Computed vibrational wavenumbers in ammonium fluoride crystals

Abstract

The vibrational wavenumbers of crystalline ammonium fluoride were calculated using the density functional method. The results support the experimental conclusions that the symmetric stretching mode in NH 4 F lies at a higher wavenumber than the asymmetric stretch, that these wavenumbers are reversed in ND 4 F and that the librational wavenumbers lie at ∼560 cm -1 .

Ammonium cyanate: a DFT study of crystal structure, rotational barriers and vibrational spectrum

“…Hence, for the lower frequency modes, only the larger displacement results are reliable. This is consistent with the results of previous calculations of the vibrational spectrum of ammonium fluoride [13]. For the high frequency modes, the results for the smaller displacement may be more accurate because of anharmonicity.…”

“…The librational frequencies of the ammonium cation are similar to the calculated frequencies in ammonium fluoride [13]. The very low frequency degenerate modes consist of a mixture of translational motion and librational motion of the anions.…”

“…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].…”

Incorporation of ammonium fluoride into clathrate hydrate lattices and its significance in inhibiting hydrate formation

“…Additionally, strong bands in the Raman spectrum appearing at around 2226 and 1980 cm À1 for the samples cannot be due to fundamentals and are ascribed to combinations involving n 2 and n 4 (asymmetric bending mode, T 2 ) with the unobserved librational modes (n L ) based on numerous Raman studies. [39][40][41][42] It was reported that the average hydrogen bond strength in the sI clathrate is weaker than that in ice-I h . 43 The half-width of the line is also considerably broader in the clathrate than in ice, due to the much larger distribution of hydrogen bond lengths.…”

The high resolution inelastic neutron scattering spectrum of ammonium fluoride

“…8 In combination with the most recent Raman study 11 the zone-centre frequencies of the expected vibrational modes have been calculated using density functional theory (DFT). 13 The calculated frequencies for the internal modes agreed well with experiment but the external mode frequencies were deemed un-reliable and hence not reported. It has not been possible to compare the results of the calculation for the librational mode frequencies with optical data as the librational modes have not been observed optically due to selection rules and the weak polarizability of the ammonium ion.…”