User`s guide
7. Exercises
perform a Vibrational Frequencies calculation at the same level of theory and with Excited State
option checked.
Complete the following table, using the scaling factor of 0.8929 for Hartree-Fock 6-31G(d)
frequency calculations.
Mode
Description
S
0
Scaled
Frequency
(cm
−1
)
S
1
Scaled
Frequency
(cm
−1
)
S
1
Literature
Frequency
(cm
−1
)
ν
1
sym CH stretch 2846
ν
2
CO stretch 1183
ν
3
CH
2
bend 1293
ν
4
out-of-plane bend --
ν
5
antisym CH stretch 2968
ν
6
CH
2
rock 904
Which vibrational modes undergo the largest change upon electronic excitation? Offer an
explanation for your result, noting that S
1
is pyramidal and that the S
0
→S
1
electronic transition is
an n
→π* transition.
N. Solutions
35. Solvation Energies of Cations
Build the ammonium cation, NH
4
+
, and perform a Geometry Optimization calculation. Be sure to
set the charge appropriately for a cation. Using the resulting geometry, perform another Geometry
Optimization calculation, but use the Advanced Options to select water as the solvent.
Repeat the gas phase and aqueous calculations for tetramethyl ammonium cation, N(CH
3
)
4
+
.
If using WebMO Pro, select the four jobs in Job Manager and click the Spreadsheet button to
obtain an immediate comparison among the four calculations. Otherwise, view the results of each
job.
Solvation energy is the energy change upon solvation, i.e., E
solvation
= E
solution
− E
gas
. The
experimental solvation energies for NH
4
+
and N(CH
3
)
4
+
are −88 kcal/mol and −60 kcal/mol,
respectively. Make a table with columns for molecule, gas phase energy, solution energy,
calculated solvation energy, and experimental solvation energy. Comment on the accuracy of the
solvation model being used.
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