Synthesis and Isolation of Titanium Metal Cluster Complexes and Ligand-Coated Nanoparticles with a Laser Vaporization Flowtube Reactor

A new laser vaporization flow reactor (LVFR) is described consisting of a laser ablation cluster source combined with a fast flowtube reactor for the production and isolation of ligand-coated metal clusters. The source includes high repetition rate laser vaporization with a 100 Hz KrF (248 nm) excimer laser, while cluster growth and passivation with ligands takes place in a flowtube with ligand addition via a nebulizer spray. Samples are isolated in a low temperature trap and solutions containing the clusters are analyzed with laser desorption time-of-flight mass spectrometry. Initial experiments with this apparatus have trapped Ti _x (ethylenediamine) _y complexes which apparently have linear metal units with octahedral ligand coordination. Other experiments have produced and isolated clusters of the form Ti _x O _y (THF) _z that apparently have linear metal oxide cores and larger (TiO₂) _x (THF) _y nanoparticle species. The isolation of these new cluster species suggest that the LVFR instrument has considerable potential for the production of new nanocluster materials.     

Evidence for rigid binding of rhodamine 6G to silica surfaces in aqueous solution based on fluorescence anisotropy decay analysis

Strong ionic binding of the cationic probe rhodamine 6G (R6G) to the anionic surface of silica particles in water provides a convenient labeling procedure to study both particle growth kinetics and surface modification by time-resolved fluorescence anisotropy (TRFA). The decays for R6G dispersed in diluted Ludox silica sols usually fit to a sum of picosecond and nanosecond decay components, along with a significant residual anisotropy component. The origin of the nanosecond decay component (phi2) is not fully understood, and has been ascribed to wobbling of the probe on the silica surface, the presence of a subpopulation of small nanoparticles in the Ludox sol, or rapid exchange between free and bound R6G. To elucidate the physical meaning of phi2, measurements were performed in various silica-based colloidal systems using different concentrations of silica. We found that the fraction of phi2 was generally higher in Ludox than in aqueous sodium silicate and decreased with increasing silica concentration; phi2 vanished upon gelation of sodium silicate at pH 7 leading to a total loss of R6G depolarization (r(t) = const). These results rule out the presence of local R6G wobbling when bound ionically to colloidal silica and support the rigid sphere model to describe the TRFA decays for R6G-Ludox. This conclusion is entirely supported by steady-state anisotropy data and structural considerations for the R6G molecule and the silica surface.     

An elementary derivation of the hard/soft-acid/base principle

The hard/soft-acid/base (HSAB) principle indicates that hard acids prefer binding to hard bases (often forming bonds with substantial ionic character) while soft acids prefer binding to soft bases (often forming bonds with substantial covalent character). Though the HSAB principle is a foundational concept of the modern theory of acids and bases, the theoretical underpinnings of the HSAB principle remain murky. This paper examines the exchange reaction, wherein two molecules, one the product of reacting a hard acid and a soft base and the other the product of reacting a soft acid with a hard base, exchange substituents to form the preferred hard-hard and soft-soft product. A simple derivation shows that this reaction is exothermic, proving the validity of the HSAB principle. The analysis leads to the simple and conceptually appealing conclusion that the HSAB principle is a driven by simple electron transfer effects.     

Variational principles for describing chemical reactions. Reactivity indices based on the external potential.

In a recent paper [J. Am. Chem. Soc. 2000, 122, 2010], the authors explored variational principles that help one understand chemical reactivity on the basis of the changes in electron density associated with a chemical reaction. Here, similar methods are used to explore the effect changing the external potential has on chemical reactivity. Four new indices are defined: (1) a potential energy surface that results from the second-order truncation of the Taylor series in the external potential about some reference, Upsilon(R(1),R(2),.,R(M)()); (2) the stabilization energy for the equilibrium nuclear geometry (relative to some reference), Xi; (3) the flexibility, or "lability", of the molecule at equilibrium, Lambda; and (4) the proton hardness, Pi, which performs a role in the theory of Br?nsted-Lowry acids and bases that is similar to the role of the chemical hardness in the theory of Lewis acids and bases. Applications considered include the orientation of a molecule in an external electric field, molecular association reactions, and reactions between Br?nsted-Lowry acids and bases.     

