Diborane Molecule Research Articles

Ab initio molecular dynamics study of the hydrolysis reaction of diborane

The hydrolysis reaction of the diborane molecule in aqueous solution has been studied by a series of Car-Parrinello Molecular Dynamics simulations in the Blue Moon Ensemble. The total reaction has been divided into two parts: one dealing with the breaking of B(2)H(6) molecule and the formation of a BH(4)(-) ion, a H(2)BOH molecule and a H(+) ion; the second leads to the formation of two hydrogen molecules and another H(2)BOH molecule, starting from BH(4)(-), two water molecules and a H(+) ion. The total reaction studied in this work has been B(2)H(6) + 2H(2)O --> 2H(2)BOH + 2H(2). We have described both structurally and electronically the reagents and the products through the radial distribution functions and the Wannier Function Center positions calculations, with attention to the solvent effects on the compounds. The free energy barrier value for the first part of the reaction and a detailed mechanisms for both parts have been reported. An interesting behavior of BH(3) and H(2) molecules in solution has been observed. They form a quite stable three center bond between the electron pair of the hydrogen molecule and the empty orbital of the boron atom in BH(3), which has been described from both a structural and electronic point of view.

The two-photon $\sf 2s\rightarrow1s$ transition of the fully-ionized muonic boron migrating in helium

In recent experiments performed at the Paul Scherrer Institute (PSI) measurements were made of the yield of the two-photon transition of the fully-ionized muonic boron formed and quenched in a gaseous mixture of diborane B2H6 and helium. In the present work this yield is calculated for an idealized case of a very low diborane density when the main 2 s state decay modes competing with the two-photon transition are due to the electron transfer from helium to the μ-ion. As the rate of this transfer depends strongly on the relative velocity, a treatment is needed of the whole kinetics of processes occurring with the μ-ion in helium. Accordingly, within the optical model with a complex potential constructed before we calculate cross-sections of the elastic scattering and electron transfer. Then the time evolution of the μ-ion energy is considered and, finally, the yield is calculated. It proves that at helium pressures Torr this yield may be written in the form: ,where the factor C is greater than unity and increases rapidly with the initial energy E0 which the μ-ion has after its formation in the diborane molecule. Thus measurement of the pressure dependence of allows E0 to be estimated. The results obtained make it possible to suggest a similar parametrization of the PSI data.

On the electron transfer in low-energy collisions of the fully-ionized muonic boron with helium atoms

In connection with recent experiments on the observation of the metastable 2s state of muonic boron (μB)4+ formed in helium with a small admixture of diborane (B2H6) we estimated the cross-section of the electron transfer from helium to the muonic ion. The collisions energy T was considered to lie between the thermal energy and 1 a. u. The muonic ion (μB)4+ was treated as a heavy beryllium isotope Be4+. It was found that the main reaction responsible for the electron transfer at the energies specified above was: Be4+ + He → Be3+(1s) + He2+ + e-. . Its mechanism is the Auger effect in the two-electron quasi-molecule (Be-He)4+. In our estimation of its cross-section we used adiabatic quasi-molecular terms obtained by diagonalizing the electronic Hamiltonian on a basis of several diabatic states constructed from two-centre orbitals. The electronic wavefunctions built with this way were employed to calculate Auger effect rates. The outgoing electron was treated to be free. The relative motion of the nuclei was considered classically. It was found that in the region 0.001 ≤ T ≤ 0.01 (in atomic units) the cross-section was a decreasing function of T and it was small (∼ 10-2 A2). Then it starts to increase and mounts to typical atomic values (1 A2) at T = 1 ÷ 2. The comparison of the reaction rate constant estimated on the basis of the obtained cross-section with experimental results establishes the upper limit of the energy of muonic boron during the 2s state lifetime (≈ 40 ns). It proves to be about 25 eV. This limit is in agreement with the typical value of 1 eV resulting from the energy transfer in the muon capture by the diborane molecule and the recoil energy acquired in the cascade. However, it does not exclude the possibility of an additional acceleration of muonic boron during its formation in the molecule.

Preparation of Si[sbnd]N[sbnd]B films by CVD techniques: effect of SiH4 addition to B2H6 and NH3 gas mixtures

Boron nitride films (B/N> 1) were obtained from diborane and ammonia gas mixtures by chemical vapour deposition (CVD) techniques. A gradual transition from the amorphous to the hexagonal (turbostratic) structure is observed when the[B2H6]/[NH3] gas flow ratio is increased from 0.05 to 0.5. The addition of SiH4 to the gas mixture (B2H6/NH3/H2) produces important changes in the reaction process. Silane molecules react with ammonia forming SiN bonds, thus limiting the amount of NH3 molecules available to react with the diborane molecules present in the reactor. From the two parallel reactions, B2H6 + NH3 and SiH4 + NH3, hard Si-N-B composites are obtained. For a given ammonia flow rate, the atomic composition of the Si-N-B system is mainly determined by the[SiH4]/[B2H6] ratio. Following these results, a series of samples with variable composition was deposited changing the[SiH4[/]B2H6] ratio. The composition and structure of the films were analysed by infrared (IR), Auger electron (AES) and X-ray photoelectron (XPS) spectroscopies. Microhardness measurements were also made, and related to the silicon content of the films.

