Measurement of extreme hyperfine fields in two-coordinate high-spin Fe2+ complexes by Mössbauer spectroscopy: essentially free-ion magnetism in the solid state.

Mössbauer studies of three two-coordinate linear high-spin Fe(2+) compounds, namely, Fe{N(SiMe3)(Dipp)}2 (1) (Dipp = C6H3-2,6-(i)Pr2), Fe(OAr')2 (2) [Ar' = C6H3-2,6-(C6H3-2,6-(i)Pr2)2], and Fe{C(SiMe3)3}2 (3), are presented. The complexes were characterized by zero- and applied-field Mössbauer spectroscopy (1-3), as well as zero- and applied-field heat-capacity measurements (3). As 1-3 are rigorously linear, the distortion(s) that might normally be expected in view of the Jahn-Teller theorem need not necessarily apply. We find that the resulting very large unquenched orbital angular momentum leads to what we believe to be the largest observed internal magnetic field to date in a high-spin iron(II) compound, specifically +162 T in 1. The latter field is strongly polarized along the directions of the external field for both longitudinal and transverse field applications. For the longitudinal case, the applied field increases the overall hyperfine splitting consistent with a dominant orbital contribution to the effective internal field. By contrast, 2 has an internal field that is not as strongly polarized along a longitudinally applied field and is smaller in magnitude at ca. 116 T. Complex 3 behaves similarly to complex 1. They are sufficiently self-dilute (e.g., Fe···Fe distances of ca. 9-10 Å) to exhibit varying degrees of slow paramagnetic relaxation in zero field for the neat solid form. In the absence of EPR signals for 1-3, we show that heat-capacity measurements for one of the complexes (3) establish a geff value near 12, in agreement with the principal component of the ligand electric field gradient being coincident with the z axis.

Thermodynamic relationship between structural isomers of the thermochromic compound bis(N-Isopropyl-5,6-benzosalicylideneiminato)nickel(II).

A pair of structural isomers was isolated at room temperature for the thermochromic nickel complex bis( N-isopropyl-5,6-benzosalicylideneiminato)nickel(II); one is a diamagnetic green form with square-planar coordination geometry (G phase), and the other is a paramagnetic brown form with a tetrahedral geometry (B phase). However, a question as to which form is thermodynamically stable was left open. To solve this problem, thermal and magnetic properties of this complex were investigated by adiabatic heat capacity calorimetry in the 6-508 K temperature range and magnetic measurements in the 2-400 K region. In addition to the two forms previously reported, two metastable crystal forms (G' and B' phases) were found. The stable phase sequence was G phase, B phase, and then liquid upon heating. The supercooled B phase gave rise to a small phase transition with nonmagnetic origin at around 50 K. By rapidly cooling the liquid, a glassy liquid state was realized below approximately 290 K. The order of thermodynamic stability at 298.15 K was revealed to be the G, B, G', and then the B' phase. The entropy, enthalpy, and Gibbs energy differences between the B and the G phases at 298.15 K were S degrees (B) - S degrees (G) = 32.8 J K (-1) mol (-1), H degrees (B) - H degrees (G) = 16.0 kJ mol (-1), and G degrees (B) - G degrees (G) = 6.25 kJ mol (-1), respectively.

Magnetostructural study of 2-(4-N-tert-butylaminoxylphenyl)benzimidazole.

The heat capacity of the title organic free radical, PhBABI, was measured over 0.3-300 K by adiabatic calorimetry and relaxation methods in the presence of external magnetic fields up to 9 T. A hump in the magnetic heat capacity was observed with a maximum at about 15 K in zero field, which did not shift at fields up to 9 T. The experimental magnetic entropy was in good agreement with the theoretical value of R ln 2 (= 5.76 J K(-1) mol(-1)) for S = 1/2 systems. The higher temperature, field-insensitive feature was fitted to several antiferromagnetic Heisenberg models. The best fits were obtained using spin ladder and coupled spin bilayer models.

Mechanochemical effect in the iron(III) spin crossover complex [Fe(3-MeO-salenEt2]PF6 as studied by heat capacity calorimetry.

