Response of cellulose nitrate track detectors to electron doses

In order to study alternative dose determination methods, the bulk etching velocity and the latent track annealing of LR 115 track detectors was studied during electron irradiation runs from a Pelletron accelerator. For this purpose alpha irradiated and blank detectors were exposed to increasing electron doses from 10.5 to 317.5 kGy. After the irradiation with electrons the detectors were etched under routine conditions, except for the etching time, that was varied for each electron dose in order to reach a fixed residual thickness. The variation of the bulk etching velocity as a function of each one of the electron doses supplied, was interpolated in order to obtain dosimetric response curves. The observed annealing effect on the latent tracks is discussed as a function of the total electron doses supplied and the temperature.     

Elastic neutron scattering and dielectric behaviour of 4% brominated betaine calcium chloride dihydrate (BCCD)

We report a combined experimental study by means of elastic neutron scattering and dielectric measurements of a partially deuterated and brominated BCCD (Betaine Calcium Chloride Dihydrate) crystal. The lowest-temperature phase is one-dimensional modulated and characterized by the coexistence of different commensurate domains (with = 1/4, 4/17, 2/9 and 1/5 on cooling), but with a clear predominance of the five-fold phase. A huge global thermal hysteresis of the wave-vector of the modulation, attaining values of about 9 K in the incommensurate phase and up to 15 K in the “harmless” low temperature part of the phase diagram, is observed up to . The role of lattice defects on this phenomenon is discussed. Similarly to the behaviour of the pure compound, the structural modulation evolves on cooling towards a soliton regime (growth of third and fifth-order satellite peaks), probably with respect to a non-stabilized non-modulated ferroelectric phase. The critical temperatures deduced from dielectric constant and pyroelectric current measurements are in very good agreement with those obtained from neutron scattering. The dielectric anomaly observed in at K, and known as the “-anomaly”, could not be related with any special feature detected in the neutron data, and in particular no correlation between this anomaly and the appearance of the soliton regime can be established. Received 26 October 1998     

Accumulation of butyltin compounds in dolphins stranded along the Mediterranean coasts

Concentrations of tributyltin (TBT) and its degradation products, monobutyltin (MBT) and dibutyltin (DBT), were determined in the liver and kidney of striped dolphins ( Stenella coeruleoalba), bottlenose dolphins ( Tursiops truncatus) and in a fetus of the common dolphin (Delphinus delphi) found stranded along the western Italian and Greek coasts in the period 1992–1994. Butyltin (BT) compounds were detected in almost all the samples analyzed and were higher in the kidney than in the liver. Total BT concentrations were 0.78–8.05 μg g⁻¹ wet wt in kidney and 0.015–1.02 μg g⁻¹ wet wt in liver of S. coeruloealba. Bottlenose dolphins had lower BT concentrations than striped dolphins. Although only one fetal sample was analyzed, it showed the highest BT concentrations of all. Unlike BT concentrations in the other adult dolphins, in the pregnant dolphin they were higher in the liver (4.35 μg g⁻¹ wet wt), suggesting that BTs are transferred from mother to fetus. No significant differences in BT concentrations were found between sexes. Of the breakdown products, DBT was predominant in most liver samples and MBT was more abundant in kidney. Although BT concentrations are known to be found in cetaceans inhabiting waters of developed countries, our observations strongly suggest that concentrations found in S. coeruleoalba were either similar to or higher than those reported for other Stenella species collected from coastal areas close to harbors or marinas. Copyright © 2000 John Wiley & Sons, Ltd.     

