Basal Face Research Articles

A study on the precipitation of radial alumina trihydrate

Alumina trihydrate is normally produced by precipitation from sodium aluminate liquors in Bayer plants. This study aims at investigating the various factors affecting the production of radial alumina trihydrate particles. Precise controlling of supersaturation and temperature as a function of time is important in producing the desirable shape and size of the particles. Morphological changes as a function of time were observed by scanning and transmission electron microscope study, and the results revealed the various particle growth mechanisms involved. In the presence of organic 3,4 dihydroxy benzoic acid (3,4 DHBA), radial seeds start to grow abnormally. Probably the organic contaminant is preferentially absorbed onto the lateral (1 0 0) + (1 1 0) faces, which promotes explosive formation of very small single-crystals or nuclei on the surfaces. The seed particle growth is simultaneously retarded. The basal (0 0 1) face of some crystals continues to grow and form long prism crystals.

Dislocation Cores and Hardness Polarity of 4H‐SiC

The hardness of opposite basal faces of 4H‐SiC single crystals has been measured in the temperature range 25°‐1200°3C. A strong hardness anisotropy between the silicon‐terminated (0001) and carbon‐terminated (0001) faces of this polar crystal has been found. Transmission electron microscopy investigation of the dislocations in the plastic zone of the 1200°3C indentations shows that they lie predominantly on the basal planes parallel to the indented face, and the extra‐half planes of the nonscrew dislocations originate from the indented face. It is also found that, when the (0001) Si‐terminated face is indented, the dislocations are either widely dissociated, with the width of the stacking fault ribbon much larger than the equilibrium value, or else they are single leading partials, with the corresponding trailing partials absent. In this case, all the leading partials are found to have a silicon core. On the other hand, the dislocations in the plastic zone of the carbon‐terminated face are in the form of dissociated dislocations, with the width of the associated stacking fault ribbons appreciably less than the equilibrium value. Moreover, the leading partials of éhese dissociated dislocations have a carbon core. The results indicate that the hardness of the polar basal faces of 4H‐SiC at elevated temperatures is partly determined by the nature of the dislocation cores nucleated by the indentation process. It is argued that this is due to the influence of the core on the generation and glide of the leading partial dislocations.

Diffusion Control during Vapour Crystal Growth

AbstractThe crystal growth rate under diffusion control is considered. Together with nutrient diffusion, the physi‐sorption of an inert gas, which impedes the surface kinetics during vapour growth, is taken into consideration. In this way the size of the diffusion field surrounding vapour grown crystals was estimated. The relations obtained are checked with data for the growth of zinc single crystals in argon atmosphere. The appearance of shallow cavities on their basal faces is used as a morphological mark for growth under diffusion control. Besides, the microstructure of the inner edge of the macroscopically flat periphery, surrounding the shallow cavities is investigated experimentally, by means of SEM.

Morphological and functional characterization of bovine oviductal epithelial cell monolayers cultured on polarizing membranes.

Several characteristics of oviductal cells, cultured under either polarizing or nonpolarizing conditions, were studied. In vitro produced bovine embryos tested the embryotrophic abilities of the respective conditioned media. Conditioned medium from the apical face of polarized cell monolayers supported higher rates of development to blastocyst and expanded blastocysts. In contrast, conditioned medium from the basal face supported embryo development only to the 8-16 cell stage; however, these embryos were able to continue development to the morula stage when cultured in medium from the apical and basal faces, indicating total cell confluence and a clear functional polarization. At the ultrastructural level, cells cultured in polarizing conditions displayed characteristics nearer to the same cells in vivo and signs of a metabolic activity higher than that in cells cultured under non-polarizing conditions. It can be concluded that cell-polarization, in our culture conditions, is beneficial to embryo development.

Epithelial cell polarization is a determinant in the infectious outcome of immunoglobulin A-mediated entry by Epstein-Barr virus.

