Triplet ground state matrix isolation spectroscopy

TMM was the first biradical to be directly characterized by low temperature, matrix isolation EPR in pioneering work by Dowd (Eq. 11.91). The spectroscopy confirmed the threefold symmetry and the triplet ground state that theory had predicted, and allowed kinetic studies under matrix isolation conditions. 

Photolysis of matrix-isolated (trimethoxysilyl)diazomethane at A, > 305 nm produced carbene 3e (equation 1), which was characterized by IR and UV-Vis spectroscopy . Under these conditions, no other species besides the carbene could be detected spectroscopically. In an 02-doped (1%) argon matrix, the carbene rapidly reacted with oxygen to give carbonyl oxide 4 which was further photoisomerized to formylsilane 5. Again, the fast reaction of 3e with 02 points to a triplet ground state of the carbene. 

UV-vis spectra of matrix-isolated intermediates are not so informative as matrix IR spectra. As a rule, an assignment of the UV spectrum to any intermediate follows after the identification of the latter by IR or esr spectroscopy. However, UV-vis spectra may sometimes be especially useful. It is well known, for example, that the energy of electronic transitions in singlet ground-state carbenes differs from that of the triplet species. In this way UV spectroscopy allows one to identify the ground state of the intermediate stabilized in the matrix in particular cases. This will be exemplified below. 

The photochemistry of HN3 in the argon and nitrogen matrices was studied for the first time by J. Pimentel et al. and the IR spectra of triplet nitrene NH and radical NH2 were recorded. A series of studies of matrix isolated NH (in ground triplet and excited singlet A states) and its deutero-substitute analogue (ND) were later performed using UV and luminescence spectroscopy. " The spectroscopy and relaxation of the lowest excited singlet state of NH / ND ( A) were studied in detail in Ne, Ar, Kr and Xe matrices... 

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