Arsenic(III) oxide intercalate with ammonium chloride: crystal structure revision and thermal characterization

Abstract

According to the crystal structure determination by Edstrand & Blomqvist [Ark. Kemi (1955), 8, 245–256], intercalate NH4Cl·As2O3·0.5H2O ({\bf Y_{{NH}_{4}Cl}}) is not isostructural with compound KCl·As2O3·0.5H2O. This is very unlikely because both NH4Br·2As2O3 and KBr·2As2O3 as well as NH4I·2As2O3 and KI·2As2O3 are isostructural. Hence, intercalate {\bf Y_{{NH}_{4}Cl}} has been studied using single-crystal X-ray diffraction in addition to attenuated total reflection Fourier transform infrared (ATR-FTIR) and 15N solid-state magic-angle spinning nuclear magnetic resonance (ssNMR) spectroscopies. These techniques indicate that revising the previous crystal structure model is necessary. Compound {\bf Y_{{NH}_{4}Cl}} crystallizes in space group P6/mmm with unit-cell parameters a = 5.25420 (10) Å and c = 12.6308 (3) Å and is isostructural with KCl·As2O3·0.5H2O. The presence of two symmetry-independent ammonium cations in the structure has been unequivocally confirmed using 15N ssNMR spectroscopy. The 15N ssNMR spectrum of intercalate {\bf Y_{{NH}_{4}Cl}} has been compared with analogous spectra of NH4Br·2As2O3 and NH4I·2As2O3 which allowed for a probable assignment of signals to ammonium cations occupying particular sites in the crystal structures. Thermogravimetry, differential scanning calorimetry and variable-temperature ATR-FTIR spectra have revealed that intercalate {\bf Y_{{NH}_{4}Cl}} is dehydrated between 320 and 475 K. Upon cooling or standing in moist air water is re-absorbed. Dehydration leads to significant shortening of the c unit-cell parameter as revealed by powder X-ray diffraction [c = 12.1552 (7) Å at 293 K]. Compound {\bf Y_{{NH}_{4}Cl}} decomposes on prolonged heating above 490 K to arsenic(III) oxide and ammonium chloride.

Anhydrous Hexagonal As2O3 Intercalates with Alkali-Metal Halides: A Comparative Study of Crystal Structures and Arsenic(III) Lone-Pair Stereoactivity Present Therein

“…Consequently, obtaining the anhydrous P CsCl and P RbCl is tricky, especially if one needs crystals suitable for single-crystal X-ray diffraction studies. We have already confirmed the earlier result of Pertlik that P NH 4 Cl cannot be obtained via a hydrothermal route . In order to have crystal structures of comparable quality, we have also grown single crystals of P KBr , P KI , P NH 4 Br , and P NH 4 I .…”

“…Apparently, chloride anion does not fit the As 6 O 12 rings as well as heavier halide anions, or the As···Cl – interactions are not strong enough to support the crystal structure properly. Hence, poorer crystal structure quality and a propensity to form hydrates in which chloride anions form fewer As···Cl – interactions, some of which are replaced with O–H···Cl – ionic hydrogen bonds. , …”

“…We have already confirmed the earlier result of Pertlik that P NHd 4 Cl cannot be obtained via a hydrothermal route. 16 In order to have crystal structures of comparable quality, we have also grown single crystals of P KBr , P KI , P NHd 4 Br , and P NHd 4 I . Crystal structures of P KCl and P RbCl have been determined recently.…”

“…Hence, poorer crystal structure quality and a propensity to form hydrates in which chloride anions form fewer As•••Cl − interactions, some of which are replaced with O−H••• Cl − ionic hydrogen bonds. 5,16 The intercalates' unit-cell volumes have been plotted as a function of the molar volumes of the salts that constitute them in Figure 2. Two groups exhibiting linear correlations can be distinguished: one comprising salts crystallizing in the NaCltype structure and the other crystallizing in the CsCl-type structure.…”