Investigation of gas phase ion-molecule reactions and determination of cation affinities using multiple-stage mass spectrometry
Abstract
Gas phase even-cation (4+2$\sp+$) Diels-Alder reactions, electrophilic aromatic additions, and cation affinities are studied with multiple-stage mass spectrometry in a pentaquadrupole mass spectrometer. The unique capability of the pentaquadrupole mass spectrometer for studying ion/molecule reactions is that it allows reactions to be performed with mass-selected reagent ions as well as allow the mass-selected products of ion/molecule reactions to be structurally characterized. Even cations including acylium ions, nitrilium and immonium ions, protonated and methylated ketone ions, generated by electron impact ionization or chemical ionization, are used in the studies of gas phase Diels-Alder cycloadditions. Tandem and multiple-stage mass spectrometry are used in studying mechanism, regioselectivity, and reactivity-energy gap relationship of the cycloadditions. Evidence for the cycloadditions are also obtained from theoretical calculations. Correlations between the cycloaddition reactivity and the HOMO/LUMO energy gap are observed for the cation systems studied when proton transfer reactions are not favored. Electrophilic brominations are studied in the reactions of BrCO$\sp+$ and CH$\sb3$NH$\sb2$Br$\sp+$ with gaseous aromatic compounds in the pentaquadrupole mass spectrometer under the nonthermal conditions. The electrophilic bromination proceeds via a $\delta$-complex to the ring as suggested by sequential product ion spectrum. Linear free energy correlations are observed when the Brown substituent constants $\delta\sp+$ are plotted against the relative yields of the electrophilic brominations. Reaction constants $\rho$ are found to be $-$0.23 and $-$0.56 for BrCO$\sp+$ and CH$\sb3$NH$\sb2$Br$\sp+$ ions, respectively. The proton affinities of free radicals and the relative cation affinities for pyridines are determined by the kinetic method. The cations studied are Cl$\sp+$, CN$\sp+$, OCNCO$\sp+$, SiCl$\sb3\sp+$, and SiCl$\sp+$. Dimers containing these cations are generated via either chemical ionization or ion/molecule reactions. Tandem and multiple-stage mass spectrometry as well as theoretical calculations show these dimers are loosely bound. Interactions between substituent(s) at the ortho-position of substituted pyridines with the cations are studied. Stereoelectronic effects are attributed to the abnormal behavior of the ortho-substituted pyridines.
Degree
Ph.D.
Advisors
Cooks, Purdue University.
Subject Area
Analytical chemistry|Organic chemistry
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