NMR spectral properties of the tetramantanes - nanometer-sized diamondoids.

Tetramantanes, and all diamondoid hydrocarbons, possess carbon frameworks that are superimposable upon the cubic diamond lattice. This characteristic is invaluable in assigning their (1)H and (13)C NMR spectra because it translates into repeating structural features, such as diamond-cage isobutyl moieties with distinctively complex methine to methylene signatures in COSY and HMBC data, connected to variable, but systematic linkages of methine and quaternary carbons. In all tetramantane C22H28 isomers, diamond-lattice structures result in long-range (4)JHH, W-coupling in COSY data, except where negated by symmetry; there are two highly symmetrical and one chiral tetramantane (showing seven (4)JHH). Isobutyl-cage methines of lower diamondoids and tetramantanes are the most shielded resonances in their (13)C spectra (<29.5 ppm). The isobutyl methylenes are bonded to additional methines and at least one quaternary carbon in the tetramantanes. W-couplings between these methines and methylenes clarify spin-network interconnections and detailed surface hydrogen stereochemistry. Vicinal couplings of the isobutyl methylenes reveal positions of the quaternary carbons: HMBC data then tie the more remote spin systems together. Diamondoid (13) C NMR chemical shifts are largely determined by α and ß effects, however γ-shielding effects are important in [123]tetramantane. (1)H NMR chemical shifts generally correlate with numbers of 1,3-diaxial H-H interactions. Tight van der Waals contacts within [123]tetramantane's molecular groove, however, form improper hydrogen bonds, deshielding hydrogen nuclei inside the groove, while shielding those outside, indicated by Δδ of 1.47 ppm for geminal hydrogens bonded to C-3,21. These findings should be valuable in future NMR studies of diamondoids/nanodiamonds of increasing size.

Quantification of naphthalenes in jet fuel with GC x GC/Tri-PLS and windowed rank minimization retention time alignment.

Comprehensive, two-dimensional gas chromatography (GC x GC) is used in conjunction with trilinear partial least squares (Tri-PLS) to quantify the percent weight of naphthalenes (two-ring aromatic compounds) in jet fuel samples. The increased peak capacity and selectivity of GC x GC makes the technique attractive for the rapid, and possibly less tedious analysis of jet fuel. The analysis of complex mixtures by GC x GC is further enhanced through the use of chemometric techniques, including those designed for use on 2-D data such as Tri-PLS. Unfortunately, retention time variation, unless corrected, can be an impediment to chemometric analysis. Previous work has demonstrated that the effects of retention time variation can be mitigated in sub-regions of GC x GC chromatograms through the application of an objective retention time alignment algorithm based on rank minimization. Building upon this previous work, it is demonstrated here that the effects of retention time variation can be mitigated throughout an entire GC x GC chromatogram with an objective retention time alignment algorithm based on windowed rank minimization alignment. A significant decrease in calibration error is observed when the algorithm is applied to chromatograms prior to construction of Tri-PLS models. Fourteen jet fuel samples with known weight percentages of naphthalenes (ASTM D1840) were obtained. Each sample was subjected to five replicate five-minute GC x GC separations over a period of two days. A subset of nine samples spanning the range of weight percentages of naphthalenes was chosen as a calibration set and Tri-PLS calibration models were subsequently developed in order to predict the naphthalene content of the samples from the GC x GC chromatograms of the remaining five samples. Calibration models constructed from GC x GC chromatograms that were retention time corrected are shown to exhibit a root mean square error of prediction of roughly half that of calibration models constructed from uncorrected chromatograms. The error of prediction is lowered further to a value that nearly matches the uncertainty in the standard percent weight values (ca. 1% of the median percent volume value) when the aligned chromatograms are truncated to include only regions of the chromatogram populated by naphthalenes and compounds of similar polarity and boiling point.

Use of antimicrobial agents in consumer products.

OBJECTIVES: To summarize available data on the effectiveness of antimicrobial ingredients in consumer products such as hand lotions and soaps and to discuss the implications of such use on antimicrobial resistance. DATA SOURCES: We searched the MEDLINE database, 1966 to 2001, using the search term resistance qualified with the terms consumer product(s), OR soap, OR lotion, OR triclosan, and LexisNexis and the World Wide Web using the search strategy antimicrobial resistance AND consumer product. DATA EXTRACTION: English-language articles were selected that provided information on the use of antimicrobial ingredients in consumer products and the effect of this use on antimicrobial resistance. DATA SYNTHESIS: Despite the recent substantial increase in the use of antimicrobial ingredients in consumer products, the effects of this practice have not been studied extensively. No data support the efficacy or necessity of antimicrobial agents in such products, and a growing number of studies suggest increasing acquired bacterial resistance to them. Studies also suggest that acquired resistance to the antimicrobial agents used in consumer products may predispose bacteria to resistance against therapeutic antibiotics, but further research is needed. Considering available data and the critical nature of the antibiotic-resistance problem, it is prudent to avoid the use of antimicrobial agents in consumer products. CONCLUSIONS: The use of common antimicrobials for which acquired bacterial resistance has been demonstrated should be discontinued in consumer products unless data emerge to conclusively show that such resistance has no effect on public health and that such products are effective at preventing infection. Ultimately, antibiotic resistance must be controlled through judicious use of antibiotics by health care professionals and the public.

2D-INADEQUATE Structural Assignment of Polybutene Oligomers.

The structure of polybutene oligomers has been unambiguously determined using NMR techniques. We used 2D-INADEQUATE to show that the major polybutene isomer synthesized via BF(3) catalysis has the expected structure with a tert-butyl group at the beginning of the chain and the expected methylvinylidene double bond at the other end. Surprisingly, we found that the main isomer of polybutene synthesized from AlCl(3) catalysis has an isopropyl group at the beginning of the chain. A trisubstituted double bond structure has been assigned at the other end of the chain. Structural information about several minor polybutene isomers has also been determined. Formation of an isopropyl group can be rationalized by assuming a protonated cyclopropane intermediate in the propagation step. Previously, the mechanism of formation of polybutene via AlCl(3) catalysis was thought to involve exclusively the tert-butyl carbenium ion. It now appears that the mechanism is more complicated than previously thought.