Multiphoton ionization and dissociation of diazirine: A theoretical and experimental study

I. Fedorov, L. Koziol, A.K. Mollner, A.I. Krylov, and H. Reisler
J. Phys. Chem. A 113, 7412 – 7421 (2009)

Multiphoton ionization and dissociation processes in diazirine have been studied experimentally via 304-325 nm two-photon absorption, and theoretically by using the EOMCCSD 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(n3px) Rydberg state, whereas in two-photon absorption at comparable energies the first photon excites the low-lying 1¹B₂ (npi*) valence state, from which the strongest absorption is to the dissociative valence 1¹A₂ (sigma_NNpi*) 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 D²Pi(v'=2)-X²Pi (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 proposed that two photon processes via the 1¹B₂ intermediate state are very efficient in this wavelength range, leading predominantly to dissociation of diazirine from the 2¹A₁ 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.

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