How good is fluorine as a hydrogen bond acceptor?

Abstract

This study is aimed at evaluating organic fluorine as a hydrogen bonding acceptor. A review of short F…H contacts from all of the organofluorine compounds deposited in the Cambridge Structural Database System (CSDS) was carried out and in parallel a theoretical estimate of the energy of such contacts with inter nuclear distance was executed. A total of 548 structures emerged which contained 1163 unique CF bonds and only 166 of these fluorine atoms posessed short CF…HX contacts of ≤ 2.35Å. Contacts between fluorine and hydrogen bound to carbon (CF…HC) represent the major category of short contacts however these were not judged to be hydrogen bonds as they are weak with energies similar to those of van der Waals complexes. Short contacts between F and the acidic hydrogens of HO or HN are rare in the CSDS with only 12 and 28 occurring respectively. There was only one contact below 2.0Å. Ab initio calculations have evaluated the relative stability and optimum distance of CF…HO bonds between water and fluoromethane and fluoroethene. It emerges that the C(sp 3)F fluorine in fluoromethane can enter into stronger hydrogen bonds than C(sp 2)F of fluoroethene. The X-ray data reinforces the conclusion that C(sp 3)F fluorine is a better hydrogen bond acceptor than C(sp 2)F fluorine. The C(sp 3)F…HO bond is less than half the strength (2.38 kcal mol −1) of a CO…HO and the C(sp 2)F…HO bond (1.48 kcal mol −1) is about half as weak again. Overall however short contacts in the Database which are consistent with an optimal F…H bond are extremely rare.