Hydrogen bonding of O—H and C—H hydrogen donors to Cl−. Results from mass spectrometric measurements of the ion–molecule equilibria RH + Cl− = RHCl−

Abstract

Equilibrium constants K1 for reaction [1] RH + Cl− = RHCl− in the gas phase were measured with a high pressure mass spectrometer under chemical ionization conditions. Data for some 40 compounds RH are presented. It is found that the binding free energies [Formula: see text] for RH = oxygen acids increase with the gas phase acidity of RH. The strongest bonds are formed with strong acids like HCO2H, CH3CO2H, and phenol. Water and alkyl alcohols give much weaker interactions. A simple relationship between gas phase acidity and binding free energy does not occur for RH = carbon acids. Carbon acids like cyclopentadiene, whose high gas phase acidity is largely due to charge derealization by conjugation in the completed anion, do not give Cl− adducts with stability commensurate with the acidity. A relationship between gas phase acidity and binding energy is found for carbon acids with carbonyl groups and for the substituted toluenes. Molecular orbital calculations with the STO-3G basis set provide insights to the bonding occurring in RHCl−. For all cases investigated, hydrogen bonding to Cl− provides the most stable structure. Generally the hydrogen bond occurs through the hydrogen which has the highest net positive charge. The hydrogen bond strength is found approximately proportional to this positive charge. Another proportionality is found between the charge transferred from Cl− to RH, on formation of RHCl−, and the strength of the hydrogen bond.