Submitted to: Pittsburg Conference on Analytical Chemistry and Applied Spectroscopy
Publication Type: Abstract Only
Publication Acceptance Date: 3/1/1998
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Binding tendencies of alkali metal cations to low molecular weight solvents have been investigated using electrospray ionization (ESI) mass spectrometry in conjunction with computer modeling. Intensities of peaks in the ESI mass spectrum corresponding to solvated alkali cations were found to decrease with increasing ionic radii of the alkali cation (Li > Na > K approx. Cs) in either dimethylacetamide or dimethylformamide. When LiCl was added to an equimolar mixture of dimethylformamide (DMF), dimethylacetamide (DMAc), and dimethylpropionamide (DMP), the intensities of gas-phase [solvent+Li]**+ peaks observed in the ESI mass spectrum decreased in the order DMP > DMAc much greater than DMF. This ranking correlates with the relative strength of the inductive effect exhibited by the alkyl group adjacent to the carbonyl function of each solvent. Electron donation thus appears to play a role in the stabiliza-- tion of the lithium cation attached to the carbonyl oxygen, a proposition that is supported by computer modeling studies of electrostatic potentials. Changes in the electrostatic potentials, however, are rather modest, and contributions from the increasing polarizability and the augmented ability to dissipate thermal energy with increasing size of the solvent molecule are postulated to act in conjunction with the inductive effect. These experiments suggest a combined role of the inductive effect, polarizability, and solvent molecule size in determining the relative intensities of solvated cation peaks in ESI mass spectra of a series of homologous solvents.