On 21 April 2014 21:57, Bernhard Rupp <hofkristall...@gmail.com> wrote:
> > > So the point is to use a meaningful qualifier that, applied as an > adjective to a space group, describes what happens if that space group acts > on a chiral object. Now the ‘enantio’ creeps in: enantio means other, > opposite, and morphos, gestalt, form or so. (Where is Tassos when you need > him…) so: The adjective of those 65 who are "not possessing improper > rotations" as "enantiomorphic", is completely illogical. They are exactly > the ones which do NOT change the ‘morph’ of any ‘enantio’. They, > logically I maintain, are ‘non-enantiogen’ because they generate no > opposite. The 11 pairs of non-enantiogenic SGs that that exist however > indeed form enantiomorphic pairs, even as groups in absence of the need to > act on a (chiral) object. One then can argue, as Ian did, that they form > chiral pairs. However, that is not necessarily a justification to call > these individual SGs themselves chiral. > > To me, the only satisfactory statement is that the 65 space groups “not > possessing improper rotations” are non-enantiogenic, and 22 of them form > enantiomorphic pairs. None of them change the handedness of a chiral object. > > > > Common use seems to be illogically “enantiomorphic” for the 65, and > semi-illogical, “chiral” for the 22 forming the 11 em pairs. Is that what > everybody including IUCr agrees upon? What does the ACA Standards > commission have to say? Who has an authoritative answer? Let there be light. > > > > Cheers, BR > > > > > > *From:* Ian Tickle [mailto:ianj...@gmail.com] > *Sent:* Sunday, April 20, 2014 4:52 PM > *To:* b...@hofkristallamt.org > *Cc:* CCP4BB@JISCMAIL.AC.UK > *Subject:* Re: [ccp4bb] Confusion about space group nomenclature > > > > > 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 > > > > > > > > >