Diseases of the nasopharyngeal epithelium due to Epstein-Barr virus (EBV) infection typically occur in chronic virus carriers with preexisting virus-specific antibodies. In vitro studies have shown that EBV-specific immunoglobulin A (IgA) promotes infection of human epithelial cells, otherwise refractory to EBV, via the polymeric immunoglobulin receptor (pIgR). To determine if EBV similarly exploits IgA transport mechanisms in vivo, we examined the fate of IgA-EBV complexes in the blood of mice, where pIgR-mediated transcytosis of IgA immune complexes through hepatocytes eliminates exogenous antigens from the circulation. By PCR analysis we showed hepatobiliary transport of IgA-EBV in viremic mice, but without detectable hepatocellular infection by immunostaining. Because efficient transport of EBV immune complexes might avert an infectious outcome, we modulated the transcytotic pathway in polarized Madin-Darby canine kidney (MDCK) cells transfected with pIgR to determine the effect on viral antigen expression. Like hepatocytes in vivo, MDCK cells in polarized monolayers translocated IgA-EBV from the basal cell face into apical medium without evidence for infection. However, when exposed to IgA-EBV as unpolarized single-cell suspensions, MDCK cells expressed EBV immediate-early and early antigens. These results suggest that pIgR-mediated transcytosis of pIgA-EBV through epithelium facilitates endogenous spread of EBV in long-term virus carriers, with infection being confined to cells with altered polarity from prior cytopathology.

Description of active sites on molybdenum oxide as detected by isotope exchange between C 18O 2 and Mo 16O 3

The oxygen exchange reaction between C 18O 2 and MO 16O 3 has been studied on well characterized MoO 3 crystallites. Temperature programmed desorption of adsorbed C 18O 2 has shown that the extent of surface oxygen exchange is higher on partially reduced samples exposing the (010) face. This has been related with the easier reducibility of the (010) face. Moreover, experiments in a recycle reactor at moderate reaction temperature (623K) suggest that surface oxygen exchange is more rapid on the basal (010) face. On the other hand, when the experiment is run at higher reaction temperature (773K), using a higher 18O(C 18O 2)/ 16O(MoO 3) ratio and for a longer period, a continuous oxygen exchange reaction occurs, specially with samples presenting lateral faces. In this latter conditions, two contributions to the exchange reaction can be distinguished. The first corresponds to a fast surface oxygen exchange. The second contribution which is more prominent over samples with (100) faces can be assigned to migration of oxygen atoms from lateral towards basal faces and/or to a slow migration of oxygen atoms from the bulk structure of MoO 3 towards the lateral faces. From the results of this study it is speculated on the importancë of a preferential oxygen migration from the bulk MoO 3 towards the lateral faces, which is related with the selective partial oxidation of olefins observed on these faces.

Fluxional Behaviour of Group 9 Metal Carbonyl Clusters

The site exchange processes of carbonyl ligands in homo- and hetero-metallic dodecacarbonyls of Ir and Rh and their derivatives take place by bridge opening and closing of CO's involving three or four metal atoms, and by rotation of terminal CO's about the apical metal centre. Substitution of Ir by Rh in the basal face of the metal framework slows down the first two processes and accelerates the third one, reflecting a significantly more electropositive character of Ir relative to Rh.

Film/Substrate Orientation Relationship in the AlN/6H-SiC Epitaxial System.

AlN has been grown on the (0001) basal and (1{bar 1}00) prism faces of 6{ital H}-SiC. High resolution transmission electron microscopy studies show that, despite the very different symmetries of these two substrate planes and their polar and nonpolar natures, the film/substrate orientation relationship is identical for both systems. It is shown that this result is consistent with a geometrical method recently proposed for determining the orientation relationship between two crystals in a bicrystal. {copyright} {ital 1996 The American Physical Society.}

Electrochemical oxidation of dihydronicotinadmide adenine dinucleotide (NADH) on single crystal gold electrodes

