Pseudoequivalent Nitrogen Atoms in Aqueous Imidazole Distinguished by Chemical Shifts in Photoelectron Spectroscopy

摘要

Rapid (picoseconds) N-H proton exchange between the two nitrogen atoms of neutral imidazole in water is shown here to give rise to distinct binding-energy peaks (chemical shifts) in both the nitrogen Is and carbon Is photoelectron spectra. Detection is possible owing to both photoelectron spectroscopy's high sensitivity to local atomic environment and the fast time scale of the photoelectron emission process (approximately 10~(17) s). In contrast, only one single ~(15)N line is observed at high pH values when using nuclear magnetic resonance (NMR) spectroscopy, which has a time resolution of approximately 10~(-5) s. Thus, in this case the two nitrogen atoms are indistinguable. Nevertheless, both isotropic and anisotropic ~(15)N NMR chemical shifts are good indicators of charge state and hydrogen-bond interactions. This was early exploited for microscale titration measurements to determine pK_a values, yielding pK_a = 6.98 for imidazole.