Equatorial Ones Research Articles

Last glacial paleoproductivity patterns in the eastern equatorial Pacific: benthic foraminifera records

This study explores the export-productivity changes for the eastern equatorial region between 100 and 115°W during the Last Glacial with the aid of benthic foraminifera accumulation rates (BFAR) from the well-dated Pleiades (PLDS) core collection (Berger and Killingley, 1981) and compares these results with western equatorial Pacific ones. We present some new data for the calibration between BFAR and the organic carbon flux to the sea-floor and discuss some of the limitations of using absolute values for this type of reconstruction. Contrary to expectation, not all export productivity to the deep ocean under the equatorial Pacific responds with the same magnitude to changes in ocean circulation during the Last Glacial period. Two export-productivity patterns emerge from this reconstruction based on benthic foraminifera accumulation. Sites slightly North of the eastern equatorial Pacific (≈1°N) show a minor increase in the organic carbon flux to the sea-floor, up to a factor of 1.2, while the reconstructed BFAR values are very similar to western equatorial Pacific ones. One of the implications of this finding is a decreased east–west equatorial export-productivity gradient relative to the modern one. In contrast, sites south of the equator (≈3°S) show more than a double-fold increase in the accumulation rates of benthic foraminifera, equivalent to an increase by a factor of 1.6–1.8 in the organic carbon flux to the sea-floor. Although there is not enough data to decide unambiguously among the different mechanisms that may have forced this pattern of export productivity, we propose that while upwelling at the equator may have been stronger during the Last Glacial, its east–west gradient was significantly decreased with respect to the modern one. Either a southern migration of the equatorial divergence zone or a relative decrease in the nutrient concentration of the upwelled waters 1°N of the equator may explain the observed lower export-productivity gradient. To the south, stronger advection of coastal upwelled nutrient-rich waters of southern origin may have enhanced the production and export of organic carbon to the deep sea, which fueled the observed higher abundance in benthic foraminifera.

Structural and electronic changes accompanying reduction of Cr(CO)4(bpy) to its radical anion: a quantum chemical interpretation of spectroelectrochemical experiments

Optimised molecular structures and charge distributions within Cr(CO)4(bpy) and its radical anion were calculated using density functional theory (DFT). It was found that, although reduction predominantly concerns the bpy ligand, its structural and electronic effects extend to the Cr(CO)4 fragment. Each equatorial and axial CO ligand was calculated to accept 7.1 and 4.8%, respectively, of the extra electron density in Cr(CO)4(bpy)˙–. This is in accordance with the IR spectroelectrochemical results which show that the corresponding CO stretching force constants decrease by 68 and 21 N m–1, respectively. The calculated spin density in Cr(CO)4(bpy)˙– resides predominantly on the bpy ligand which behaves spectroscopically as bpy˙–. The spin density is delocalised to both axial and equatorial pairs of CO ligands by mixing of π*(CO) orbitals with the, predominantly π*(bpy), SOMO. In addition, part of the spin density is delocalised selectively to the axial CO ligands by an admixture of their σ orbitals into the SOMO. This σ–π* contribution is responsible for isotropic EPR hyperfine splitting which was observed from the axial 13C(CO) atoms only. Accordingly, the isotropic hyperfine splitting constants correlate with calculated Fermi contact terms instead of total spin densities. Complete active space self-consistent field (CASSCF)-calculated changes in charge distribution upon a Cr→bpy MLCT excitation show that the electron density localised on the bpy ligand increases by about the same amount upon reduction or MLCT-excitation of Cr(CO)4(bpy). The axial CO ligands are depopulated by MLCT excitation ca. 1.6 times more than the equatorial ones. These conclusions can be generalised and applied to other coordination and organometallic complexes of low-valent metals which contain a reducible or radical-anionic ligand.