The electro-oxidation of dihydronicotinamide adenine dinucleotide (NADH) on three low-index basal single crystal gold faces was studied in a phosphate buffer solution. The reaction was found highly structure-sensitive; the Au(100) and Au(111) faces showed a high electrocatalytic activity, while the Au(110) face was much less active. The high electrocatalytic effect was attributed to the oxidation-mediator ability of the partially discharged OH (1 − r) − species on the gold surface. The structural effect of the gold faces on the electro-oxidation of NADH may result from the difference in the adsorption of NAD +, via its pyridine moiety as the adsorption site, on the different gold faces. The possibility of developing a highly sensitive NADH sensor based on the electrochemical faceting technique is suggested.

Tetranuclear and Pentanuclear Vanadium(IV/V) Carboxylate Complexes: [V(4)O(8)(NO(3))(O(2)CR)(4)](2-) and [V(5)O(9)X(O(2)CR)(4)](2-) (X = Cl(-), Br(-)) Salts.

The syntheses and properties of tetra- and pentanuclear vanadium(IV,V) carboxylate complexes are reported. Reaction of (NBzEt(3))(2)[VOCl(4)] (1a) with NaO(2)CPh and atmospheric H(2)O/O(2) in MeCN leads to formation of (NBzEt(3))(2)[V(5)O(9)Cl(O(2)CPh)(4)] 4a; a similar reaction employing (NEt(4))(2)[VOCl(4)] (1b) gives (NEt(4))(2)[V(5)O(9)Cl(O(2)CPh)(4)] (4b). Complex 4a.MeCN crystallizes in space group P2(1)2(1)2(1) with the following unit cell dimensions at -148 degrees C: a = 13.863(13) Å, b = 34.009(43) Å, c = 12.773(11) Å, and Z = 4. The reaction between (NEt(4))(2)[VOBr(4)] (2a) and NaO(2)CPh under similar conditions gives (NEt(4))(2)[V(5)O(9)Br(O(2)CPh)(4)] (6a), and the use of (PPh(4))(2)[VOBr(4)] (2b) likewise gives (PPh(4))(2)[V(5)O(9)Br(O(2)CPh)(4)] (6b). Complex 6b crystallizes in space group P2(1)2(1)2(1) with the following unit cell dimensions at -139 degrees C: a = 18.638(3) Å, b = 23.557(4) Å, c = 12.731(2) Å, and Z = 4. The anions of 4a and 6b consist of a V(5) square pyramid with each vertical face bridged by a &mgr;(3)-O(2)(-) ion, the basal face bridged by a &mgr;(4)-X(-) (X = Cl, Br) ion, and a terminal, multiply-bonded O(2)(-) ion on each metal. The RCO(2)(-) groups bridge each basal edge to give C(4)(v)() virtual symmetry. The apical and basal metals are V(V) and V(IV), respectively (i.e., the anions are trapped-valence). The reaction of 1b with AgNO(3) and Na(tca) (tca = thiophene-2-carboxylate) in MeCN under anaerobic conditions gives (NEt(4))(2)[V(4)O(8)(NO(3))(tca)(4)] (7). Complex 7.H(2)O crystallizes in space group C2/c with the following unit cell dimensions at -170 degrees C: a = 23.606(4) Å, b = 15.211(3) Å, c = 23.999(5) Å, and Z = 4. The anion of 7 is similar to those of 4a and 6b except that the apical [VO] unit is absent, leaving a V(4) square unit, and the &mgr;(4)-X(-) ion is replaced with a &mgr;(4),eta(1)-NO(3)(-) ion. The four metal centers are now at the V(IV), 3V(V) oxidation level, but the structure indicates four equivalent V centers, suggesting an electronically delocalized system. Variable-temperature magnetic susceptibility data were collected on powdered samples of 4b, 6a, and 7 in the 2.00-300 K range in a 10 kG applied field. 4b and 6a both show a slow increase in effective magnetic moment (&mgr;(eff)) from approximately 3.6-3.7 &mgr;(B) at 320 K to approximately 4.5-4.6 &mgr;(B) at 11.0 K and then a slight decrease to approximately 4.2 &mgr;(B) at 2.00 K. The data were fit to the theoretical expression for a V(IV)(4) square with two exchange parameters J = J(cis)() and J' = J(trans)() (H = -2JS(i)()S(j)()): fitting of the data gave, in the format 4b/6a, J= +39.7/+46.4 cm(-)(1), J' = -11.1/-18.2 cm(-)(1) and g = 1.83/1.90, with the complexes possessing S(T) = 2 ground states. The latter were confirmed by magnetization vs field studies in the 2.00-30.0 K and 0.500-50.0 kG ranges: fitting of the data gave S(T) = 2 and D = 0.00 cm(-)(1) for both complexes, where D is the axial zero-field splitting parameter. Complex 7 shows a nearly temperature-independent &mgr;(eff) (1.6-2.0 &mgr;(B)) consistent with a single d electron per V(4) unit. The (1)H NMR spectra of 4b and 6a in CD(3)CN are consistent with retention of their pentanuclear structure on dissolution. The EPR spectrum of 7 in a toluene/MeCN (1:2) solution at approximately 25 degrees C yields an isotropic signal with a 29-line hyperfine pattern assignable to hyperfine interactions with four equivalent I = (7)/(2) (51)V nuclei.

