Direct formation of CH2 (b̃ 1B1) in the near-UV photodissociation of diazirine

Lim, Soon-Mi; Kim, Taek‐Soo; Lim, Goo‐Il; Kim, Sang Kyu; Choi, Young Sik

doi:10.1016/s0009-2614(98)00370-4

Cited by 6 publications

(4 citation statements)

References 16 publications

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“…Gas-phase photolysis of parent diazirine excited at the origin (322.96 nm) of the S 0 -S 1 transition has been shown to produce excited singlet methylene which fluoresces at λ > 575 nm with a lifetime of ∼16 µs. 31 This experiment demonstrates once again that UV photolysis of a carbene precursor can generate a carbene in an electronically excited state. To the best of our knowledge, the corresponding experiment has not been reported with diazomethane.…”

Section: Discussionmentioning

confidence: 73%

Ultrafast Study of p-Biphenylyldiazomethane and p-Biphenylylcarbene

2006

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p-Biphenylyldiazomethane was excited by femtosecond pulses of UV light in acetonitrile, in cyclohexane, and in methanol. Ultrafast photolysis produces a singlet excited state of p-biphenylyldiazomethane with lambdamax = 490 nm, and lifetimes of less than 300 fs in acetonitrile, in cyclohexane, and in methanol. The decay of the excited state is accompanied by the growth of transient absorption with lambdamax = 360 nm. The carrier of this transient absorption is attributed to singlet p-biphenylylcarbene, a result that is consistent with the predictions of TD-DFT calculations. The singlet carbene lifetimes are 200 and 77 ps in acetonitrile and cyclohexane, respectively, and are controlled by intersystem crossing to the lower energy triplet state. The transient absorption does not decay to baseline in acetonitrile, because of the formation of nitrile ylide. The equilibrium mixture of singlet and triplet p-biphenylylcarbene reacts with acetonitrile to form a nitrile ylide (lambdamax = 370 nm), and with cyclohexane by C-H insertion 1-20 ns after the laser pulse. The singlet carbene lifetime is only 7.9 ps in methanol, owing to a rapid reaction with the solvent. Reaction with the solvent gives rise, in part, to a p-biphenylylbenzyl cation (lambdamax = 450 nm, tau = 6.3 ps) in methanol.

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Section: Discussionmentioning

confidence: 73%

Ultrafast Study of p-Biphenylyldiazomethane and p-Biphenylylcarbene

2006

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“…8. The detailed experimental setup had been described in our previous publication 10. The premixed diazirine/He gas (3 %, 1 atm) was expanded into vacuum through 0.5 mm diameter nozzle to generate the molecular beam.…”

Section: Methodsmentioning

confidence: 99%

“…However because of the lack of experimental studies, even the structure and dephasing mechanism of diazirine in the first electronically excited state (S 1 ) are still the subject of controversy 9. The only molecular‐beam experimental work on diazirine (S 1 ) is the report that the excited singlet state of methylene, 1 CH 2 (B̃), is produced from the S 1 –S 0 origin band of diazirine 10…”

Section: Mode‐dependent Fano‐fitting Parameters With Band Assignmentsmentioning

confidence: 99%

Mode‐Dependent Fano Resonances Observed in the Predissociation of Diazirine in the S₁ State

et al. 2010

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Resonance in chemical reactions is a very intriguing quantum mechanical phenomenon and provides essential information about the detailed shape of the potential-energy surfaces especially near the nuclear configuration that is critical for dynamical constraints. [1] Fano-type resonance is particularly interesting since it results from the interference of two distinct reactive pathways leading to the same product channel. [2] When an atom is excited to a super-excited state above the ionization threshold energy, and for instance, the isoenergetic continuum is coherently excited at the same time, this results in the interference of direct and indirect ionization processes to give the asymmetric line shape in the ionization cross section. [3] Fano resonance has often been found in the ionization cross section of atoms, whereas its observation in the molecular systems is very rare, especially in gas-phase dynamics. Only a few cases have been reported to date in vibrational autoionization of H 3 [4] or predissociation of H 2 , [5] Cs 2 , [6] and FNO. [7] The multidimensionality of the internal coordinate of polyatomic molecules generally makes the observation of the Fano resonance more difficult since the coherent excitation of bound and continuum states is less probable when the number of internal degrees of freedom increases. The intrinsic spectral congestion of polyatomic molecules may also hamper the experimental observation of asymmetric resonance. Consequently, Fano resonances in polyatomic systems have rarely been interrogated in chemical reactions. Herein, we report the observation of Fano profiles in the partial photodissociation cross section of the diazirine. The yield of the nascent 1 CH 2 (B ) fragment from the S 1 state of diazirine has been monitored as a function of the excitation energy by detecting the total fluorescence emitting in the 1 CH 2 (B ) to 1 CH 2 (ffi) transition. All of the Franck-Condon active bands show asymmetric line shapes, giving the modedependent dynamic parameters, such as homogeneous line width, center of resonance frequency, and the asymmetry parameter. The symmetric and asymmetric C À N stretching modes exhibit quite different Fano profiles, giving qualitative and yet essential information about the shape of the potentialenergy surface in the vicinity of the transition state. Especially since the interference in the reaction pathways could be further utilized to control the outcomes of chemical reactions, [7] the experimental finding of the Fano resonance in a polyatomic molecule may open the possibility of controlling the reaction of complicated molecular systems in multidimensional coordinates through the manipulation of the phase involved in the excitation process.Diazirine (CH 2 N 2 ), as a clean source of carbene and a precursor of diazomethane, [8] has been intensively studied theoretically. However because of the lack of experimental studies, even the structure and dephasing mechanism of diazirine in the first electronically excited state (S 1 ) are still the subject o...

