Proton Dispersion Forces and Continuous Energy Level Distribution of Protons in the Hydrogen Bonds of Semiprotonated Poly-ʟ-histidine and with Model Substances

Abstract

IR spectroscopic investigations were conducted with poly-L-histidine (PLH) films, hydrous imidazole and pyrazole solutions and anhydrous N-methylimidazole with respect to protonation. Continuous absorption arises with increasing protonation and is maximally intensive with PLH, imidazole and N-methylimidazole when 50% protonated. The absorption indicates the formation of ΝΗ^⊕･·･·･·Ν bonds. The protons in these bonds are found in continuous energy level distribution. This continuity is due to the extremely large polarizability of symmetrical hydrogen bonds with double minimum potential well in which the proton tunnels. Thus on the one hand proton dispersion forces can arise, and on the other hand the anion fields can exert considerable influence. Both cause an energy level shift. The influence of these tunneling protons on the NH groups in PLH, imidazole and pyrazole is under discussion. The Cl^⊖ ions are added to these NH groups via NH · · · Cl^⊖ bonds. The CH groups in the N-methylimidazole are so acidic that CH · · · Cl^⊖ bonds are formed. In the case of PLH, imidazole and N-methylimidazole, the loss of the continuous absorption at more than 50% protonation is explained by the fact that each imidazole ring then possesses a Cl^⊖ ion and thus the NH^⊕· · · N bonds are ruptured. With pyrazole, the continuous absorption continues to increase at 50% protonation. This is due to the fact that its pKa value is considerably less than that of the other substances. Hence H₅O₂ ^⊕ groupings are already formed preferentially before the NH^⊕· · · Cl^⊖ groups. The rise of the continuous absorption in the case of the imidazole solution at a 100% or higher degree of protonation can be explained likewise. The corresponding rise with N-methylimidazole is accounted for by the formation of Cl^⊖H^⊕· · ·Cl^⊖ bonds.