A global empirical model of effects of large-scale internal gravity waves in the night-time ionosphere

On the basis of an analysis of a large amount of data from the vertical ionospheric sounding stations located in the Northern half of the Eastern hemisphere, a global empirical model of variations of f 0 F 2 and the F 2 layer height associated with large-scale internal gravity waves (IGW) during magnetospheric substorms and the first few hours of magnetic storms is constructed. The model is valid for night hours in the interval from subauroral latitudes to equatorial ones for all seasons and for different levels of solar activity. The model allows one to estimate the probability, delay relative to substorm onset, amplitude and time evolution of the IGW effects in the ionospheric F region. Using this model, the main features of the IGW effect in the ionospheric F region are derived depending on season, latitude, and levels of solar and magnetic activities. It is found that the IGW probability is quite high; it increases with increases of substorm intensity and solar activity. For substorms with AE max > 1000 nT in years of solar maximum, this probability amounts to as much as 100%. The IGW amplitude also increases with increases of substorm intensity and solar activity. The IGW effect amplitude depends on solar activity nonlinearly: it increases appreciably when F 10.7 changes from 75 to 150 and almost does not vary when F 10.7 grows from 150 to 200. At high solar activity, seasonal variations are clearly exhibited: the IGW effect amplitude is noticeably greater in winter and the equinoxes than in summer. In years of solar minimum, seasonal variations are negligible. For all seasons and levels of solar and magnetic activities, the effect amplitude is greatest at subauroral latitudes and then decreases smoothly as the latitude is reduced, with a local maximum near geomagnetic latitude φ = 20 . Interpretations of some features of the effect are suggested.

Synthesis and X-ray Structural Characterization of Two Unbridged Diplatinum(III) Compounds: cis- and trans-Bis[bis(1-imino-1-methoxyethane)trichloroplatinum(III)]. Transient Species in the Oxidation of Platinum(II) to Platinum(IV)

Unbridged PtIII dimers, cis- and trans-[PtCl3L2]2 (L = (E)-HNC(OMe)Me, cis-2 and trans-2), are formed, as transient species, in the oxidation with excess chlorine of the PtII compounds cis- and trans-[PtCl2L2] (cis-1 and trans-1) to the corresponding PtIV species, cis- and trans-[PtCl4L2] (cis-3 and trans-3). Complexes cis-2 and trans-2 are rather stable at low temperature (<0 and −20 °C for cis-2 and trans-2, respectively), while at higher temperature (20 °C) they readily oxidize to PtIV (τ1/2 = 80 and 5 min for cis-2 and trans-2, respectively). The structures of the PtIII species have been solved by single-crystal X-ray diffraction. The compounds are neutral dimers composed of two square-planar PtCl2L2 units perpendicularly connected by a Pt−Pt bond and capped by axially coordinated chloride ligands. Bond distances and angles within the equatorial PtCl2L2 units are normal. The axial Pt−Cl bond lengths are ca. 0.14 A longer than the equatorial ones, indicating a strong trans influence of the intermetalli...

ChemInform Abstract: Unexpected Stability of δ‐Lactones with Axial Substituents Rather than Equatorial Ones. Conformational Evaluation by Molecular Mechanics and Molecular Orbital Calculations.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Unexpected stability of δ-lactones with axial substituents rather than equatorial ones. Conformational evaluation by molecular mechanics and molecular orbital calculations

A 1:1 mixture of the trisubstituted δ-lactones 5a and 5b was subjected to thermodynamically equilibrated conditions to give predominantly 5b with axial substituents rather than 5a with all equatorial ones, in contrast to the Prelog-Djerassi lactone derivatives 3a and 3b. Further surprisingly, it has been found that the disubstituted lactone 15 also adopts a half-chair conformation with axial substituents. The conformational analyses of these compounds by molecular mechanics and molecular orbital calculations have emphasized gauche effects and electrostatic interactions as a cause of the preference for axial conformers.

