Heterogeneous proton-bound dimers with a high dipole moment monomer: How could we experimentally observe these anomalous ionic hydrogen bonds?

Electronic structure calculations (CBS-QB3 and G3MP2) have been used to predict a suitable method to experimentally observe the anomalous structure which is predicted to exist in a proton-bound dimer with a high dipole moment monomer. The enthalpy associated with forming the proton-bound dimer from its protonated and neutral monomers is shown to be linearly related to the difference in proton affinities which has been observed experimentally. However, unlike previous experimental studies, the linear correlation is not predicted to depend strongly, if at all, on whether the basic sites are C=O, C=N, or O(H) n-donor bases. Thermochemical measurements, then, are probably not the best method to distinguish between the structures of heterogeneous proton-bound dimers. It has been shown that a suitable method to experimentally observe the anomalous structure of proton-bound dimers containing, a high dipole moment monomer (or very polar monomer) is by spectroscopic measurement. The O-H+-O asymmetric stretch is probably not the best infrared band to try to correlate with structure. The best band to observe is one which is in a region of the spectrum not masked by other absorptions and is also sensitive to the proximity of the binding proton. For example, it is shown that the methanol-free O-H stretch is very sensitive to the O-H+ bond distance for a series of heterogeneous proton-bound dimers containing methanol. It is predicted that the free O-H stretch of the methanol/acetonitrile proton-bound dimer is more closely related to the O-H stretch in protonated methanol than the O-H stretch in neutral methanol. Observations of these bands should confirm that the proton is closer to methanol in the methanol/acetonitrile proton-bound dimer despite acetonitrile having a higher proton affinity.