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“…By combining high-level electronic structure calculations and photofragment laser spectroscopic experiments in molecular beams, we were able to investigate the photodissociation of c-CH 2 N 2 following two-photon absorption. In the only previous molecular beam study, diazirine was excited to the 1 1 B 2 ← 1 1 A 1 origin band, and fluorescence, tentatively assigned to emission from high vibronic levels of the excited 1 1 B 2 state of the singlet methylene product, was observed …”

Section: Introductionmentioning

confidence: 95%

Multiphoton Ionization and Dissociation of Diazirine: A Theoretical and Experimental Study

et al. 2009

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Multiphoton ionization and dissociation processes in diazirine have been studied experimentally via 304-325 nm two-photon absorption and theoretically by using the EOM-CCSD and B3LYP methods. The electronic structure calculations identified two excited valence states and four Rydberg states in the region 4.0-8.5 eV. In one-photon excitation, the strongest absorption is to the 2(1)A(1)(3p(x) <-- n) Rydberg state, whereas in two-photon absorption at comparable energies the first photon excites the low-lying 1(1)B(2) (pi* <-- n) valence state, from which the strongest absorption is to the dissociative valence 1(1)A(2) (pi* <-- sigma(NN)) state. The diazirine ion is calculated to be rather unstable, with a binding energy of only 0.73 eV and a geometry that resembles a weakly bound CH(2)(+)...N(2) complex. In the experimental studies, resonance-enhanced multiphoton ionization (REMPI) experiments show no ions at the parent diazirine mass but only CH(2)(+) ions from dissociative photoionization. It is proposed that weak one-photon absorption to the 1(1)B(2) state is immediately followed by more efficient absorption of another photon to reach the 1(1)A(2) state from which competition between ionization and fast dissociation takes place. Strong signals of CH(+) ions are also detected and assigned to 2 + 1 REMPI via the D(2)Pi (v' = 2) <-- <-- X(2)Pi (v'' = 0) two-photon transition of CH fragments. Velocity map CH(+) images show that CH(X, v'' = 0, N'') fragments are born with substantial translational energy, indicating that they arise from absorption of two photons in diazirine. It is argued that two-photon processes via the 1(1)B(2) intermediate state are very efficient in this wavelength range, leading predominantly to dissociation of diazirine from the 1(1)A(2) state. The most likely route to CH(X) formation is isomerization to isodiazirine followed by dissociation to CH + HN(2). In agreement with other theoretical papers, we recommend revisions of the heats of formation of diazirine and diazomethane.

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Direct formation of CH2 (b̃ 1B1) in the near-UV photodissociation of diazirine

Cited by 6 publications

References 16 publications

Ultrafast Study of p-Biphenylyldiazomethane and p-Biphenylylcarbene

Ultrafast Study of p-Biphenylyldiazomethane and p-Biphenylylcarbene

Mode‐Dependent Fano Resonances Observed in the Predissociation of Diazirine in the S₁ State

Multiphoton Ionization and Dissociation of Diazirine: A Theoretical and Experimental Study

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Direct formation of CH2 (b̃ 1B1) in the near-UV photodissociation of diazirine

Cited by 6 publications

References 16 publications

Ultrafast Study of p-Biphenylyldiazomethane and p-Biphenylylcarbene

Ultrafast Study of p-Biphenylyldiazomethane and p-Biphenylylcarbene

Mode‐Dependent Fano Resonances Observed in the Predissociation of Diazirine in the S1 State

Multiphoton Ionization and Dissociation of Diazirine: A Theoretical and Experimental Study

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Mode‐Dependent Fano Resonances Observed in the Predissociation of Diazirine in the S₁ State