Triplet ground state spectroscopic studies

The putative benzyloxynitrene can be intercepted with tetramethylethylene to form the expected aziridine. Time-resolved IR spectroscopy was unable to detect the 0-nitrene, but detected the presence of PhCH2N=0 formed with a time constant of 250 ns after the laser pulse. Thus, the lifetime of benzyloxynitrene is also 250 ns. The TRIR spectroscopic studies indicated that benzyloxynitrene reacts with oxygen a rate constant of 10 M s. This value strongly suggests that the 6>-nitrene, in contrast to the Al-nitrenes has a triplet ground state. 

In an effort to produce diarylcarbenes with singlet ground states rather than the more usual triplet ground states, the diazoparacyclophane derivatives (49) were prepared and irradiated. Triplet signals were detected spectroscopically for three of the five carbenes (50) studied, but no definite conclusions could be drawn regarding the multiplicities of these ground states. 

Pioneering work of Dowd on trimethylene methane [142], 1,8-naphthoquinodi-methane diradicals were among the first examples of non-Kekule 7r,7r-diradicals that were found to be persistent in their triplet ground state at cryogenic temperatures and were thoroughly characterized by various spectroscopic methods [143], Time resolved studies of diradicals have provided lifetimes and reactivities of numerous diradical intermediates in solution [144], y-irradiation of CAN, 68, in haloalkane glasses at 77 K yielded radical cation 68 + its electronic absorption spectrum has been obtained and represents first report of absorption spectrum of an ionized diradical [145]. Irradiation at >640 nm converted it into an isomer identical with that formed by ionization of 1,4-dihydro-l,4-ethanonaphtho[l,8-rfe][l,2]diazepine 70, and subsequent irradiation of the radical cation at A > 540 nm. Results identified the photo isomer of 68 + to be 69 + (Scheme 4). 

Until recently iodocarbenes such as CHI and CI2 were believed to have triplet ground states, largely on the basis of negative ion photoelectron spectroscopic smdies [19, 20, 65]. Recent theoretical studies [18, 20, 23-25], including the current one, have clearly contradicted this, as discussed above. Moreover, the spectroscopic work of Tao et al. [21, 22] on CHI has confirmed that its ground state is indeed singlet. 

Most stable ground-state molecules contain closed-shell electron configurations with a completely filled valence shell in which all molecular orbitals are doubly occupied or empty. Radicals, on the other hand, have an odd number of electrons and are therefore paramagnetic species. Electron paramagnetic resonance (EPR), sometimes called electron spin resonance (ESR), is a spectroscopic technique used to study species with one or more unpaired electrons, such as those found in free radicals, triplets (in the solid phase) and some inorganic complexes of transition-metal ions. 

Photochemical addition of furan to Pummerer s ketone (121) affords the two (4+2)-cycloadducts (122) and (123). The structures of both of these products has been established by X-ray crystallography. Spectroscopic studies on the enone (121) suggest that the triplet state is highly twisted and the authors suggest that the addition to furan results in the formation of two biradicals which lead to the observed products. No evidence for the involvement of a trans-ground state enone was obtained. ... 

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