Nuclear spin–spin coupling constants; equilibrium and kinetic studies for fluoroberyllate complexes in solution

Abstract

19F nuclear magnetic resonance signals of aqueous solutions containing beryllium and fluoride ions have been used to study the equilibrium and kinetic processes occurring. Detailed studies of line widths, intensities, chemical shifts, and coupling constants have been made. This leads to an equilibrium constant determination Kc = 8.0 ± 2.0 × 10−2 mole kg−1 for the dissociation reaction[Formula: see text]The line width studies show that in addition to the above reaction there is fluoride ion exchange involving all three species. Rate constants at 33 °C are: k1 = 23.8 (s−1), k−1 = 3 × 102 mole−1 s−1, k2 = 4.3 × 102 mole−1 s−1 (for F− ion exchange with BeF42−), k3 = 4.5 × 102 mole−1 s−1 (for F− ion exchange with BeF32−), k4 = 4.5 × 102 mole−1 s−1 (for F− ion exchange between BeF32− and BeF4−). These values are determined within a factor of three. The equilibrium constant Kc is independent of temperature in the range 2 to 50 °C. An entropy of reaction ~ −5 e.u. is indicated.Studies of BeF2 solutions show that at least two fluoroberyllate complexes are present and these are assigned as monomeric BeF2 and (BeF2)n. Values of coupling constants and line widths in the absence of exchange processes strongly suggest the structures BeF3(H2O)− and BeF2(2H2O) for the tri- and di-fluoride species.The reduced coupling constants jX−F in the isoelectronic series BeF42−, BF4−, CF4, and NF4+correlate with atomic number in the same manner as the series TeF6, SbF6−, and SnF62−. A change in sign of the coupling constant jX–F is suggested between NF4+, CF4, BF4− as a group and BeF42−. The variation of coupling constants j and |Ψ2ns(0)| with atomic number is presented for a wide range of elements. The question of the origin of the sign of the coupling in relation to the atomic number dependence is related to a recent theory of Pople and Santry (16).