Polymerization of 2 16 silicic acid molecules, H4Si04, has been studied using the molecular dynamics computer simulation technique. Multibody potentials which reproduce bulk and surface structures of silica and silicate glasses, as well as molecular configurations containing Si, 0, and H ions, were used in the simulations. Results of the simulations are consistent with experimental data of sol-gel systems. Chains form at the early stages of polymerization, followed by ring formation, consistent with interpretations of N M R data and semiempirical molecular orbital quantum calculations. The activation energy for formation of branching Qn species is 12 kcal/mol, consistent with the experimental data of 12 kcal/mol for gelation. The relative time evolution of the various Qn species is consistent with experimental NMR data.
The mechanism of the OH-initiated oxidation of isoprene in the presence of NO and O 2 has been investigated using a discharge-flow system at and 2 torr total pressure. 298 K OH radical concentration profiles were measured using laser-induced fluorescence as a function of reaction time. The rate constant for the reaction of was measured to OH ϩ isoprene be cm 3 In the presence of NO and O 2 , regeneration of OH(1.10 Ϯ 0.05) ϫ 10 mol s . radicals by the reaction of isoprene-based peroxy radicals with NO was measured and compared to simulations of the kinetics of this system. The results of these experiments are consistent with an overall rate constant of cm 3 (with an uncertainty factor Ϫ12 Ϫ1 Ϫ19 ϫ 10 mol s of 2) for the reaction of isoprene-based hydroxyalkyl peroxy radicals with NO.
The impact of improved irrigation and nutrient practices on ground water quality was assessed at the Nebraska Management System Evaluation Area using ground water quality data collected from 16 depths at 31 strategically located multilevel samplers three times annually from 1991 to 1996. The site was sectioned into four 13.4‐ha management fields: (i) a conventional furrow‐irrigated corn (Zea mays L.) field; (ii) a surge‐irrigated corn field, which received 60% less water and 31% less N fertilizer than the conventional field; (iii) a center pivot‐irrigated corn field, which received 66% less water and 37% less N fertilizer than the conventional field; and (iv) a center pivot‐irrigated alfalfa (Medicago sativa L.) field. Dating (3H/3He) indicated that the uppermost ground water was <1 to 2 yr old and that the aquifer water was stratified with the deepest water ∼20 yr old. Recharge during the wet growing season in 1993 reduced the average NO3–N concentration in the top 3 m 20 mg L−1, effectively diluting and replacing the NO3‐contaminated water. Nitrate concentrations in the shallow zone of the aquifer increased with depth to water. Beneath the conventional and surge‐irrigated fields, shallow ground water concentrations returned to the initial 30 mg NO3–N L−1 level by fall 1995; however, beneath the center pivot‐irrigated corn field, concentrations remained at ∼13 mg NO3–N L−1 until fall 1996. A combination of sprinkler irrigation and N fertigation significantly reduced N leaching with only minor reductions (6%) in crop yield.
Biological N2 fixation is achieved under ambient conditions by enzymatic catalysis. The enzyme nitrogenase has been studied extensively, but the N2 chemical reduction step is, by far, not rate limiting and hard to examine. A new method was developed that allows studying the reduction transition state within the enzyme's complex kinetic cascade by means of the 15N kinetic isotope effect on the reaction's second-order rate constant, V/K. A value of 1.7% +/- 0.2% was measured.
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