[xc630]

Hello,

how do you identify molecules with an axis of improper rotation?

For instance these are the answers for the following molecules but WHY?

alanine: no

glycine: yes

acetic acid: yes

TeF2Br2Cl2 (w/ Te in middle & identical halogen atoms occupying adjacent sites in an octahedral arrangement around Te
no

BBRCLF
yes

THanks!

[Alpha-Omega]

An Improper Axis of Rotation is designated by the symmetry element s_n which is a compound operation. 

In order to answer those questions you will have to draw out the molecule or use simulation software to model the molecules.  Then you will have to perform the symmetry operations that compose the s_n (Improper Rotation) element.

Symmetry operations are acts of rotating or reflecting a molecule or figure. Symmetry elements are the symmetry operations that can be done on a molecule to leave it indistinguishable from the way it was when you started.


Please see the following links:

http://www.reciprocalnet.org/edumodules/symmetry/operations/improper.html

Symmetry Tutorial-Introduction:  http://symmetry.otterbein.edu/jmol/index.html

Symmetry Tutorial Improper Rotation:   http://symmetry.otterbein.edu/jmol/improper.html

Symmetry Tutorial Proper Rotation:  http://symmetry.otterbein.edu/jmol/proper.html

Basic Symmetry Tutorial (may be more difficult):  http://www.chem.swin.edu.au/modules/mod3/symmetry.html