Predictions Based on Formal Charge
A Comparison of Geometry Predictions Based on Formal Charge Considerations With Results From Computer Calculations
James E. Finholt
This discussion was written for instructors. It was written to provide a starting point for the development of a fully detailed set of student instructions to fit a particular course, available hardware and available software. The exercise has been tested on a computer using a Windows 95
The formal charge concept is a crude, but useful concept which can be used to make predictions about the relative stability of isomers and resonance structures. In this exercise predictions based on formal charge considerations
Use Lewis theory to draw all resonance forms (include triple bond structures) of NNO, NON, OOC, OCO, and OOO (ozone). Calculate the formal charge of each atom in each resonance structure. Pick out the most stable resonance form of each molecule and explain the basis for your choice. Predict the relative stability of NNO and NON and explain. Predict the relative stability of OOC and OCO and explain. Predict the geometry of each molecule and explain.
Use the computer to build each of these species. Beautify and then optimize the geometry using AM1 parameters. Note the heat of formation of each molecule and use this information to determine the most stable isomers. Determine the bond angle in each case. Compare the calculated results with your predictions and try to explain any differences.
Diatomic nitrogen has a triple bond and hydrazine (H2NNH2) has a single bond between the nitrogen atoms. Calculate the NN bond distance in NNO, N2, and H2NNH2 and use the results to provide additional support for your prediction about the most stable resonance form of NNO.
Conduct some research on other molecules and ions to compare formal charge predictions with those obtained using the computer. See if you can find any cases where the two approaches give different results.