Reaction of diborane and ammonia gas mixtures in a chemical vapor deposition hot-wall reactor

Boron nitride has been deposited by chemical vapor deposition from diborane and ammonia gas mixtures, in the form of either films or powders depending on the pressure in the reactor. IR analysis of the boron nitride deposits has been used to obtain the composition of the films. In the temperature range studied, 600-850 o C, the effect of the deposition temperature on the reaction kinetics has been interpreted by assuming a change in the relative concentration of activated species arising from diborane and ammonia molecules. This change modifies the reaction path followed in the boron nitride synthesis through the formation of different intermediate compounds: aminodiborane in the low-temperature range (<700 o C) and borazine at the higher temperatures (≥775 o C)

Near-edge and photoemission studies of condensed diborane. Evidence for intermolecular interactions in the solid phase

Boron K near-edge X-ray absorption fine structure (NEXAFS) and photoemission spectra for condensed diborane and argon containing five percent B 2H 6 (matrix isolated case) were obtained. The near-edge spectrum obtained for the condensed diborane differs from that obtained for the matrix isolated case which resembles that reported for the gas phase. The valence band photoemission from condensed B 2H 6 appears more similar to that published for B 4H 10 than that for gaseous diborane. From the spectroscopic data, it is concluded that diborane molecules are affected by intermolecular interactions and undergo a change in electronic structure.

Bond orders in three-centre bonds: an analytical investigation into the electronic structure of diborane and the three-centre four-electron bonds of hypervalent sulphur

Analytical calculations are performed for the bond orders in two- and four-electron three-centre model systems. It is shown that the presence of the two three-centre two-electron bonds in diborane leads to bond orders of about 0.5, not only between the “bridging” hydrogens and each boron atom, but also between the two borons; the corresponding attractive boron-boron exchange interaction stabilizes the diborane molecule against the dissociation into two BH 3 molecules. In accordance with the actual observations and Musher's theory, it is demonstrated that appreciable hypervalent bond order can be formed only with rather electronegative substituents withdrawing a considerable electron population from the central sulphur. It is concluded that the bond orders and bond lengths of such hypervalent bonds must be very sensitive to any changes in the group-electronegativity of the substituents; the increase in the electronegativity of one substituent leads to the strengthening of the hypervalent bond formed with the other, in full accord with the empirical observations.

Structure factors calculated from molecular wavefunctions. II. Analysis of the molecular charge distribution in diborane

The electron density distribution in the crystal of diborane has been determined using structure factors based on molecular densities. The libration-correction and third-cumulant terms have also been included in the expression for the temperature factor. The equilibrium re values for BH bond lengths obtained by this treatment are about 0.02Åshorter than the spectroscopic ones. A total librational motion of the diborane molecule is determined: the root-mean-square oscillations about inertial axis are 11.0° (8), 9.6° (5) and 7.2° (5), respectively. The dynamic theoretical deformation density shows a three-center BHB bond picture for the bridge structure.

A simple refinement of density distributions of bonding electrons. III. Experimental static electron densities for the diborane molecule

From recently refined models of the electron density distribution in the diborane molecule, static density sections are calculated and presented as difference densities ρ(molecule at rest) - ρ(isolated atoms at rest). The sections obtained are compared with corresponding ones derived from quantum-chemical calculations by Laws, Stevens and Lipscomb.

Reaction of diborane with potassium, rubidium, and cesium borohydrides in diglyme

1. Diborane is absorbed by suspensions of the borohydrides of potassium, rubidium, and cesium in diglyme, with their simultaneous transition into solution. The solubility of the MBH4 increases in the order K to Cs. The ratio of the dissolved diborane and borohydride molecules corresponds to the formula MBH4 ·xBH3, where x ≈ 2. 2. The borohydrides separate from solution when precipitated with diethyl ether.

The electronic structure of the diborane molecule

The electronic structure of the diborane molecule is examined within a simple group function model. The group functions are taken to be a product of singly-occupied non-orthogonal orbitals. This model yields a molecular wavefunction which provides a simple interpretation while incorporating a substantial amount of chemically significant electron correlation. The resulting description makes possible a direct quantitative verification of the qualitative valence-shell electron-pair repulsion theory of directed valence.