Reactions of iridium and ruthenium complexes with organic azides

Interaction of N 3 R with Ir(mes) 3 (mes = mesityl, C 6 H 2 Me 3 -2,4,6) gave products dependent on the nature of the azide. When R = mes, the tetrazenido amide complex 1 is obtained in which dehydrogenative coupling of the mesityl groups via the o-methyls has occurred; thermolysis of 1 in toluene resulted in cleavage of the tetrazene ring and formation of amide complex 2. When R = Ph, the aryl tetrazenido amide complex 3 is formed. Photolysis of a mixture of N 3 (mes) and [RuCl 2 (PPh 3 ) 3 ] followed by phosphine exchange gave the tetrazene complex [Ru II Cl 2 {N 4 (mes) 2 }(PMe 3 ) 2 ] 4. Thermal reaction of [RuCl 2 H 2 (PPr i 3 ) 2 ] with N 3 (mes) gave the triazenophosphorane complex [RuCl 3 (PPr i 3 ){N 3 (mes) PPr i 3 }] 5. The ruthenium allyl amide [Ru(PMe 3 ) 3 {NHC 6 H 3 Pr i (η 3 -CH 2 CCH 2 )}] 6 bearing a new hybrid ligand was obtained by interaction of trans-[RuCl 2 (PMe 3 ) 4 ] with Li[NH(C 6 H 3 Pr i 2 -2,6)] in di-n-butyl ether. Plausible reaction mechanisms accounting for the formation of the new compounds are proposed. Finally, the crystal structures of the complexes 1–6 have been determined. Complexes 1 and 2 have pseudo-square planar geometries involving the olefin formed by the coupled methyl groups of two mesityls and three (1) or two (2) amide nitrogens and a chlorine atom (2). Compound 3 has a trigonal bipyramidal metal centre with the axial Ir–N amide bonds longer than the equatorial ones; 4 has an octahedral structure with a bidentate tetragonal ligand and trans phosphines whilst 5 is distorted octahedral with a N,N-chelating phosphazide ligand. Complex 6 is also octahedral with the allyl groups occupying cis sites and the three Ru–P bonds in a facial arrangement.

Comparison Between the Performance of Ti:LiNbO3 and H:LiNbO3 Rotated Optical Axis Waveguides - Summary

Guided and leaky mode propagation in three-layered and five-layered c-rotated anisotropic waveguides is examined. The considered dielectric tensor configurations are the polar, longitudinal and equatorial ones. The modal characteristics of titanium diffused lithium niobate (Ti:LiNbO3 ) and proton exchanged lithium niobate (H:LiNbO3 ) waveguides are compared. The experimental results for a three layered Y-cut Ti:LiNbO3 waveguide agree very well with the calculated ones.

Correlation curves for equatorial-axial interactions

Metal-ligand distances for hexacoordinate complexes show a mutual interrelation manifested in the fact that with increasing axial distances the equatorial ones decrease and vice versa. These R a vs R e dependences have been registered for certain chromophores and also for hexafluoro, hexachloro, hexanitro and hexaaqua complexes. Based on the anharmonic shape of the adiabatic potential surface, a theoretical analysis yields the form of the R a vs R e dependence which, in most simple form, is a hyperbolic function.

The sodium and potassium atmosphere of the moon and its interaction with the surface

Observations of lunar atmospheric sodium and potassium from May 1988 to July 1991 are reported and analyzed. Densities at 80° north and south are less than equatorial ones by a factor of 2–3. For our observations, which do not reach above 800 km from the limb, the apparent scale heights for the intensity are 119–611 km for Na, and 85 and 154 km for K; most of these are much larger than would be expected for atoms thermalized to the surface temperature. However, the intensity drops off with increasing radius at a much greater rate than would be observed for an atmosphere that is mostly escaping. We interpret our data using both single- and two-component analyses. We amplify an earlier suggestion that source atoms are quickly redistributed into thermal and suprathermal populations by “competing release mechanisms” acting at the surface. The suprathermal distributions are produced by solar radiation releasing atoms adsorbed on the surface ( photodesorption). We present reasons why the energy distribution seems to mimic a Maxwellian. The competing release mechanisms explain an obvious trend of decreasing apparent scale heights toward the subsolar point, where the density in the lowest 100 km appears to be dominated by thermally desorbed atoms. The suprathermal component is expected to appear at greater altitudes, but the early subsolar data do not extend high enough to reveal it. Six of the data sets are tentatively resolved into thermal and suprathermal components. The variation with latitude is naturally explained if a larger fraction of the atoms at large solar zenith angles are adsorbed to the surface, rather than being visible in the atmosphere. Migration to the dark side may also play a role. It is shown that at most a very small fraction of the observed atoms below a few hundred km altitude can be on escape trajectories. We apply these ideas to the budget of atomic oxygen. We suggest that the inventory of oxygen atoms is greatly reduced because they stick to the surface with high efficiency similar to that of the alkalis, and subsequently recombine with each other or with partially reduced oxides of such atoms as Mg, Fe, Al, and Si.

