Experiment #44 - Addition of Singlet Difluorocarbene to Ethylene

Optimize the geometry & calculate the HUMO, LUMO & ELPOT of CF2. Construct CF2 using the EXPERT BUILDER. Choose -X- with bent bonds & C. Click in the screen area. Choose -X & F. Click at the ends of the yellow bond lines. MINIMIZE. SAVE AS cf2. [Or, you can make a directory & save the file in it by saving the molecule as expt44/cf2. This commands creates the expt44 directory & saves molecule cf2 in it.] Use the SETUP menu. Choose SEMIEMPIRICAL. Give the calculation a title, choose AM1 & GEOMETRY OPTIMIZATION. [The MULTIPLICITY defaults to SINGLET]. SAVE & SUBMIT the calculation. DISPLAY the OUTPUT. Record the heat of formation.

SETUP the calculation of the HOMO, LUMO, & ELPOT all at once using the ADD command. In the SETUP menu, choose SURFACES. Choose SURFACE: HOMO, PROPERTY: NONE. ADD. This shows the job as "pending" in the window. Choose SURFACE: LUMO, PROPERTY:NONE. ADD. The window now shows 2 jobs pending. Choose SURFACE: DENSITY, PROPERTY:ELPOT. ADD. The box now shows 3 jobs pending. SAVE & SUBMIT. All 3 jobs will run. When they have completed, go to the DISPLAY menu & choose SURFACES. The box will show the 3 jobs listed as "completed". Use the mouse to click on the HOMO job. Choose DISPLAY SURFACE, STYLE: MESH. View & rotate the structure. CLOSE the FILE. OPEN the file, cf2, again. In the DISPLAY menu, choose SURFACES & select the LUMO job. Choose DISPLAY SURFACE, STYLE: MESH. View & rotate the structure. CLOSE the file. OPEN cf2 again, choose SURFACES, select DENSITY, & DISPLAY SURFACE. Click on MAP PROPERTY choosing the ELPOT job. View & rotate, comparing a MESH surface view to a SOLID surface view. CLOSE.

Construct the transition state model. Select NEW in the FILE menu. From RINGS, select CYCLOPROPYL. Click on the screen. Cyclopropane appears. Replace the H's with F's in the appropriate locations. In the GEOMETRY menu, choose CONSTRAIN DISTANCE. Choose two C atoms. A window opens. Click on CONSTRAIN DISTANCE. It will list the present distance between the two atoms. Change the distance to the required value. Repeat for the other two sets of C atoms. SAVE AS ts (or expt44/ts) In the SETUP menu, choose SEMIEMPIRICAL. Give the calculation a title, choose AM1, GEOMETRY OPTIMIZATION, CONSTRAINTS. Type in HESS=UNIT in the OPTIONS box. SAVE & SUBMIT. DISPLAY the OUTPUT. Record heat of formation.

Now do a normal mode analysis on the optimized structure. In the SETUP menu, choose SEMIEMPIRICAL. Give a TITLE, choose SINGLE POINT ENERGY, AM1, CONSTRAINTS. In the OPTIONS box, put FREQ. SAVE. In the SETUP menu, choose PROPERTIES. Click on FREQUENCIES. SAVE & SUBMIT the job. DISPLAY the OUTPUT. The imaginary frequency is indicated by a negative number & corresponds to the reaction coordinate. The existence of only one negative frequency means a transition state has been located. It's important to check this in the output. If you get more than one (or less than one) imaginary frequency, you have not found the transition state (unless the other negative numbers are very close to zero). In the DISPLAY menu, choose VIBRATION. All the frequencies will be listed. Click on the imaginary one & view the vibrational motion. View the others.

To complete the study, carry out geometry optimizations of ethylene & 1,1-difluorocyclopropane. Construct & optimize the geometries as above and calculate D H_rxn.

(1) Do the HOMO & LUMO resemble the qualitative pictures given in the lab write-up?

(2) What does the electrostatic potential suggest about the interaction of CF₂ with ethylene?

(3) Is the transition state "early" or "late"? Is this consistent with the Hammond Postulate?