Boron Nuclei Research Articles

Anion radicals as intermediates in the cathodic reduction of β-diketoboronates (cyclic voltammetry, EPR and uv—vis)

In β-diketoboronates (2-R 1, 4-R 2-C 3HO 2BX 2), with R 1, R 2, Me, Ph, substituted-Ph and PhCHCH and with 2X: C 10H 6O 2 — —, C 6H 4O 2 — —, oxalate, 2AcO and fluorine 2F −, an increase of the cyclovoltammetric cathodic peak potentials, E pc, generally, to the negative values was observed for various substituents (R 1, R 2): E pc(Me, Me) > (Me, Ph) > (Ph, Ph) > (PhCHCH, PhCHCH) and for various anions (X): E pc (AcO, AcO) > (C 6H 4O 2 — —) > (C 10H 6O 2 — —) > (oxalate). The highest degree of reversibility in cyclic voltammetry and simultaneously stable anion radicals in EPR investigations were found if R 1 = R 2 = Ph, less for R 1 = Me, R 2 = Ph, irreversible and without anion radicals if R 1 = R 2 = Me. According to the analysis of EPR spectra the anion radicals generated have a π-character. Their spin distribution of the unpaired electron was described by the following splitting constants (expressed in mT): a H(CH 3) = 0.4–0.7, phenyl protons a m H ≅ 0.1, a 0 H ≅ 0.25, a p H ≅ 0.3, boron nucleus a 11 B = 0.04–0.1 and fluorine a F = 0.2–0.4. A linear correlation between evaluated frequency (ν max) from the longest wavelength absorption band and cathodic peak potentials ( E pc) was found.

Microwave spectrum of dichloroborane (BHCl2)

The microwave spectrum of dichloroborane has been observed and the rotational constants of four isotopic species are determined as follows: A = 46911.09(7), B = 3185.937(10) and C = 2980.425(14) MHz for the normal species, A = 46747.14(8), B = 3099.543(14) and C = 2904.037(14)MHz for BHCl37Cl, A = 49302.05(24), B = 3185.536(32) and C = 2989.368(51) MHz for 10BHCl2 and A = 35153.18(9), B = 3186.026(15) and C = 2918.233(11) MHz for BDCl2. The following complete rs structure was determined: rs(BH) = 1.184(2) Å, rs(BCl) = 1.735(2) Å and ∠ ClBCl = 120.4(2)°. The hyperfine structure due to the two chlorine and one boron nuclei has been analysed.

Magnetic properties of the BH molecule

The magnetic susceptibility of the BH molecule, as well as the magnetic shielding at the boron nucleus, have been calculated using CASSCF wavefunctions. A variety of active spaces have been employed, thus including more and more dynamical correlation. With very large active spaces, nearly all the valence correlation can be retrieved. The effect of core correlation is discussed in a second series of calculations and found to be small. Final results are +12.5 ppm cgs for the susceptibility and −175 ppm for the magnetic shielding at the boron nucleus.

Point defects in lithium triborate (LiB3O5) crystals

Electron paramagnetic resonance (EPR), electron-nuclear double resonance, optical absorption, and thermoluminescence have been used to investigate radiation-induced point defects in a single crystal of lithium triborate (LiB3O5). Two prominent defects are observed after irradiation near liquid-nitrogen temperature with 60 kV x rays. A four-line EPR spectrum, with 12.2 G splittings, is assigned to a trapped-hole center, and another four-line EPR spectrum, with 120 G splittings, is assigned to a trapped-electron center. In each case, the nucleus responsible for the observed hyperfine is 11B. The trapped hole is localized on an oxygen ion and has a weak hyperfine interaction with one neighboring boron nucleus, whereas the trapped electron is localized primarily on a boron ion with a correspondingly larger hyperfine interaction. Both defects become thermally unstable near 125 K, and their decay (i.e., recombination) correlates with an intense thermoluminescence peak at this same temperature. An optical absorption peak at 300 nm is produced by the x rays and thermally decays at the same temperatures as the EPR spectra.