The crystal and molecular structures of the N,N’-diisopropylbutyramide (DIPIBA) and N,N-dicyclohexylacetamide (DCA) complexes [Th(NCS) 4(DIPIBA) 3] and [Th(NCS) 2Cl 2(DCA) 3

The X-ray crystal structures of [Th(NCS) 4(DIPIBA) 3] ( 1) [DIPIBA Me 2CH CON(CHMe 2) 2] and [Th(NCS) 2Cl 2(DCA) 3]( 2) [DCA MeCON(cycloC 6H 11) 2] have been determined from three-dimensional X-ray diffraction data. The different steric constraints of the ligand are not sufficient for different coordination geometries. In 1 and 2 the coordination polyhedron about the seven-coordinate thorium atom is a pentagonal bipyramid geometry which is becoming rather common for thorium(IV) compounds. In these compounds two chlorine ( 2) and two thiocyanate ( 1) anions are in axial positions with a significant shortening of the axial Th—NCS bonds with respect to the equatorial ones, while in the equatorial plane a neutral ligand lies between two anionic ligands, the remaining coordination positions being occupied by two other neutral ligands. The average bonding distances are Th O = 2.34(1)Å, Th N ax = 2.42(1) and Th N eq = 2.46(1)Åin 1, and Th O = 2.357(5), Th N eq = 2.479(6) and Th Cl ax = 2.695(3)Åin 2.

Ab initio calculations of structural features not easily amenable to experiment: Part 74. Conformational analysis of cyclotrimethylene-trinitramine (RDX)

The structures of four forms of cyclotrimethylene-trinitramine (CH 2NNO 2) 3 (RDX) were determined by SCF/4-21G ab initio gradient optimization. The forms considered are the chair forms AAA, AAE, AEE and EEE, in which the NO 2 groups are either in the axial (A) position or in the equatorial (E) position, except for the EEE form which is nearly planar. The form AAA was identified as the most stable conformation, in agreement with gas phase and solution studies of the compound. Calculations of vibrational frequencies yielded imaginary values for all forms with equatorial NO 2 groups. Thus, the latter are not energy minima in SCF/4-21G space. Even though the 4-21G calculations are rather approximate for compounds of this type, they make it possible to describe characteristic structural trends involving axial and equatorial NO 2 groups. Most importantly, axial N-N bonds are considerably longer (by about 0.05 Å) than equatorial ones. This finding is in agreement with the hypothesis by Stals that axial N-NO 2 bonds in secondary nitramines are weaker than equatorial ones.

Synthese und flüssigkristalline Eigenschaften von Multiethern des myo‐ und scyllo‐Inosits

