Modified Spectral Editing Methods for C CP/MAS Experiments in Solids

The spectral editing approach of Zilm and coworkers utilizes polarization, polarization inversion, and spin depolarization methods for enhancing or suppressing NMR spectral lines in solids. The proposed pulse sequences allow nonprotonated C, CH, CH₂, and CH₃ types of carbon resonances to be separated from one another and identified accordingly. The former method tentatively separates the nonprotonated C and CH₃ peaks with a cutoff shift of 35 ppm. This shift is a reasonable demarcation shift for a preponderance of organic molecules, but exceptions do exist that could constitute a serious drawback in a few instances. The new approach separates the nonprotonated C and CH₃ carbon peaks unequivocally using modified pulse sequences similar to those of Zilm. Further, both the CH only and CH₂ only spectra, respectively, can be acquired directly from combining so called (+) and (−) sequences using different spectral delay periods and pulse parameters. The (+) and the (−) pulse sequences produce signals for the nonprotonated and methyl carbons that have essentially the same amplitude but opposite phases. These spectra, combined with the previously reported CH₃ and nonprontonated C only spectra, offer a complete spectral editing technique for solid samples. Examples of these spectral editing methods are provided for 3-methylglutaric acid, fumaric acid monoethyl ester, and two complex natural products: methyl o-methylpodocarpate and 10-deacetylbaccatin III.     

Electron density modeling and reconstruction of infinite periodic minimal surfaces (IPMS) based phases in lipid-water systems. II. Reconstruction of D surface based phases

This is the second of two papers dealing with the structure of lipid-water phases based on Infinite Periodic Minimal Surfaces (IPMS). The first paper describes mathematical modeling of such phases. In this paper, a new reconstruction method, called the methyl trough search, is described and used to solve the structures based on powder pattern X-ray diffraction data. Structures are derived for both a single chain lipid-water system (mono-olein) and a diacyl phospholipid-water system (2-2 methyl butyl 16:0 phosphatidylcholine). The methyl trough search uses the low electron density of the lipid methyl tails to determine the correct phasing for the electron density reconstruction. The data are consistent with a structure based on the IPMS D surface. The results are compared to other methods used to solve the mono-olein structure; the structure of the diacyl lipid has never before been solved. We discuss the subtleties involved in reconstruction of D surface based phases and the substantial artifacts that arise in low-resolution reconstructions of hydrocarbon lipids lacking heavy-atom sites. Received 15 January 1999 and Received in final form 20 October 1999     

3,7-Dimethylguanine, a new purine from a Philippine sponge Zyzzya fuliginosa.

A new purine 3,7-dimethylguanine (1) has been isolated from the marine sponge Zyzzya fuliginosa, along with the known metabolites, makaluvamines A, C, K (2--4), 4-hydroxyphenylacetic acid (5), methyl ester of 4-hydroxyphenylacetic acid (6), 4-hydroxyphenethyl alcohol (7), L-phenylalanine (8) and L-tryptophan (9). The structure of 3,7-dimethylguanine (1) was elucidated by analysis of 1D and 2D (one- and two-dimensional) NMR [HMQC (heteronuclear multiple quantum coherence), gHMBC (heteronuclear multiple bond connectivity), 1H-15N gHMBC] data, mass spectroscopy data, and by comparison with 3,7-dimethylisoguanine (10).     

Some remarks on normalized matching

A new class of LYM orders is obtained, and several results about general LYM orders are proved. (1) Let A₁ ? A₂ ? … ? A_r be a chain of subsets of [n] = {l,…,n}. Let 〈a_i〉 and 〈b_i〉 be two nondecreasing sequences with a_i ? b_i for l ? i ? r. Then {X ? [n]: a_i ? | ∩ A_i|? b_i}, ordered by inclusion, is a poset having the LYM property. (2) The smallest regular covering of an LYM order has M(P) chains, where M(P) is the least common multiple of the rank sizes. (3) Every LYM order has a smallest regular covering with at most || ? h(P) classes of distinct chains, where h(P) is the height of P. To obtain (3), we discuss “minimal sets” of covering relations between two adjacent levels of an LYM-order.     

Pyrolysis of 2-butyne/vinylacetylene mixtures between 350 and 450°C

Vinylacetylene and 2-butyne mixtures were pyrolyzed at 350–450°C in the absence and presence of O₂ and NO. The major product of the reaction is a polymer, but o-xylene is also produced and was studied as the species of interest. The C₈H₁₀ formation rate is first-order in C₄H₄ and C₄H₆. The rate coefficient is best fitted by though it is not inconsistent with where R is the ideal gas constant in kJ/mol-K. O-xylene formation occurs by two processes: a concerted molecular mechanism (?67%) and a singlet diradical mechanism (?33%).     

Medium loss optical fibres and cables

Many of the uses to which fibres will be put in the next few years will require at most 100 MHz bandwidth and relatively short fibre lengths. Research has therefore been concentrated on producing a high numerical aperture fibre using normal lead silicate glasses. Details are given for commercially available fibres having attenuation less than 100 dB km^-1 in the range 800–900 nm. These are available in both bundle and single fibres, the former sheathed in pvc and the latter in polypropylene.     

Autocrosslinkable methyl-2-phenylethylpolysiloxane stationary phase for capillary column gas chromatography

Methyl-2-phenylethylpolysiloxane polymers have been synthesized for comparison with methylphenylpolysiloxane stationary phases for gas chromatography. The 50% 2-phenylethyl polysiloxane was found to be autocrosslinkable at 260°C without addition of free redical initiator. Although the selectivity of this phase appears to be similar to the 50% phenyl polysiloxane, its thermal stability is not as high.     

Identification of dominant odor chemicals emanating from explosives for use in developing optimal training aid combinations and mimics for canine detection

Despite the recent surge in the publication of novel instrumental sensors for explosives detection, canines are still widely regarded as one of the most effective real-time field method of explosives detection. In the work presented, headspace analysis is performed by solid phase microextraction (SPME)/gas chromatography-mass spectrometry (GC-MS), and gas chromatography-electron capture detection (GC-ECD), and used to identify dominant explosive odor chemicals seen at room temperature. The activity of the odor chemicals detected was determined through field trials using certified law enforcement explosives detection canines. A chemical is considered an active explosive odor when a trained and certified explosives detection canine alerts to a sample containing that target chemical (with the required controls in place). A sample to which the canine does not alert may be considered an inactive odor, but it should be noted that an inactive odor might still have the potential to enhance an active odor's effect. The results presented indicate that TNT and cast explosives share a common odor signature, and the same may be said for plasticized explosives such as Composition 4 (C-4) and Detasheet. Conversely, smokeless powders may be demonstrated not to share common odors. The implications of these results on the optimal selection of canine training aids are discussed.