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P3_Reaction Energies

Overall errors for b3lyp models do not change

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Unformatted text preview: -311+G** basis sets actually yield slightly lower overall errors for this particular set of comparisons than the corresponding models with larger “limiting” basis sets. Overall errors for B3LYP models do not change significantly with basis set, while those for MP2 models with the two smaller basis sets are larger than those for the larger sets. Individual errors show wider deviations (see Excel spreadsheet). The B3LYP/6-31G* and B3LYP/6-311+G** models show the best overall performance, although those of the corresponding Hartree-Fock models are quite similar. 26 Table P3-4: Basis Set Summary of Deviations from G3 of Hartree-Fock, B3LYP and MP2 Energies of Structural Isomers (kJ/mol) Hartree-Fock B3LYP MP2 6-31G* 9 8 17 6-311+G** 10 7 12 16 7 cc-pVTZ cc-pVQZ 27 Acidity of Propyne: Loss of hydride anion from propyne can either occur from the sp hybridized carbon or from the sp3 hybridized carbon. The former might be expected as acetylene is a much stronger acid than ethane, while the latter might be expected as it would lead to a delocalized anion. Which deprotonation is thermodynamically favored? H3 C C C– H3C C CH H2C C C H2 – Use the B3LYP/6-311+G** model to obtain equilibrium geometries for the two anions. Which is lower in energy? Is the less stable anion likely to be detectable in an equilibrium mixture at room temperature? (Assume a threshold of 5%.) Isomers of Carboranes: Carboranes are compounds of carbon, boron and hydrogen. Depending on stoichiometry, they exhibit caged (closo), partially caged (nido from the Latin word for nest) or open (arachno from the Greek word for spider) structures. are B4C2H6 and B10C2H12 simple examples of closo structures. The former can exist in one of two isomers, and the latter in one of three isomers. H C HB H B BH HB BH C H HB CH HB BH C H ChemDraw B10C2H12for isomers Use the B3LYP/6-31G* model to obtain equilibrium geometries for the two isomers of B4C2H6 and the three isomers of B10C2H12. Examine carbon-carbon bond lengths in the appropriate isomer of each. Do these appear to be “normal” single bonds or are the significantly shorter or longer? If the latter provide a rationalization? Identify the lowestenergy isomer for B4C2H6 and B10C2H12 and provide a rationale for the observed preferences. Performance of Practical Models for Hydrogenation Reactions: A hydrogenation reaction maintains overall bond count but does not maintain individual bond counts. For example, the products of hydrogenation of ethane have the same number (eight) of σ bonds as the reactants but two new CH bonds have replaced a C-C bond and H-H bond. CH3-CH3 + H-H → 2CH4 Calculate hydrogenation energies for ethane, hydrazine, hydrogen peroxide and fluorine using the Hartree-Fock, B3LYP and MP2 models with both the 6-31G* and 6-311+G** 28 basis sets (six models in total). Correct these for zero-point energy and finite temperature using the data in Appendix X. Compare these to the results from the G3(MP2) recipe. CH3-CH3 H2N-NH2 HO-OH F- F -60 kJ/mol -185 kJ/mol -347 kJ/mol -548 kJ/mol Which (if any) models yield hydrogenation energies that are within +/-12 kJ/mol of the G3(MP2) values? Which model provides the best results overall? Energy Content of Hydrazine Fuels: According to the B3LYP/6-31G* model, which fuel delivers the greater energy on a per gram basis, hydrazine or tetramethylhydrazine? The products of oxidation are N2 and water for hydrazine and N2, water and CO2 for tetramethylhydrazine. Make certain to include the mass of the oxidizer (O2) in your calculations. How does the better of the two fuels compare with molecular hydrogen on a per gram basis? Combustion of Hydrocarbons and Fluorocarbons: Hydrocarbons are commonly used as fuels but fluorocarbons are not. Of course, fluorocarbons cannot be mined or drilled from the earth, but is there also a fundamental reason? Use the B3LYP/6-31G* model to calculate the energy of complete combustion of methane, ethane and propane (to CO2 and water) and tetrafluoromethane, hexafluoroethane and decafluoropropane (to CO2 and OF2). Point out differences between energies of combustion for the hydrocarbon and analogous fluorocarbon and relate this to their use as fuels. “Combustion” of Silanes: Silicon-oxygen polymers (“sand”) result from combustion (in oxygen) of silanes and other silicon-containing compounds. This makes it difficult to assign heats of formation. One clever solution is to “burn” such compounds in fluorine (F2) rather than in oxygen. This leads only to gaseous products (SiF4 and HF) the amounts of which may easily be determined, for example, combustion of silane. SiH4 + 4F2 SiF4 + 4HF Use the B3LYP/6-31G* model to determine energies of complete “combustion” (in F2) of silane, disilane, trisilane, 2-silyltrisilane and 2,2-disilyltrisilane (resulting in only SiF4 and HF as products). On a per gram basis, combustion of which of these produces the greates amount of heat? H SiH4 H3 S i SiH3 H3 S i Si S i H2 H3 S i S i H3 SiH3 H3 S i SiH3 Si SiH3 H3 S i SiH3...
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