Semiempirical quantum chemical calculations (AM1/CI and PM3/CI) confirm the qualitative perturbational prediction that electron-withdrawing groups on the ring nitrogen of a 3,4-dimethylenepyrrole should diminish the energy separation of the singlet and triplet states to near zero. Syntheses of a series of precursors of such biradicals have been developed. Study of the chemistry and spectroscopy of the biradicals has revealed persistent singlet states for the cases where the substituent is methyl, isobutyryl, and pivaloyl. In the cases of N-arenesulfonyl-3,4dimethylenepyrroles, both a singlet and a triplet form can be observed as persistent species. In this paper, the properties of the singlets in this series are described. Although energy transfer from the excited triplet state of the sensitizer xanthone to the diazene precursor of N-p-toluenesulfonyl-3,4-dimethylenepyrrole is observed by nanosecond timeresolved spectroscopy, the chemical behavior of the biradical intermediate is the same as that observed in the direct photolysis or thermolysis of the diazene. The reactive form of the biradical under these conditions appears to be the singlet.
Experimental Design. Tuning the Multiplet Gap in N-Substituted-3,4-dimethylenepyrrole Biradicals.Prediction of the spacings of the lowest energy multiplets of π-conjugated non-Kekulé molecules is a stringent test of theory. Although in most cases these separations are difficult to measure, the effect of structure on the qualitative trends of their direction and magnitude should be estimable by theoretical calculation. We are particularly attracted to the use of computationally designed systematic structural variations as means of tuning the spacings and hence providing experimental tests of such estimates. The necessary elements of this approach are a parent system with near-degenerate multiplet states, a variable structural feature that can be used to perturb their separation, and a theoretical means for selecting potentially fruitful structural alterations. Since the singlet-triplet gap in such species can be made quite small, the tuning process ultimately might offer the power of choice over the multiplicity of the ground state.The disjoint hydrocarbon biradical tetramethyleneethane (TME, 1) provides a useful starting structure for such a process. Although the size of the energy separation between the lowest triplet and singlet states of TME is not yet known from experiment, theory 2 predicts a very small gap (E T -E S ∼ -1 to +2 kcal/mol). A heteroatom bridge connecting two termini of the TME molecule, as in previous work 3 from this laboratory on 3,4-dimethylenefuran and 3,4-dimethylenethiophene, introduces a variable source of electronic perturbation.The present paper describes the preparation and characterization of the singlet states of several such biradicals, and an accompanying paper 11b reports results of the search for the corresponding triplet states.Our present computational approach was implicit in theoretical considerations by Du, Hrovat, and Borden, 4 whic...
The ability to monitor and control plant nutrient ions in fertigation solutions, on an ion-specific basis, is critical to the future of controlled environment agriculture crop production, be it in traditional terrestrial settings (e.g., greenhouse crop production) or as a component of bioregenerative life support systems for long duration space exploration. Several technologies are currently available that can provide the required measurement of ion-specific activities in solution. The greenhouse sector has invested in research examining the potential of a number of these technologies to meet the industry's demanding requirements, and although no ideal solution yet exists for on-line measurement, growers do utilize technologies such as high-performance liquid chromatography to provide off-line measurements. An analogous situation exists on the International Space Station where, technological solutions are sought, but currently on-orbit water quality monitoring is considerably restricted. This paper examines the specific advantages that on-line ion-selective sensors could provide to plant production systems both terrestrially and when utilized in space-based biological life support systems and how similar technologies could be applied to nominal on-orbit water quality monitoring. A historical development and technical review of the various ion-selective monitoring technologies is provided.
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