William
F. Coleman
Wellesley
College
| The majority of Introductory Chemistry texts provide students with an adequate introduction to the visual aspects of the molecular orbital model for homonuclear diatomic molecules. The treatment of heteronuclear diatomic and polyatomic molecules is less uniform. Heteronuclear diatomics, when mentioned, are invariably treated as being derived from homonuclear diatomics. While the atomic orbital energy level differences in heteronuclear diatomics is sometimes pictured, the consequences of those differences for the resultant molecular orbitals are rarely discussed. The discussion of polyatomic molecular orbitals in these texts is limited to showing that parallel p-orbitals produce delocalized pi molecular orbitals. The molecules typically mentioned in this context are benzene, nitrate ion and carbonate ion. However, It is rarely pointed out that the six p-orbitals in benzene would form 6 pi molecular orbitals, and that only one of these orbitals would look like the picture in the text. Clicking on any of the molecule names below takes one to a page where interactive molecular orbitals may be viewed. In order to view the orbitals you must first install the HyperChem Web Viewer plug-in, available at the HyperChem web site,and browse with Internet Explorer 6.0 or higher running on a PC, or on a Mac running Virtual PC. (A version of these files using the older HyperChem Web Browser for Netscape may be available soon).
When a file is first opened the lowest energy molecular orbital is displayed. Right-clicking and dragging resizes the orbital. Left-clicking and dragging rotates the orbital. Right-clicking twice brings up a menu for altering various parameters of the orbital display. You can alter the molecular display, change the background color etc. The key feature of the menu is the orbital submenu. Here you can select the orbital display mode, and in the parameters section, set the orbital contour level, the transparency, the mesh size and the orbital to be displayed. For example, since there are 30 valence orbitals in benzene, there should be 30 molecular orbitals. As there are also 30 valence electrons in this molecule, 15 of those molecular orbitals should be filled. That means that the molecular orbital that is 15th highest in energy should correspond to the highest occupied molecular orbital. The HyperChem Web Browser numbers the molecular orbitals starting from zero (the lowest energy molecular orbital) so orbital 14 corresponds to the highest occupied molecular orbital. In water, this highest occupied molecular orbital would be orbital 3. The orbitals being displayed here result from PM3 semi-empirical calculations. The numerical results of the calculations are summarized in the log file generated by HyperChem. Comparing the log file to the orbital images provides students with a convenient way to connect the molecular orbitals to their atomic orbital parentage. Here is a description of the log file and a brief discussion of how to read such a file. Clicking on a molecule name in the table below takes you to a page with both the orbitals and the relevant portion of the log file for that calculation (no log file is included for C60, impressive though it may be).
A word of caution. The numbering systems for orbitals in the log file starts with 1, while that in the web browser plug-in starts at 0. Consequently, the orbital images must always be compared to an orbital in the log file one number higher. For example, the highest occupied molecular orbital in ethene would be 6 in the log file and 5 in the browser - please do not shoot the messenger. Lastly, here is a page that allows students to drag hydrogen and oxygen atomic orbitals onto a water molecule and then compare the overlaps they predict to the computed molecular orbitals. This enables students to begin to see the role that orbital symmetry plays in molecular orbital formation.
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