Magnetic and thermal properties of the iron(III) spin crossover complex [Fe(3MeO-salenEt)(2)]PF(6) are very sensitive to mechanochemical perturbations. Heat capacities for unperturbed and differently perturbed samples were precisely determined by adiabatic calorimetry at temperatures in the 10-300 K range. The unperturbed compound shows a cooperative spin crossover transition at 162.31 K, presenting a hysteresis of 2.8 K. The anomalous enthalpy and entropy contents of the transition were evaluated to be Delta(trs)H = 5.94 kJ mol(-1) and Delta(trs)S = 36.7 J K(-1) mol(-1), respectively. By mechanochemical treatments, (1) the phase transition temperature was lowered by 1.14 K, (2) the enthalpy and entropy gains at the phase transition due to the spin crossover phenomenon were diminished to Delta(trs)H = 4.94 kJ mol(-1) and Delta(trs)S = 31.1 J K(-1) mol(-1), and (3) the lattice heat capacities were larger than those of the unperturbed sample over the whole temperature range. In spite of different mechanical perturbations (grinding with a mortar and pestle and grinding in a ball-mill), two sets of heat capacity measurements provided basically the same results. The mechanochemical perturbation exerts its effect more strongly on the low-spin state than on the high-spin state. It shows a substantial increase of the number of iron(III) ions in the high-spin state below the transition temperature. The heat capacities of the diamagnetic cobalt(III) analogue [Co(3MeO-salenEt)(2)]PF(6) also were measured. The lattice heat capacity of the iron compounds has been estimated from either the measurements on the cobalt complex using a corresponding states law or the effective frequency distribution method. These estimations have been used for the evaluation of the transition anomaly.

Spin crossover phenomenon accompanying order-disorder phase transition in the ligand of [FeII(DAPP)(abpt)](ClO4)2 compound (DAPP = bis(3-aminopropyl)(2-pyridylmethyl)amine, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) and its successive self-grinding effect.

The spin crossover phenomenon of the recently described spin crossover complex [FeII(DAPP)(abpt)](ClO4)2 [DAPP = bis(3-aminopropyl)(2-pyridylmethyl)amine, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] accompanying an order-disorder phase transition of the ligand was investigated by adiabatic heat capacity calorimetry, far-IR, IR, and Raman spectroscopies, and normal vibrational mode calculation. A large heat capacity peak due to the spin crossover transition was observed at T(trs) = 185.61 K. The transition enthalpy and entropy amounted to Delta(trs)H = 15.44 kJ mol-1 and Delta(trs)S = 83.74 J K-1 mol-1, respectively. The transition entropy is larger than the expected value 60.66 J K-1 mol-1, which is contributed from the spin multiplicity (R ln 5; R: the gas constant), disordering of the carbon atom of the six-membered metallocycle in the DAPP ligand, and one of the two perchlorate anions (2R ln 2), and change of the normal vibrational modes between the high-spin (HS) and low-spin (LS) states (35.75 J K-1 mol-1). The remaining entropy would be ascribed to changes of the lattice vibrations and molecular librations between the HS and LS states. Furthermore, [Fe(DAPP)(abpt)](ClO4)2 crystals disintegrated and became smaller crystallites whenever they experienced the phase transition. This may be regarded as a successive self-grinding effect, evidenced by adiabatic calorimetry, DSC, magnetic susceptibility, and microscope observation. The relationship between the crystal size and the physical quantities is discussed.

Alkyl group as entropy reservoir in an MMX chain complex, Pt2(n-PenCS2)4I.

Heat capacity of halogen-bridged one-dimensional binuclear metal complex (so-called MMX chain) having four n-pentyl groups, Pt2(n-PenCS2)4I, was measured by adiabatic calorimetry. A first-order phase transition was observed at 207.4 K when measurement was made after cooling from room temperature. The enthalpy and entropy of transition were determined to be 10.19 kJ mol(-1) and 49.1 J K(-1) mol(-1), respectively. A monotropic phase transition was observed at 324 K on heating, and the entropy of transition was essentially null. The sample once heated above 324 K never returned to the initial phase at room temperature and underwent a higher-order phase transition at 173 K and a first-order phase transition at 220.5 K. The enthalpy and entropy of the first-order phase transition were estimated to be 11.6 kJ mol(-1) and 52.4 J K(-1) mol(-1), respectively. The magnitude of the entropy gain at the phase transition from the initial room-temperature phase to the high-temperature phase at 324 K shows that in Pt2(n-PenCS2)4I a large amount of entropy reserved in alkyl chain is transferred to dithiocarboxylato groups upon the phase transition, as in the cases of Pt2(n-PrCS2)4I and Pt2(n-BuCS2)4I.

