Preparation,crystal structure and reactions of a new spin-labelling reagent,cis-3,5-dibromo-4-oxo-2,2,6,6- tetramethylpiperidin-1-yloxy : Crystal structure of a derived bis-nitroxide

Methods for the preparation of cis - 3,5 - dibromo - 4 - oxo - 2,2,6,6 - tetramethylpiperidin -1 - yloxy (2b) and its use as a convenient, selective spin-labelling reagent for amino functions, are described. The crystal and molecular structures of 2b and the bis-nitroxide 4 - [(2',2',5',5' - tetramethylpyrrolin -1 - yloxy) - 3' - carbonyloxy] - 2, 2,6,6 - tetramethyl - 3,5 - dibromo - 3,4 - dehydropiperidin - 1 - yloxy (3) [obtained on treatment of 2b with triethylamine] have been determined by conventional heavy-atom techniques. In 2b the individual molecules have mirror symmetry with the bromine atoms equatorial. The N-O bond length is 1.278(5) Å, CN?C is 125.1(3)° and the N-O bond makes an angle of 24.2° with the CN?C plane. In 3 there are two different nitroxide groups. The pyrrolinyloxy ring is virtually planar with a N-O bond length of 1.252(13) Å and CN?C of 113.1(10)°. The 6 -membered ring exists in a sofa conformation with a pseudoaxial Br atom. Its nitroxide group is tetrahedrally distorted. The N-O bond length is 1.268(15) Å, CN?C is 121.7(10)° and the N-O bond makes an angle of 16.0° with the CN?C plane. The packing of 3 is dictated by a short Br... .O secondary bond of 3.09(1) Å, 0.28 Å less than the sum of the van der Waal's radii resulting in dimeric units related by a centre of symmetry.     

Reversibility in the protonolysis of a beta-diketiminate stabilised calcium bis(trimethylsilyl)amide with benzylamine

Protonolysis of the beta-diketiminate calcium bis(trimethylsilyl)amide [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe3)2}(THF)] with benzylamine is reversible and forms a quantifiable equilibrium mixture.     

Parallel Integer Optimization for Crew Scheduling

Performance aspects of a Lagrangian relaxation based heuristic for solving large 0-1 integer linear programs are discussed. In particular, we look at its application to airline and railway crew scheduling problems. We present a scalable parallelization of the original algorithm used in production at Carmen Systems AB, Göteborg, Sweden, based on distributing the variables. A lazy variant of this approach which decouples communication and computation is even useful on networks of workstations. Furthermore, we develop a new sequential active set strategy which requires less work and is better adapted to the memory hierarchy properties of modern RISC processors. This algorithm is also suited for parallelization on a moderate number of networked workstations.     

The preparation and characterization of 5-substituted-4-chloro-1,2,3-dithiazolium salts and their conversion into 4-substituted-3-chloro-1,2,5-thiadiazoles

A series of monosubstituted acetonitriles were treated with disulfur dichloride at room temperature in CH2Cl2 to afford 5-substituted-4-chloro-1,2,3-dithiazolium chlorides 1.Where the 5-substituent was not a good leaving group the chloride salts were converted into the corresponding perchlorate salts 2 which were sufficiently stable and soluble to provide both (1)H- and (13)C-NMR and cyclic voltammetry data. Several of the dithiazolium chlorides were converted into their corresponding 4-substituted-3-chloro-1,2,5-thiadiazoles 13 on treatment with aqueous ammonia. Mechanisms for all reactions are proposed.     

Synthesis of the beta 2 agonist (R)-salmeterol using a sequence of supported reagents and scavenging agents

[formula: see text] The enantioselective synthesis of (R)-salmeterol has been achieved by using a sequence of supported reagents and sequestering agents. The saligenin core was installed by a regiospecific alkylation and a chiral auxilliary approach was employed to introduce the desired stereochemistry via a diastereoselective reduction.     

