Boranes and Azaborane with QMC.ppt

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1、Boranes and Azaborane with QMC,Francesco Fracchia, Dario Bressanini, Gabriele Morosi Universit dellInsubria, Como, ITALY,QMC in the Apuan Alps VI 2010 TTI Vallico Sotto,Hydrides,Hydrides are potential hydrogen storage materials If H is bound to light elements (B, N,) materials with a high hydrogen c

2、ontent per unit weight can be obtained Accurate thermodynamic data are required Some thermodynamic properties are not known, or their accuracy is not good enough,A short history of B2H6,1912: Boranes first studied by Alfred Stock 1925: X-ray, diborane, ethane-like structure? 1934: diborane is diamag

3、netic 1937: Bauer (electron diffraction), ethane-like only 12 valence electrons. What is the structure? (Valence Bond theory) Many hypothesis,A short history of B2H6,But diborane does not easily release protons,Ethane-like, but with unpaired electrons,A short history of B2H6,1940: resonant structure

4、s with bridging hydrogens proposed,1941: added ionic structures,A short history of B2H6,1941: Schlesinger in a letter to Pauling,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

5、 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,Paulings reply:,A short history of B2H6,I do not feel very frie

6、ndly toward the structure which you mention in your letter for 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 stronge

7、r than would be expected for a structure of this type, in which there is no direct B-B bond,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,Dimerization energy of BH3 - experiments

8、,Dimerization energy BH3 + BH3 B2H6 difficult to measure,Dimerization energy of BH3 ab initio,FN-DMC calculations for BH3, B2H6, BH3CO, BH2+ Orbitals from B3LYP always slightly better than HF,QMC calculations,The Y could not be improved by adding determinants,Intermezzo,A single determinant can be i

9、mproved 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,Intermezzo,If the 2-nodal regions conjecture is true: The Kohn-Sham wave function, ev

10、en 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?,Dimerization energy of BH3 QMC,This work FN-DMC -37.04(1) Experimental (-39.1 -34.3) 2,Azabo

11、rane,Azaborane BH3NH3 is a stable, nontoxic solid 19.5% by weight is hydrogen. High melting point (110-114C) due to dihydrogen bond BH-+HN Can release hydrogen reversibly nBH3NH3 BH2NH2n+nH2,Azaborane,However No efficient regeneration process is known Lack of experimental data Dissociation energy ha

12、s not been measured BH3NH3 BH3 + NH3 Few ab initio calculations,Azaborane,FN-DMC Single determinant, B3LYP orbitals + Jastrow ZPE and thermal corrections computed with GAMESS Comparison with other calculations,NH3, comparison,Total energy of NH3,Other calculations (NH3,NH4+),Protonic Affinity of NH3 NH3 + H+ NH4+ Effect of diffuse functions in basis set,Other calculations (NH3,NH4+),Conclusions,Boron compounds are chemically very interesting There is still “work to do” Rather small molecules QMC seems well suited, even for larger boranes,