Water structures of differing order and mobility in aqueous solutions of schizophyllan, a triple-helical polysaccharide as revealed by dielectric dispersion measurements.

Dielectric dispersion measurements were made on aqueous solutions of a triple-helical polysaccharide schizophyllan over a wide concentration range 10-50 wt % at -45 to +30 degrees C. In the solution state, three different water structures with the different relaxation times tau were found, namely, bound water (taul), structured water (taus), and loosely structured water (tauls) in addition to free water (tauP). Structured water is less mobile and loosely structured water is nearly as mobile as free water, but bound water with taul is much less mobile, thus taul >> taus >> tauls greater, similar tauP. The order-disorder transition accompanies the conversion between structured water and loosely structured water. However, the species with taus remains even in the disordered state and constitutes part of bound water in the entire temperature range. In the frozen state, in addition to bulk water formed by partial melting, two mobile species existed, which were assigned to liquidlike bound water and found to be a continuation of bound water in the solution state. These relaxation time data are discussed in connection with the entropy levels of the four structures deduced from heat capacity data (cf. Yoshiba, K.; et al. Biomacromolecules 2003, 4, 1348-1356).

Static water structure detected by heat capacity measurements on aqueous solutions of a triple-helical polysaccharide schizophyllan.

Heat capacity measurements were made on aqueous solutions of a triple-helical polysaccharide schizophyllan by precision adiabatic calorimetry over a wide range of concentrations 30.45-90.93 wt % at temperatures between 5 and 315 K. The heat capacity curves obtained were divided into four groups depending on the weight fraction of schizophyllan w regions I-IV. In region I, triple-helices with the sheath of bound water, structured water, and loosely structured water forming layers around the helix core are embedded in free water. In region II, there is no free water, and loosely structured water decreases until it vanishes, but structured water stays constant with increasing w. In region III, bound water remains unaffected, but structured water decreases with increasing w by overlapping each other. Finally, in region IV, only schizophyllan and bound water exist, the latter decreasing upon increasing w. The maximum thickness of each layer is 0.18(3) nm for bound water, 0.13(4) nm for structured water, and 0.23(6) nm for loosely structured water, and these layers of water are at the enthalpy levels of 53%, 93.7%, and nearly 100%, respectively, between ice (0%) and free water (100%).

Alkyl chains acting as entropy reservoir in liquid crystalline materials.

The roles played by the conformational disordering of alkyl chains in determining the aggregation states of matter are reviewed for liquid crystalline materials from a thermodynamic perspective. Entropy, which is one of the most macroscopic concepts but which has a clear microscopic meaning, provides crucial microscopic information for complex systems for which a microscopic description is hard to establish. Starting from structural implication by absolute (third-law) entropy for crystalline solids, the existence of successive phase transitions caused by the successive conformational melting of alkyl chains in discotic mesogens is explained. An experimental basis is given for the "quasi-binary picture" of thermotropic liquid crystals, i.e., the highly disordered alkyl chains behave like a second component (solvent). A novel entropy transfer between the "components" of a molecule and the resulting "alkyl chains as entropy reservoir" mechanism are explained for cubic mesogens.

Ordering in aqueous polysaccharide solutions. II. Optical rotation and heat capacity of aqueous solutions of a triple-helical polysaccharide schizophyllan.

