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of “simplest molecules” that contain the same bonds. For example, the bond separation
reaction for acrolein, relates it to ethane, ethylene and formaldehyde, the simplest
molecules with carbon-carbon single and double bonds and a carbon-oxygen double
bond. Two molecules of methane need to be added to the left hand side to achieve
H2C=C-C(H)=O + 2CH4 → H2C=CH2 + H3C-CH3 + H2C=O Aside from the conformation of the reactant, the energy of a bond separation reaction is
well-defined and unique as long as the Lewis structure is well defined and unique.
Because it maintains both total and individual bond counts, it might be expected to be
reasonably well described even by models that provide poor account of electron
Bond separation reactions may be used to determine whether two substituents bonded to
carbon interact constructively (bond separation reaction is endothermic), destructively
(bond separation reaction is exothermic) or not at all.
XCH2Y + CH4 → CH3X + CH3Y Use the HF/6-31G* model to obtain equilibrium geometries and energies for all
molecules involved in bond separation reactions of molecules, CH2X2 (X=CH3, CMe3,
CN, F, SiH3). Interpret the calculated reaction energies both in terms of steric interactions
and the σ and π donor/acceptor properties of the individual substituents:
σ donor, π donor
σ acceptor, π acceptor
σ acceptor, π donor
σ donor, π acceptor Me, CMe3
SiH3 Which of the substituents interact favorably? Which interact unfavorably? Provide a
rationale for each.
Compare bond distances in CH3X and CH2X2 compounds. Do the changes parallel the
interaction energies? Elaborate. 37 Bond Stengths vs. Bond Separation Energies in Fluoromethanes: In an earlier
problem (see Chapter P2), you found that CF bond energies in fluoromethanes, CFnH4-n
(n=1-4), increase with increasing number of fluorines. Is this trend reflected in the bond
separation energies? Use the HF/6-31G* model to calculate energies for all reactants and
products for the three bond separation reactions below.
CH2F2 + CH4 → 2 CH3F
CF3H +2CH4 → 3CH3F
CF4 + 3CH4 → 4CH3F Do the bond energies of these reactions (normalized for the number of CF bonds) parallel
the bond dissociation energies calculated previously?
Repeat your calculations and analysis for bond separation reactions of fluorosilanes,
Repeat your calculations and analysis for bond separation reactions on both carbon and
silicon compounds substituted by chlorine instead of fluorine, CClnH4-n and SiClnH4-n,
(n=1-4). 38 Ion Molecule Reactions
Quantum chemical calculations are not restricted to uncharged molecules,
but may also be applied to cations and anions. While there are virtually no
experimental data relating to the geometries of in the gas phase, there is a
wealth of data relating to their energies. The two most common sources are
proton transfer reactions, that is, acidities and basicities, and electron
transfer reactions, that is, ionization potentials and electron affinities.
There are an enormous number of X-ray crystal structures of ions (together with their
counterions). Discussion has already been provided in Chapter P2. As discussed earlier in this chapter, relative acidities and basicities are most
commonly determined by ion-cyclotron-resonance (ICR) spectroscopy.
What is actually measured is the equilibrium abundance of the ions involved
in a proton transfer reactions between two bases (or two acids) of similar
strength, for example, that between methylamine and ethylamine (bases) or
between acetic acid and propanoic acid (acids). The neutral molecules are
MeNH2 + EtNH3+ MeNH3+ + EtNH2
MeCO2H + EtCO2- MeCO2- + EtCO2H
Measurements are all finally related to a single standard base (ammonia) and
standard acid (???), giving rise to absolute basicities and acidities.
Relative Acidities of Propene and Propyne: Is propene or propyne the stronger acid in
the gas phase? Use the B3LYP/6-311+G** model to calculate the geometries for the
reactants and product of the reaction. Be certain to consider all possible anions resulting
from deprotonation of propene and propyne.
propene + propyne-H+ propene-H+ + propyne Rationalize your result. Is it anticipated by comparison of electrostatic potential maps for
propene and propyne? Elaborate.
Why is Cyclopentadiene a Strong Acid? Cyclopentadiene is a much stronger acid than
1,3-pentadiene in the gas phase, that is, the following reaction is highly exothermic.
cyclopentadiene + 1,3-pentadiene - H+ 39 cyclopentadiene-H+ + 1,3-pentadiene Use the B3LYP/6-31G* model to obtain equilibrium geometries for cyclopentadiene and
its deprotonated form (cyclopentadienyl anion). Note any structural changes that have
occurred to cyclopentadiene as a result of deprotonation, and use these to rationalize its
Gas and Aqueous-Phase Basicities of Amines: The relative base strengths (proton
affinities) of free amines are known to differ significantly from those in water. For
example, in the gas-phase,...
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- Spring '09