This preview shows page 1. Sign up to view the full content.
Unformatted text preview: (n=1-4).
Repeat your calculations and analyses on both carbon and silicon compounds substituted
with chlorine instead of fluorine, CClnH4-n and SiClnH4-n (n=1-4).
Singlet and Triplet Carbenes: Molecules incorporating divalent carbon are referred to
as carbenes or methylenes. The parent compound, CH2 (methylene), is known to possess
a triplet ground state, with one unpaired electron residing in an in-plane σ orbital and the
other in an out-of-plane π orbital. The lowest-energy singlet state (with both electrons in
the σ orbital) is known experimentally to be approximately 42 kJ/mol higher in energy.
Because triplet methylene and other triplet carbenes have one fewer electron pair than the
corresponding singlets, Hartree-Fock models will always bias in favor the former. Thus,
the (estimated) Hartree-Fock limiting (cc-pVQZ basis set) energy difference for triplet
and singlet methylene (triplet favored) is 118 kJ/mol. B3LYP and MP2 models show no
such bias. Limiting (cc-pVQZ basis set) singlet-triplet energy splittings are 48 and 60
kJ/mol (in favor of the triplet). These two models appear to correctly assign the ground
state, and (in the very few cases for which experimental data exist) provide a good
account of singlet-triplet energy differences.
Use the B3LYP/6-31G* model to obtain equilibrium geometries of both singlet and
triplet states of methylene, difluoromethylene (CF2), dichloromethylene (CCl2) and
dibromomethylene (CBr2). Adjust the calculated singlet-triplet energy differences for
CF2, CCl2 and CBr2 to account for the error in the singlet-triplet difference for CH2 and
assign the ground state for each. Rationalize any change in preferred ground state
(relative to the parent compound) that you uncover. 24 Singlet and Triplet Cyclobutadiene: While cyclobutadiene, C4H4, is a very short-lived
molecule, the cyclobutadienyl ligand is common throughout organometallic chemistry.
Here, the coordinated metal may change the number of π electrons from 4 (meaning that
cyclobutadiene is formally an antiaromatic molecule) to a lesser or greater number. Does
cyclobutadiene itself possess a singlet or triplet ground state? To tell, obtain equilibrium
geometries for both singlet and triplet cyclobutadiene using the B3LYP/6-311+G**
model, starting from structures that are not square. Describe the geometries. Is either or
both square? If not, provide a rational as to why not?
Calculate the singlet-triplet energy difference in cyclobutadiene, and correct it to account
for the error in the corresponding difference in methylene (experimentally the triplet is
favored by 42 kJ/mol). Which state is favored and by how much?
Cyclopropylidene and Tropylidene Use the B3LYP/6-31G* model to obtain equilibrium geometries for both singlet and
triplet state of cyclopropylidene. Correct the calculated singlet-triplet energy difference to
account for the error in the corresponding difference in methylene (experimentally the
triplet is favored by 42 kJ/mol). Is the ground state different from that for methylene?
Examine both the carbon-carbon bond distances in singlet cyclopropylidene as well as its
highest-occupied molecular orbital (use cyclopropene as a reference). Speculate on the
cause behind the state preference
Repeat the calculations and analysis for tropylidene. Use cycloheptatriene as a reference
and locate the molecular orbitals in the carbene that correspond to the three occupied π
orbitals in cycloheptatriene.
Dissociation of Krypton Difluoride: While numerous compounds of xenon are known,
the only neutral krypton compound to be reported is the difluoride, XeF2. Is such a
species thermodynamically stable with regard to dissociation into xenon atom and
fluorine molecule? To tell, use the B3LYP/6-31G* model to obtain equilibrium structures
of XeF2, F2 and (the energy of) Xe.
Sulfur-Sulfur Linkages in Proteins: Nearby cysteine residues in proteins may form
sulfur-sulfur linkages. In so doing they impose geometrical constraints on the protein
which in turn affects its secondary structure. How strong are sulfur-sulfur bonds? To tell,
consider the model reaction.
H3CS-SCH3 2 CH3S
Use the B3LYP/6-311+G* model to calculate equilibrium geometries for dimethyl
disulfide and thiomethoxy radical, and calculate the energy of SS bond cleavage. How
does it compare with other bond energies (see Excel spreadsheet AH and AB bond
dissociation energies for typical examples)? 25 The next comparison involves energy differences among structural isomers.
Here, the total number of electron pairs is conserved but the numbers of
individual bond types are not maintained. G3(MP2) serves as a reference. As
with the previous bond energy comparisons, only a summary of mean
absolute errors is provided (Table P3-4).
An Excel spreadsheet containing structural isomer energies for Hartree-Fock, B3LYP and
MP2 models with 6-31G*, 6-311+G**, cc-pVTZ and cc-pVQZ basis sets is provided on
the CD-ROM accompanying this text (structural isomer energies). Hartree-Fock models with the 6-31G* and 6...
View Full Document
- Spring '09