The nuclear quadrupole coupling of 10B and 11B in the carboranes B10C2H12

The nuclear quadrupole couplings of 10B and 11B in p-carborane, p-B10C2H12, were measured by the field cycling technique (nuclear quadrupole double resonance) at 77 K. The couplings are e2qQ/h(10B)=2980±40 kHz, e2qQ/h(11B)=1430±20 kHz, and η=0.49±0.03. Ab initio calculations of the electric field gradient tensors at the boron nuclei in o-, m-, and p-carborane were performed. The results based on the Gaussian basis set (9s5p/4s)/[4s2p/2s] are in good agreement with the experimental couplings for p-carborane and those for o- and m-carborane obtained earlier by us. There are no reorientational jumps of the globular carborane molecules at 77 K on the nuclear quadrupole resonance time scale. The valence bond view of bonding in the carboranes is confirmed by the good correlation of experimental quadrupole coupling constants and those derived from the valence bond structures.

Synthesis of icosahedral boranes containing bismuth

The first bismaboranes, 1,2-EBiB 10 H 10 (E=O, As, Sb, Bi), have been prepared and characterized. The compound 1,2-Bi 2 B 10 H 10 has also been characterized by a single-crystal X-ray structure determination. Two-dimensional ( 11 B− 11 B) NMR studies have been used to assign boron nuclei to specific resonances. The anion B 11 H 11 Bi − was prepared from B 11 H 14 − , BiCl 3 , and n-butyllithium. While these compounds decompose slowly at room temperature in solution, in the solid state they are surprisingly stable to heat

Multinuclear magnetic resonance studies on serine protease transition state analogues.

31P Nuclear Magnetic Resonance (NMR) studies were performed on mono- and diisopropylphosphoryl derivatives of alpha-chymotrypsin, trypsin, and subtilisin. Questions addressed included the pKa of the active center Asp...His...Ser triad in both species. While the pKa in the diisopropylphosphoryl derivatives is near 7.4 (found in this and other laboratories earlier) and reflects a nearly normal imidazolium titration curve, the apparent pKa in the monoisopropylphosphoryl enzymes (obtained by "aging" of the diisopropylphosphoryl derivatives and monitored by 31P NMR) is between 9.7 and 11.4 depending on the protease. This latter "titration" of the 31P NMR signal is reversible and presumably reflects the interaction of the imidazolium positive charge with the monoanionic phosphodiester. Of the two tetrahedral intermediates, the properties of the monoisopropylphosphoryl enzyme are probably more representative of the tetrahedral oxyanionic intermediate invoked during peptide hydrolysis. The same NMR technique was used to determine the action of PAM (pyridine-2-aldoxime methiodide, a known "antidote" for acetylcholinesterase inactivated by diisopropylfluorophosphate), on the inactivated enzymes. It was clear that the "antidote" could reverse the diisopropylphosphorylation but was ineffective on the monoisopropylphosphoryl ("aged") enzyme. 11B NMR studies were performed on phenylboronic (PBA) acid and 3,5-bis-trifluoromethylphenylboronic acid in the absence and presence of chymotrypsin and subtilisin. At 22 degrees C the former, but not the latter, compound was in fast exchange between the free and enzyme bound states. The relaxation parameters could be calculated for the bound PBA in chymotrypsin and the fluorinated analogue in subtilisin and clearly indicated that the boron nucleus was tetrahedral in the active centers, a good analogue for the tetrahedral oxyanionic intermediate.

Hyperfine fields at iron and carbon nuclei in Gd 2Fe 14C

Nuclear magnetic resonance and 57Fe Mössbauer spectroscopy were used to investigate hyperfine fields at the six crystallographically different iron sites and the single carbon site in the compound Gd 2Fe 14C. Shifts in the hyperfine fields at iron sites to values smaller than those determined in Gd 2Fe 14B provide evidence of a stronger electron transfer from the carbon atoms to the iron atoms. The hyperfine field at the carbon nuclei was found to be 2.38 T at maximum spin-echo amplitude which is the same value as that determined for the hyperfine field at the boron nuclei in Gd 2Fe 14B.

