This overlap density contour plot shows the xy-plane for the Z form of acetic acid after an optimized RHF/6-311G computation with GamessPC using the wine(980913) WindowsNT emulator under Linux (Suse 6.3, kernel 2.2.13). The plotting software is Molden V.3.4. The full red circles in the plane HCCOOH are the Kimball radii of the C(1s2) and O(1s2) cores, while the open red circles depict the Kimball spheres of the CH, CC, C=O(s), C-O, and OH bonds. The superposition on the overlap density gives an idea of the simplification of the Kimball model. The out-of-plane CH-bonds, lone pairs on the O-atoms, and the p-bond of C=O are not drawn.
The optimization started from a Kimball structure, shown superimposed on the wireframe of the final structure with the above model chemistry at left. The main difference is a flattening out of the O-C=O angle while all the other structural parameters are nearly equal (demonstrated by the Kimball spheres in the top picture). With actual Kimball.exe the proton in the O-H bond is not shielded enough which causes a stronger attraction to the nearest lone pair of the C=O oxygen (incipient hydrogen bond formation, look at the density plot again, to observe the strong accumulation of electron density around the proton, also in the C-H bond. This causes much better shielding than the flat density of the Kimball spheres). Since the HOC angle is stiffer than the OCO angle (bending freq. 1295 cm-1 versus 620 cm-1), the latter is mainly deformed. Similar structural defects can be seen with Nitric Acid, and S-2-Aminobutyric Acid in the Table (go back one frame), whereas E- or trans-Nitrous Acid does not exhibit it. One of the ongoing improvements of Kimball software is the introduction of better and individual shielding parameters (now the same is used for all doubly occupied clouds).
Feb. 28, 2000 by ES