Deuterium oxide solutions of schizophyllan, a triple-helical polysaccharide, undergoing an order-disorder transition centered at 17 degrees C, were studied by optical rotation (OR) and heat capacity (C(p)) to elucidate the molecular mechanism of the transition and water structure in the solution and frozen states. The ordered structure at low temperature consisted of the side chains and water in the vicinity forming an ordered hydrogen-bonded network surrounding the helix core and was disordered at higher temperature. In the solution state appeared clearly defined transition curves in both the OR and C(p) data. The results for three samples of different molecular weights were analyzed theoretically, treating this transition as a typical linear cooperative transition from the ordered to disordered states and explained quantitatively if the molecular weight polydispersity of the sample was considered. The excess heat capacity C(EX)(p) defined as the C(p) minus the contributions from schizophyllan and D(2)O was estimated. In the frozen state it increased with raising temperature above 150 K until the mixture melted. This was compared with the dielectric increment observed in this temperature range and ascribed to unfreezable water. From the heat capacity and dielectric data, unfreezable water is mobile but more ordered than free water. In the solution state, the excess heat capacity originates from the interactions of D(2)O molecules as bound water and structured water, and so forth. Thus the schizophyllan triple helix molds water into various structures of differing orders in solution and in the solid state.

Degree of disorder in cubic mesophases in thermotropics: thermodynamic study of a liquid crystal showing two cubic mesophases.

Heat capacity of a thermotropic mesogen ANBC(22) (4(')-alkoxy-3(')-nitrobiphenyl-4-carboxylic acid with 22 carbon atoms in alkyl chain) showing two cubic mesophases was measured by adiabatic calorimetry between 13 and 480 K. Excess enthalpies and entropies due to phase transitions were determined. A small thermal anomaly due to the cubic Im3m-->cubic Ia3d phase transition was successfully detected. Through an analysis of chain-length dependence of the entropy of transition, the sequence of two cubic mesophases (with space groups Ia3d and Im3m) is deduced for thermotropic mesogens ANBC(n). It is shown that the disorder of the core arrangement decreases in the order of Sm-C-->cubic (Im3m)-->cubic (Ia3d) while that of the chain in the reverse order cubic (Ia3d)-->cubic (Im3m)-->Sm C.

Thermodynamic investigation of the charge-ordered insulating state of the quasi-one-dimensional organic system (DI-DCNQI)2Ag.

Heat capacity measurements of a charge-ordered organic conductor (DI-DCNQI)2Ag have been performed in a temperature range between 0.3 and 14 K. We found no thermal anomaly at the Néel temperature ( T(N) = 5.5 K) but instead a T-linear term suggestive of the spin excitations of one-dimensional character in the charge-ordered insulating state. The analysis of the T-linear term and the excess entropy indicates that the charge fluctuations in the charge-ordered state influence the growth of spin excitations at elevated temperatures, which seems to be a peculiar aspect of a 1D charge-ordered system.

Ordering in aqueous polysaccharide solutions. I. Dielectric relaxation in aqueous solutions of a triple-helical polysaccharide schizophyllan.

Deuterium oxide solutions of a triple-helical polysaccharide schizophyllan, undergoing an order-disorder transition centered around 17 degrees C, were studied by the time-domain reflectometry (TDR) to obtain dielectric dispersions in the solution and frozen states. In the solution state, the dispersion below the transition temperature is resolved in three dispersions (relaxation times at 0 degrees C) ascribed to side chain glucose residue (1; 102 ns), structured water (s; 2.0 ns) and bulk water (h), respectively, from low to high frequencies. Bulk water is divided into slow water (h2; 0.04 ns) and free or pure water (h1; 0.02 ns). Above the transition temperature structured water almost disappears and is compensated by slow water. Structured water is similar to bound water for proteins but different from it because of this transition behavior. Another dispersion (l) seen at the lowest frequency is assigned to the rotation of side-chain glucose residue coupled with hydrated water. Parts of this dispersion and structured water are suggested to constitute bound water. In the frozen state were observed a major dispersion (h; 0.14 ns) and a minor one (m; 28 ns), which were ascribed to considerably mobile and less mobile waters. They are similar to but not exactly the same as that for unfreezable water in bovine serum albumin solutions argued by Miura et al. (Biopolymers, 1995, Vol. 36, p. 9). Water is molded into different structures by the triple helix.