Two-dimensional boron-11-boron-11 NMR spectroscopy: coupling through bridging hydrogen atoms

Application of B- B two-dimensional correlated spectroscopy (COSY) to a variety of boranes and substituted boranes has shown coupling between B nuclei linked by bridging hydrogen atoms to be more common than previously observed. Coupling between the hydrogen-bridged boron nuclei in decaborane(14), 2-(CH3)B5H8, B4H10, (Me2N)2B4H8, ( -Me2NCH2)B5H8, and ( -Me3CCH=N)B5H8 are reported. The factors that contribute to the detection of coupling between hydrogen-bridged boron atoms are discussed.

The microwave spectrum of the HBF+ ion

Rotational transitions of the HBF+ ion were observed in the region up to 380 GHz by using a source-modulated microwave spectrometer combined with a hollow cathode free space cell. The HBF+ (or DBF+) ion was generated by a dc discharge in a mixture of BF3 and H2 (or D2). The ion was identified on the basis of the linear-molecule spectral pattern with the nuclear hyperfine structure of the boron nucleus, supported by the spectral-intensity lowering by an external magnetic field. The rotational constant, the centrifugal distortion constant, and the nuclear quadrupole coupling constant were precisely determined for three isotopic species: H11BF+, H10BF+, and D11BF+. From the observed ground-state rotational constants the combined rs parameters were obtained; rs(BH) =1.173 47(2) Å and rs(BF) =1.210 28(2) Å.

Rotational spectrum and structure of a linear B2H6–H/DF dimer

Rotational spectra have been observed at 4–18 GHz for several isotopic species of a diborane–hydrogen fluoride complex, using a Flygare, Fourier transform microwave spectrometer with a pulsed supersonic nozzle as the molecular source. The dimer is a near symmetric prolate top (κ=−0.9995); it has a linear BB–H/DF equilibrium structure with the H/D end of the H/DF attracted axially to one of the diborane’s terminal BH2 groups. The B, C, DJ, and DJK rotational constants are: for 11B2H6–HF, 2111.601(1) and 2091.308(1) MHz, and 5.83(3) and 46.1(5) kHz; for 11B2H6–DF, 2092.991(2) and 2072.772(2) MHz, and 5.34(13) and 56.6(11) kHz; and for 10B11BH6–HF, 2176.086(1) and 2154.566(1) MHz, and 6.28(3) and 56.1(5) kHz, respectively. The spectra are insensitive to the value of A, which is assumed to be that of diborane itself (79.6 GHz). The hyperfine structure of the J=0→1 transitions for 11B2H6–HF and 10B11BH6–HF shows that the outer boron nucleus in the dimer has a very small quadrupole coupling constant (‖χaa‖ ≲15 kHz for 11B) while that for the inner boron is appreciable (χaa ∼−220 kHz for 11B). The implications of these results are considered. The hyperfine structure also gives an average torsional amplitude for the HF of 27° with respect to the a axis. In-plane torsional amplitudes of 13.5° were determined for the 11B2H6 and 10B11BH6 from the rotational constants for the HF dimers. With allowance for the torsional oscillations, the B⋅⋅⋅H distance is 2.5032 and 2.5038 Å in the dimers. A somewhat smaller B⋅⋅⋅D distance of 2.4955 Å is estimated for 11B2H6–DF. The attractive potential between B2H6 and HF is estimated from DJ to have a well depth ε of ∼530 cm−1, which is comparable with that reported for the N2–HF dimer. The novel structure of the B2H6–H/DF complex is discussed.

