Reorientational Dynamics and Solid-Phase Transformation of Ammonium Dicyanamide into Dicyandiamide:  A ²H Solid-State NMR Study

Abstract

The reorientational dynamics of ammonium dicyanamide ND 4 [N(CtN) 2 ] and the kinetics as well as the mechanism of the solid-state isomerization reaction from ammonium dicyanamide into dicyandiamide (Nt CsNdC(NH 2 ) 2 ) was studied by means of 2 H and 14 N solid-state NMR spectroscopy in a temperature range between 38 and 390 K. Whereas in previous investigations the mechanism of the solid-state transformation was investigated by means of vibrational and magic angle spinning solid-state NMR spectroscopy as well as neutron diffraction, we here present a comprehensive 2 H study of the ammonium ion dynamics prior to and during the course of the reaction, thereby highlighting possible cross correlations between dynamics and reactivity involving the ammonium ion. The ND 4 + group was found to undergo thermally activated random jumps in a tetrahedral potential, which is increasingly distorted with increasing temperature, giving rise to an asymmetrically compressed or elongated tetrahedron with deviations from the tetrahedral angle of up to 6°. The correlation time follows an Arrhenius law with an activation energy of E a ) 25.8(2) kJ mol -1 and an attempt frequency of τ 0 -1 ) 440(80) THz. The spin-lattice relaxation times were fitted according to a simple Bloembergen-Purcell-Pound type model with a T 1 minimum of 4 ms at 230 K. Temperature-dependent librational amplitudes were extracted by line-shape simulations between 38 and 390 K and contrasted with those obtained by neutron diffraction, their values ranging between 5 and 28°. The onset and progress of the solid-phase transformation were followed in situ at temperatures above 372 K and could be classified as a strongly temperature-dependent, heterogeneous two-step reaction proceeding with rapid evolution of ammonia and comparatively slow subsequent reintegration into the solid. On the microscopic level, this correlates with a rapid proton transferspossibly triggered by a coupling between the ammonium ion dynamics and phonon modes on the terahertz time scalesand an essentially decoupled nucleophilic attack of ammonia at the nitrile carbon, giving rise to significantly differing time constants for the two processes.

In situ solid-state 1H NMR studies of hydration of the solid acid catalyst ZSM-5 in its ammonium form

“…Indeed, the shift of the 1 H NMR resonance for the ammonium cations to lower field is consistent with the formation of N-H?O hydrogen bonds [24]. We note that ammonium cations in solid-state environments often undergo reorientational dynamic processes [25][26][27][28][29][30] (including cases of hydrogen-bonded solids in which all four N-H bonds of the ammonium cation are engaged in strong hydrogen bonds and at temperatures lower than those studied here), and it is reasonable to propose that the ammonium cations in NH 4 -ZSM-5 may also undergo significant dynamics at the temperatures studied in the present work. As a consequence, it is likely that the observed 1 H NMR resonance for an ammonium cation engaged in hydrogen bonding with water may represent the average over a dynamic process in which all four N-H bonds are involved in hydrogen bonding during the timescale of the measurement.…”

Nonpolar-to-Polar Phase Transition of a Chiral Ionic Plastic Crystal and Switch of the Rotation Symmetry

“…A simple Lorentzian-shape peak of the 2 H NMR spectrum is associated with a globular symmetry of the 2 H atom or free rotation in a period of the time scale of NMR measurement. On the other hand, an asymmetric environment of the 2 H atom leads to Pake doublet shape in 2 H NMR peak Figure a.…”

Salts and Ionic Liquids of Resonance Stabilized Amides

“…Both isomers are separated by only 1 kcal mol À1 and can be regarded as conformers with respect to rotation about the N(CN) À CHO bond ( Synthesis and properties of alkali dicyanoamides have been studied intensively over the last decades. [10][11][12][13][14][15][16][17][18][19][20][21][22][23] We want to focus here on covalently bound dicyanoamides, for which little is known. Alkali dicyanoamides are stable in neutral aqueous solution; however, they tend to polymerize at lower pH values.…”