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Boranes and Azaborane with QMC

Boranes and Azaborane with QMC. Francesco Fracchia, Dario Bressanini , Gabriele Morosi Università dell’Insubria, Como, ITALY. QMC in the Apuan Alps VI 2010 – TTI Vallico Sotto. Hydrides. Hydrides are potential hydrogen storage materials

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Boranes and Azaborane with QMC

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  1. Boranes and Azaborane with QMC Francesco Fracchia, Dario Bressanini, Gabriele Morosi Università dell’Insubria, Como, ITALY QMC in the Apuan Alps VI 2010 – TTI Vallico Sotto

  2. Hydrides • Hydrides are potential hydrogen storage materials • If H is bound to light elements (B, N,…) materials with a high hydrogen content per unit weight can be obtained • Accurate thermodynamic data are required • Some thermodynamic properties are not known, or their accuracy is not good enough

  3. A short history of B2H6 • 1912: Boranes first studied by Alfred Stock • 1925: X-ray, diborane, ethane-like structure? • 1934: diborane is diamagnetic • 1937: Bauer (electron diffraction), ethane-like • only 12 valence electrons. What is the structure? (Valence Bond theory) • Many hypothesis

  4. A short history of B2H6 But diborane does not easily release protons Ethane-like, but with unpaired electrons

  5. A short history of B2H6 • 1940: resonant structures with bridging hydrogens proposed • 1941: added ionic structures

  6. A short history of B2H6 As a result of our work on the metallo borohydrides I definitely feel that a structure for diborane quite different from those generally proposed, would aid in correlating many of the observations we have made […] The structure I have in mind is a bridge structure, in which the two boron atoms are joined to each other through an unusual type of hydrogen bond, perhaps best represented by the following formula • 1941: Schlesinger in a letter to Pauling

  7. A short history of B2H6 I do not feel very friendly toward the structure which you mention in your letterfor the diborane molecule. So long as the suggested structure remains vague and indefinite, it is not easy to say that it is eliminated by electron data or other data. However, the force constant for the B-B vibration is I think much stronger than would be expected for a structure of this type, in which there is no direct B-B bond • Pauling’s reply: • Later work by Longuet-Higgins, Hedberg and Shomaker, and especially Lipscomb confirmed the 2 electron 3 center “banana” bond • 1976: Nobel prize in Chemistry to Lipscomb

  8. Dimerization energy of BH3 - experiments • Dimerization energy BH3 + BH3 → B2H6 difficult to measure

  9. Dimerization energy of BH3 – ab initio

  10. QMC calculations • FN-DMC calculations for BH3, B2H6, BH3CO, BH2+ • Orbitals from B3LYP always slightly better than HF • The Y could not be improved by adding determinants

  11. Intermezzo • A single determinant can be improved using a Backflow Transformation • However using Backflow alone, on a single determinant Y is not sufficient, since the topology is still wrong We still must add determinants or use a different functional form

  12. Intermezzo • If the 2-nodal regions conjecture is true: • The Kohn-Sham wave function, even with the exact exchange-correlation, has the wrong nodal topology (Nodal regions > 2) • Wrong Y but exact density and energy • In QMC we use Y from DFT • Should we trust it? • Why does it give better nodes than HF?

  13. Dimerization energy of BH3 – QMC This work FN-DMC -37.04(1) Experimental (-39.1 ~ -34.3) ±2

  14. Azaborane • AzaboraneBH3NH3is a stable, nontoxic solid • 19.5% by weight is hydrogen. • High melting point (110-114°C) due to dihydrogen bond B−Hδ-···δ+H−N • Can release hydrogen reversibly • nBH3NH3→ [BH2NH2]n+nH2

  15. Azaborane • However • No efficient regeneration process is known • Lack of experimental data • Dissociation energy has not been measured • BH3NH3 → BH3 + NH3 • Few ab initio calculations

  16. Azaborane • FN-DMC • Single determinant, B3LYP orbitals + Jastrow • ZPE and thermal corrections computed with GAMESS • Comparison with other calculations

  17. NH3, comparison • Total energy of NH3

  18. Other calculations (NH3,NH4+) • Protonic Affinity of NH3 • NH3 + H+ → NH4+ • Effect of diffuse functions in basis set

  19. Other calculations (NH3,NH4+)

  20. Conclusions • Boron compounds are chemically very interesting • There is still “work to do” • Rather small molecules • QMC seems well suited, even for larger boranes

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