An Experimental Charge Density Study of the N‐H⁺…O⁻Bonds in a Proton‐Transfer System in Comparison with Regular N‐H…O and O‐H…N Hydrogen Bonds

An experimental charge density study has been carried out on N‐H + …O − bonds in the proton‐transfer compound piperazinium oxalate as well as on the normal O‐H…N bonds in the thiodiglycolic acid‐4,4′‐bipyridine complex, based on X‐ray diffraction measurements at 130 K. The results are compared with those of the normal N‐H…O bonds in 5‐nitrouracil. In spite of the different crystal geometries, the N‐H + …O − and the N‐H…O bonds exhibit similar values of electron density and the associated curvatures at the hydrogen‐acceptor bond critical points (BCPs). Surprisingly, the polarization parameters of the two BCPs are comparable (˜20%). The pseudoatomic charge on the protonated N atom (in N‐H + …O − ) is, however, much smaller (‐0.05 e ) in comparison to the unprotonated case (˜ ‐0.2 e ). The O‐H…N hydrogen bonds exhibit shorter H…N distances compared to the charged and the uncharged H…O bonds. Accordingly, the electron densities and the curvatures evaluated at the H…N critical points are also much higher.