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University of Alaska Fairbanks
Department of Chemistry and Biochemistry

Updated 6-15-2019
 

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

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.
                                           
                                       
Chem 105 June 2019

    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 is a review of Lewis formulas, formal charges, and how to run WebMO.
--In Part II, students draw a Lewis formula for each of three molecules whose molecular formula and topology (whether it is linear or branched, and which atom is central) are given. 
--Predict the geometry of the central atom using VSEPR theory (use the table shown below). Then go into WebMO and build the molecule per your Lewis formula. Using the PM7 quantum program calculate the most stable geometry. Compare with your prediction.
-- A fourth, 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 represent 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.

VSEPR geometry table PDF

Alternate WebMO server. If for some reason the UAF chem dept WebMO servers are down or unreachable, the "Working Demo" website at Hope College, MI, can run MOPAC calculations. 

https://www.webmo.net/demoserver/cgi-bin/webmo/login.cgi

Login as guest, pwd guest. Be sure to include an identifier in the 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 at 490,000). Jobs on this server are limited to 1 min.

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