Stability of Dibenzalacetone Isomers


Stability of Dibenzalacetone Isomers:

In the synthesis of dibenzalacetone by aldol condensation of acetone with benzaldehyde1 the primary product melts at 110-112° C and has a intense uv absorption at 330 nm. Two other isomers are known.

Experimental data on the three isomers are summarized in the table below.

Isomer #

melting point ° C

UV spectrum l (nm)

UV spectrum, e














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.


  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.




Statement (fragment) to be changed

Change to:

perform geometry optimization


change to: "do-optimization"

report heat of formation


change to:


report dipole moment

None; add a line after "total energy" or "heat-of-formation" request.

add this line:



1. Williamson, K.W. and Feiser, Experimental Organic Chemistry

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