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.
SiMe(Cme3)2 Si M e3Si
M e3Si C Me OSiMe3
Si (1-adamantyl) 1 2
Si C (CMe3)Me2Si C C M e3Si
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 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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