2 O·BF 3 or with ISOX·BF 3 under our conditions, but bpy does not. None of these chelating ligands can displace pyridine from (py) 2 BF 2 + ·PF 6 − under conditions that are effective with tertiary-amine chelating donors. (1,10-Phen)BF 2 + but not the other (DD)BF 2 + species is attacked by dimethylsulfoxide (DMSO), to form first (DMSO) 2 BF 2 + and finally DMSO·BF 3 .

Several possible methods of synthesis of chelated fluoroboron cations are explored, using the tert-amines N,N,N',N'-tetramethylethylenediamine (Me4en) and N,N,N',N",N"-pentamethyldiethylenetriamine (Me5dien) as model chelating ligands. Both ligands displace pyridine from (pyr)2BF2+ (as its PF6- salt) to form the bidentate (Me4en)BF2+ and (Me5dien)BF2+ cations. The same cations, as well as the corresponding BFCl+ and BFBr+ cations, can also be prepared by displacement of the donor molecule (D = pyridine or isoxazole) and the heavy halide ion (Cl- or Br-) from the neutral D·BF2X and D·BFX2 adducts. The central nitrogen of Me5dien becomes chiral when it and one terminal nitrogen are coordinated, and the prochiral and magnetically nonequivalent fluorines of (Me5dien)BF2+ give 19F NMR signals separated by 1.2 ppm. In (Me5dien)BFCl+ the boron is a second chiral centre and the two diastereomers, distinguishable by NMR with 19F chemical shifts differing by 3.0 ppm, form in a 3:1 ratio. The bidentate BFBr+ cations of Me4en and Me5dien are insoluble in non-coordinating solvents but have been detected by positive ion FAB mass spectrometry and 11B MAS NMR. The tridentate complex (Me5dien)BF+2 does not form under our conditions.Key words: N,N,N',N'-tetramethylethylenediamine, N,N,N',N",N"- pentamethyldiethylenetriamine, chelated fluoroboron cations, fluorine-19 NMR, boron-11 NMR, pyridine, isoxazole, chiral, magnetically nonequivalent.

Optically clear sol−gel-derived organic−inorganic hybrid materials were prepared by a sonication method suitable for entrapment of biological compounds. Sonication at pH 2.5 hydrolyzed mixtures of tetraethyl orthosilicate (TEOS) and one of the organosilanes methyltriethoxysilane (MTES), propyltrimethoxysilane (PTMS), or dimethyldimethoxysilane (DMDMS). Buffer solutions containing the fluorescent probes 7-azaindole or prodan, or the proteins human serum albumin (HSA) or lipase, were then added to promote gelation and the resulting materials were aged at 4 °C over several months. The optical clarity, hardness, and degree of cracking were examined, in conjunction with solid-state 29Si and 13C NMR of the materials and fluorescence spectra of the entrapped probes. These studies revealed that MTES can be added to TEOS up to a level of 20% (v/v) with retention of good physical characteristics, thus allowing control over the hydrophobicity and cross-linking within the matrix. Materials with more than 20% MTES, or...

ZnCl2–aluminosilicate catalysts were prepared via a sol–gel route involving fluoride‐catalyzed hydrolysis of aluminum and silicon alkoxides in the presence of NaF, KF, NH4F and ZnF2. The catalysts were characterized by employing 29Si, 27Al and 19F solid‐state MAS NMR. The dependence of the activities of the catalysts on the nature and amount of fluoride present in the catalysts were investigated using Friedel–Crafts alkylation reaction of benzene with benzyl chloride.

Mixed-donor difluoroboron and chlorofluoroboron cations DD'BF2+ and DD'BFCl+ involving at least one C=N donor molecule can be formed by Cl- displacement from the mixed boron trihalide adducts D·BF2Cl and D·BFCl2, provided that the steric hindrance of D' and especially D is not too great. The DD'BF2+ cations can also be formed by selective displacement of one donor molecule in certain D2BF2+ cations, present in acetone as their PF6- salts, allowing isolation of the first salt of a mixed-donor difluoroboron cation, (quinuclidine)(pyridine)BF2+·PF6-. In less favourable cases, however, either no reaction occurs or the reaction continues to give D'2BF2+. Reactivities and 19F and 11B NMR parameters of mixed boron trihalide adducts and fluoroboron cations of a number of C=N donors are reported.Key words: fluoroboron cations, mixed-ligand, fluorine-19 NMR, boron-11 NMR, oxazolines, thiazolines, C=N donors.

