Real‐time monitoring of the ring‐opening polymerization of rac‐lactide with in situ attenuated total reflectance/Fourier transform infrared spectroscopy with conduit and diamond‐composite sensor technology

Polymerization rates were proportional to initial Sn(Oct)₂ concentration at low [Sn(Oct)₂]₀/[PrOH]₀ values, but began to level off at higher values. When [Sn(Oct)₂]₀/[PrOH]₀ was significantly greater than unity, the opposite behavior occurred. Tin(II) alkoxide concentration became limited by the initial PrOH concentration and independent of initial Sn(Oct)₂ concentration. Addition of 2‐ethylhexanoic acid caused polymerization rate retardation, without affecting molecular weight. A control polymerization was conducted in the absence of PrOH. The molecular weight of the resulting polymer was consistent with the measured water content (3.7 wt % by Karl Fisher titration) of the as‐received Sn(Oct)₂. The polymerization rate in the absence of PrOH was slow, and this suggested that water is less efficient than an alcohol in creating polymerization‐active stannyl ether bonds. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6238–6247, 2004     

Role of pore size on the electrochemical oxidation of 5-hydroxytryptophan in a silica sol–gel matrix

Silica sol–gels that are prepared by room-temperature processing are suited as matrices for voltammetry in the absence of a contacting liquid phase because the electrolyte encapsulated within the pore volume supports charge transport. We hypothesized that when the electrode process involves a coupled chemical reaction, the pore size can influence the product distribution. This hypothesis was tested with 5-hydroxytryptophan (5-HTPP) as the electroactive species because the cation radical formed by its oxidation can react to form a simple dione or larger dimer, trimer, and oligomer products. Microporous and mesoporous silica sol–gels, which have respective pore diameters in the Å and nm ranges, were the solid-state electrolytes. Glassy carbon (GC) electrodes were used initially; however, experiments in aqueous solution showed that passivation precluded applications in a solid electrolyte. Boron-doped diamond (BDD) electrodes yielded the same voltammetric signature as that of GC but passivated more slowly. In accord with the hypothesis, the oxidation of 5-HTPP at BDD in microporous silica yielded voltammetry suggesting that dione formation was the primary oxidation pathway. With mesoporous silica, the analogous experiment suggested dimer formation. The voltammetric interpretation was consistent with the results obtained by chemical oxidation of 5-HTPP in these matrices followed by product identification by mass spectrometry.     

An analysis of the rotational, fine and hyperfine effects in the (0, 0) band of the AΠ-XΣ transition of manganese monohydride, MnH

High-resolution (±0.003 cm⁻¹), laser induced fluorescence (LIF) spectra of a supersonic molecular beam sample of manganese monohydride, MnH, have been recorded in the 17500-17800 cm⁻¹ region of the (0, 0) band of the A⁷Π-X⁷Σ ⁺ system. The low-N branch features were modeled successfully by inclusion of the magnetic hyperfine mixings of spin components within a given low-N rotational level using a traditional ‘effective’ Hamiltonian approach. An improved set of spectroscopic constants has been extracted and compared with those from previous analyses. The optimum optical features for future optical Stark and Zeeman measurements are identified.     

Carbon Monoxide Activation by a Molecular Aluminium Imide: C−O Bond Cleavage and C−C Bond Formation

Anionic molecular imide complexes of aluminium are accessible via a rational synthetic approach involving the reactions of organo azides with a potassium aluminyl reagent. In the case of K₂[( NON )Al(NDipp)]₂ ( NON =4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethyl-xanthene; Dipp=2,6-diisopropylphenyl) structural characterization by X-ray crystallography reveals a short Al−N distance, which is thought primarily to be due to the low coordinate nature of the nitrogen centre. The Al−N unit is highly polar, and capable of the activation of relatively inert chemical bonds, such as those found in dihydrogen and carbon monoxide. In the case of CO, uptake of two molecules of the substrate leads to C−C coupling and C≡O bond cleavage. Thermodynamically, this is driven, at least in part, by Al−O bond formation. Mechanistically, a combination of quantum chemical and experimental observations suggests that the reaction proceeds via exchange of the NR and O substituents through intermediates featuring an aluminium-bound isocyanate fragment.     

