Spectroscopic studies of chemically liberated "free" -OH groups in aqueous N₂H₄, NH₃, and CH₃NH₂ solutions

As a contribution to understanding and identifying the nature of the disputed "broken" hydrogen bond in aqueous solutions we have sought to compare the spectroscopic character of "free" -OH groups produced by chemical means at constant temperature with that of the putative "free" -OH produced in pure water alcohols, etc. by temperature increases. "Free", or at least very weakly bonded, -OH groups have been produced quasi-stoichiometrically at room temperature by displacing bound -OH groups from OH⋯O bonds previously free with previously free -NH groups, the competitive status of which has been abruptly increased by protonation of the amine group. The sharp overtone bands of the unbonded -NH groups necessarily present in amine + H₂O solutions (due to the excess of protons over lone pairs) disappear stoichiometrically on protonation using HClO₄, and an equivalent number of weakly bonding -OH groups are produced. The spectroscopic signature of the chemically liberated -OH group is almost indistinguishable from that of the group liberated thermally, and a well-defined isosbestic point at 1442 nm (close to the approximate isosbestic point at 1440-1450 nm in the heated water spectra) implies that the same mechanism of "exciting across the centroid" (a (strong bond) ↔ (weak bond) exchange within a continuum model) is involved in each case.