I have used a achiral supramolecular host and inserted a enantiopure chiral guest by simple stirring in DCM. The host-guest complex formed was chiral (confirmed by CD spectroscopy).
However, single cyrstal xrd data showing achiral space group P -1 for the same complex. R value is 0.07 after solution. There is only one molecule in the asymmetric unit. I have also tried to solve data by chiral space group P1 but not able to locate all atoms and R value is also high (R=12%). What is the possible reason behind crystallizing in achiral space group.
Mr. Dhamija,
What you have obtained using the PLATON (http://www.cryst.chem.uu.nl/spek/platon/) validation report?
Two possible explanations can be offered for the observation of P-1 space group
i) The sample used for CD spectrum and that for X-ray crystllography may not be one and the same.
ii) The space group assignment can be incorrect.
A compound containing a chiral component should crystallize in one of the 65 Sohncke space groups. Such a compound can not crystallize in P-1 (Space group No. 2).
Please note that the Sohncke space group P1 is achiral and NOT a chiral space group as often described. However the crystal structure which is packed in this space group (P1) is chiral. For details pl. read the 2003 article "Chiral and achiral crystal structures" by H.D. Flack in Helvetica Chim Acta (especially page 914 & 915) of this article . Article Chiral and Achiral Crystal Structures
Is there really a correlation? Vice versa is OK, i.e. if you have an chiral group the basis needs to be chiral as well, but I do not see any reason why the opposite should be valid. There would be only a problem, if the structure would be described as chiral and you would measure an achiral but this is not your question... therefore again, why do you expect achiral group? Only, because you combined a chiral guest in an achiral host?
Of the 230 space groups only 22 (11 pairs) are chiral. The remaining are all achiral. Any solid containing a chiral moiety can crystallize in any of the 65 Sohncke space groups which include the 22 chiral space groups and also 43 achiral space groups for example P1, P21, P212121 etc. So in principle, a host-guest compound (containing a chiral guest) can crystallize in an achiral space group but only those space groups which can accomodate chiral crystal structures namely those which have no mirror planes and or inversion centres. In view of the CD spectral result which shows chirality, the centrosymmetric P-1 space group is unacceptable.
The assignment of P-1 space group (if correct) can also explain that there is no chiral moiety in the structure due to both enantiomers being present in equal amounts which has taken place during the stirring in the reaction medium. This is one reason why it is recommended that in such cases (chiral molecules) the same crystal that was used for structure refinement be examined by CD spectrum.
when the chiral molecule crystallizes as enantiopure material it crystallises in one of the Sohncke space group, that is a space group without mirror or inversion symmetry (like P21 or P212121). But when the same chiral molecule crystallizes as a Racemic form, then there is no space group restriction, i.e., it can crystallize in P21/c or P-1 or other space groups.
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