Displacement and distortion of the ammonium ion in rotational transition states in ammonium fluoride and ammonium chloride

Abstract

Ab initio density functional calculations have been carried out on ammonium fluoride to determine the equilibrium structure and the transition state for rotation of the ammonium ion. The calculated equilibrium geometry agrees satisfactorily with crystallographic data. Optimization of the crystal geometry in the transition state for rotation results in significant distortion and displacement of the ammonium ion within the unit cell. Upon reexamination of the rotational transition states in ammonium chloride, similar distortion and displacement of the ammonium ion are found. The rotation process is similar to a carousel motion, in which the ion moves along the rotation axis and changes shape as it rotates. These results show that rigid ion models for ammonium ion rotational dynamics in crystals are, at best, incomplete.

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“…There are no large changes in the bond angles. The geometry of the cation in the transition state is very similar to that predicted in ammonium fluoride, for the case in which alternate cations are rotated [11]. One of the anions is slightly tilted away from the c axis as the cation rotates.…”

“…In the transition state for cation rotation about an N H bond, the cation is symmetrically situated with respect to one of the mirror planes, and so the geometry of the transition state could be optimized within this constraint, as was done for ammonium fluoride and chloride [11]. Only one of the two cations in the unit cell was placed in the transition state, while the other remained in the most stable orientation.…”

“…The first computational method used is similar to that described previously [10,11], namely the FEMD method of Alavi et al [15]. In this method, a free energy functional is optimized, yielding at self-consistency the electronic free energy.…”

“…In this configuration, two of the hydrogen atoms are located on a mirror plane, and the other two are symmetrically placed on opposite sides of this plane, so the forces perpendicular to the mirror plane are zero. In the calculations, an ion placed in this configuration remains at the transition state while the atomic positions in the unit cell are optimized [11]. If one of the two cations in each unit cell is rotated to this configuration, then over the whole crystal alternate ions are at the transition state.…”

“…The density functional approach has been used previously to study the structure and dynamics of several ammonium salts [10][11][12][13], and was successful in predicting the structures of NH 4 Cl and NH 4 F, and vibrational spectra of the latter salt. In addition, it was shown that at the transition state for rotation about an N H bond, the ammonium ion is subjected to distortion and displacement as it rotates [11,12]. The breaking of three hydrogen bonds allows the fourth to pull the whole cation off its normal position in the crystal, with N H bond extension and partial transfer of a proton from the (acidic) ammonium ion to the (basic) anion.…”