Effects of Iron and Aluminium Oxides on the Colloidal and Surface Properties of Kaolin

AbstractThe association between clay silicates, and iron and aluminium oxides has a major influence on the chemical and physical properties of soils. In this work the interaction of a kaolin substrate with iron and aluminium oxides and/or hydroxides obtained by basification of solutions of the metal ions was compared to that of quartz. Both precipitates were obtained in the presence of the substrates.The aluminium precipitates had higher crystallinity, and thus led to smaller increases in specific surface area than those of iron, and were more effective modifiers of the surface electrical properties of the kaolin-oxide mixtures. At concentrations as low as 0.43% Al (g/100 g of substrate) the point of zero charge (PZC) of the components with variable charge was measurable, while Fe required 2.23% and gave lower PZCs than those of corresponding concentrations of Al. In both cases the PZCs shifted to higher pH as metal concentration was increased, as did the flocculation interval of colloidal suspensions of kaolin, which were close to the PZCs (where these were evaluated).The Al and Fe oxides precipitated on quartz had higher crystallinities. Both metals increased the specific surface area to a similar extent, with an almost linear relationship to metal concentration. Samples containing ca. 6.5% Fe or Al had similar or slightly higher PZCs than corresponding kaolin samples.The results were interpreted by assuming, in the case of kaolin, the union of the metal precipitate with the basal faces of the substrate, so decreasing the negative charge at this surface; and in the case of quartz, the formation of a hydroxide coating that neutralizes the negative charge on the silica surface. The difference between the results obtained for each metal was attributed to the different morphologies of their oxide precipitates.

A theoretical study of Ziegler-Natta olefin polymerization on the TiCl 3 crystalline surface

Olefin insertion into Ti-methyl bonds of methyltitanium chloride clusters on a TiCl 3 crystalline surface has been studied by using paired interacting orbitals (PIO). We have shown that at least two types of active sites can be assumed on the edge of the basal face of violet TiCl 3 crystallites: a edge type active site which possesses a dangling bonded Cl atom, four bridged Cl atoms and a Cl vacancy and a corner type active site which possesses a dangling bonded Cl atom, three bridged Cl atoms and two Cl vacancies. The size effects on these active site have been estimated by changing the number of titanium atoms of the TiCl 3 model clusters. The most important interaction in the olefin coordinated state is electron delocalization from the occupied Ti d xz orbital of the methyltitanium chlorides to the π∗ orbital of the coordinated olefin molecule, while in the transition state is electron delocalization from the occupied C p and Ti d xz orbital of the methyltitanium chloride species to the π∗ orbital of the olefin. These interactions are compactly shown in PIO's. The coordination energy and the activation energy for the olefin insertion are almost independent of the cluster size when the number of Ti atoms is greater than three. The corner type active site has been shown to be stabilized at the transition state by the absence of one of the Cl atoms which are located orthogonal to the insertion plane. Thus, the olefin insertion on the corner type active site is more favorable than that on the edge type active site. In the case of propylene insertion, syn-orientation of propylene is favored. This regioselectivity should be lower on the corner type active site than that on the edge type active site.

