Crystal structures of the acid salts of some monobasic acids. Part XIV. Neutron-diffraction studies of potassium hydrogen bis(trifluoroacetate) and potassium deuterium bis(trifluoroacetate): crystals with short and symmetrical hydrogen bonds

The crystal structures of potassium hydrogen bis(trifluoroacetate)(I) and its deuterium analogue (II) have been determined by neutron-diffraction analysis. For (I)a= 8·773(5), b= 10·169(6), c= 9·255(6)Å, β= 98·85(7)°, Z= 4, space group I2/a, final R 5·2% for 849 non-zero data; for (II), a= 8·784(4), b= 10·177(5), c= 9·264(8)Å, β= 98·83(10)°, Z= 4, space group I2/a, final R 5·0% for 1048 data. The hydrogen bond, lying across a centre of inversion, has O ⋯ H ⋯ O 2·437(4) in (I), and O ⋯ D ⋯ O 2·437(3)Å in (II). The absence of significant isotope effect suggests that this may be a genuinely symmetrical hydrogen bond, with the hydrogen nucleus vibrating (anharmonically) in a single potential-energy well. This is strongly supported by spectroscopic measurements, as well as by analysis of the vibrational motions of the proton or deuteron, within its O ⋯ O environment. Some of the problems of studying hydrogen bonds in crystallographically symmetrical sites by diffraction methods are discussed.