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Re: Absolute structure



Dear Brian,
  After e-discussion with Tony Linden, I have added two extra
_enumeration_details for _chemical_absolute_configuration. I think I
managed to conserve your formatting.
Best wishes,
  Howard

data_chemical_absolute_configuration
    _name                      '_chemical_absolute_configuration'
    _category                    chemical
    _type                        char
    loop_ _enumeration           
          _enumeration_detail    rm     
;                                        absolute configuration established by
                                         the structure determination of a
                                         compound containing a chiral reference
                                         molecule of known absolute configuration
;
                                 ad     
;                                        absolute configuration established by
                                         anomalous dispersion effects in
                                         diffraction measurements on the
                                         crystal
;
                                 rmad   
;                                        absolute configuration established by
                                         the structure determination of a
                                         compound containing a chiral reference
                                         molecule of known absolute
                                         configuration and confirmed by
                                         anomalous dispersion effects in
                                         diffraction measurements on the
                                         crystal.
;
                                 syn   
;                                        absolute configuration has not been
                                         established by anomalous dispersion effects
                                         in diffraction measurements on the
                                         crystal. The enantiomer has been assigned
                                         by reference to an unchanging chiral centre
                                         in the synthetic procedure.
;
                                 unk   
;                                        absolute configuration is unknown
                                         there being no firm chemical evidence
                                         for its assignment to hand and it
                                         having not been established by
                                         anomalous dispersion effects in
                                         diffraction measurements on the
                                         crystal. An arbitrary choice of
                                         enantiomer has been made.
;
                                 .      'inapplicable'
    _definition
;              In 'Enantiomers, Racemates, and Resolutions' by Jean-Jacques
               Andre Collet and Samuel H. Wilen: John Wiley & Sons, New
               York, it is stated that "the absolute configuration of a chiral
               substance is known when an enantiomeric structure can be
               assigned to an optically active sample of a given sign." It is
               thus always recommended to report the optical activity in
               solution of the molecule(s) using _chemical_optical_rotation.

               Sufficient conditions for the assignment of
               _chemical_absolute_configuration are as follows, in which
               Set_E is defined to be the set of non-centrosymmetric crystal
               classes {1, 2, 3, 4, 6, 222, 32, 422, 622, 23, 432}, x(u) is
               the value of the Flack(1983) parameter as given by
               _refine_ls_abs_structure_Flack, NEAR = 1.6 and FAR = 5.0.

               If the crystal class is NOT in Set_E then
               _chemical_absolute_configuration is 'inapplicable' and must
               take the value '.' if present.

               For 'rm' to be valid the crystal class must be in Set_E and the
               source of the chiral reference substance of known absolute
               configuration must be reported.

               For 'ad' to be valid the crystal class must be in Set_E and
               |x/u| < NEAR and |(1-x)/u| > FAR.

               For 'rmad' to be valid the conditions of both 'rm' and 'ad' must
               be fulfilled.

               For 'syn' to be valid the crystal class must be in Set_E and the
               chiral centre used to assign the enantiomer must be reported.

               For 'unk' to be valid the crystal class must be in Set_E.

               Ref: Flack, H. D. (1983). Acta Cryst. A39, 876-881.
;


-- 
Howard Flack        http://www.unige.ch/crystal/ahdf/Howard.Flack.html
Laboratoire de Cristallographie               Phone:(+41 22) 702 62 49
24 quai Ernest-Ansermet             mailto:Howard.Flack@cryst.unige.ch
CH-1211 Geneva 4, Switzerland                   Fax:(+41 22) 781 21 92


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