Ultrafast time resolved and computational studies of diazo and diazirine excited states, and of carbenes.

摘要

Ab initio quantum calculations and ultrafast time-resolved laser flash photolysis techniques have been used to study singlet carbenes and the photochemistry of diazirines and diazo compounds. After a brief introduction of carbene chemistry in Chapter 1, the photophysics and photochemistry of aryldiazirines are investigated in Chapters 2 through 6. Detailed theoretical calculations begin with parent phenyldiazirine and its isomer phenyldiazomethane. The structures of the ground and electronic excited states (S1, S2, and S3) of both compounds are optimized with RI-CC2 and DFT methods. The denitrogenation of both phenyldiazirine and phenyldiazomethane to produce singlet phenylcarbene, and the isomerization between both compounds, are investigated mechanistically on their potential energy surfaces. These predictions support the spectroscopic assignment in ultrafast studies of arylalkyldiazirines in chapters 3 -- 6 and the accuracy of these theoretical methods are calibrated by the excellent agreement with experimental data. In Chapter 3 we present the first direct observation of singlet phenylcarbene and measurement of its lifetime in solution using ultrafast time-resolved infrared spectroscopy. In Chapter 4 we provide the first direct observation of the S1 excited state of para-methoxy-3-phenyl-3-methyl diazirine (p-CH3OC 6H4CN2CH3) with both IR and UV--vis detection techniques. The S1 state of the diazirine decays into the diazo compound directly. The S2 excited state is populated with 270 nm light and decays directly into singlet arylcarbene and diazo compound, as well as the S1 state, via internal conversion. A Hammett study of the S1 excited states is discussed in Chapter 5. An excellent linear correlation is obtained between the S1 lifetimes of arylchlorodiazirines and their para- substituent sigmap + parameters. The effect of substitution of beta-hydrogens on the S1 state lifetimes is examined in Chapter 6 and is consistent with the RIES mechanism. The wavelength dependence effect on the photochemistry of aryldiazirines was discussed.;In Chapter 7 we present the first direct observation of a singlet vinylcarbene and study its cyclization to a cyclopropene product in solution. Calculations predict that singlet vinylcarbene is highly delocalized over the C=C double bond.;The photochemistry of N,N-diethyldiazoacetamide is detailed in Chapter 8. We concluded that the excited state of the diazoamide precursor undergoes direct intramolecular C-H insertions in forming both beta- and gamma-lactams, as well as denitrogenation to produce singlet carbene. The relaxed singlet carbene decays by isomerizing into gamma-lactam in chloroform, and in methanol, this path is suppressed by intermolecular OH insertion reactions.