Analyzing Fluorine-Fluorine interactions in the Solid State

Abstract

The crystallisation of a series of perfluorinated mono and di-carboxylic acids and the co-crystallisation of these perfluorinated acids with some additives such as; phenanthroline, bipyridine, naphthalene and perfluoronaphthalene were investigated. All of these compounds have been structurally characterised using single crystal X-ray crystallography and quantum theory of atoms in molecules (QTAIM) in order to investigate the presence of F…F interactions in the solid state. A number of F…F contact distances less than twice the van der Waals radius of fluorine (2.94 Å) were extracted from the X-ray results of the perfluorinated mono and di carboxylic acids ranging from 2.665(5) Å in the structure of C4F8(COOH)2.2H2O to 2.924(7) Å in CF3(CF2)5COOH.H2O. The F…F interactions observed in the co-crystallised perfluorinated acid systems ranged from 2.765(3) Å in C16H3F24O8.3(C12H8N2).2(C8H2F12O4).C12H9N2 to 2.929(16) Å in the compound 2(C6H2F8O4).2(C6HF8O4).2(C12H9N2).2(C12H8N2). Converting the perfluorinated mono carboxylic acids of CF3(CF2)6COOH, CF3(CF2)3COOH and CF3(CF2)5COOH into sodium salts resulted in CF3(CF2)6COONa, CF3(CF2)5COONa and CF3(CF2)3COONa which when co-crystallised with phenanthroline showed F…F interactions ranged from 2.719(4) to 2.925(4) Å in 2CF3(CF2)6COO.C24H22N4Na2O3. Other inter-molecular interactions also observed and discussed in this work include hydrogen bonds and π-π stacking. QTAIM calculations were used in this work to study the electron densities involved in these interactions and were used to confirm the short distances as closed-shell interactions. The highest electron density involved in the observed F…F interactions was 0.0088 a.u. and the lowest one was 0.0042 a.u. while the electron densities involved in the O…H hydrogen bonds were found to be in the range 0.0717 a.u. in the crystal structure of COOH(CF2)2COO.C12H9N2 to 0.049 a.u. in the crystal structure of 2(C6H2F8O4).2(C6HF8O4).2(C12H9N2).2(C12H8N2).

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Alan Brisdon (Supervisor)