Van der waals and polar intermolecular contact distances: quantifying supramolecular synthons

Crystal structures are viewed as being determined by ranges and constraints on interatomic contact distances between neighboring molecules. These distances are considered to arise from environment-dependent atomic sizes, that is, larger sizes for isotropic, van der Waals type contacts and smaller sizes for more-polar, possibly ionic contacts. Although the idea of different, or anisotropic, radii for atoms is not new, we developed a method of obtaining atomic sizes that is based on a theoretical framework. Using different atomic sizes for the same atom in different environments, we were able to rationalize some structural observations and anomalies. For example, benzene with the Pbca structure may be described in terms of two types of C···H interactions: a longer contact largely of the van der Waals type, and a shorter, structure-determining type (Cδ -···Hδ +), which we term "n-polar". Our approach is illustrated with three examples: 1) the equivalence in crystal packing of fluorobenzene, benzonitrile, pyridine N-oxide, and pyridine/HF 1:1 molecular complex, all of which take the not-so-common tetragonal P41212 space group and are practically isomorphous; 2) the similarity of the Pa3 acetylene and Pbca benzene crystal structures; and 3) the equivalence between an increase in pressure and an increase in the "n-polar"contacts in Pbca benzene; in other words, the equivalence between hydrostatic pressure and chemical pressure. In the context of crystal engineering, we describe a method whereby the topological information conveyed in a supramolecular synthon is recast in a more quantitative manner. A particular synthon, and in turn the crystal structure to which it leads, is viable within small ranges of distances of its constituent atoms, and these distances are determined by chemical factors

A magnetic susceptibility study of spin-state transitions in rare-earth trioxocobaltates(III)

Rare-earth trioxocobaltates(III), Ln[CoO3], with Ln=Pr, Nd, Tb, Dy, and Yb exhibit low-spin to high-spin transitions of cobalt characterised by a maximum in the Δχ−1 against temperature plots where Δχ is the cobalt contribution to the magnetic susceptibility. The susceptibility behaviour is distinct from that of La[CoO3] which shows a plateau in the χ−1-T curve accompanied by a structural transition. The temperature at which the Δχ−1-T curve shows a maximum increases with the decrease in the size of the rare-earth ion. The susceptibility behaviour of solid solutions of La1−xNdxCoO3 has been investigated to see how the behaviour characteristic of Nd[CoO3] changes to that of La[CoO3]

Optical Activity From Extra Dimension

Optical activity, like Faraday effect, is a rotation of the plane of polarization of propagating light in a medium and can be attributed to different sources with distinct signatures. In this note we discuss the effect of optical activity {\it{in vacuum}} due to Kaluza-Klein scalar field $\phi$, in the presence of an external electro-magnetic field. The astrophysical implication of this effect is indicated. We also point out the possibility of observing the same in laboratory conditions.Comment: Four Page

Atomic sizes from atomic interactions

We obtain an atomic size, rnZc , in the presence of an interaction (represented by an electron-hole pair, e-h+ as in vacuum polarization techniques) as a sum of contribution, rnv, from the interaction of nval valence s- and p-electrons and a contribution rRG from inner filled shell electrons with rare-gas configuration. The method is applicable to all elements for a given electron configuration that is usually available simply from the position of the elements in the periodic table. The sizes thus obtained are close to other "core sizes" obtained in the literature. The transition metal elements are treated as group II elements. This method gives sizes for the actinide and trans-actinide elements without requiring relativistic corrections. The importance of these sizes in interpreting interatomic distances in terms of electronic configuration is illustrated for actinide elements

A simple biosynthetic method for the preparation of glycerol-labelled phosphatidylcholine

A convenient method is described for the preparation of glycerol-labelled phosphatidylcholine with very high specific activity. It involves germination of soybean seeds in the dark at 37°C for 48 h in the presence of labelled glycerol, followed by extraction and purification of the phospholipid

Bond length variations: electron number profiles and transferable atomic sizes

A profile of the number of electrons with distance along the M-X bond in gas-phase diatomic molecules has been obtained from electron density plots calculated using DFT B3LYP 6-311G∗∗ method for some representative molecules. This "number profile" is compared with that expected from the partitioning of the 1D bond-distance into atom-specific transferable "hub" or core atomic sizes of the M and X atoms and another "axle" size which is associated with a pair of (bonding) electrons. The "hub" size is proportional to a core atom-specific size, rnZc with rnZc(M) ≥ rnZc(X). For "single bonds", the "hub" size for M atom is CMrnZc(M) and for X atom is CXrnZc(X). The "axle" size, DMX, is usually the ordinary (~4aH/3 where aH is the Bohr radius of the hydrogen atom) or elongated (~2aH) bond length of the hydrogen molecule. The "hub" and "axle" sizes could be characterized "charge-transfer" (CM = π2/3 = 2.144; CX = π4/3/2 = 2.300 and DMX = 4aH/3) or "neutral" (CM or CX = 1, 2, ... and DMX = 2aH). We use a new "static" or "peripatetic" classification for the core sizes which is derived from a new condition for metallization in elements based on atomic size. The charge-transfer distance, dMX±, is usually found for "static" conditions while the "neutral" description is usually found when X = F or for "peripatetic" conditions. Such a partitioning is seen to agree with that from the plot of the total number of electrons, Nel, vs r along a bond axis. The Nel vs r plots from each atom are described by a simple hydrogen-atom-like function which differ away ("out") or towards ("in") the M-X bond. Thus Nin,out(M,X) = (ZM,X ± 1)exp (-r/Bin,out) where the minus sign is associated with M and plus sign with X and Bin,out being related inversely to the Slater orbital exponent

The mechanism of intestinal absorption of phosphatidylcholine in rats

1. The mechanism of absorption of phosphatidylcholine was studied in rats by injecting into the intestine phosphatidylcholine specifically labelled either in the fatty acid or in the glycerol moiety or with (32)P, when considerable amounts of 1-acyl-lysophosphatidylcholine were found in the intestinal lumen. 2-([(14)C]Acyl)phosphatidylcholine gave markedly more radioactive unesterified fatty acids in the lumen, compared with the 1-([(14)C]acyl) derivative. Some of the radioactivity from either the fatty acid or the glycerol moiety of the injected phosphatidylcholine appeared in the mucosal triacylglycerols. 2. Injection of (32)P-labelled phosphatidylcholine or (32)P-labelled lysophosphatidylcholine led to the appearance of radioactive glycerylphosphorylcholine, glycerophosphate and P(i) in the mucosa. 3. Rat mucosa was found to contain a highly active glycerylphosphorylcholine diesterase. 4. It was concluded that the dietary phosphatidylcholine is hydrolysed in the intestinal lumen by the pancreatic phospholipase A to 1-acylglycerylphosphorylcholine, which on entering the mucosal cell is partly reacylated to phosphatidylcholine, and the rest is further hydrolysed to glycerylphosphorylcholine, glycerophosphate, glycerol and P(i). The fatty acids and glycerophosphate are then reassembled to give triacylglycerols via the Kennedy (1961) pathway