Organometallic polymers. XXIV. Radical-initiated copolymerization of ferrocenylmethyl acrylate and methacrylate with acrylonitrile,maleic anhydride,and N-vinyl-2-pyrrolidone

Ferrocenylmethyl acrylate (I) and ferrocenylmethyl methacrylate (II) have been readily copolymerized with maleic anhydride in benzene–ethyl acetate solutions. Similarly, II has been copolymerized with both acrylonitrile and N-vinyl-2-pyrrolidone in benzene solutions to give higher molecular weight copolymers in high yields. In all cases azobisisobutyronitrile has been the initiator. Based on e values obtained, the metal carbonyl substituent acts as an electron-withdrawing group. Over a wide range of comonomers (N-vinyl-2-pyrrolidone, styrene, vinyl acetate, methyl acrylate, acrylonitrile, and maleic anhydride) I and II exhibit r₁ values lower than (and r₂ values higher than) similar copolymerizations with methyl acrylate or methyl methacrylate. Further more, the Q values found for I (0.03–0.11) and II (0.08–0.18) are smaller than those for methyl acrylate (0.46) and methyl methacrylate (0.74). Thus, I and II are less reactive than expected, presumably due to steric effects.     

Diastereoselective alkylation of beta-amino esters: structural and rate studies reveal alkylations of hexameric lithium enolates

Alkylation of beta-amino ester enolates proceeds with high diastereoselectivity. Single crystal, powder, and solution X-ray diffraction studies of the enolate show that the racemic enolate forms prismatic hexamers. 6Li NMR spectroscopic studies on partially racemic enolates reveal complex mixtures of homo- and heterochiral hexamers. An implicit fit of the aggregate populations to the Boltzmann distribution provides the free energy differences and equilibrium constants for the ensemble. Rate studies show that enolate alkylation occurs directly from the hexamer with participation by THF. A mechanism based on the alkylation of a ladder-like aggregate is proposed.     

Information Theory, the Shape Function, and the Hirshfeld Atom

Following the work of Nalewajski and Parr, there has been a surge of interest in the use of information theory to describe chemical bonding and chemical reactions. However, the measure of “information” used by Nalewajski and Parr is not any of the usual conventional entropies, chiefly because the electron density is not normalized to one. The consequences of this are discussed, and a solution is constructed using the shape function and an “entropy of mixing” term. The same revision, however, cannot be made when if the Tsallis entropy, instead of the Shannon form, is used. This serves to emphasize that the Hirshfeld atom is a very specific result, associated only with logarithmic measures of information. A less specific derivation due to Nalewajski provides one resolution to this quandary; this derivation is analyzed in detail.     

Scalable Syntheses of Both Enantiomers of DNJNAc and DGJNAc from Glucuronolactone: The Effect of N‐Alkylation on Hexosaminidase Inhibition

The efficient scalable syntheses of 2‐acetamido‐1,2‐dideoxy‐D ‐galacto‐nojirimycin (DGJNAc) and 2‐acetamido‐1,2‐dideoxy‐D ‐gluco‐nojirimycin (DNJNAc) from D ‐glucuronolactone, as well as of their enantiomers from L ‐glucuronolactone, are reported. The evaluation of both enantiomers of DNJNAc and DGJNAc, along with their N‐alkyl derivatives, as glycosidase inhibitors showed that DGJNAc and its N‐alkyl derivatives were all inhibitors of α‐GalNAcase but that none of the epimeric DNJNAc derivatives inhibited this enzyme. In contrast, both DGJNAc and DNJNAc, as well as their alkyl derivatives, were potent inhibitors of β‐GlcNAcases and β‐GalNAcases. Neither of the L ‐enantiomers showed any significant inhibition of any of the enzymes tested. Correlation of the in vitro inhibition with the cellular data, by using a free oligosaccharide analysis of the lysosomal enzyme inhibition, revealed the following structure–property relationship: hydrophobic side‐chains preferentially promoted the intracellular access of iminosugars to those inhibitors with more‐hydrophilic side‐chain characteristics.