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

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• 作者：Igor Fedorov ; Lucas Koziol ; Andrew K. Mollner ; Anna I. Krylov ; Hanna Reisler
• 刊名：Journal of Physical Chemistry A
• 出版年：2009
• 出版时间：July 2, 2009
• 年：2009
• 卷：113
• 期：26
• 页码：7412-7421
• 全文大小：286K
• 年卷期：v.113,no.26(July 2, 2009)
• ISSN：1520-5215

文摘

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 2¹A₁(3p_x ← n) Rydberg state, whereas in two-photon absorption at comparable energies the first photon excites the low-lying 1¹B₂ (π* ← n) valence state, from which the strongest absorption is to the dissociative valence 1¹A₂ (π* ← σ_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 CH₂⁺···N₂ complex. In the experimental studies, resonance-enhanced multiphoton ionization (REMPI) experiments show no ions at the parent diazirine mass but only CH₂⁺ ions from dissociative photoionization. It is proposed that weak one-photon absorption to the 1¹B₂ state is immediately followed by more efficient absorption of another photon to reach the 1¹A₂ 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 D²Π (v′ = 2) ← ← X²Π (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 1¹B₂ intermediate state are very efficient in this wavelength range, leading predominantly to dissociation of diazirine from the 1¹A₂ state. The most likely route to CH(X) formation is isomerization to isodiazirine followed by dissociation to CH + HN₂. In agreement with other theoretical papers, we recommend revisions of the heats of formation of diazirine and diazomethane.