Mechanism of the view-dependence of ultrasonic backscatter from normal myocardium

Anisotropy of ultrasonic scattering and attenuation in heart tissue depends on the specific orientation of myofibers with respect to angle of insonification. The present study was designed to delineate the effect of the angle of insonification with respect to the alignment of myofibers on measurements of integrated backscatter. A transmural cube of myocardium was cut from the anterior wall of the left ventricles from 5 normal canine hearts and their transmural scattering behavior was studied with the use of reflection acoustic microscopy at 50 MHz. A theoretical model for scattering based on the Born approximation (weak scattering) was employed to predict the relationship between backscatter and angle of insonification. Insonification of the basal face (basal view) demonstrated a wide transmural variation in integrated backscatter (/spl sim/15 dB), while insonification of the lateral face (lateral view) had much reduced variation (/spl sim/4 dB), despite an equivalent overall shift in transmural fiber angle of /spl sim/85/spl deg/ across the ventricular wall. Integrated backscatter was greatest in the midmyocardium when the basal face was viewed and least in the midmyocardium when the lateral face was viewed. The backscatter in the subepicardial and subendocardial regions was similar for both views. The maximum difference in backscatter from basal and lateral views at the midmyocardial level was approximately 18 dB, which represents a 64-fold change in the intensity of ultrasonic backscatter. The mathematical model developed for scattering based on the Born approximation (weak scattering) predicted the observed relationship between backscatter and angle of insonification. The rapid angular variation of integrated backscatter perpendicular to the fiber direction and the slow variation at parallel incidence observed experimentally were predicted by the model. This angular variation is due to the specific shape and elastic properties assumed for the predominant myocardial scatterer. There was a strong relationship between backscatter and fiber orientation, indicating that the view chosen for insonification of myocardium in clinical imaging may influence the estimation of scattering behavior. The mathematical model utilized here predicts the anisotropic behavior of scattering and suggests that the principal scattering structure in normal myocardium may be a stiff collagen shell surrounding a more compliant myocyte. This model might provide a valid approach for the study of material properties of the heart with the use of ultrasound. >

Synthesis of [Ir3Rh(CO)12] and Fluxional Behaviour of Some of Its Substituted Derivatives

