What are the structures of these three isomers? Does the most stable
isomer predominate in the product mixture? We will use molecular
mechanics and semiempirical methods to evaluate the relative
stabilities of the isomers of dibenzalacetone. We begin with the three
geometric isomers, each of which has a number of stable conformations.
Once these are identified, the energies may be evaluated by a batch
method using Microsoft Excel.
By considering the relative stability of each isomer, the ease with
which it would pack into a crystalline lattice, and the planarity of
its conjugated electronic system you will be able to associate a
structure with each of the isomers for which data are available.
Batch Calculations on Sets of Molecules
If you have access to Microsoft Excel and wish to automate
operations in HyperChem, the good news is that you can! HyperChem comes
equipped with this capability as well as a spreadsheet (PLOT.XLS) and a
macro(PLOT.XLM) all set up to run MM Single Point Energy calculations
on a set of molecular structure files that also come with HyperChem.
Once you see how these pieces work, you can modify them to suit your
own needs or create entirely new applications. Some facility in the use
of multiple applications in Windows and with Microsoft Excel is needed.
Procedure
- Build the three geometric isomers of dibenzalacetone. Rotate about
the single C-C bonds to generate conformational isomers of each of
these. Save each unique structure (and remember the names!). Set up
(but do not carry out) a molecular mechanics, MM calculation.
- Start up Excel. Beginning at about line 5 of a new spreadsheet,
enter the names of the dibenzaldehyde structure files in a column,
omitting the ".HIN" extension. These names will be read in sequentially
and the results of calculations will be recorded in the row next to
each filename.
- Now get the macro that runs the calculations and records the
results. Under FILE choose OPEN and look in the \hyper\ directory for
PLOT.XLM. Refer to the HyperChem manual, "Getting Started", for a
line-by line guide to the commands in this macro.
- To initiate the calculation, return to the spreadsheet window and
place the cursor in the cell that contains the first structure file
name. Click once to select this cell as the active cell. Then under the
(Excel) Macro menu choose Run. In the dialog box pick PLOT.XLM!Compute.Results to run the macro.
- Numbers will be recorded in the spreadsheet. What are they? Refer
back to PLOT.XLM to find out. After the last "EXECUTE" command in the
macro are a series of data transfer statements, each beginning with
"FORMULA.ARRAY(REQUEST..." . From these we see that the first column
reports the total energy, followed by: stretch-energy, bend-energy,
torsion-energy, nonbond-energy, and electrostatic energy. Enter column
headings to identify these contributions to the MM total energy.
Compare the MM energies to identify the most stable conformer. It
may be instructive to rank the conformers in order of decreasing
stability, and to compare (perhaps overlay) structures whose energies
are close.
Recall that this calculation did not perform a geometry optimization
on any of the structures. To perform the same batch calculation with an
energy minimization/geometry optimization, or to otherwise modify the
calculation, read on!
Customizing the Calculation.
1. Changing the Computational Method. Force Field, Semi-empirical, or ab-initio?
The simplest change to make does not involve any modification of the
macro at all. Recall that it was in HyperChem, under "Set Up" that the
MM method was selected. To perform a semi-empirical calculation,
simply go back to the SetUp menu and make another choice! Then proceed
as before. If you are not doing an MM calculation, you may wish to
delete the lines in PLOT.XLM that ask HyperChem for stretch, bend, etc.
contributions to the energy.
2. Further Changes - Batch Calculations on Sets of Molecules
By making simple changes in the macro PLOT.XLM, you can tailor the
calculations and the output to suit your needs. BE SURE TO SAVE PLOT.XLM "AS IS" BEFORE MAKING CHANGES. Save each modified macro under a different name.