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...

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In contrast to the photolyses of N-methyl- or N-pivaloylpyrrolo[3,4]dihydropyridazines in low-temperature matrices, which give the blue, ESR-inactive singlet 3,4-dimethylenepyrrole biradical at all wavelengths, the photolyses of the N-arenesulfonyl derivatives in this series are strongly wavelength-dependent. In three separate instances, both singlet and triplet N-arenesulfonyl-3,4-dimethylenepyrrole biradicals have been observed. In each case, one or two other triplets are seen by ESR spectroscopy after irradiation under special conditions. These are tentatively assigned conformationally isomeric diazenyl biradical structures. The possible origins of the slow intersystem crossing rates for singlet−triplet interconversions in N-arenesulfonyl-3,4-dimethylenepyrrole biradicals are examined. Near-zero ionic character of the singlet wave function is considered to be a less likely cause than conformational control of the spin state. The latter circumstance would require that intersystem crossing be coupled to conformational change, which can be very slow at low temperatures. The zero-field splittings determined from the ESR spectra of N-tosyl-3,4-dimethylenepyrrole biradical are found to be temperature-dependent, which could be caused by librations and/or internal rotational (conformational) motions of the biradical in the matrix.

The EPR spectra of carbonand boron-attached carboranylnitrenes are described. The zero-field splitting parameter \D/hc\ shows that there is little delocalization of the nonbonding electrons into the cage.Carbon-substituted azides have been known since 1864 and are so common as to be fitting subjects of extensive reviews.2-3 Boron-substituted azides have also been known for many years in a variety of structural types.4 Borazides with simple alkyl5 and aryl6 substituents can be found, as well as those attached to borazines7 and relatively exotic boron clusters.8 Alkyl and other azides are common sources of monovalent nitrogen-containing intermediates, the nitrenes.9 Although concerted rearrangements are prominent in the chemistry of the boron-substituted azides,