AbstractThe redox condensation of [Ir(CO)4]–, [Ir(cod)(THF)2]+, and [Rh(cod)(THF)2]+ (cod = cycloocta‐1,5‐diene) followed by saturation with CO (1 atm) in THF afforded the first synthetic route to pure [Ir3Rh(CO)12] (1). Substitution of CO by monodentate ligands gave [Ir3Rh(CO)8(μ2‐CO)3L] (L = Br–,2; I–, 3; bicyclo[2.2.1]hept‐2‐ene, 4; PPh3, 5). Clusters 2–5 have Cs symmetry with the ligand L bound to the basal Rh‐atom in axial position. They are fluxional in solution at the NMR time scale due to two CO scrambling processes: the merry‐go‐round of basal CO's and changes of basal face. An additional process takes place in 5 above room temperature: the intramolecular migration of PPh3 from the Rh‐ to a basal Ir‐atom. Substitution of CO by polydentate ligands gave [Ir3Rh(CO)7–x(μ2‐CO)3(η4‐L)x] (L = bicyclo[2.2.1]hepta‐2,5‐diene (= norbornadiene; nbd), x = 1, 6; L = nbd, x = 2, 13; L = cod, x = 1, 7; L = cod x = 2, 15), [Ir3Rh(CO)7(μ2‐CO)3(η2‐diars)] (diars = 1,2‐phenylenebis‐(dimethylarsine); 8), [Ir3Rh(CO)7(μ2‐CO)3(η4‐L)] (L = methylenebis(diphenylphosphine), bonded to 2 basal Ir‐atom (9a) or one Ir‐ and one Rh‐atom (9b)), [Ir3Rh(CO)6(μ2‐CO)3(η4‐nbd)PPh3] (12), and [Ir3Rh(CO)6(μ2‐CO)3(μ3‐L)] (L = 1,3,5‐trithiane, 10; L = CH(PPh2)3, 11). Complexes 6–8, 9a, 10, and 11 have Cs symmetry, the others C1 symmetry. They are fluxional in solution due to CO scrambling processes involving 1, 3, or 4 metal centres as deduced from 2D‐EXSY spectra. Comparison of the activation energies of these processes with those of the isostructural Ir4 and Ir2Rh2 compounds showed that substitution of Ir by Rh in the basal face of an Ir4 compound slows the processes involving 3 or 4 metal centres (merry‐go‐round and change of basal face), but increases the rate of carbonyl rotation about an Ir‐atom.

Halo polarization profiles and the interfacial angles of ice crystals

Polarization and radiance of various types of refraction halo in ice-crystal swarms that extend to ground level were measured as a function of scattering angle. Simultaneously, samples of the crystals that produce these halos were collected and replicated. The halo polarization peaks are wider than the Fraunhofer theory of diffraction predicts for the observed size distribution of the replicated crystals. The explanation we put forward is that the angles between crystal prism faces are not always exact integer multiples of 60°, and the basal faces are not always exactly parallel, as is usually assumed. The collected crystals confirm this. The widths of the halo polarization peaks can be explained if the distributions of the interfacial angles around their means reach their half-maximum values at a deviation of 0.49° ± 0.05°. This corresponds to a deviation of 0.35° ± 0.03° of the face normals from their crystallographic positions. The presence of variation in interfacial angles in low-level halos seems to ari e from the fact that the crystals are growing. Some hitherto unexplained features in halo displays can be understood by considering variations in the interfacial angles.

Intramolecular Dynamics of Tetranuclear Iridium Carbonyl Cluster Compounds. Part V. Polysubstituted derivatives of [Ir4(CO)12

AbstractThe dynamic behaviour of twelve polysubstituted derivatives of [Ir4(CO)12] has been investigated in solution, using 2D‐EXSY, and VT‐31P‐ and 13C‐NMR. [Ir4(CO)6(μ2‐CO)3(η4‐diarsine) PPh3] and [Ir4(CO)6(μ2‐CO)3(η4‐nor‐bornadiene)(PMePh2)] exhibit two isomeric forms in solution, which interconvert through an intramolecular change of basal face. The related cluster [Ir4(CO)6(μ2‐CO)3(η4‐norbornadiene)PPh3] exists as a single isomer in solution. It displays rotation of CO ligands about the apical Ir‐atom, followed by two consecutive changes of basal face. The tetrasubstituted clusters with two chelating ligands [Ir4(CO)5(μ2‐CO)3(η4‐diolefin)2] also exhibit rotation of apical CO's, the activation energy increases with greater steric hindrance of the radical ligands. A quantitative analysis of the 31P‐ and 13C‐2D‐EXSY spectra followed by simulation of the corresponding VT‐NMR spectra of [IR4(CO)5(μ2‐CO)3(μ2‐L)2] (L = bis(diphenylphosphino)methane and 1,3‐bis(diphenylphosphino)propane) revealed a pairwise averaging of the P‐atoms, caused by two parallel changes of basal face averaging all CO ligands. In addition, the restricted rotation of ligands about the apical Ir‐atom was identified at higher temperatures. The remaining clusters are either rigid on the NMR time scale, or display CO‐scrambling about a single Ir‐atom.