Quadrupole coupling constants of deutron and boron nuclei in solid KBD 4

Spin-lattice relaxation times T 1 for deuterons and boron nuclei were measured in solid KBD 4. With the assumption of a vamishing asymmetry parameter the values of 106, 54 and 29 kHz were obtained for the electric quadrupole coupling constant e 2 qQ/ h of deuteron and 10B and 11B nuclei, respectively.

Beiträge zur Chemie des Bors, 124 [1] Tris(trimethylsilyl)silyl-borane und Tris(trimethylsilyl)silyl-borate / Contributions to the Chemistry of Boron, 124 [1] Tris(trimethylsilyl)silyl Boranes and Tris(trimethylsilyl)silyl Borates

Abstract The reactions of several boron halides, methoxides, alkyls and hydrides with tris(tri-methylsilyl)silyllithium, which was isolated as the solvate [(CH3)3Si]3SiLi · 3C4H8O, 1(TMSSLi) have been studied. (CH3)2B−Si[Si(CH3)3]3 and 9−[(CH3)3Si]3Si-9-BBN, were isolated, while [(CH3)3Si]3Si-B(tC4H9)2 could only be detected by 11B NMR. In addition TMSS-B[N(CH3)2]2, (TMSS)2BNMe2 and (TMSS)2BOCH3 were prepared. The methoxy-boranes (CH3)3-nB(OCH3)n add to 1 forming silylborates; however, no OCH3/Si[Si(CH3)3]3 substitution occurs. The hydrogen bridge in 9-BBN is cleaved symmetrically. The results can be explained by the basicity and the steric requirements of the TMSS group. The TMSS group exhibits a deshielding effect at the boron nucleus relative to the (CH3)3Si group for silylboranes as well as for silylborates.

NMR investigation of chemical exchange and quadrupole resonance in Y(BH4)3�2C4H8O

1. The1H and11B NMR spectra of Y(BH4)3·2C4H8O indicate the existence of three processes which influence the shape of the lines; quadrupole relaxation of boron nuclei, intermolecular exchange of BH4, groups, and intramolecular proton exchange within each boron hydride group. 2. The authors have compiled a program and made theoretical calculations of the spectra, taking account of the influence of relaxation and exchange, and have determined the activation energies of the processes. The boron hydride protons are equivalent at −80°C, showing that there is only a low barrier to intramolecular proton exchange.

Formation of amorphous micro-outgrowths during the chemical vapour deposition of boron

The formation of three-dimensional amorphous micro-outgrowths during the chemical vapour deposition of boron from hydrogen-boron trichloride gas mixtures was investigated. The micro-outgrowths originate from growths of amorphous boron nuclei. Two mechanisms for the formation of the nuclei are proposed. (1) Preferential impurity adsorption on certain surface sites which prevents incident adatoms from being captured at these sites is assumed to occur in the high adsorption system H 2-BCl 3. This causes accumulation of the adatoms and an appreciable build-up of supersaturation. Nucleation can then occur. The separate nuclei formed in this way grow and coalesce to give a continuous layer. The nucleaus density is a function of the distance between the accumulation sites of the boron adatoms which is affected by the impurity adsorption, the surface diffusion distance and the deposition rate, and is discussed in terms of these parameters. (2) Since amorphous boron can nucleate on crystalline boron, it is assumed to nucleate on rhombic-shaped micro-areas containing boron in a transitional stage between the amorphous and the crystalline states. This nucleation mechanism is assumed to be valid particularly near the borderline separating the stability regions of amorphous and crystalline boron. The micro-outgrowths are uniformly distributed over the primary boron nodules. Repeated nucleation, growth and coalescence cause surface roughness. At this stage the micro-outgrowths become non-uniformly distributed over the higher-order nodules because of a local variation in the deposition rate.

