title: Use of intensity quotients and differences in absolute structure refinement.
abstract: Several methods for absolute structure refinement were tested using single-crystal X-ray diffraction data collected using Cu Kα radiation for 23 crystals with no element heavier than oxygen: conventional refinement using an inversion twin model, estimation using intensity quotients in SHELXL2012, estimation using Bayesian methods in PLATON, estimation using restraints consisting of numerical intensity differences in CRYSTALS and estimation using differences and quotients in TOPAS-Academic where both quantities were coded in terms of other structural parameters and implemented as restraints. The conventional refinement approach yielded accurate values of the Flack parameter, but with standard uncertainties ranging from 0.15 to 0.77. The other methods also yielded accurate values of the Flack parameter, but with much higher precision. Absolute structure was established in all cases, even for a hydrocarbon. The procedures in which restraints are coded explicitly in terms of other structural parameters enable the Flack parameter to correlate with these other parameters, so that it is determined along with those parameters during refinement.
authors.affiliation: EaStCHEM School of Chemistry and Centre for Science at Extreme Conditions, The University of Edinburgh, King's Buildings, West Mains Road, Edinburgh EH9 3JJ, Scotland. s.parsons@ed.ac.uk
authors.forename: Simon
authors.initials: S
authors.lastname: Parsons
authors.forename: Howard D
authors.initials: HD
authors.lastname: Flack
authors.forename: Trixie
authors.initials: T
authors.lastname: Wagner
mesh_terms: D000409:Alanine / Q000737:chemistry; D001499:Bayes Theorem; D003460:Crystallization; D018360:Crystallography, X-Ray / Q000379:methods*; D005973:Glutamine / Q000737:chemistry; D008333:Mandelic Acids / Q000737:chemistry; D015394:Molecular Structure
chemical_list: D008333:Mandelic Acids; D005973:Glutamine; C037938:mandelic acid; D000409:Alanine
keywords: absolute structure refinement; intensity quotients