Substituent effects on imidazole basicity and photoelectron spectroscopy determined ionization energies

VUV photophysics and dissociative photoionization of pyrimidine, purine, imidazole and benzimidazole in the 7–18eV photon energy range

“…There are steps in the parent ion yield curve at ( [23] as well as by Ramsey [22]. The parent ion yield curve also exhibits a fall off at 13.48 eV which is close to the onset of the HCN loss channel (m/z = 91) whose AE = 13.2 ± 0.2 eV, and is most probably related to the opening of this channel.…”

“…The ion signal increases rapidly from 8.37 ± 0.01 eV, so that the region between 8.20 and 8.37 eV can be considered as a tail. We note that the first band in the PES of benzimidazole [22], which peaks at 8.44 eV, has a tail in this energy region, as does that of Kovac et al [23], who reported the peak to be at 8.45 eV. The NIST compilation [30] gives the adiabatic IE = 8.0 eV from the PES data of Ramsey [22] and of Kovac et al [23], evidently from an estimation of the beginning of the tail in the PES band.…”

“…8). Reported vertical ionization energy values are: PI IE(vert) = 8.81 ± 0.01 eV [28]; EI IE(vert) = 9.12 eV [61]; PES IE(vert) = 8.96 eV [22], 8.78 eV [60]. In the PIMS study by Main-Bobo et al [28], who used a many-line hydrogen lamp as photon source, the photon wavelength resolution was 17 meV at 120 nm (10.33 eV).…”

“…Apart from photoelectron spectral studies [21][22][23][24] and some early mass spectrometry work [25][26][27][28] there has been relatively little previous investigation of ionization phenomena of the molecules under study here. Except for the work on imidazole of Main-Bobo et al [28], no dissociative photoionization studies have been reported.…”

Direct determination of the ionization energy of histidine with VUV synchrotron radiation

“…We tentatively assign m/z 82 as 4-methyl-1H-imidazole. The vertical ionization energy of 1-methyl-1H-imidazole and 2-methyl-1H-imidazole have been measured to be 8.66 and 8.50 eV, respectively, with photoelectron spectroscopy [34]. As a first approximation, the appearance value of 8.5 eV for m/z 82 suggests that it is indeed methyl imidazole that is being detected and 8.5 eV is the ionization energy for that particular fragment.…”