P3_Reaction Energies

For now we probe their thermochemical stability

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Unformatted text preview: , all of the compounds that do exist have large bulky groups shielding the double bond, thereby making it difficult for reagents to approach. Me SiMe3 SiMe(Cme3)2 Si M e3Si M e3Si C Me OSiMe3 Si (1-adamantyl) 1 2 CMe3 (CMe3)Me2Si Si C (CMe3)Me2Si C C M e3Si Si C CMe3 3 C (CMe3)Me2Si 4 We examine the kinetic reactivity of silaolefins elsewhere in this text. For now we, probe their thermochemical stability, specifically the thermochemistry of hydrogenation relative to that of normal olefins. Obtain equilibrium geometries for 1,1-dimethylsilaethylene (Me2Si=CH2) and isobutene, as well as their hydrogenation products, trimethylsilane and isobutene. Use the B3LYP/6-31G* model. Evaluate the energy of the reaction comparing hydrogenation energies for 1,1-dimethylsilaethylene and isobutene. Me2Si=CH2 + Me3CH → Me3SiH + Me2C=CH2 What does it tell you about the strength of the SiC double bond in the silaolefin relative to the CC double-bond strength in isobutene? Repeat your calculations for the germanium analogue of isobutene, that is, evaluate the energy of the reaction. Me2Ge=CH2 + Me3CH → Me3GeH + Me2C=CH2 Do you conclude silicon or germanium forms the stronger π bond to carbon? Elaborate any assumptions that you have made to reach this conclusion. Finally, repeat both sets of calculations using geometries obtained from the HF/3-21G model (in lieu of the B3LYP/6-31G* model), following these with energy calculations with the B3LYP/model. Have the results changed significantly. Estimate the computation cost of this approach relative to that employing “exact” geometries. Incorporating Atoms and Molecules into BuckyBall: The cavity inside Buckyball, aka, Buckmeister fullerene, fullerene of C60), is not large enough to incorporate anything but atoms and very small molecules (atomic inclusion compounds are common but only a few examples of molecular complexes have been reported). 54 Host-guest complexes need to be formed by “assembling” the fullerene in the presence the atoms/molecules that are to be incorporated, for example, a helium complex needs to be formed by assembling it helium gas. The fullerene “mesh” is very tight, and entrapped atoms and molecules cannot escape. Therefore, it is not clear if complex formation is thermodynamically driven force or whether complexes exist because fullerene does not provide an exit for its guest. Quantum chemical calculations may be employed to decide. These are large molecules and “full” treatments with models capable of producing reliable binding energies may be prohibitive in terms of computer time. An alternative is to obtain equilibrium geometries with a simple model and follow these with energy calculations with a better (and computationally more expensive) model. In this example, the HF/3-21G model serves the first role and the B3LYP/6-31G* model the second role. Even so, calculations of this magnitude are not suitable as “homework exercises”, and the results have been provided to examine and ponder. No quantum chemical calculations are required. Results for C60 and for helium, neon, hydrogen molecule, methane and water complexes of C60 obtained form the B3LYP/6-31G* model based on HF/3-21G equilibrium geometries are provided in C60 and C70 complexes. Also provided results for C70 and for the C70-hydrogen molecule complex. Calculate binding energies for the five C60 complexes. fullerene (host) + guest host-guest complex Which if any of the complexation reactions are thermodynamically driven? Are your results consistent with electrostatic potential maps for the inside surface of fullerene examined in Chapter P1? Elaborate. Calculate the binding energy of hydrogen molecule into C70. Is the guest more or less inclined thermodynamically to associate with the inside of the larger C70 host than it is with the smaller C60 host? Finally, calculate binding energies for hydrogen molecule inside both C60 and C70 cages obtained from B3LYP/6-311+G** energies again assuming HF/3-21G equilibrium geometries. Do you see a marked change either in the absolute numbers and/or in the difference between binding energies for the smaller and larger hosts from you previous results? Elaborate. Results for both C60 and C70 and together with their complexes with hydrogen molecule, obtained form the B3LYP/6-311+G** model based on HF/3-21G equilibrium geometries are provided in C60 and C70 hydrogen molecule complexes. 55 56 Thermochemical Recipes This chapter has focused on the calculation of energies of diverse chemical reactions. One lesson we have learned is that reactions that involve explicit bond making or breaking require “better” quantum chemical models than reactions that maintain bonding and only change local environment. Another lesson is that even the “best” of the models that are presently routinely applicable to molecules of moderate size may lead to errors of unacceptable magnitude. It would certainly be desirable to have a single practical model that would be able to provide energies for any reaction to within 4-8 kJ/mol and be applicable to molecules with molecular weights up t...
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