Synthesis and Liquid‐Crystalline Properties of myo‐ and scyllo‐Inositol Multi EthersSelectively alkylated myo‐ and scyllo‐inositol ethers were obtained by multi‐step syntheses starting from commercially available myo‐inositol. Necessary conversions of axial hydroxy groups into equatorial ones were achieved using either potassium superoxide (KO2) or potassium nitrite (KNO2) as oxygen nucleophiles for the transformation of myo‐ into scyllo‐ inositol derivatives. Polarizing microscopic and thermoanalytical studies of numerous members of three families (6, 13, and 20) of inositol ethers reveal for most of them thermotropic liquid‐crystalline behavior. Furthermore, in case of the two long‐chain scyllitol tetraethers 13d and e an unusual thermopolymorphism has been found. The number, position, and stereochemical arrangement of alkyl chains attached to the inositol cores strongly influence the mesogenic properties of these biomolecule derivatives, chemically vicinal diols and tetrols. The self‐assembly of the studied inositol multi ethers to columnar mesophases of “supramolecular structures” is discussed considering geometrical features of these molecules and their capabilities to aggregate by hydrogen bonding.

On the numerical dispersion relation of equatorial waves

Numerical dispersion relations for equatorial wave modes are computed two ways: from equations for the pressure, p, and for the meridional velocity, v. These are compared with both the continuous and the analytic finite difference dispersion relations for the u‐v‐p system of equations on an Arakawa C‐grid derived by D. W. Moore (personal communication, 1990). For a particular meridional wave structure and grid resolution, the dispersion relation is approximated more accurately via the finite‐difference form of the v‐equation than by either the p‐equation or the u‐v‐p system. However, the finite difference v‐equation possesses an undesirable wave solution which has unbounded u and p, and it also lacks the equatorial Kelvin wave. On the other hand, the p‐equation, is found to possess additional numerical solutions due to the conversion of the apparent singularities of the continuous equation at the inertial latitudes (yil = ±σ/β) into regular singularities of the discrete equation due to numerical truncation. These additional solutions are well separated in dispersion space from the equatorial ones except at selected frequencies. The frequencies at which the wavelengths of these additional solutions to the p‐equation approach the continuous dispersion relations are found to be sensitive to the distance from the inertial latitude to the nearest gridpoint.

Unprecedented stability of δ-lactones with axial substituents rather than equatorial ones; comparison with the Prelog–Djerassi lactone derivative

A 1 : 1 mixture of the trisubstituted δ-lactones 11a and 11b was subjected to thermodynamically equilibrated conditions to give predominantly 11b with axial substituents rather than 11a with all equatorial ones, in contrast to the Prelog–Djerassi lactone derivatives 3a and 3b, and, further surprisingly, it has been found that the disubstituted lactone 10 also adopts a chair conformation with axial substituents.

Liquid-Crystalline Inositol Ethers, their Syntheses and Columnar Mesophases

Abstract Syntheses of selected multi-O-alkylethers of the stereoisomeric myo- and scyllo-inositols are described. Necessary conversions of axial hydroxyl groups into equatorial ones have been performed with either potassium superoxide (KO2) or potassium nitrite (KNO2) as powerful oxygen nucleophiles for this purpose. Most of the novel cyclitol ethers are thermotropic liquid crystals. Depending on the number, positions, and stereochemical arrangement of alkyl chains attached to the inositol core, these biomolecule mesogens, chemically vicinal diols and a tetrol, show interesting modes of molecular organizations in their columnar liquid-crystalline phases. The architectures of these mesophases are based on hydrogen-bridged multimers. Models for their self-assembled “supramolecular structures” are presented and discussed.

Analysis of the coordination geometry in copper complexes

Geometrical parameters associated with a metal coordinated system have been analysed using data from 75 structures of copper complexes involving 370 coordination bonds. Coordination bond length and coordination bond angle, and deviation of the metal atom from the plane of equatorial ligand atoms have been analysed in detail for both oxygen and nitrogen ligand atoms. At the ligand end, the parameters defined closely follow that of the hydrogen bonded system and are related to the lone pair orbital directions. We find that (1) for oxygen ligands, the axial bond lengths are distinctly longer than the equatorial ones in octahedral (OCT) and square pyramidal (PY) geometries. The situation is reversed for trigonal bi-pyramidal (TBP) geometry with nitrogen ligands. (2) The coordinnation bond angle involving axial atoms varies between 130 and 180° for the OCT case and 170 and 180° for TBP geometry. Angles lying around 90° range from 75–105° with a few exceptions in OCT geometry. (3) The metal atom lies very nearly in the equatorial plane in TBP, while it deviates significantly in others. The extent of deviation can be explained qualitatively with the pull of the metal atom by the axial atoms. (4) The distribution of the ligand end parameters indicate that the bond direction strongly tends to cling to the associated orbital. When the ligand atom has two orbitals, the influence of the non-associated orbital is not significant and is far less as compared to hydrogen bonding.

