Application of multistage mass spectrometry to structural analysis and ion-molecule reaction chemistry
Abstract
Gas-phase reaction pathways were elucidated and isomeric ions distinguished using ion-molecule reactions and CAD. These studies were uniquely facilitated when using a pentaquadrupole mass spectrometer, which contains three spatially separate reaction regions, the ion source and two collision/reaction quadrupoles. The ability to mass-analyze ions before and after each collision quadrupole provides a powerful set of MS/MS/MS capabilities. Four isomeric C$\sb4\rm H\sb4\sp{+.}$ ions were found to react with neutral conjugated dienes allene, isoprene, furan, and thiophene by (2+2) and (2+4) cycloaddition and, in several cases, adduct fragments were observed to undergo a second addition of neutral diene. Information on the structure and reaction/fragmentation pathways was obtained using MS/MS/MS sequential product and reaction intermediate scans. The interesting cyclobutadiene radical cation was observed to be the most efficient dienophile. Electrophilic aromatic Cl$\sp{+}$ addition and CO$\sp{+\sp\cdot}$ substitution onto aromatic compounds was studied using NH$\sb3\rm{Cl}\sp{+}$, Cl-C$\equiv$O$\sp+,$ protonated CH$\sb3$Cl, and Cl$\sp{+}$ reagent ions. Comparisons of MS$\sp3$ sequential product spectra of mass-selected product ions with the MS/MS spectra of model ions allowed the determination of the addition sites. These sites were found to have the highest Cl$\sp{+}$ affinities by semi-empirical AM1 molecular orbital calculations. Conditions under which $\rm{C\sb2 H\sb4 N\sp{+}}$, $\rm{C\sb3 H\sb6\ N\sp{+}}$, and related nitrilium and immonium ions undergo polar Diels-Alder cycloaddition to the neutral diene isoprene were established. Cycloaddition occurs when the difference in the LUMO(ion) and HOMO(isoprene) energies is small and competing reactions are endothermic. The kinetic method was employed to order the relative gas phase chlorine cation affinities of substituted pyridines by fragmenting Cl$\sp{+}$-bound dimers of two pyridines. Unlike proton affinities, Cl$\sp{+}$ affinities reveal intramolecular steric effects between ortho groups and the bridge ion which significantly decrease the affinities for the larger Cl$\sp{+}$ ion in the cases of hindered molecules. These steric effects agree well with the corresponding S$\sp0$ parameters obtained by kinetic measurements in solution. Semi-empirical AM1 molecular orbital calculations were used to place the relative Cl$\sp{+}$ affinities on an absolute scale. Br$\sp{+}$ and I$\sp{+}$ bridge ions were also examined and discussed.
Degree
Ph.D.
Advisors
Cooks, Purdue University.
Subject Area
Analytical chemistry
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.