NMR study of proton and boron dynamics in KB3H8

1H and 11B second moments and spin-lattice relaxation times (in the laboratory frame) in powdered KB3H8 have been measured as a function of temperature (430>T>50K). The existence of a two-stage proton exchange mechanism has been verified and the B3 triangle has been found to execute a twofold reorientation in a temperature region below the phase transition at 316K. Above the phase transition the molecule executes isotropic reorientations. The relaxation results show that the 1H and 11B spin reservoirs are effectively decoupled by the large quadrupolar spin relaxation of the boron nuclei. Activation energies for the reorientation processes have been obtained from the relaxation data.

ESR of X-ray irradiated lithium boracites

ESR spectra of paramagnetic defects induced by X-ray irradiation in lithium chloroboracites and bromoboracites have been measured for the first time, at 9 GHz and room temperature. ESR parameters are deduced by computer simulations of the spectra for polycrystalline powders and a single crystal. The paramagnetic center is characterized by an isotropic g — factor and an anisotropic hyperfine interaction with a boron nucleus. It is shown to be an unpaired electron trapped in a vacant site of the network and localized on a sp 3 boron orbital. The defect is similar to the boron electron center (BEC) detected in borate glasses.

11B nuclear quadrupole interaction in metal hexaborides (MB 6)

The magnitude of the electric field gradient at the boron nucleus, ¦ eq¦, in divalent- and trivalent-metal hexaborides (MB 6) as well as in mixed-valent SmB 6 has been measured by the nuclear magnetic resonance method. In each group of divalent- and trivalent-metal hexaborides, ¦ eq¦ decreases remarkably with increasing lattice parameter. At a given lattice parameter, ¦ eq¦ for the trivalent-metal hexaborides is smaller than that for the divalent-metal hexaborides. The value of ¦ eq¦ for SmB 6 is situated between those for the divalent- and trivalentmetal hexaborides. On the basis of these results, the electronic structure of the hexaborides is discussed.

The temperature dependence of the PMR spectra of Sc(BH4)3 � 2C4H8O and the quadrupole relaxation of the boron and scandium nuclei

1. The NMR spectra of Sc(BH4)3 · 2C4H8O indicate equivalence of the boron hydride protons and indicate that on the time scale of NMR there is rapid proton exchange within each BH4 group. 2. On fall of temperature, there is averaging out of the spin-spin coupling of the protons with the boron and scandium nuclei owing to quadrupole relaxation of these nuclei. 3. The authors make a theoretical calculation of the spectra with allowance for the influence of quadrupole relaxation of the boron and scandium nuclei, and determine the activation energies of molecular rotation and the quadrupole binding constants of boron and scandium in Sc(BH4)3 · 2C4H8O.

Solvation spectra. Part 53.—Infrared and nuclear magnetic resonance studies of the tetrahydroborate anion in various pure solvents and binary aqueous mixtures

Borohydride salts have been studied in various solvents by infrared and n.m.r. spectroscopy. Solutions in water containing HOD in low concentration exhibit an O—D stretching frequency profile indicative of relatively strong hydrogen bonding to the anion, comparable with that for chloride ions. The asymmetric B—H stretching band for BH–4(v3) is a broad singlet in water (∼2270 cm–1) but shifts strongly to lower frequencies as basic aprotic solvents, such as hexamethylphosphoramide, are added. This is again indicative of “hydrogen-bonding” to the anions by water.The proton resonance for water shifts linearly on adding borohydride salts and from this shift and previously estimated cation shifts, a molal shift for BH–4 in water of + 0.08 p.p.m. has been derived. This falls between the shifts induced by chloride and bromide ions. The 1H resonance for the anion in water (a multiplet from coupling to the two types of magnetic boron nuclei, 10B and 11B) moves up-field as basic aprotic solvents are added after a small initial down-field shift or plateau. Heating also causes an up-field shift. We conclude that hydrogen-bonding induces a down-field shift which parallels the low-frequency shift in v3. Possible structures for the hydrogen-bonded complex are considered.In contrast, addition of t-butyl alcohol or triethylenediamine to aqueous borohydride solutions induces a rapid down-field shift. This is interpreted in terms of clathrate cage formation.