Hi Bernhard My understanding, gleaned from ITC-A and ITC-B is that the 65 space groups listed here: http://www.ccp4.ac.uk/dist/html/alternate_origins.html that I assume you are referring to, are "enantiomorphic", which is defined as "not possessing improper rotations" (see http://pd.chem.ucl.ac.uk/pdnn/symm2/enantio1.htm). The non-superposable mirror image of a chiral object is called its enantiomorph, from Latin meaning "opposite form". The chiral object by itself is one of a pair of enantiomers, each being the enantiomorph of the other.
You need to be clear when talking about chirality whether you are referring to the space-group (or point-group) diagrams or to the contents of the unit cell. Not all the 65 enantiomorphic space group diagrams are chiral, even though the unit cells may be (you can have a non-enantiomorphic molecule crystallising in an enantiomorphic space group, but not vice versa). For example no triclinic, monoclinic or orthorhombic enantiomorphic SG diagrams are chiral (they are superposable on their mirror images), so enantiomorphic space group diagrams such as those of P1, P2, P21, P222, P212121 etc. do not have enantiomorphs (they can be regarded as their own enantiomorphs). However enantiomorphic space group diagrams containing 3, 4 or 6-fold screw axes are all chiral so do have enantiomorphs, e.g. there are enantiomorphic pairs P31 & P32, P41 & P43, P41212 & P43212 etc. HTH! Cheers -- Ian On 20 April 2014 00:35, Bernhard Rupp <hofkristall...@gmail.com> wrote: > Hi Fellows, > > > > because confusion is becoming a popular search term on the bb, let me > admit to one more: > > What is the proper class name for the 65 space groups (you know, those): > > > > Are > > (a) these 65 SGs the chiral SGs and the 22 in the 11 enantiomorphic > pairs the enantiomorphic SGs? > > Or > > (b) the opposite? > > > > In other words, is (a) enantiomorphic a subclass of chiral or (b) chiral > a subclass of enantiomorphic? > > Small molecule crystallography literature seems to tend to (b) whereas in > macro I often find (in terms of number of class members) chiral > > enantiomorphic. Interestingly, did not find an authoritative definition in > ITC-A. > > > > Logical is neither. The 65 are perhaps enantiostatic because they do not > change handedness (as opposed to enantiogen), and the 22 are enantiodyadic > (or so). I am sure Tassos will enlighten us on that one…. > > > > So, (a) or (b) or ? > > > > Happy Easter, BR > > > > > > >
