On hydrogen and deuterium bonds as well as on Li⁺, Na⁺ and Be²⁺ bonds: IR continua and cation polarizabilities

Authors:

• Georg Zundel,
• Bogumił Brzezinski,
• Jerzy Olejnik

Abstract

Experimentally observed and calculated infrared (IR) continua of the homoconjugated hydrogen bonds in H₅O⁺₂ or deuterium bonds in D₅O⁺₂, both of which show large polarizability, are compared. With N⁺H⋯N⇌N⋯H⁺N bonds, for instance, the far IR continua give information on how strongly these bonds are polarized to one or the other side. In the case of heteroconjugated AH⋯B ⇌ A^-⋯H⁺B bonds it is shown how the reaction field creates two minima within these bonds. The wavenumber-dependent intensity distribution of the IR continua is discussed as a function of the length and strength of the hydrogen bonds. Li⁺, Na⁺ and Be²⁺ bonds also show cation polarizabilities, due to shifts of these cations within the bonds formed by them. The wavenumber-dependent far IR intensity distribution of the continua caused by homoconjugated Li⁺ bonds is discussed as a function of the length and strength of these bonds. Furthermore, it is shown that with O^-Li⁺⋯ON ⇌ O^-⋯Li⁺ON bonds the equilibria can be shifted as a function of the electron density at the acceptor or donor atoms, respectively. These shifts are analogous with such shifts in hydrogen bonds as a function of ΔpK_a. The wavenumber-dependent far IR intensity distribution with homoconjugated polarizable O^-Na⁺⋯^-O ⇌O^-⋯Na^+-O bonds is also discussed. In the case of heteroconjugated Na⁺ bonds it is shown that the fluctuation frequency may be so small that the coalescence point is in the far IR region. It is demonstrated that large cation polarizabilities due to collective cation motion are not specific for chains of hydrogen bonds, but occur also in chains of two Li⁺ or Na⁺ bonds. H⁺,D⁺,Li⁺ and Na⁺ bonds with large cation polarizabilities are compared in terms of the spectral regions in which the continua of these bonds occur and in terms of the size of their cation polarizabilities. Finally, the Be²⁺ polarizability due to the motion of Be²⁺ between four acceptors is discussed with three types of system. It is shown that the broad absorption of the Be²⁺ bonds is shifted toward higher wavenumbers. Because of the high affinity of Be²⁺ ions for the acceptor groups, the potential has a steep slope at the acceptors and is relatively narrow. © 1993.