Germanium(II)-Komplexe mit Dithio-at-Chelatliganden / Germanium(II) Complexes with Dithio-ate Chelating Ligands

Stable germanium(II)-dithiocarbamate complexes Ge[S2C(NR2)]2 (R = Me, Et) have been obtained from GeCl2 · dioxane and Na[S2C(NR2)]. As an intermediate, Ge(Cl)[S2C(NEt2)] also could be isolated. Likewise, Ge[S2PMe2]2 was synthesized from GeCl2 · dioxane and Na[S2PMe2] and characterized by X-ray structure analysis. The germanium atom acts as a spiro-center for four sulfur atoms in a distorted ψ-trigonal bipyramidal configuration which is established by two anisobidentate dithiophosphinate ligands. The axial Ge—S bonds are significantly longer (mean: 2.722 A) than the equatorial ones (mean: 2.415 A). The variation in the axial bonds is more pronounced (0.058 A) than in the equatorial (0.013 A) Ge—S bonds, but this effect is much smaller than in the comparable Ge[(PMe2)2CPMe2]2 complex.

The Crystal and Molecular Structure of Di-μ-hydroxo-bis[pentaaquascandium(III)] Benzenesulfonate Tetrahydrate, [{(H2O)5Sc}(OH)2{Sc(H2O)5}](C6H5SO3)4·4H2O: A Dimeric Complex in Pentagonal-Bipyramidal Hepta-Coordination

Abstract The crystal and molecular structure of the mew title complex has been determined by means of a single crystal X-ray diffraction method. It is triclinic with the space group P\bar1, Z=2, a=15.518(5), b=19.827(7), c=7.493(2) Å, α=100.90(3), β=103.94(2), γ=82.85(3)°. Both of the metal atoms have almost the same coordination geometry, pentagonal bipyramidal which coordinates the oxygen atoms of five water molecules (2 axial and 3 equatorial ones) and the two hydroxide ions which bridge the both metal atoms. The Sc(1)···Sc(2) distance is 3.403(2) Å. The average Sc–O(H2O) bond length is 2.194 Å, which is much longer than that of Sc–O(OH), 2.072 Å. Benzenesulfonate ions and the other water molecules do not coordinate to the metal atoms, though their oxygen atoms as well as those of the coordinated water are connected with each other by a network involving hydrogen bonding.

The Crystal and Molecular Structures of Four Homologous, Mesogenictrans,trans-4,4′-dialkyl-(1 α,1 ′-bicyclohexyl)-4ß-carbonitril (CCN's)

Abstract The crystal and molecular structures of four homologous (R′(CN)-C6H9-C6H10-R, CCN's) were determined using single crystal x-ray diffraction data. In all four structures the cyano groups occupy axial positions whereas the alkyl groups occupy equatorial ones. The cyclohexane rings are approximately perpendicular to each other in CCN33 (R˭R′˭C 3 H7), in CCN35 (R˭C 3 H7, R′˭C 5 H11) and in CCN38 (R˭C 3 H7, R′˭C 8 H17) leading to a room filling of the free space produced by the cyano groups. More than that, the hydrogen-hydrogen contacts between the cyclohexane rings are minimal in such an arrangement. However, in CCN46 (R˭C 4 H9, R′˭C 6 H13) the molecules contain nearly coplanar bicyclohexyl moieties. These and some other structural features are discussed with regard to the question of the extent to which the crystalline state can be interpreted as a precursor of liquid crystalline states.