Novel mesoporous silicas have been prepared via a sol-gel route involving fluoride-catalyzed hydrolysis of tetraethylorthosilicate (TEOS). Incorporation of zinc chloride by sol-gel synthesis gives a range of mesoporous materials with significantly higher catalytic activity than the commercially available catalyst, Clayzic, in a model Friedel-Crafts alkylation reaction. The dependence of catalytic activity and physical structure on the amounts of solvent, water, zinc chloride, and potassium fluoride used in the preparation are explored, and the materials have been further characterized by nitrogen adsorption to determine surface areas, total pore volumes, and pore-size distributions and by 29Si and 19F MAS NMR spectroscopy. The most active catalysts generally have the highest total pore volumes, with pore-size distributions larger than 8 nm.Key words: sol-gel, Friedel-Crafts, Clayzic, silica.

Aluminosilicates have been prepared from aluminium tri-sec-butoxide and tetraethyl orthosilicate in butanol incorporating transition metal pentane-2,4-dionate complexes, which on calcination give active Friedel–Crafts alkylation catalysts having texturalmesoporosity. Addition of β-diketonates to the aluminosilicate sol–gel results in complexation to aluminium which, on calcination,generates a mesoporous aluminosilicate with increased surface area and pore volume. This provides a suitable framework upon which zinc chloride may be supported at higher loadings than for either the unmodified material or K10, the support for the commercial catalyst Clayzic, to give materials with significantly higher catalytic activity in a Friedel–Crafts alkylation reaction.

Aluminosilicate catalyst supports have been prepared from aluminium tri-sec-butoxide and tetraethyl orthosilicate in butanol using sol–gel methodologies, thereby providing useful supports for zinc chloride, to generate heterogeneous Friedel–Crafts alkylation catalysts. The optimum loading for these sol–gel-derived materials is higher than that of K10, the support for the commercially available catalyst ‘Clayzic’, and gives significantly higher catalytic activity in a standard test reaction. A more convenient, ‘one-pot’ approach, in which zinc chloride is added during the sol–gel synthesis, generates catalysts which also show significantly higher activity than Clayzic. The resulting materials have been characterized by 29 Si cross-polarization magic-angle spinning (CP-MAS) and 27 Al MAS NMR, IR spectroscopy and by surface area and pore volume analysis. The Si:Al ratio, drying temperature and relative amounts of water and (non-aqueous) solvent have pronounced effects on catalytic activity. The most active catalysts show relatively high levels of framework aluminium species compared with non-framework sites, and also have higher surface areas and pore volumes than materials with lower activity.

Carbothermal formation of low-z β′-sialon (z=0.5) from halloysite clay has been carried out using two different forms of the necessary additional silicon, namely, finely divided SiO 2 (quartz) and elemental Si. XRD and solid-state 29 Si and 27 Al MAS NMR studies of the early stages of the reaction at 1000–1400 °C show that the SiO 2 forms both Si 3 N 4 , via a series of oxynitrides, and SiC. These then react with the mullite produced by thermal decomposition of the clay, to form β′-sialon. The addition of 3 mass% Y 2 O 3 to the reaction mix does not change the reaction sequence, but facilitates the formation of both mullite and β′-sialon, in which it also promotes the formation of Al–N units. Using elemental Si, some SiC is formed, which reacts with the mullite and remaining Si to form O′-sialon, which may then react further with mullite to produce β′-sialon. The addition of Y 2 O 3 forms transient Y 2 Si 2 O 7 at lower temperatures, but in the later stages facilitates the formation of β′-sialon with respect to O′-sialon.

Amidines as strong Lewis bases react with amidine–mixed boron trihalide adduct systems D•BFnCl3−n (n = 0–3) in the presence of excess boron trihalide to give complex mixtures of products including the mixed tetrahaloborate anions BFnCl4−n− and the fluoroboron cations D2BF2+ and D3BF2+, which coexist with the neutral adducts D•BFnCl3−n. Excess amidine rapidly displaces chloride ion from the chlorofluoroborate anions and neutral mixed boron trihalide adducts to give high yields of the fluoroboron cations. The D3BF2+ cations of the amidines 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) and 1,5-diazabicyclo[4,3,0]non-5-ene (DBN) are inert and have been isolated as their hexafluorophosphate salts. (DBN)2BF2+ is inert and has been isolated as its hexafluorophosphate salt, but (DBU)2BF2+ is highly reactive. Key words: amidines, fluoroboron cations, mixed boron trihalide adducts, redistribution reactions, fluorine-19 NMR, boron-11 NMR, 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU), 1,5-diazabicyclo[4,3,0]non-5-ene (DBN)

