User`s guide
7. Exercises
C
C
C
H
H
F
H
H
H
C
CC
F
H
H
H
H
H
C
CC
H
H
H
H
F
H
Do not do a comprehensive cleanup, as the Add Hydrogens function uses rules that are
applicable to stable molecules, not transition states. Instead, manually add the hydrogens,
including a hydrogen that is bonded to both C1 and C3. Adjust the hybridization of C2 to sp
2
, and
clean up the geometry only. Adjust the H-C2 bond angle appropriately so it is symmetric with
respect to the ring. Perform an Optimize Transition State PM3 calculation on the molecule. If the
job was successful, view the result to see if the molecule looks as expected. If the job failed,
rebuild the transition state but try adjusting the C-C-C bond angle to approximately 100° while
maintaining the symmetry of the ring (i.e., increase C1-C2-C3 by 15°, and then increase C3-C2-
C1 by 15°).
Create a new job using the optimized transition state geometry. Perform a Vibrational
Frequencies PM3 calculation on the previously optimized geometry. View the transition state.
Insert a picture of the nuclear motion at the transition state. How do you know that it is a
transition state?
Build and optimize the geometries of the reactant and product for this reaction. Make a table with
columns for species and energy, and include the energies of the reactant, transition state, and
product. What is the reaction barrier for 1,3 hydrogen shift of fluoropropene in kcal/mol?
18. Thermochemistry of CH
3
CHO → CO + CH
4
in the gas phase
Build and perform Geometry Optimization and Vibrational Frequencies (or Optimize+Vib Freq)
Hartree-Fock 6-31G(d) calculations on acetaldehyde, carbon monoxide, and methane. To speed
up your calculation, first pre-optimize the acetaldeyhde geometry using PM3 theory or a smaller
basis set.
Make a table of calculated values with columns for molecule, C
v
, S, E
298
, H
298
, and G
298
.
Visit the NIST webbook (http://webbook.nist.gov/chemistry) and make a table of experimental
values with columns for molecule, C
p
, S, and ∆
f
H (kcal/mol).
Compute C
p
from your calculations for each species by using the relationship that C
p
= C
v
+ R for
an ideal gas.
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