CF3 groups in crystal structures are often disordered. However, there are also examples with ordered CF3 groups. My impression is, that the crystal structure might be determining. E.g. I have several examples in which the CF3 groups of individual species in the crystal form layers, face-to-face to each other. In this case disorder seems to be favoured. If this is true, CF3 groups facing other groups, especially H donor groups might be ordered. Is my thinking reasonable and are there references for this?
Attached the CF3-group of an elastase inhibitor that is well ordered. In that case only one of the flourines forms a hydrogen bond (to the catalytic serine of the enzyme).
Possible solution: Structure of CF3 groups in crystal structure depends of number of paired or unpaired electrons in Carbon atoms. When Carbon atom is with four unpaired electron than if lose one electron than this CF3 group as Methylium or trifluoro CF3+ is named. When there are only two unpaired electrons in Carbon orbitals, with one empty cell, than two Fluorine atoms are interconnected with Carbon with covalent bonds. The third Fluorine atom is connected with Carbon atom with donor-acceptor mechanism. Fluorine donates two electrons in Carbon empty cell while one unpaired Fluorine atom act as radical. This group is usually named Carbontrifluorideradical (CF3). Unpaired electron is from Fluorine not from Carbon atom. Such two radical groups can join together with covalent bond making high ordered groups, face to face to each other, through Fluorine atoms. Next CF3 group, which Carbon atom has one empty cell, can join this group with same donor-acceptor mechanism to any free Fluorine electrons pair, and so on.
Simply, if F can form any stronger attractive electrostatic interaction, the structure will be most probably ordered.
If not or when the repulsive forces between partially negatively charged fluorine atoms are present, the structure will be most probably disordered.
Dear Karsten, Aliakbar and Erik, thank you; Are there any references for this?
Dear Axel,
the elastase/Inhibitor complex was published here:
http://www.ncbi.nlm.nih.gov/pubmed/21549129
or
http://www.sciencedirect.com/science/article/pii/S0022283611004761
The Protein Data Bank reference is 3Q77:
http://www.rcsb.org/pdb/explore/explore.do?structureId=3Q77
Best wishes from
Karsten
It is almost textbook knowledge (Steed & Atwood: Supramolecular Chemistry for example), but I have seen an article about using C-F...X interactions in crystal engineering only several months ago, maybe in IUCrJ/Acta Cryst. B/CrystEngComm/Crystal Growth and Design. However I cannot recall it and apparently I haven't stored it in my Mendeley :( so, search for "fluorine" AND "crystal engineering" in Scopus and limit to above mentioned journals, it should give you basic orientation.
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