Carbon-13 chemical shifts of alkene carbons are observed in the ranges 78–109 ppm (Cα) and 154–164 ppm (Cβ) for a series of 11 2-acylidene-3,5-diaryl-2,3-dihydro-1,3,4-thiadiazoles, 3 2-acylidene-3-alkyl-2,3-dihydrobenzothiazoles, and 2 2-acylidene-3-alkyl-2,3-dihydrobenzoselenazoles of known geometry, indicating appreciable charge polarization in these compounds as in other push–pull olefins. Substitution that promotes more extensive charge delocalization results in the Cα signal shifting to the higher-frequency end of the chemical shift range. The observed shifts are compared with those calculated according to the Pretsch scheme. Key words: carbon-13 NMR, chemical shifts, push–pull olefins, 1,3,4-thiadiazoles, benzothiazoles, benzoselenazoles.

The adducts pyr•BF2Br and pyr•BFBr2 (pyr = pyridine) form fluoroboron cations by displacement of Br− by excess pyridine, the ease of cation formation being pyr2BF2+ » pyr2BFBr+ » pyr3BF2+•Cl− can be displaced from pyr•BF2Cl and pyr•BFCl2, but much less readily, to form pyr2BF2+, pyr2BFCl+, and, under forcing conditions, a few percent of pyr3BF2+. Non-fluorine-containing mixed boron trihalide adducts of pyridine also form haloboron cations by heaviest-halide-ion displacement, for example pyr•BClI2 giving pyr2BClI+, the ease of displacement always being I− > Br− > Cl−, and displacement always occurring more readily from mixed boron trihalide adducts than from unmixed-halogen adducts. The mechanistic implications of this are discussed. ortho Substituents greatly reduce the ability of pyridine to displace heavy halide ion, so 2-methylpyridine gives 2-Mepyr2BF2+ and 2-Mepyr2BFBr+ but not 2-Mepyr2BFCl+ or 2-Mepyr3BF2+, while 2,6-dimethylpyridine does not form any haloboron cations. 19F spin-lattice relaxation times of the fluoroboron cations are much shorter than those of neutral boron trihalide adducts in the same solution, and provide a further diagnostic test for their presence. Key words: fluoroboron cations, pyridines, mixed boron trihalide adducts, fluorine-19 NMR, boron-11 NMR.

A simple empirical relationship has been found between silicate mineral structure and 29 Si chemical shift, which can be used to calculate chemical shift from atomic positions. It is based on the magnetic susceptibility anisotropy of the bonds to second nearest neighbours modified by the bond valence and the angle at the bridging oxygen. Calculations on theoretical models of structures show an almost linear relationship between the length of the bond to the second nearest neighbour and chemical shift, with the rate of change being dependent on the angle at the bridging oxygen. The relationship is used to investigate Al-Si ordering in zunyite

The abundance of chain- (Q2), sheet- (Q3), and framework-like (Q4) tetrahedral units, and the distribution of Al among those units in CaOAl2O3SiO2 and MgOAl2O3SiO2 glasses, has been investigated by 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR). In each system two compositional series were studied: (i) average polymerization is held constant at one non-bridging oxygen per tetrahedron (Q3), and (ii) silica content held constant at 50 mol% (average polymerization varying from Q2.67 to Q4). The results indicate that (1) Al is entirely tetrahedrally coordinated and is largely in Q4 sites, except in Al-rich Q3 glasses where it is also in Q3 sites, and (2) at constant polymerization of Q3 the ‘reaction’ Q2 + Q4 = 2Q3 goes to the right with increasing Al content. 29Si chemical shifts calculated from 27Al peak positions assuming that all Al is tetrahedrally coordinated and in Q4 sites, and that only Q2, Q3, and Q4 sites are present in the glass, give reasonable agreement with measured values (±2 ppm), but this model applies only to sufficiently silicon-rich compositions in which there is negligible Al in Q3 sites.

