Computational Investigation of Condensed Phase Properties of Ionic Systems
Type of Degreedissertation
Chemistry and Biochemistry
MetadataShow full item record
Four computational studies have been performed with various computational techniques including density functional theory, post-HF theory, and implicit solvation modeling to investigate the dehydrogenation mechanism of LiNH2BH3(s), acid dissociation constants (pKa1, pKa2, and pKa3) of polyprotic acids, the dissolution free energy of alkali-metal dianion salts (M2X1), and redox potentials of polyboranes. Theoretical background and computational methodology in these studies are introduced in chapter 1. Chapter 2-5 present four different studies exploring condensed-phase properties of inorganic materials including reaction mechanisms. Chapter 2 reports on the formation of LiNH2BH3 from (LiH)4 and NH3BH3 and their subsequent dehydrogenation. The free energy of activation for loss of H2 is reduced from 37.2 kcal/mol in NH3BH3 to 11.0 kcal/mol in (LiH)4 + NH3BH3. Further, H2 elimination from the (LiNH2BH3)2 dimer is predicted to be much easier than from the monomer which may suggest that a cooperative H2-loss mechanism is possible in solid LiNH2BH3. Chapter 3 reports a systematic study of ΔGaq/pKa for monoprotic, diprotic, and triprotic acids based on DFT/aug-cc-pVTZ combined with CPCM and SMD solvation modeling. All DFT/cavity set combinations considered showed similar accuracy for ΔGaq1/pKa1 (70% within ±2.5 kcal/mol of experiment) while only the M05-2X/Pauling cavity combination gave reasonable results for ΔGaq2/pKa2 when both pKa values are separated by more than three units (70% within ±5.0 kcal/mol of experiment). Chapter 4 reports on the dissolution Gibbs free energies (ΔGodiss) of salts (M2X1) using the Conductor-like Polarizable Continuum Model (CPCM) solvation modeling. The absolute solvation free energies of the alkali metal cations (ΔGsolv(M+)) come from the literature, which coincide well with half reduction potential versus SHE data. Lattice free energies (ΔGlatt) of salts were determined by three different approaches: (1) volumetric, (2) a cohesive Gibbs free energy (ΔGcoh) plus gaseous dissociation free energy (ΔGgas), and (3) the Born-Haber cycle. Only the M05-2X/Pauling combination with the three different methods for estimating ΔGlatt yields the expected negative dissolution free energies (ΔGodiss) of M2SO4. Chapter 5 reports the reduction potentials (EoRed versus SHE) of hypercloso boron hydrides BnHn (n=6-13) and B12X12 (X=F, Cl, OH, and CH3) with CPCM and SMD solvation modeling. The EoRed of BnHn-/2- (n=6-12) with the G4/M06-2X/Pauling (energy/solvation/cavity) combination agrees within 0.2 V of experimental values. The experimental oxidative stability (E1/2) of BnXn2- (X=F, Cl, OH, and CH3) is usually located between the values predicted using the B3LYP and M06-2X functionals. The disproportionation free energies (ΔGdpro) of 2BnHn- → BnHn + BnHn2- reveal that the stabilities of BnHn- (n=6-13) to disproportionation decrease in the order B8H8- > B9H9- > B11H11- > B10H10-. The spin densities in B12X12- (X=F, Cl, OH, and CH3) tend to delocalize on the boron atoms rather than on the exterior functional groups. The partitioning of ΔGsolv(BnHn2-) over spheres allows a rationalization of the nonlinear correlation between ΔGE.A. and EoRed for B6H6 /2-, B11H11-/2-, and B13H13-/2-.