Proton magnetic resonance studies on methyl and chloro substituted diboranes

Detailed proton NMR analyses were made on the diboranes, B 2H 6, CH 3B 2H 5, 1,1-(CH 3) 2B 2H 4, 1,2-(CH 3) 2B 2H 4, 1,1,2-(CH 3) 3B 2H 3, 1,1,2,2-(CH 3) 4B 2H 2 and ClB 2H 5. The bridge hydrogen chemical shift moves −0.40 ppm per methyl group on the diborane molecule and −1.1 ppm with a chlorine substituent. From 11B-decoupled proton spectra long-range homonuclear proton coupling could be observed in the methyl substituted diboranes: J H T−H μ = 7.5—8.7 HZ, J CH μ−H μ = 2.4−3.5HZ, J CH 3−HT( gem) =5.0—5.3 HZ, Both the terminal proton (on methyl and chloro diboranes) and the methyl proton chemical shifts are moved significantly upfield with substitution on the far boron atom.

Geometry of Molecules. II. Diborane and Ethane

The diborane molecule has been investigated in its equilibrium D2h geometry, in the D3d (ethane) structure, and three intermediate configurations. Charge-contour diagrams show that the electron distribution between the boron nuclei is considerably less concentrated than in the boron—bridge-hydrogen bonds. Calculations for ethane in both equilibrium and bridged-hydrogen configurations show that the carbon p functions of C2H6 play a much greater role in molecular binding than those of boron in B2H6 and this fact explains their different geometry.

The Problems of Classical Vibrations on Diborane

AbstractA brief survey of the methods of applying various kinds of symmetry coordinates for the calculation of normal vibration frequencies for symmetrical molecules with special reference to the more recent method of constructing the symmetry coordinates in terms of the internal coordinates of the molecular system. The importance of forming redundant symmetry coordinates when there are more internal coordinates than the vibrational degrees of freedom has been given. The present vibrational analyses have been applied to the diborane molecule after giving a short account regarding the molecular structure.

Kinetics of Two Exchange Reactions Involving Diborane

The kinetics of the exchange between deuterium and diborane were studied over a temperature range of 25°—75°C, and it was found that the reaction was 32 order with respect to diborane and zero order with respect to deuterium at high pressures. At low pressures the zero-order dependence of the deuterium changed to first-order. The activation energy of the zero-order reaction was 20.4±2 kcal/mole. A mechanism is proposed in which the borine radical reacts with deuterium on the wall and is followed by a rate determining bimolecular collision between the partially deuterated borine radical and a diborane molecule. The closely related exchange reaction between normal and completely deuterated diborane was also investigated. The total order of this reaction was 32, and it had an activation energy of 21.8±3 kcal/mole. A mechanism is proposed which involves the collision between a borine radical and a diborane molecule as the rate determining step.

Structure of Boron Hydrides

THE chief contention put forward in our earlier paper was that in diborane and similar molecules there were pairs of hydrogen atoms which occupied a special position in linking together different parts of the molecule. We agree with Dr. Burawoy that the formulation of these links as resonance hybrids represents only one way of describing this state of affairs: for example, a molecular orbital treatment of the system would not involve the use of the resonance concept. On the other hand, we doubt whether this linkage can be regarded as an electrostatic 'hydrogen bond' of the type met with in HF, H2O and NH3. The tendency to this type of dipole association decreases rapidly along the series FH, —OH, $$ NH, C$$H, and it would be remarkable if it appeared again to a high degree in $$BH. It is, of course, conceivable that the BH link has a considerable dipole with the negative end on the hydrogen atom, but it is not likely that this would lead to the formation of a stable dimer showing no tendency to further polymerization. A much more likely result would be the formation of an indefinite series of polymers, as is the case with H2O, HF and NH3. It is improbable that the small size of the boron atom is a determining factor, since the hydrogen compounds of aluminium and gallium dimerize as well as those of boron.

Infra-Red and Raman Spectra of Polyatomic Molecules XV. Diborane

The infra-red spectrum of diborane has been studied on a prism spectrometer from 400 to 4000 cm—1. Absorption occurs over much of the region investigated. A reliable and complete analysis of the infra-red and Raman spectra of diborane must await further work, but two alternative tentative analyses of the main features are presented which are in accord with the heat capacity of gaseous diborane from 100 to 300°K. One of these analyses is based on the assumed existence of a singlet electronic state at 412 cm—1 above the ground state and requires the potential barrier to internal rotation within the molecule to be of the order of 15,000 calories per mole. The alternative analysis assumes that no electronic state lies less than 1600 cm—1 above the ground state and requires a potential barrier of about 5000 calories per mole. Both analyses assume an ethane-like structure for the diborane molecule.

The Gaseous Heat Capacity and Restricted Internal Rotation of Diborane

The heat capacity of gaseous diborane has been measured from 95° to 324°K with the low pressure thermal conductivity apparatus described earlier. The bracketing method was applied over most of this temperature range, with acetylene and ethylene as comparison gases. At 95°K, ethylene and ethane were used as comparison gases and the accommodation coefficient ratios were assumed to be unity. The heat capacity at low temperatures indicates that internal rotation in the diborane molecule is restricted by a potential barrier whose height is probably between 4000 and 6000 cal./mole if the barrier is assumed to be a sinusoidal function of the internal rotational angle.