The Theoretical Basis for the Parameterization of Ice Crystal Habits: Growth by Vapor Deposition

Abstract A theoretical analysis of surface kinetic and gas-phase diffusional effects permits the growth rates and habits of ice crystals to be specified in a self-consistent way. The analysis makes use of the fact that the difference between the condensation coefficients of the prism and basal faces determines the primary crystal habits, whereas the spatial variations of the vapor density contribute to the secondary habits. The parameterization scheme that results from the theoretical analysis yields a power law relationship between the a and c axial lengths that matches earlier empirical formulas derived from observational data for the temperature range of −30° to 0°C. Through application of this adaptive parameterization in a microphysical model that categorizes ice particles according to both their masses and shapes, it is shown that deviations from the power law relationship may develop if the crystals experience significant variations in the air temperature and in their inherent growth habits. A mass...

Crystal structure and intramolecular dynamics of [Ir 3Rh(CO) 8(η 4-cycloocta-1,5-diene) 2

The redox condensation of [Ir(CO) 4] −, [Ir(COD)(THF) 2] + and [Rh(COD)(THF) 2] − (COD = cycloocta-1, 5-diene) yields [Ir 3Rh-(CO) 8(η 4-COD) 2] ( I). X-ray diffraction shows that I contains three edge- bridging CO's defining a basal Ir 2Rh face of the metal tetrahedron, and two COD chelating the Rh atom and one basal Ir atom, respectively. In solution this structure is static on the NMR time scale up to ca. 220 Y, Above that temperature CO scrambling occurs due to the rotation of three terminal CO's about a local C 3 axis on the apical Ir atom. The reaction of I with CO (1 arm) in THF affords the first synthetic route to pure [Ir 3Rh(CO) 12].

Synthesis, Reactivity, and Fluxional Behaviour of [Ir2Rh2(CO)12], and Crystal Structure of [Ir2Rh2(CO)8(norbornadiene)2

AbstractThe synthesis of [Ir2Rh2(CO)12] (1) by the literature method gives a mixture 1/[IrRh3(CO)12] which cannot be separated using chromatography. The reaction of [Ir(CO)4]− with 1 mol‐equiv. of [Rh(CO)2(THF)2]+ in THF gives pure 1 in 61% yield. Crystals of 1 are highly disordered, unlike those of its derivative [Ir2Rh2(CO)5(μ2‐CO)3(norbornadiene)2] which were analysed using X‐ray diffraction. The ground‐state geometry of 1 in solution has three edge‐bridging CO's on the basal IrRh2 face of the metal tetrahedron. Time averaging of CO's takes place above 230 K. The CO site exchange of lowest activation energy is due to one synchronous change of basal face, as shown by 2D‐ and VT‐13C‐NMR. Substitution of CO by X− in 1 takes place at a Rh‐atom giving [Ir2Rh2(CO)8(μ2‐CO)3X]− (X = Br, I). Substitution by bidentate ligands gives [Ir2Rh2(CO)7(μ2‐CO)3(η4‐L)] (L = norbornadiene, cycloocta‐1,5‐diene) where the ligand L is chelating a Rh‐atom of the basal IrRh2 face. Carbonyl substitution by tridentate ligands gives [Ir2Rh2(CO)6(μ2‐CO)3(μ3‐L)] (L = 1,3,5‐trithiane, tripod) with L capping the triangular basal face of the metal tetrahedron. Carbonyl scrambling is also observed in these substituted derivatives of 1 and is mainly due to the rotation of three terminal CO's about a local C3 axis on the apical Ir‐atom.

Thickness and anisotropy of the ice-water interface

Measurements of the ellipticity of the ice-water interface are reported for both the basal and prism faces of ice. The sign of the ellipticity indicates anisotropy of the water molecules in the interface region, and the data are consistent with a thickness - 1 nm and roughly twice as much anisotropy for the prism as for the basal face.