Precursors for MOCVD and ALD of rare earth oxides-complexes of the early lanthanides with a donor-functionalized alkoxide ligand

Complexes of the early lanthanides with the donor-functionalized alkoxide ligand mmp (Hmmp = HOCMe(2)CH(2)OMe, 1-methoxy-2-methylpropan-2-ol) are excellent precursors for Metal Organic Chemical Vapor Deposition (MOCVD) and Atomic Layer Deposition (ALD) of lanthanide oxides; however, their coordination chemistry, which is the subject of this paper, is rather complex. Precursors for MOCVD and ALD of lanthanide oxides are prepared by the reaction of [Ln{N(SiMe(3))(2)}(3)] with 3 equiv of the alcohol Hmmp in toluene in the presence of 1 equiv of tetraglyme and are indefinitely stable in solution. Reaction of [Ln{N(SiMe(3))(2)}(3)] with 3 equiv of Hmmp in the absence of stabilizing Lewis bases gives complex condensed products with empirical formula [{Ln(mmp)(3-n)}(2)O(n)]. These condensed products show poor volatility and are unsatisfactory precursors for MOCVD or ALD of oxides. The cluster complex [La(3)(mu(3),kappa(2)-mmp)(2)(mu(2),kappa(2)-mmp)(3)(mmp)(4)] has been prepared by careful reaction of [La{N(SiMe(3))(2)}(3)] with 4 equiv of Hmmp and has been characterized by single-crystal X-ray diffraction. Salt metathesis reactions using M(mmp) (M = Li or Na) are unreliable routes to [Ln(mmp)(3)]. Crystals of the heterometallic cluster complex [NaLa(3)(mu(3)-OH)(mu(3),kappa(2)-mmp)(2)(mu(2),kappa(2)-mmp)(4)(mmp)(3)] were isolated from the reaction of [La(NO(3))(3)(tetraglyme)] with 3 equiv of Na(mmp), and crystals of [Li(kappa(2)-Hmmp)Pr(mu(2),eta(2)-mmp)(4))LiCl] were isolated from the reaction of PrCl(3) with 3 equiv of Li(mmp); both of these complexes have been characterized by single-crystal X-ray diffraction.     

Spin amplification in solution magnetic resonance using radiation damping

The sensitive detection of dilute solute spins is critical to biomolecular NMR. In this work, a spin amplifier for detecting dilute solute magnetization is developed using the radiation damping interaction in solution magnetic resonance. The evolution of the solvent magnetization, initially placed along the unstable -z direction, is triggered by the radiation damping field generated by the dilute solute magnetization. As long as the radiation damping field generated by the solute is larger than the corresponding thermal noise field generated by the sample coil, the solute magnetization can effectively trigger the evolution of the water magnetization under radiation damping. The coupling between the solute and solvent magnetizations via the radiation damping field can be further improved through a novel bipolar gradient scheme, which allows solute spins with chemical shift differences much greater than the effective radiation damping field strength to affect the solvent magnetizations more efficiently. Experiments performed on an aqueous acetone solution indicate that solute concentrations on the order of 10(-5) that of the solvent concentration can be readily detected using this spin amplifier.     

Interdicting nuclear material on cargo containers using knapsack problem models

This paper introduces a framework for screening cargo containers for nuclear material at security stations throughout the United States using knapsack problem, reliability, and Bayesian probability models. The approach investigates how to define a system alarm given a set of screening devices, and hence, designs and analyzes next-generation security system architectures. Containers that yield a system alarm undergo secondary screening, where more effective and intrusive screening devices are used to further examine containers for nuclear and radiological material. It is assumed that there is a budget for performing secondary screening on containers that yield a system alarm. This paper explores the relationships and tradeoffs between prescreening, secondary screening costs, and the efficacy of radiation detectors. The key contribution of this analysis is that it provides a risk-based framework for determining how to define a system alarm for screening cargo containers given limited screening resources. The analysis suggests that highly accurate prescreening is the most important factor for effective screening, particularly when screening tests are highly dependent, and that moderately accurate prescreening may not be an improvement over treating all cargo containers the same. Moreover, it suggests that screening tests with high true alarm rates may mitigate some of the risk associated with low prescreening intelligence.