Assignment #4. Reaction barrier and enthalpy
Calculate the enthalpy of reaction and the barrier enthalpy for acetaldehyde
going to vinyl alcohol at the HF/3-21G, HF/6-31G(d) and B3LYP/6-31G(d)
levels of theory. At each level of theory, optimize the geometries
for the reactant, product and transition state. In an Excel spreadsheet
report for each level of theory the total enthalpies in atomic units (can
be found in the output file, just after the frequencies, on the line “Sum
of electronic and thermal Enthalpies”) and the reaction enthalpy and barrier
enthalpy in kcal/mol (1 atomic unit = 627.5095 kcal/mol). Also report
the C-C, C-O, C-H1, and O-H1 bond lengths for each structure at each level
of theory. Take careful note of the conformations indicated below.
For the transition structure, start with acetaldehyde and change the geometry
as indicated; in the job set-up select “Locate a Transition State” and
“Calculate the Force Constants Initially”, and in the Additional Keywords
window, enter “Opt=NoEigenTest”. For other calculations, this window should
always be blank. You can also use the QST2 / QST3 method to optimize the
transition state. To speed up the computations, it is best to start
a higher level calculation from the geometry optimized at a lower level.
