TY - JOUR
T1 - Polarizability and inductive effect contributions to solvent-cation binding observed in electrospray ionization mass spectrometry
AU - Striegel, André M.
AU - Piotrowiak, Piotr
AU - Boué, Stephen M.
AU - Cole, Richard B.
N1 - Funding Information:
This work was sponsored by the U.S. Department of Agriculture under USDA, ARS Cooperative Agreement 58-6435-2-110. PP acknowledges the support from the U.S. Department of Energy, DE-EG02-97ER 14756. Mass spectrometry support was provided by the National Science Foundation through grant no. CHE-9512155 and by the W. M. Keck Foundation. We thank Dr. Timothy Schatz (University of New Orleans) for his assistance, and would like to acknowledge the important contributions of the late Dr. Judy D. Timpa to all aspects of this work.
PY - 1999
Y1 - 1999
N2 - Electrospray ionization (ESI) mass spectrometry (MS) has been used in conjunction with computer modeling to investigate binding tendencies of alkali metal cations to low molecular weight solvents. Intensities of peaks in ESI mass spectra corresponding to solvent-bound alkali metal cations were found to decrease with increasing ionic radii (Li+ > Na+ > K+ > Cs+) in either dimethylacetamide (DMAc) or dimethylformamide (DMF). When a lithium or sodium salt was added to an equimolar mixture of DMF, DMAc, and dimethylpropionamide (DMP), the intensities of gas-phase [solvent + alkali cation]+ peaks observed in ESI mass spectra decreased in the order DMP > DMAc ≫ DMF. A parallel ranking was obtained for alkali metal cation affinities in ESI-MS/MS experiments employing the kinetic method. These trends have been attributed to a combination of at least three factors. An inductive effect exhibited by the alkyl group adjacent to the carbonyl function on each solvent contributes through-bond electron donation to stabilize the alkali metal cation attached to the carbonyl oxygen. The shift in the partial negative charge at the oxygen binding site with increasing n-alkyl chain length (evaluated via computer modeling), however, cannot fully account for the mass spectrometric data. 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. Further evidence of these contributions to solvent-cation binding in ESI-MS is given by the relative intensities of [solvent + Li]+ peaks in mixtures containing equimolar quantities of alcohols, indicating preferential solvation of Li+ in the order n-propanol > ethanol > methanol. These experiments suggest a combined role of polarizability, the inductive effect, and solvent molecule size in determining relative intensities of solvated cation peaks in ESI mass spectra of equimolar mixtures of homologous solvents.
AB - Electrospray ionization (ESI) mass spectrometry (MS) has been used in conjunction with computer modeling to investigate binding tendencies of alkali metal cations to low molecular weight solvents. Intensities of peaks in ESI mass spectra corresponding to solvent-bound alkali metal cations were found to decrease with increasing ionic radii (Li+ > Na+ > K+ > Cs+) in either dimethylacetamide (DMAc) or dimethylformamide (DMF). When a lithium or sodium salt was added to an equimolar mixture of DMF, DMAc, and dimethylpropionamide (DMP), the intensities of gas-phase [solvent + alkali cation]+ peaks observed in ESI mass spectra decreased in the order DMP > DMAc ≫ DMF. A parallel ranking was obtained for alkali metal cation affinities in ESI-MS/MS experiments employing the kinetic method. These trends have been attributed to a combination of at least three factors. An inductive effect exhibited by the alkyl group adjacent to the carbonyl function on each solvent contributes through-bond electron donation to stabilize the alkali metal cation attached to the carbonyl oxygen. The shift in the partial negative charge at the oxygen binding site with increasing n-alkyl chain length (evaluated via computer modeling), however, cannot fully account for the mass spectrometric data. 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. Further evidence of these contributions to solvent-cation binding in ESI-MS is given by the relative intensities of [solvent + Li]+ peaks in mixtures containing equimolar quantities of alcohols, indicating preferential solvation of Li+ in the order n-propanol > ethanol > methanol. These experiments suggest a combined role of polarizability, the inductive effect, and solvent molecule size in determining relative intensities of solvated cation peaks in ESI mass spectra of equimolar mixtures of homologous solvents.
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U2 - 10.1016/S1044-0305(98)00140-8
DO - 10.1016/S1044-0305(98)00140-8
M3 - Article
AN - SCOPUS:0033427354
SN - 1044-0305
VL - 10
SP - 254
EP - 260
JO - Journal of the American Society for Mass Spectrometry
JF - Journal of the American Society for Mass Spectrometry
IS - 3
ER -