Comparison of BLADE and conventional T2-TSE sequences for the sagittal visualization of the cervical spinal cord in multiple sclerosis patients — A case report

The purpose of this study is to report the significant differences found in the identification of lesions in cervical spinal cord of two patients with multiple sclerosis when using the BLADE T2-TSE and BLADE T2-TIRM sequences as opposed to the conventional T2-TSE and T2-TIRM sequences for sagittal acquisition at 1.5 T. In both patients, one more lesion was identified with the BLADE sequences than with the conventional ones. Consequently, we suggest the use of BLADE T2-TSE and BLADE T2-TIRM sequences in place of conventional ones for sagittal examination of the cervical spinal cord of multiple sclerosis patients. The advantages of ΤΙRΜ to reveal the pathology of the cervical spinal cord and the advantage of BLADE sequences to improve image quality should be combined in a sequence that could be ideal for cervical spinal cord examinations.     

Electron affinities of a homologous series of tertiary alkyl radicals and their C-H bond dissociation energies (BDEs)

Heats of formation have been derived from G3(MP2)//B3LYP and G3MP2B3(+) atomization energies for tert-butyl radical (6R), cubyl radical, bicyclooctyl radical (1R), and tricyclo[3.3.n.0^3,7]alk-3(7)-yl (n=0-3, 2R-5R) radicals, and their respective anions (1A-6A) and hydrocarbons (1H-6H). The electron affinity (EA) of 6R is estimated at 1.5±2 kcal/mol and tert-butyl anion (6A) is likely to be bound. In the homologous series 2R-5R the EAs range from 3.4±2 to 13.5±2 kcal/mol. The computed enthalpies of the acidities of the tricyclic hydrocarbons 1H-5H are in the range 407-411 kcal/mol. Their C-H bond dissociation energies (BDEs) are in the range 97-110 kcal/mol. The increase of the BDEs in the homologous series 2H-5H and the increase of EAs of 2A-5A is attributed to the enhanced pyramidalization induced in radicals 2R-5R by the shortening of the methylene chain connecting carbons C₃ and C₇.     

Crystal engineering with 2,1,3-benzoselenadiazole and mercury(II) chloride

The reactions of HgCl₂ with 2,1,3-benzoselenadiazole (bsd) in methanol afforded the polymeric complexes [HgCl₂(bsd)₂]_n (1) and [HgCl₂bsd]_n (2) in good yields. The crystal structures of 1 and 2 have been determined by single-crystal X-ray crystallography. Complex 1 is an one-dimensional (1D) polymer and consists of (HgCl₂)_n chains running parallel to the c-axes. Complex 2 is a two-dimensional (2D) polymer and consists of (HgCl₂)_n chains running parallel to the a-axes and being further bridged by bsd molecules to create a layer lying parallel to the ab plane. Both crystal structures are dominated by π···π interactions between the bsd molecules, while the presence of N···Se interactions increases the dimensionality in 1. Characteristic IR data are discussed in terms of the nature of bonding in the structures of the two complexes.     

Interdependence of in-cell xenon density and temperature during Rb/129Xe spin-exchange optical pumping using VHG-narrowed laser diode arrays

The (129)Xe nuclear spin polarization (P(Xe)) that can be achieved via spin-exchange optical pumping (SEOP) is typically limited at high in-cell xenon densities ([Xe](cell)), due primarily to corresponding reductions in the alkali metal electron spin polarization (e.g. P(Rb)) caused by increased non-spin-conserving Rb-Xe collisions. While demonstrating the utility of volume holographic grating (VHG)-narrowed lasers for Rb/(129)Xe SEOP, we recently reported [P. Nikolaou et al., JMR 197 (2009) 249] an anomalous dependence of the observed P(Xe) on the in-cell xenon partial pressure (p(Xe)), wherein P(Xe) values were abnormally low at decreased p(Xe), peaked at moderate p(Xe) (~300 torr), and remained surprisingly elevated at relatively high p(Xe) values (>1000 torr). Using in situ low-field (129)Xe NMR, it is shown that the above effects result from an unexpected, inverse relationship between the xenon partial pressure and the optimal cell temperature (T(OPT)) for Rb/(129)Xe SEOP. This interdependence appears to result directly from changes in the efficiency of one or more components of the Rb/(129)Xe SEOP process, and can be exploited to achieve improved P(Xe) with relatively high xenon densities measured at high field (including averaged P(Xe) values of ~52%, ~31%, ~22%, and ~11% at 50, 300, 500, and 2000 torr, respectively).