Intermolecular versus intramolecular hydrogen bonding: Metastable unimolecular and collision induced dissociation studies on ion clusters of bifunctional compounds

Protonated bifunctional compounds are known to form intramolecular hydrogen bonded cyclic structures despite ring strain and geometric constraints. However, for the case of a protonated cluster, the overall stability of the internal hydrogen bond changes as the degree of solvation increases. We have generated protonated monomer and dimer ions of methoxyethanol (CH₃OCH₂CH₂OH) and dimethoxyethane (CH₃OCH₂CH₂OCH₃) in the ion source of a triple quadrupole mass spectrometer and obtained their metastable dissociation (MSD) spectra and collision induced dissociation (CID) spectra, in order to probe the hydrogen bonding interaction within these ionic species. The MSD spectrum of H⁺(CH₃OCH₂CH₂OH) exhibits a peak corresponding to loss Of H₂O while the CID spectrum shows peaks corresponding to the loss of either H₂O or CH₃OH. Ab initio Hartree-Fock (HF) calculations predict the lowest energy conformation of H⁺(CH₃OCH₂CH₂OH) to be a cyclic structure which is stabilized by an intramolecular hydrogen bond. HF calculations also predict the loss of H₂O from this cyclic structure to be exothermic and the loss of CH₃OH to be endothermic. Activation energies, obtained at the HF level, favour the loss of H₂O over CH₃OH, in agreement with the observed MSD spectrum. In the case of the protonated dimer (H⁺(CH₃OCH₂CH₂OH)₂) these loss channels are absent, due to preferential loss of monomer. Possible mechanisms for the formation of these and other various ions are presented.