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.
Ab initio density functional calculations have been carried out on the ordered phase of ammonium chloride and the results compared with experimental data. Powder neutron diffraction measurements have been carried out on 14 ND 4Cl and 15 ND 4Cl at a temperature of 5 K; the lattice parameter is 3.8224(4) A and the N—D bond length is 1.0318(8) A. Calculated structural and spectroscopic quantities agree well with the results of the neutron diffraction refinements, and with measured spectroscopic data. The calculated barrier to reorientation of the ammonium ion about the fourfold axis is 29 kJ mol -1 , and the calculated compressibility is 3.78 ◊ 10 -11 Pa -1 . The octopole moment of the ammonium ion in the crystal has been estimated from the calculated electron density, and is about 10-20% greater than for the free ion. The octopole moment does not vary appreciably with the orientation of the ammonium ion in the lattice. There is no evidence of any covalent interaction between the ammonium and chloride ions.
A new Rietveld refinement of previously reported powder neutron diffraction data on ND 4 ReO 4 taken over a wide range of temperatures [Powell, Brown, Harnden & Reid (1993). Acta Cryst . B 49 , 463–468] is reported. Rigid-body constraints are applied to the cation geometry and occupation of a second cation orientation at higher temperatures is permitted. The resulting structure at higher temperatures is to be preferred to the previous structure with a single cation orientation and supports the pseudo-spin theory of cooperative disordering without a phase transition.
An analysis of the heat capacity of deuterated and undeuterated NH4ReO4 has been carried out in which the effects of the anisotropy of the thermal expansion have been considered, an approach hitherto not used for ammonium compounds. In the ammonium scheelites, the axial thermal expansion coefficients are very large, but of opposite sign, and as a result the volume of the scheelite lattice is nearly independent of temperature. It is shown that the correction from constant stress to constant strain results in a major contribution to the heat capacity of this highly anisotropic lattice. The difference between the experimental and calculated values of heat capacity, referred to as ΔCp, is expressed as the sum of the contributions from the anisotropy and the rotational heat capacity. The results of the analysis show that the rotational contribution is much smaller then previously thought. However, the exact contribution of the anisotropy cannot be calculated at this time because the elastic constants are not known. In calculating the heat capacity, maximum use has been made of external optical mode frequencies derived from spectroscopic measurements.
ADVERTISEMENT RETURN TO ISSUEPREVLetterNEXTDoes Molecular Mass Greatly Affect Boiling and Melting Points? (author response)R. J. C. Brown View Author Information Department of Chemistry, Queen''s University, Kingston, Ontario K7L 3N6, CanadaCite this: J. Chem. Educ. 2004, 81, 5, 643Publication Date (Web):May 1, 2004Publication History Received3 August 2009Published online1 May 2004Published inissue 1 May 2004https://doi.org/10.1021/ed081p643.3RIGHTS & PERMISSIONSArticle Views234Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (65 KB) Get e-AlertsSUBJECTS:Transition temperature Get e-Alerts
Abstract Measurements of the spin-lattice relaxation time for rhenium in KReO 4 and NH 4 ReO 4 are reported; relaxation rates are extremely fast due to the large quadrupole moment of Re. Calculations of the electric field gradient in KReO 4 show that the EFG is a very sensitive function of the orientation of the anion in the unit cell as well as of the unit cell dimensions, and that geometrical effects make a major contribution to the temperature dependence. Neutron diffraction in these two salts has been employed to determine the temperature dependence of structural parameters over a wide range of temperature, and preliminary results are reported.
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