The formation of the magnesium silicate minerals forsterite, enstatite, and roedderite by heating of amorphous “protosilicate” gels precipitated from aqueous solution has been studied by 29Si MAS nmr. Gentle drying of the hydrogels at 110° C gives materials with broad nmr signals that do not differ appreciably with preparation conditions, but the minerals formed by heating at 750° C or higher are greatly dependent on the precipitation and washing conditions of the original gel. The rare mineral roedderite, best known from studies of unequilibrated enstatite chondrite meteorites, becomes a major species along with forsterite when the hydrogels are washed with sodium hydroxide solution before drying and heating to 750° C.

Georgia kaolinite dehydroxylated at 950°C for 24 h, then leached with KOH at 90°C for 1 h, shows a well-resolved 29 Si MAS NMR resonance at -77.5 ppm, within 2 ppm of the calculated position for Si incorporated in γ-alumina spinel. From a knowledge of the total SiO 2 in the leached sample and its partitioning between the spinel and residual grain boundary phases, a maximum of 3.9 wt% is estimated to be present in the spinel.

Despite an apparently successful empirical correlation, it is incorrect to ascribe variations in 29Si and 13C chemical shifts in silicon carbide polytypes to diamagnetic anisotropy effects Keywords: silicon carbide, long-range shielding, polytypism, MAS NMR.

Nuclear magnetic resonance (NMR) spin lattice relaxation is highly sensitive to the nature, amount, and homogeneity of unpaired-electron-containing impurities in silicon carbide and similar solids, and is a promising tool for the study of dopants and impurities. In nitrogen-doped 6H silicon carbide, 13C and 29Si NMR spin lattice relaxation is highly site-dependent. Not only do carbon sites relax much more rapidly than the corresponding silicon sites, but also there are unprecedented differences in relaxation efficiency among the different carbon (and silicon) sites, consistent with much higher unpaired electron density at the higher-symmetry (Types A and B) sites than at the lowest-symmetry (Type C) site. In contrast, all sites relax at equivalent rates in undoped samples and in commercial abrasive grade material with high levels of impurities, although there are large differences in relaxation efficiency among samples. The change in the nature of the relaxation process, from exponential in high-purity to stretched exponential in lower-purity samples, is apparently related to changes in unpaired-electron mobility.

Boron trihalide adducts of tetramethylurea (1) and of 1,1-bis(dimethylamino)ethylene (2) show similar complexation shifts and restricted rotation about the central C—N bonds. Rotation becomes more hindered as the Lewis acid strength increases in the series BF3 3 3. A large high-field complexation shift of the CH2 protons of 2 corresponds to loss of the olefinic structure on formation of the BF3 adduct.

Halogen redistribution in the sparingly soluble boron trihalide adducts of Me3P, Me3PO, and Me3PS gives mixed boron trihalide adducts of these bases. Only small proportions of the fluorine-containing mixed boron trihalide adducts are present at equilibrium in the Me3PS adduct system, in accord with previous studies of adducts of other sulfur donors. The effects of the donor on F,Cl and F,Br halogen redistribution are discussed.

13C nmr confirms that coordination of metal ions by pyridoxine is through the C-3 and C-4′ oxygens in aqueous solution. Nitrogen appears to become more effective as a donor site in water–dimethylsulfoxide mixtures, while with increasing proportions of DMSO some cations are coordinated by DMSO rather than by pyridoxine. Changes in 13C spin–lattice relaxation times on metal ion coordination are more informative about metal binding sites than changes in 13C chemical shift. Some analogous results are reported for pyridoxamine.

Tetramethylthiourea and tetramethylselenourea form 1:1 BF3 adducts which are partially converted to ionic structures (donor)2BF2+.BF4− in solution. The degree of ionization is small except at low temperatures when BF3 is in excess; ionization is essentially complete at −90 °C when a sufficient excess of BF3 is present. Formation of B2F7− from BF4− and BF3 at low temperatures is responsible for this. Hence under these conditions the 1:1 adduct is replaced by (donor)2BF2+.B2F7−. The existence of ionic adducts is further confirmed by the identification of BF3Cl− and BF2Cl2− in the mixed-halogen system tetramethylthiourea–BF3–BCl3.Methyl group nonequivalence in the proton spectrum below −70 °C indicates restricted rotation about the central C—N bonds of coordinated tetramethylthiourea.

