Hi Fellows,
thanks for the comments. Some of them agree with what I found through more (small mol) literature search. Let me explain why I am pestilent about this: If people who are already in the know use a weird term but have common understanding what it means, be it. If I introduce it in a textbook or introductory article, not so. It needs to make sense to someone who hears this term the first time. As it stopped making sense to me, I guess they’d be confused too. An important point made, was to distinguish between objects that can be chiral (i.e. have a certain defined handedness, χείρ cheir, hand), and space groups, which inherently are just a mathematical concept and in essence a set of instructions of how to deal with an object, and not chiral themselves. Ian’s space group diagrams, in contrast, are objects and they can display chirality and not be superimposable (i.e. superimpossible?). Space groups just act upon objects, be they chiral or not. 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
