Vibrational behavior of matrix-isolated ions in Tutton compounds. II. Infrared spectroscopic study of and ions included in copper sulfates and selenates

Electronic properties and vibrational spectra of (NH4)2M″(SO4)2·6H2O (M = Ni, Cu) Tutton's salt: DFT and experimental study

The complex crystals of the family of the Tutton's salt have been investigated through the numerous experimental and theoretical studies to understand their physical properties and their potential technological applications. In spite of the more than 60 years of research, there are very few studies about the electronic properties of Tutton's salt. In our present work, we have calculated the stability, electronic properties and the first theoretical study of band structure of the three different crystals of the Tutton's salt, ammonium nickel sulfate hexahydrate ((NH₄)₂Ni(SO₄)₂·6H₂O), ammonium nickel-copper sulfate hexahydrate ((NH₄)₂Ni_0.5Cu_0.5(SO₄)₂·6H₂O) and ammonium copper sulfate hexahydrate ((NH₄)₂Ni(SO₄)₂·6H₂O) with the help of periodic ab-initio calculations based on density functional theory (DFT). In addition to this, the internal Raman and FTIR modes of the ionic fragments [Ni(H₂O)₆]²⁺, [Cu(H₂O)₆]²⁺ NH₄⁺ and SO₄^2- of the sample crystals were obtained by employing the ab initio and the orientation of the molecular vibrations of the ionic fragments have been presented in picturized form. Furthermore, the Raman and FTIR spectroscopy of the sample crystals were measured in the range 100-4000 cm^-1 and 400-4000 cm^-1 respectively, and the internal vibrational modes obtained from experimental measurement have been compared with those obtained from DFT calculations.

Infrared spectroscopic study of SO₄²⁻ ions included in M'₂M''(SeO₄)₂⋅6H₂O (Me'=K, NH₄⁺; M''=Mg, Co, Ni, Cu, Zn) and NH₄⁺ ions included in K₂M(XO₄)₂⋅6H₂O (X=S, Se; M''=Mg, Co, Ni, Cu, Zn)

Infrared spectra of Tutton compounds, M'₂M''(SeO₄)₂⋅6H₂O (M'=K, NH₄⁺; M''=Mg, Co, Ni, Cu, Zn; X=S, Se), as well as those of SO₄²⁻ guest ions included in selenate host lattices and of NH4(+) guest ions included in potassium host lattices are presented and discussed in the regions of ν₃ and ν₁ of SO₄²⁻ guest ions, ν₄ of NH₄⁺ guest ions and water librations. The SO₄²⁻ guest ions matrix-isolated in selenate matrices (approximately 2 mol%) exhibit three bands corresponding to ν₃ and one band corresponding to ν₁ in good agreement with the low site symmetry C₁ of the host selenate ions. When the larger SO₄²⁻ ions are replaced by the smaller SO₄²⁻ ions the mean values of the asymmetric stretching modes ν₃ of the included SO₄²⁻ ions are slightly shifted to lower frequencies as compared to those of the same ions in the neat sulfate compounds due to the smaller repulsion potential of the selenate matrices (larger unit-cell volumes of the selenates). It has been established that the extent of energetic distortion of the sulfate ions matrix-isolated in the ammonium selenates as deduced from the values of Δν₃ and Δν₃/νc is stronger than that of the same ions matrix-isolated in the potassium selenates due to the formation of hydrogen bonds between the SO₄²⁻ guest ions with both the water molecules in the host compounds and the NH₄⁺ host ions (for example, Δν₃ of the sulfate guest ions have values of 30 and 51 cm(-1) in the nickel potassium and ammonium compounds, and 33 and 49 cm(-1) in the zinc potassium and ammonium compounds, respectively). The infrared spectra of ammonium doped potassium sulfate matrices show three bands corresponding to Δν₄ of the included ammonium ions in agreement with the low site symmetry C₁ of the host potassium ions. However, the inclusion of ammonium ions in selenate matrices (with exception of the magnesium compound) leads to the appearance of four bands in the region of ν₄. At that stage of our knowledge we assume that some kind of disorder of the ammonium ions included in selenate lattices occurs due to the different proton acceptor capability of the SO₄²⁻ and SO₄²⁻ ions. The latter ions are known to exhibit stronger proton acceptor abilities. This fact will facilitate the formation of polyfurcate hydrogen bonds of the ammonium ions in the selenate matrices, thus leading to increasing in the coordination number of these ions, i.e. to a disorder of the ammonium guest ions. The strength of the hydrogen bonds formed in the title Tutton compounds as well as that of the hydrogen bonds in potassium compounds containing isomorphously included ammonium ions as deduced from the wavenumbers of the water librations are also discussed. The bands corresponding to water librations in the spectra of the mixed crystals K₁.₈(NH₄)₀.₂M(XO₄)₂⋅6H₂O (M=Mg, Co, Ni, Cu, Zn; X=S, Se) broaden and shift to lower frequencies as compared to those of the potassium host compounds, thus indicating that weaker hydrogen bonds are formed in the mixed crystals. These spectroscopic findings are owing to the decrease in the proton acceptor capacity of the SO₄²⁻ and SO₄²⁻ ions due to the formation of hydrogen bonds between the host anions and the guest ammonium cations additionally to water molecules (anti-cooperative or proton acceptor competitive effect). Furthermore, the band shifts in the spectra of the selenate matrices are generally larger than those observed in the spectra of the respective sulfates due to the stronger proton acceptor ability of the selenate ions.