Detailed studies of nickel atom – benzyl halide cocondensation reactions, with and without a second organic ligand, have been carried out. Benzyl halides (X = Cl, Br) when cocondensed with nickel atoms at −196 °C give no isolable organonickel product but only bibenzyl and nickel(II) halide, indicative of an initial unstable benzylnickel compound. Allyl chloride/benzyl chloride mixtures do not give the stable η3-allylnickel chloride, but instead a highly-unstable sublimable yellow solid which apparently is the η3-benzylnickel chloride dimer. All of this work indicates complexities in the initial metal atom interactions with the ligands. A further case is reported in which the presence of aluminosilicate insulating wool drastically changes the products obtained.

The formation of four-coordinate haloboron cations from aliphatic tertiary amine adducts of the mixed boron trihalides by heavy halogen displacement has been systematically studied by 19F and 11B nuclear magnetic resonance and positive ion fast atom bombardment mass spectrometry (FAB). Low-steric-hindrance donor molecules readily displace bromide ion from tertiary amine – bromodifluoroborane adducts, D•BF2Br, to form difluoroboron cations D2BF2+ and DD′BF2+, but the corresponding dibromofiuoroborane and tribromoborane adducts are highly resistant to bromide ion displacement. Bis(tertiary amine)dichloroboron and -chloroiodoboron cations can be obtained by selective iodide displacement from D•BCl2I and D•BClI2. Fast atom bombardment mass spectrometry selectively detects the haloboron cations in preference to the neutral adducts in mixtures, and is a valuable complement to nmr in monitoring formation of the haloboron cations as well as any ionic by-products. Keywords: difluoroboron cations, dihaloboron cations, NMR, 11B, 19F, fast atom bombardment (FAB), ligand substitution, redistribution reactions.

Boron trihalide adducts of acetate and benzoate esters readily exchange halogen to form mixed boron trihalide adducts D·BF n Cl 3−n (n = 0–3). Benzoate esters, but not acetate esters, also form difluoroboron cations D 2 BF + 2 , probably by displacement of chloride ion from the BF 2 Cl adducts. In esterBF 3 BBr 3 systems reaction of the bromine-containing adducts is competetive with halogen redistrubution; a major decomposition product is the D 2 BF + 2 ion, in accord with bromide ion being a better leaving group than chloride ion.

1,1-Bis(piperidinyl)ethylene coordinates to the boron trihalides through the CH2 carbon, but the covalent adducts D.BX3 undergo rearrangement to give predominantly the ionic adducts D2BX2+.BX4−. Striking variations in 11B and 13C nmr peak widths support our proposed structures. The disappearance of peaks due to excessive broadening can be misleading in the interpretation of such systems.

Nuclear magnetic resonance (NMR) spectra from 23Na and 35C1 in fluid inclusions in samples of quartz and beryl show the potential of NMR as a powerful analytical technique for the study of fluid inclusions.

The reaction 2 hmpa·BF3⇌ BF2(hmpa)2++ BF4– occurs spontaneously in the hexamethylphosphoramide (hmpa)–BF3 system. The degree of ionization for the above reaction is 0.13 in CDCl3 solution at 27 °C. Factors favouring the relatively rare ionic form of boron trifluoride adducts are discussed.

Protonation shifts and 13C T1 values in the title systems are reported. Evidence is found for predominant O-protonation and for enhanced intermolecular association in the protonated forms of the amides.

Halogen redistribution in the sparingly soluble boron trihalide adducts of Me3P, Me3PO, and Me3PS gives mixed boron trihalide adducts of these bases. Only small proportions of the fluorine-containing mixed boron trihalide adducts are present at equilibrium in the Me3PS adduct system, in accord with previous studies of adducts of other sulfur donors. The effects of the donor on F,Cl and F,Br halogen redistribution are discussed.

Low-temperature n.m.r. studies of the Me3N + F3P·B(BF2)3 system indicate the formation of adducts F3P·B-(BF2NMe3)n(BF2)3–n(n= 1–3). The compound B8F12 is very reactive toward Me3N in dichloromethane solution at –90 °C and the only product tentatively identified is Me3N·B(BF2)3.

In methyl acetate and its mono- and di-thioanalogues, donation to the boron trihalides to form 1 : 1 complexes is from the carbonyl or thiocarbonyl group. Thus preference for the doubly bonded donor atom outweighs preferences previously noted between oxygen and sulphur as donor atoms. Complexes involving carbonyl and thiocarbonyl donation show contrasting behaviour which appears to be typical of oxygen and sulphur donation.

Boron trihalide adducts of tetramethylurea (1) and of 1,1-bis(dimethylamino)ethylene (2) show similar complexation shifts and restricted rotation about the central C—N bonds. Rotation becomes more ...