MM failure - Br)(Br webpage

From WikidChem

[edit]

Slide 29

Just as we have seen with other tools that we use, Molecular Mechanics is very useful, but it is not completely correct. It approximates atoms as spherical, but as we see in the next two slides, this is not the case.

[edit]

Slide 30

This slide shows how two Br atoms would be arranged if they were actually spherical.

[edit]

Slide 31

X-ray diffraction experiments were performed using crystals to measure the positions of Br for many different organic compounds. It was determined that Br atoms are not actually spherical, and to balance attractive and repulsive forces of neighboring molecules, the atoms are actually closer than expected. Furthermore, the exact distance of the atoms from one another varies depending on the angle: Br atoms closer to the C-Br bond are farther away, while those farther from the C-Br bond are closer. Thus, Molecular Mechanics programs only give us an estimate of the actual molecule since it does not take into account variation of position with distance.