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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Can. J. Chem. 69, 104 (1991). a-Guaiacoxyacetoverahone, a lignin model compound, has been studied using a combination of laser flash photolysis and steady state quantum yields. Triplet lifetimes range from 175 to 600 ns while the quantum yields of photodegradation vary from 0.06 to 0.6, depending on the solvent. Singlet processes (@ -0.1), previously unobserved for aromatic ketones, are implicated in the mechanism, and account for most or all of the degradation in aprotic solvents.

'The photochemistry of various mono-and dimethoxy-substituted a-bromoacetophenones has been investigated by laser flash photolysis in organic solvents. The short-lived excited singlet states cleave to yield bromine atoms and the corresponding methoxyphenacyl radicals with quantum yields ranging from 0.13 to 0.35. With the exception of 4-methoxy-a-bromoacetophenone (6), all other substrates yield readily detectable triplet states; these have .rr,.rre character and are very poor hydrogen abstractors. Triplet decay does not contribute to methoxyphenacyl radical formation. While methoxyphenacyl radicals may have a spin density as high as 0.3 at the carbonyl oxygen, which accounts for the alkoxy-like visible band in their absorption spectrum, their chemical reactivity is dominated by the radical character at the carbon site. Methoxyphenacyl radicals are modest hydrogen abstractors; for example, reaction of 6 with 1,4-cyclohexadiene occurs with a rate constant of 2.6 x 10-5 M-I s-I, while for addition to the double bond in I,]-diphenylethylene the rate constant is 9.4 x lo7 M-' s-'. Additions to other double bonds are likely to be slower (e.g., 12 + 1,3-cyclohexadiene, k 5 10' M-' s-I). In contrast, reaction with oxygen occurs with kc] = 2.5 x 10%~-' s-I. Under the conditions relevant for their participation in paper yellowing, the methoxyphenacyl radicals will be converted to peroxyl radicals. These probably play a key role in the oxidative photodegradation of lignin.Key rvords: methoxyphenacyl radicals, a-bromoacetophenones, laser flash photolysis. Rksum6: Faisant appel ii la photolyse laser Cclair, on a ttudit la photochimie de diverses mono-ou dimCthoxy-a-bromoacCtophCnones dans des solvants oganiques. Les &tats singulets excitCs de courte durCe de vie se brisent pour donner des atomes de brome et les radicaux mCthoxyphCnacyles correspondants, avec des rendements quantiques allant de 0,13 ii 0,35. A I'exception de la 4-mCthoxy-a-bromoacCtophCnone (6), tous les autres substrats fournissent des ttats triplets facilement dCtectables; ils posskdent un caractkre .rr,.rr:"t une trks mauvaise habilitC ii enlever des hydrogknes. La dCgCnCrescence de 1'Ctat triplet ne contribue pas la formation du radical mCthoxyphCnacyle. Alors que les radicaux mCthoxyphCnacyles peuvent possCder une densite de spin allant jusqu'ii 0.3 au niveau de I'oxygkne du carbonyle, ce qui explique la bande semblable ii celle des alcoxy dans leur spectre d'absorption, leur rCactivitC chimique est dominee par le caractere radicalaire au site carbonC. L'habilitC des radicaux mCthoxyphCnacyles ii enlever des hydrogknes est modeste; par exemple, la rCaction du composC 6 avec le cyclohexa-1,4-dikne se produit avec une constante de vitesse de 2,6 x 10' M-' s-', alors que, pour l'addition ii la double liaison du 1,l -diphCnylCthylkne, la constante de vitesse est de 9,4 x lo7 M-' s-I. 11 est probable que les additions sur d'autres doubles liaisons seront plus lentes (par exemple, pour 12 + cyclohexa-l,3-dikne, k 5 10-5 M-' s-I). Par ailleurs, la rC...

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