Peter N. Nelson

Theories and experimental investigations of the structural and thermotropic mesomorphic phase behaviors of metal carboxylates

Investigations on the phase behaviors and structural properties of mono-, di- and poly-valent metal carboxylates are reviewed with reference to developments in experimental and theoretical concepts surrounding their liquid crystalline properties. The main methods of structural investigation such as X-ray diffraction, infrared and 13C-NMR spectroscopies are examined in detail on the basis of common synthetic routes leading to the isolation of pure compounds. A detailed review of the thermal behaviors of several metal carboxylates is presented along with proposed theories and molecular models for odd–even alternation, chain length effects, phase structures and mesophase formation. Theories explaining the effects of metal ion radii and chain unsaturation are also discussed. Proposed degradation mechanisms resulting in the formation of various products and kinetic studies are also considered. Though this review highlights a number of investigations on the structural and phase properties of the mostly widely studied carboxylates, the results presented here strongly indicate that there is room for further studies on some of these systems.

Powder X-ray diffraction, infrared and 13C NMR spectroscopic studies of the homologous series of some solid-state zinc(II) and sodium(I) n-alkanoates

A comparative study of the room temperature molecular packing and lattice structures for the homologous series of zinc(II) and sodium(I) n-alkanoates adduced from Fourier transform infrared and solid-state (13)C NMR spectroscopic data in conjunction with X-ray powder diffraction measurements is carried out. For zinc carboxylates, metal-carboxyl bonding is via asymmetric bridging bidentate coordination whilst for the sodium adducts, coordination is via asymmetric chelating bidentate bonding. All compounds are packed in a monoclinic crystal system. Furthermore, the fully extended all-trans hydrocarbon chains are arranged as lamellar bilayers. For zinc compounds, there is bilayer overlap, for long chain adducts (nc>8) but not for sodium compounds where methyl groups from opposing layers in the lamellar are only closely packed. Additionally, the hydrocarbon chains are extended along the a-axis of the unit cell for zinc compounds whilst for sodium carboxylates they are extended along the c-axis. These packing differences are responsible for different levels of Van der Waals effects in the lattices of these two series of compounds, hence, observed odd-even alternation is different. The significant difference in lattice packing observed for these two series of compounds is proposed to be due to the difference in metal-carboxyl coordination mode, arising from the different electronic structure of the central metal ions.

Solid state 13C-NMR, infrared, X-ray powder diffraction and differential thermal studies of the homologous series of some mono-valent metal (Li, Na, K, Ag) n-alkanoates: A comparative study

A comparative study of the molecular packing, lattice structures and phase behaviors of the homologous series of some mono-valent metal carboxylates (Li, Na, K and Ag) is carried out via solid state FT-infrared and (13)C-NMR spectroscopes, X-rays powder diffraction, density measurements, differential scanning calorimetry, polarizing light microscopy and variable temperature infrared spectroscopy. It is proposed that, for lithium, sodium and potassium carboxylates, metal-carboxyl coordination is via asymmetric chelating bidentate bonding with extensive intermolecular interactions to form tetrahedral metal centers, irrespective of chain length. However, for silver n-alkanoates, carboxyl moieties are bound to silver ions via syn-syn type bridging bidentate coordination to form dimeric units held together by extensive head group inter-molecular interactions. Furthermore, the fully extended hydrocarbon chains which are crystallized in the all-trans conformation are tilted at ca. 30°, 27°, 15° and 31° with respect to a normal to the metal plane, for lithium, sodium, silver and potassium carboxylates, respectively. All compounds are packed as lamellar bilayer structures, however, lithium compounds are crystallized in a triclinic crystal system whilst silver, sodium and potassium n-alkanoates are all monoclinic with possible P1 bravais lattice. Odd-even alternation observed in various physical features is associated with different inter-planar spacing between closely packed layers in the bilayer which are not in the same plane; a phenomenon controlled by lattice packing symmetry requirements. All compounds, except silver carboxylates, show partially reversibly first order pre-melting transitions; the number of which increases with increasing chain length. These transitions are associated, for the most part, with lamellar collapse followed by increased gauche-trans isomerism in the methylene group assembly, irrespective of chain length. It is proposed that the absence of mesomorphic transitions in their phase sequences is due to a lack of sufficient balance between attractive and repulsive electrostatic and van der Waals forces during phase change. The evidence presented in this study shows that phase behaviors of mono-valent metal carboxylates are controlled, mainly, by head group bonding.

Chain Length and Thermal Sensitivity of the Infrared Spectra of a Homologous Series of Anhydrous Silver(I) n-Alkanoates

The thermal and chain length sensitivity of the infrared spectra of some solid state anhydrous silver(I) salts (n-octanoate to n-eicosanoate, inclusive) are discussed. At ambient temperature, the tilted alkyl chains, anchored to the metal planes via chelating bidentate coordination to the silver ions, are crystallized in the fully extended all-trans conformation. Interestingly, though all compounds are crystallized in a monoclinic crystal system, their lateral chain packing, van der Waals effects, and hence vibrational features are chain length-dependent. This is a direct result of electrostatic effects of the COO group in addition to vibrational coupling between CH2, CH3, and COO modes, an effect which varies significantly with chain length. Variable temperature infrared measurements indicate significant irreversible changes in the metal-carboxyl coordination sphere, most likely due to bond fission. For long chain adducts (), thermally induced crystal system switching, monoclinic to triclinic, indicates greater thermal sensitivity of their alkyl chains. During heating, the regions of the hydrocarbon chains, furthest from the COO, become increasingly molten and mobile as the stepwise melt advances towards the solid COO moieties. This solid-liquid melting behaviour is responsible for mesophase formation in metal carboxylate systems.