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https://hdl.handle.net/2142/70267
Description
Title
Time-Resolved Studies of Aryl Carbenes
Author(s)
Brauer, Beth-Ellen
Issue Date
1984
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Abstract
Picosecond transient absorption spectroscopy (using a Nd:YAG laser with 355 nm excitation and 440 nm to 640 nm probe) was used to study three aryl carbenes generated by photolysis of their parent diazo compounds in fluid solution. Irradiation of 9-diazofluorene generates a transient with an absorption maximum at 470 nm which is believed to be triplet fluorenylidene. The rate constant for formation of this intermediate was found to be 2.4 (+OR-) 0.7 x 10('9) s('-1) in acetonitrile, 2.0 (+OR-) 0.5 x 10('9) s('-1) in hexafluorobenzene, and > 7 (+OR-) 1 x 10('9) s('-1) in cyclohexane. An additional transient absorbing at 470 nm and 500 nm which is attributed to the 9-fluorenyl radical appears with a rate constant of 7.2 (+OR-) 1.9 x 10('8) s('-1) in cyclohexane and 3.0 (+OR-) 1.8 x 10('8) s('-1) in cyclohexane-d(,12). A broad weak absorption (450 nm to 550 nm) can be detected in under 100 ps after irradiation of 10-diazo-9-mesityl-9,10-dihydro-9-bora-anthracene in cyclohexane. This absorption is most likely due to triplet boraanthrylidene. Transients are rapidly formed (under 100 ps) following irradiation of 11-diazobenzo{b}fluorene in acetonitrile, benzene, and cyclohexane. The transient absorption maximum is at roughly 460 nm in acetonitrile or benzene but is apparently 5 nm to 10 nm less in cyclohexane. These absorptions can be attributed to a carbene(s). A similarly absorbing transient was studied by nanosecond flash photolysis in several solvents and in the presence of a variety of quenchers. EPR studies indicate that irradiation of 11-diazobenzo{b}fluorene in methylcyclohexane (T (LESSTHEQ) 55) generates a triplet ground state carbene (Curie law followed within error).
Products due to photolysis of 11-diazobenzo{b}fluorene at room temperature were also studied. Direct or triplet sensitized photodecomposition of this compound in the presence of methanol appears to form a photolabile ether product in roughly 80% yield or more. This result and the high rate constant obtained for reaction of the 460 nm absorbing transient with methanol (3.9 (+OR-) 0.4 x 10('9) M('-1)s('-1)) may suggest that the free energy difference between the carbene singlet and triplet levels is very small. Flash photolysis studies of two diazoketones, 2-diazoacenaphthenone and 9,10-diazophenanthrane were unsuccessfully attempted.
Results from studying these carbenes indicate that reactions of carbenes in solution could depend upon solvent, intersystem crossing rates, and energy differences between the singlet and triplet states.
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