Abstract
The Flack Parameter is now almost universally reported for all chiral materials characterized by X-ray crystallography. Its elegant simplicity was an inspired development by Howard Flack, and although the original algorithm for its computation has been strengthened by other workers, it remains an essential outcome for any crystallographic structure determination. As with any one-parameter metric, it needs to be interpreted in the context of its standard uncertainty.
Highlights
IntroductionThe formation of Howard Flack (1943–2017) was as a chemist, but he was an able mathTehmeaftoicrmianatiownhoof Htouwrnaerdd Ftlhacisk (1sk94il3l–2to017c)rwysatsaallsoagrcahpemhiicst,pbruotbhleemwsas (aFlsigouarneab1l)e.mMatohsetmoatficihains wcohnotrtiubrunteiodnths itsosckriyllsttoalclroygsrtaapllohgyrwapilhlipcapsrsoublnenmosti(cFeidgubryec1h).eMmoissttsorfehlyisincgonotnriXbu-rtaioynasntaolcyrsyissttaollroogbruapsthlyy wchiallrapcatsesriusnennoetwicemd abtyercihaelsmainstds raerleyipnrgoboanbXly-rnayotawnaellylskisntoowrnobeuvsetnlytcohmaraanctyerpisreofneeswsiomnaaltesrtiraulsctaunrde aanrealpyrsotsb.aHbleydniodtnwoetlgl rkinnodwonuteavnenentodlmesasnsytrperaomfeossfipoanpael rsst,rubcuttuirnesatenaadlypsrtso.dHuceeddida snmotagllrninudmobuetraonf ceanrdelfeuslslystwreraimtteonf, pcaarpeefrusl,lybutht oinusgtheatdouptrowdourckesdina tshmeadllonmuaminbseorfodf actaareafcuqlulyiswitiroitnte, nsp, caacreegfurolluypththoueogrhyt aonudt wcroyrsktsalilnogthraepdhoimc tawininsnoifndga. ta acquisition, space group theory and crystallographic twinning
It does not seem to be widely appreciated that the heavy atom does not have to be part of the material under investigation
X-ray crystallography can provide a robust indication of the absolute structure of the bulk of the single crystal used in the experiment, not the absolute structure of the bulk of a batch crystalline sample, and it can only provide an estimate of the enantiopurity
Summary
The formation of Howard Flack (1943–2017) was as a chemist, but he was an able mathTehmeaftoicrmianatiownhoof Htouwrnaerdd Ftlhacisk (1sk94il3l–2to017c)rwysatsaallsoagrcahpemhiicst,pbruotbhleemwsas (aFlsigouarneab1l)e.mMatohsetmoatficihains wcohnotrtiubrunteiodnths itsosckriyllsttoalclroygsrtaapllohgyrwapilhlipcapsrsoublnenmosti(cFeidgubryec1h).eMmoissttsorfehlyisincgonotnriXbu-rtaioynasntaolcyrsyissttaollroogbruapsthlyy wchiallrapcatsesriusnennoetwicemd abtyercihaelsmainstds raerleyipnrgoboanbXly-rnayotawnaellylskisntoowrnobeuvsetnlytcohmaraanctyerpisreofneeswsiomnaaltesrtiraulsctaunrde aanrealpyrsotsb.aHbleydniodtnwoetlgl rkinnodwonuteavnenentodlmesasnsytrperaomfeossfipoanpael rsst,rubcuttuirnesatenaadlypsrtso.dHuceeddida snmotagllrninudmobuetraonf ceanrdelfeuslslystwreraimtteonf, pcaarpeefrusl,lybutht oinusgtheatdouptrowdourckesdina tshmeadllonmuaminbseorfodf actaareafcuqlulyiswitiroitnte, nsp, caacreegfurolluypththoueogrhyt aonudt wcroyrsktsalilnogthraepdhoimc tawininsnoifndga. ta acquisition, space group theory and crystallographic twinning. Just as the image of an object remains visible when a mirror is rotated about its normal, so diffraction continues to occur as the crystal is rotated about the normal to the diffracting plane This means that on a suitably engineered instrument, (e.g., a four circle diffractometer), the rotational orientation (ψ) of the crystal can be varied for a given Miller index. Modern instruments are normally programmed to make between 5 and 20 observations of equivalent reflection at different ψ values. These independent measurement of the same quantity are termed redundancy, or multiplicity of observation, MoO. Measurements on a typical organometallic crystal might take a less than an hour on a laboratory instrument, or just a few minutes at a synchrotron source, yielding a data set of tens of thousands of observations
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