User`s guide
7. Exercises
View the results of the saddle calculation, choose
New Job Using This Geometry. Perform a
PM3 Transition State Optimization calculation.
View the results of the transition state optimization job, and choose New Job Using This
Geometry. For cosmetic purposes, open the molecule in the WebMO Editor and adjust the bonds
in the HOCC 4-membered rings to single bonds. Perform a PM3 Vibrational Frequencies
calculation. Verify that there is a single negative (imaginary) frequency, and report its frequency.
Verify that the corresponding vibrational mode corresponds to proton migration. What other
motions are involved at the transition state?
Optionally, verify that this geometry is a transition state by performing Forward and Reverse IRC
calculations, optimizing the resulting geometries, and comparing them to the reactant and product.
M. Excited States
33. UV-Vis Spectra of Conjugated Aldehydes
Build and perform Geometry Optimization calculations for the following molecules. Using the
resulting geometry, perform UV-Vis Spectra calculations for each molecule.
molecule
O
2-propenal
O
2-butenal
O
2,4-pentadienal
Abs Max (nm) 209 221 251
The first excited state (A″) corresponds to a forbidden n→π* transition and is therefore extremely
weak. The second excited state (A
′) corresponds to an allowed π→π* transition and is strongly
absorbing. Compare the computed absorption maximum to the experimentally observed
maximum in a table with columns for molecule, computed maximum, experimental maximum,
and difference. Comment on any trends that are observed.
34. Excited State Optimization and Vibrational Frequencies of
Formaldehyde
Build ground state (S
0
) formaldehyde, H
2
CO, and perform a Hartree-Fock 6-31G(d) Geometry
Optimization calculation. Using the resulting geometry, perform a Vibrational Frequencies
calculation at the same level of theory.
The first excited singlet state (S
1
) of formaldehyde is pyramidal. Build S
1
formaldehyde, and
move the O atom out of the molecular plane by adjusting the O-H-H-C dihedral angle to 20
degrees. Perform a Hartree-Fock 6-31G(d) Geometry Optimization calculation, but use the
Advanced Options dialog box and check the Excited State option. Using the resulting geometry,
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