Trans Influence of Triphenylstibine: Crystal Structures of cis‐[PtBr2(SbPh3)2], trans‐[PtBr(Ph)(SbPh3)2], [NMe4][PtBr3(SbPh3)], and cis‐[PtBr2(SbPh3)(PPh3)]

The work reports the unexpected reaction of diphenyldibromo antimonates (III) with PtCl₂ and cis‐[PtCl₂(PPh₃)₂]. The reaction gives triphenylstibine containing Pt^II complexes viz. cis‐[PtBr₂(SbPh₃)₂] ( 1 ), trans‐[[PtBr(Ph)(SbPh₃)₂] ( 2 ), [NMe₄][PtBr₃(SbPh₃)] ( 3 ), and cis‐[PtBr₂(PPh₃)(SbPh₃)] ( 4 ). All the complexes were characterised by elemental analyses, IR, Raman, ¹⁹⁵Pt NMR, FAB mass spectroscopy and X‐ray crystallography. A plausible mechanism via the phenyl migration is proposed for the formation of these complexes. The average Pt–Br distance in 1 is 2.456(2) Å, in 2 2.496 Å(trans to Ph) while in 3 it is 2.476 Å (trans to Sb) implying a comparable trans influence of Ph₃Sb and Ph₃P.     

Synthesis of Conformationally Restricted Carbocyclic Nucleosides: The Role of the O(4′)-Atom in the Key Hydration Step of Adenosine Deaminase

Conformationally restricted carbocyclic nucleosides with either a northern(N)-type conformation, i.e., N-type 2′-deoxy-methanocarba-adenosine 8 ((N)MCdAdo), or a southern(S)-type conformation, i.e. S-type 2′-deoxy-methanocarba-adenosine 9 , ((S)MCdAdo), were used as substrates for adenosine deaminase (ADA) to assess the enzyme's preference for a fixed conformation relative to the flexible conformation represented by the carbocyclic nucleoside aristeromycin ( 10 ). Further comparison between the rates of deamination of these compounds with those of the two natural substrates adenosine (Ado; 1 ) and 2′-deoxyadenosine (dAdo; 2 ), as well as with that of the conformationally locked nucleoside LNA-Ado ( 11 ), which, like the natural substrates, has a furanose O(4′) atom, helped differentiate between the roles of the O(4′) anomeric effect and sugar conformation in controlling the rates of deamination by ADA. Differences in rates of deamination as large as 10000 can be attributed to the combined effect of the O(4′) atom and the enzyme's preference for an N-type conformation. The hypothesis proposed is that ADA's preference for N-type substrates is not arbitrary; it is rather the direct consequence of the conformationally dependent O(4′) anomeric effect, which is more efficient in N-type conformers in promoting the formation of a covalent hydrate at the active site of the enzyme. The formation of a covalent hydrate at the active site of ADA precedes deamination. A new and efficient synthesis of the important carbobicyclic template 14a , a useful intermediate for the synthesis of (N)MCdAdo ( 8 ) and other conformationally restricted nucleosides, is also reported.     

Polyphosphinoborane Block Copolymer Synthesis Using Catalytic Reversible Chain-Transfer Dehydropolymerization

An amphiphilic block copolymer of polyphosphinoborane has been prepared by a mechanism-led strategy of the sequential catalytic dehydropolymerization of precursor monomers, H₃B ⋅ PRH₂ (R=Ph, n-hexyl), using the simple pre-catalyst [Rh(Ph₂PCH₂CH₂PPh₂)₂]Cl. Speciation, mechanism and polymer chain growth studies support a step-growth process where reversible chain transfer occurs, i.e. H₃B ⋅ PRH₂/oligomer/polymer can all coordinate with, and be activated by, the catalyst. Block copolymer [H₂BPPhH]₁₁₀-b-[H₂BP(n-hexyl)H]₁₁ can be synthesized and self-assembles in solution to form either rod-like micelles or vesicles depending on solvent polarity.     

Mechanochemical Desymmetrization of Unbiased Bis- and Tris-alkynes to Access 3,5-Isoxazoles-Alkyne Adducts and Unsymmetrical Bis-3,5-isoxazoles**

A mechanochemical desymmetrization of symmetrical bis- and tris-alkynes by a controlled 1,3-dipolar cycloaddition reaction using nitrile oxide dipoles (NOs). This operationally simple protocol allows access to 3,5-isoxazole-alkyne adducts from easily prepared or commercially available symmetrical bis- and tris-alkynes in moderate to excellent yields. In addition, we have highlighted the synthetic utility of 3,5-isoxazole-alkyne by developing a route to access, for the first time, β-ketoenamine-alkyne derivatives, and unsymmetrical bis-3,5-isoxazoles.