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University of Alaska Fairbanks
Department of Chemistry and Biochemistry

Updated 12-20-2019
 

A Pi molecular orbital of PABA, and hyperlink to login page
To login, click this graphic.

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The WebMO login site is now restricted to connections from the University of Alaska Fairbanks network. If you are off-campus, VPN to UAF first, then click the graphic to log in.
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This site provides quick access to the MOPAC semi-empirical quantum mechanics program via WebMO, a graphical web-based interface used by chemists to construct molecular models, carry out quantum calculations, and view calculation results.
      Experiment 5. Testing Lewis Formulas and Valence Shell Electron   
            Repulsion (VSEPR) Models with Quantum Theory

Obtain the experiment handout and pre-lab sheet on the Chem 105 Blackboard site.

--Part I: Review of Lewis formulas, formal charges, and how to run WebMO.
--Part II: Draw Lewis formulas for two molecules whose formula and topology ("linear" or "branched", and which is the cental atom) are given. Predict the geometry of the central atom using VSEPR theory (use the table shown below).
--Part III:  Log into WebMO and build the molecules per your Lewis formula. Using the PM7 quantum program "geometry optimization" option to calculate the most stable geometry. Compare with your prediction.
--Part IV: Plastic models
--Part V:  A resonance stabilized, molecule is given. Draw two (or some cases, three) Lewis formulas representing the resonance bonding. Build and optimize each one using WebMO and PM7. Compare their energies, bond angles, and bond distances. This will hopefully provide convincing evidence that these formulas actually are different representations of a single thing.

--YouTube videos (see links below) show how to to do the various parts.
 
Login to WebMO. Start by clicking the red and blue graphic at the top right. Enter the username and password assigned to your class or lab section. If you forgot it, log on as guest, password webmo. Chem 105 jobs are limited to 10 min (but they should finish in seconds).

YouTube videos illustrating Experiment 5 :
   Parts II and III. Lewis formula. Testing VSEPR geometry prediction
   Part V. Investigating resonance formulas
   Apologies: The audio commentaries refer to a previous version of this experiment where what is now Part II was called Part I, and so on.
   Part V. Displaying 3D images of a pi-MO of a resonance-stabilized ion.

VSEPR geometry table PDF


Alternate WebMO servers for off-campus users.
If for some reason the UAF chem dept WebMO servers are down or unreachable, the following sites can be used (by anyone, anywhere):
(1) The WebMO "Working Demo" at Hope College, MI, can run MOPAC and Gaussian calculations. 
          https://www.webmo.net/demoserver/cgi-bin/webmo/login.cgi
Login as guest, pwd guest. Be sure to include an identifier in your job name, otherwise it may be difficult to find your output, since jobs are being submitted continuously from around the world (the job number currently is over 500,000). Jobs on this server are limited to 1 min.

(2)  A Google Cloud Services installation of WebMO and MOPAC2016. https://34.82.243.146/~webmo/cgi-bin/webmo/login.cgi  If your browser says it is unsafe or insecure, connect anyway, but do not enter any personal information. Username: guest, password: webmo. Jobs on this server are limited to 10 min.
 

More details.
Contact: jwkeller /at/ alaska.edu