Triplet carbenes matrix isolation spectroscopy

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. 

On the contrary. Chapman and McMahon found that H-transfer in o-tolylcar-bene can be directly observed at very low temperatures, through several different spectroscopic methods. Irradiation of Ar matrix isolated diazo compound 50 at 4.2 to 10 K gave triplet carbene 51, which could be characterized by EPR, IR, and UV/VIS spectroscopy. The various spectra of 51 slowly decayed at temperatures as low as 4.2 K (tin ca. 64 h), and o-xylylene (52) could be observed to grow correspondingly... 

The observational techniques used are spectroscopic in all cases. Electronic and vibration-rotation spectroscopy have been used for the simplest structures such as methylene and the halomethylenes the phase in which the carbene is examined does not seem to have much influence on the observed spectra (Bass and Mann, 1962). For more complicated carbenes, structural information has been largely gleaned from EPR spectroscopy using the matrix isolation technique, and this of necessity restricts studies to triplet states. 

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. 

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