The photoionisation of two phenylcarbenes and their diazirine precursors investigated using synchrotron radiation

Abstract

Two phenylcarbenes, chlorophenylcarbene (CPC) and trifluoromethylphenylcarbene (TFPC), were generated by jet flash pyrolysis of diazirine precursors. Their photoionisation was studied by photoelectron–photoion coincidence spectroscopy using synchrotron radiation. For CPC we determined an adiabatic ionisation energy (IE) of 8.15 eV and a vertical IE of 9.3 eV. For TFPC we obtained an adiabatic IE of 8.47 eV and a vertical IE of 8.95 eV. The photoelectron spectra are broad and unstructured due to a large increase in the angle between the phenyl group, carbene centre and the substituent (Cl or CF₃). The geometry change upon ionisation is more pronounced for CPC. Being a singlet arylcarbene, CPC is more strongly bent in the neutral ground state than the triplet TFPC. In addition, the bond between the carbenic centre and the chlorine atom shortens upon ionisation, because the radical cation is stabilised by the non-bonding electrons of the Cl through a mesomeric effect. The photoionisation and dissociative photoionisation of the diazirine precursors are also explored. The CPC precursor, 3-chloro-3-phenyldiazirine, undergoes complete dissociative photoionisation and only the CPC⁺ fragment is observed above 8.8 eV. For 3-trifluoromethyl-3-phenyldiazirine some molecular ions can be observed above 9.05 eV. However, dissociative photoionisation also sets in right at the ionisation threshold. A fit to the data yields an appearance energy of AE (0 K) ≈ 9.27 eV.