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Unformatted text preview: econdary carbocation center. Use the B3LYP/6-31G* model to obtain equilibrium geometries for the two isomers of
adamantyl cation. Are the CCC bond angles at the cation center in 2-adamantyl cation
significantly distorted from ideal (120o)? Which cation is lower in energy? Does it appear
that the need to accommodate a planar cation is more important that the benefit achieved
in going from a secondary to tertiary center? Elaborate. The final comparisons (Table P3-5) exemplify reactions in which both the
total number of electron pairs and the numbers of individual bond types are
conserved, specifically comparisons of regio and stereoisomers. As with
previous comparisons, data from G3(MP2) calculations serve as a reference. 34 Table P3-5: reference Deviations from G3(MP2) of Hartree-Fock, B3LYP and MP2
Energies of Regio and Stereoisomers (kJ/mol) isomer Hartree-Fock
6-31G* 6-311+G** 6-31G* 6-311+G** 6-31G* 6-311+G** G3 1,3-butadiene 1,2-butadiene 50 53 39 43 47 49 49 2-butyne 1-butyne 28 24 33 27 23 18 22 cyclobutene methylenecyclopropane 29 26 25 20 32 31 44 isobutene trans-2-butene
17 cyclopentene methylenecyclobutene 86 85 79 79 89 89 85 2-methyl-1,
62 methyl vinyl trans-2-butenal
4 mean absolute error 5 5 8 8 4 4 – 1,4-pentadiene
1,2-pentadiene 35 Isomers of Pentavalent Phosphorus Halides, PFnCl5-n: While pentavalent phosphorus
halides adopt trigonal bipyramidal geometries with distinct axial and equatorial
positions, these positions rapidly interconvert via pseudorotation (see discussion in
Chapter P5). Therefore, observed properties, for example, the dipole moment, will be
those of an equilbrium mixture of all possible isomers and depend on temperature.
Use the B3LYP/6-31G* model to obtain equilibrium geometries for the two isomers of
SF4Cl (with Cl in either an equatorial or axial position). Which isomer is lower in
energy? Why? Calculate the dipole moment of a sample at room temperature. Is it
different at 50 K? At 1000 K?
Repeat the calculations for SF3Cl2 (3 isomers), SF2Cl3 (3 isomers) and SFCl4 (2 isomers).
Are the results consistent with those for SF4Cl? Elaborate. Compute dipole moments at
50 K, room temperature and 1000 K.
Oxy Acids of Phosphorus: The oxy acids of phosphorus can exist in one of two isomeric
(tautomeric) forms, one in which the phosphorus is trivalent and the other in which it is
R OH R' P
R H The experimental evidence points strongly to the pentavalent species as the dominant
form. Specifically, the infrared spectra of phosphorus oxy acids contain lines
characteristic of P=O and PH bond stretches and no evidence of OH stretches (except
where R or R’ is OH). The one exception appears to be the bis(trifluoromethyl)
compound (R=R’=CF3). Despite the apparent preference for pentavalent forms, the
known “chemistry” of the phosphorus oxy acids demands involvement of the trivalent
structure. This suggests that the two must be in equilibrium, meaning that they two are
close in energy.
Use the B3LYP/6-31G* model to obtain equilibrium geometries for both trivalent and
pentavalent forms of dimethylphosphonate (R=R’=OMe). Precede your calculations by a
search of possible conformers using molecular mechanics (discussion of conformational
searching will be provided in Chapter P5). Which form is lower in energy? Is your result
consistent with the experiment? Elaborate. What temperature would be required for the
higher energy structure to be present as 5% of an equilibrium mixture?
Repeat your calculations with the bis(trifluoromethyl) compound (R=R’=CF3). If, as the
experimental data suggest, there is a change in preferred structure, propose a reason why.
Adamantene: The six atoms involved in a carbon-carbon double bond prefer to lie in the
same plane, for example, the two carbons and four hydrogens in ethylene lie in the same
plane. This does not appear to be possible for adamantene, the olefin formed from lss of
hydrogen from the stable hydrocarbon, adamantane.
- H2 36 Use the B3LYP/6-31G* model to obtain the equilibrium geometry for adamantene. Does
the molecule actually incorporate a double bond, that is, with a carbon-carbon distance in
the usual range of 1.30 to 1.34Ǻ? Is one or both of the carbons involved in the bond
puckered? Are they twisted relative to each other? Display the HOMO of adamantene,
and describe how it differs from the HOMO of a “normal” alkene.
Relate the hydrogenation energy of adamantene to that of 2-methyl-2-butene, a molecule
with similar “substituents” on the double bond. (Use the B3LYP/6-31G* model to obtain
geometries for 2-methylbutane and 2-methyl-2-butene.)
adamantene + 2-methylbutane adamantane + 2-methyl-2-butene
What does this tell you about the strengths of the two π bonds?
Bond Separation Reactions and Interaction of Substituents: A bond-separation
reaction relates any molecule that can be written in terms of a Lewis structure to th...
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This note was uploaded on 02/22/2010 for the course CHEM N/A taught by Professor Head-gordon during the Spring '09 term at Berkeley.
- Spring '09