Dear Bernhard (and others), I was looking for catchy combinations of "chiral" or "enantio" and Latin or Greek words for "support" or "allow" -- until I realized there is already a name for this very concept, used widely in chemistry: I think the precise and correct term applicable to the "65" should be pro-chiral spacegroups. They are not chiral by themselves, but addition of "something" /allows/ for the creation of a chiral object (i.e. the crystal).
Cheers, Jens On Tue, 2014-04-29 at 16:12 +0200, Bernhard Rupp wrote: > Response to off-board mail: > > >How about [calling them] non-centro-symmetric space groups, as I often tell > >my students? > > Almost, but not exact enough..... > > The 65 are only a subset of non-centrosymmetric space groups: > > Not all enantiogenic (not elements of the 65-set) space groups are > centrosymmetric. Simplest example Pm. > According to above definition Pm (and many more lacking a center of > inversion) would be a ok space group for chiral motifs. > > (when a space group has the 'center at ....' annotation in the Tables, it > has a coi and is a centrosymmetric space group). > > This implies that there are actually three types of crystal structures (cf. > Flack): > > (a) chiral (non-centrosymmetric) crystal structures > (b) centrosymmetric crystal structures > (c) achiral non-centrosymmetric crystal structures > > And just as a reminder, the substructure inversion for 3 members of the 65 is > not about the origin (0,0,0): I41, I4122, F4132 > are their own enantiomorph, so for them there is no enantiomorphic pair (eg. > I41 and I43), in fact no separate space > group I43 is even necessary - look at the SG diagram #80 - both, 41 and 43 > axes appear in the same SG. (2005 Erice paper of George explains more) > > Enough yet? > > Cheers, BR