Growth, structure and spectral studies of a novel mixed crystal potassium zinc manganese sulphate

Mixed crystals of K2Zn0.84 Mn0.16(SO4)2·6H2O were grown from an equimolar aqueous solution of Tutton's salt, K2 Zn(SO4)2·6H2O and MnSO4 by slow evaporation solution growth technique. The crystal composition as determined by single crystal XRD analysis reveals the co-existence of zinc and manganese in the mixed crystal. The surface morphological changes are observed by scanning electron microscopy. Small variations in cell parameter values, slight shifts in characteristic vibrational patterns in FT-IR and reduction in intensities observed in XRD confirm the crystal stress as a result of formation of mixed crystal. High resolution XRD diffraction estimates the crystalline perfection of the mixed crystal with predominantly vacancy type of defects. It belongs to P21/c space group with cell parameter values, a=6.1530 Å, b=12.2230 Å, c=9.0430 Å, α=β=ν=90°, V=657.56 Å(3), Z=4. High transmittance in the visible region is observed.

Infrared spectroscopy reveals the nonsynchronicity phenomenon in the glassy to fluid micellar transition of DSPE-PEG2000 aqueous dispersions

One challenging question regarding the phase transition mechanism of amphiphiles is to seek the roles individual groups/portions of the amphiphilic molecule play during the transformation. To address this question, we selected a poly(ethylene glycol)-grafted phospholipid, distearoylphosphatidylethanolamine-N-[methoxy(poly(ethylene glycol))-2000] (DSPE-PEG2000), to study its glassy to fluid micellar phase transition by using differential scanning calorimetry and Fourier transform infrared (FTIR) spectroscopy. FTIR results revealed that during the glassy to fluid micellar transition, the lipid acyl tails have evident conformational rearrangements but undergo only slight modifications in the packing state. For the lipid interface region, small changes in the hydration state of C=O groups were observed, whereas for the lipid headgroups (NHCO and PO(4)(-)), their conformation and hydration states remain unchanged. Thus, the head, the interface, and the tail regions of DSPE-PEG2000 molecules change nonsynchronously during the transition. As to the bulky PEG corona residing at the outer micellar surface, no evident hydration state change was observed upon heating, and its behavior is almost the same as that of the hydrated free PEG2000 molecules. Such a nonsynchronous change in different parts of the self-assembled amphiphilic aggregates undergoing phase transition could be a common phenomenon that needs to be widely recognized.