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An accurate gas-phase structure determination of cis-hex-3-ene-1,5-diyne (1), the parent molecule for the Bergman cyclization reaction, has been achieved through a combination of Fourier transform microwave spectroscopy and computational quantum chemistry. The microwave spectra of seven isotopes of enediyne 1 have been observed using a pulsed-nozzle Fourier transform spectrometer. The ground-state rotational constants were corrected for vibrational effects and the equilibrium structure (R e ) was deduced. The equilibrium structure displays excellent agreement with that obtained from high-level ab initio calculations (CCSD(T)/cc-pVTZ). Both approaches confirm that the molecule has C 2V symmetry with slight deviations of the alkyne units (C-CtC-H) from linearity. These groups distort away from each other in contra-distinction to their distortion during the Bergman cyclization reaction. The C1-C6 distance (4.32 Å), which has been viewed as a critical parameter governing reactivity in the Bergman cyclization, is notably longer than an earlier, widely publicized value (4.12 Å). An important role of the macrocyclic ring in enediyne antibiotics is to overcome an intrinsic distortion of the enediyne moiety in the direction opposite that of the reaction coordinate for Bergman cyclization.

Field-scale observations of two upland soils derived from contrasting granite and basalt bedrocks are presented to hypothesize that redox activity of rhizospheres exerts substantial effects on mineral dissolution and colloidal translocation in many upland soils. Rhizospheres are redox-active microsites and in the absence of O 2 , oxidation of rhizodeposits can be coupled by reduction of redoxactive species such as Fe, a biogenic reduction that leads to Fe translocation and oxidation, accompanied by substantial proton flux. Not only do rhizogenic Fe-C redox cycles demonstrate a process by which the rhizosphere affects an environment well outside the near-root zone, but these redox processes are also hypothesized to be potent weathering systems, such that rhizogenic redox-reactions complement acid-and ligand-promoted reactions as major biogeochemical processes that control crustal weathering. The potential significance of Fe-C redox cycling is underscored by the deep and extensive rooting and mottling of upland subsoils across a wide range of plant communities, lithologies, and soil-moisture and temperature regimes.

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Rhodamine WT (RWT), a fluorescent xanthene dye, is often used as a conservative tracer in aquifer characterization and as a surrogate or sorbing tracer for contaminant fate and transport. Quantitative tracing employing RWT is confounded by the presence of two major fluorescent constituents in the tracer-grade mixture. Here, we have confirmed that the two constituents are isomers of RWT, elucidated their molecular structures and their percent mass distribution in the tracer-grade mixture, and examined their individual sorption behavior onto soil solids. The energy-minimized geometry of the meta isomer indicates that it possesses a greater potential for (i) hydrophobic exclusion from bulk solution, (ii) electrostatic attraction to the solid phase, and (iii) surface complexation with surface-bound Al and Fe ions as compared with the para isomer. Hence, the meta isomer consistently sorbs to a higher extent onto the mineral phases examined. The para isomer has the potential to be a nearly conservative tracer, and the meta isomer has the potential to be a nonconservative tracer. To facilitate RWT use as a conservative tracer and comparison of tracer tests at different locations, we recommend modification of the RWT manufacturing process toward production of 100% of the para isomer. Alternatively, appropriately designed tests with tracer-grade RWT have the potential for simultaneous estimation of hydraulic parameters and contaminant fate and transport.

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Accurate equilibrium structures have been determined for (Z)-pent-2-en-4-ynenitrile (8) and maleonitrile (9) by combining microwave spectroscopy data and ab initio quantum chemistry calculations. The microwave spectra of 10 isotopomers of 8 and 5 isotopomers of 9 were obtained using a pulsed nozzle Fourier transform microwave spectrometer. The ground-state rotational constants were adjusted for vibration-rotation interaction effects calculated from force fields obtained from ab initio calculations. The resultant equilibrium rotational constants were used to determine structures that are in very good agreement with those obtained from high-level ab initio calculations (CCSD(T)/cc-pVTZ). The geometric parameters in 8 and 9 are very similar; they also do not differ significantly from the all-carbon analogue, (Z)-hex-3-ene-1,5-diyne (7), the parent molecule for the Bergman cyclization. A small deviation from linearity about the alkyne and cyano linkages is observed for 7-9 and several related species where accurate equilibrium parameters are available. The data on 7-9 should be of interest to radioastronomy and may provide insights on the formation and interstellar chemistry of unsaturated species such as the cyanopolyynes.

Experimental data on the stereoselectivity of base-catalyzed 1,2-elimination reactions that produce conjugated carbonyl compounds are scarce in spite of the importance of these reactions in organic and biochemistry. As part of a comprehensive study in this area, we have synthesized stereospecifically-deuterated beta-tosyloxybutanoate esters and thioesters and studied the stereoselectivity of their elimination reactions under non-ion pairing conditions. With the availability of both the (2R*,3R*) and (2R*,3S*) diastereomers the innate stereoselectivity could be determined unambiguously. (1)H and (2)H NMR data show that these substrates produce 5-6% syn elimination, the usual amount for acyclic substrates undergoing E2 reactions. Contrary to earlier suggestions, activation by a carbonyl group has virtually no influence upon the stereoselectivity. Elimination of the (2R*,3R*) diastereomer of the beta-tosyloxyester and thioester produces 21-25% of the (Z)-alkene, much more than observed with a poorer beta-nucleofuge. A relatively large amount of (Z)-alkene product seems to be a good marker for an E2 pathway, in which the transition state is E1cB-like, rather than an E1cB(irrev) mechanism. Syn KIE values were higher than those for anti elimination for the esters as well as the thioesters. Experimental challenges to the synthesis of stereospecifically-deuterated beta-tosyloxyesters are discussed.