Study of organic cations in the gas phase by tandem mass spectrometry.

Abstract

The structure elucidation of gas-phase isomeric species $\rm C\sb2H\sb4X\sp+$ (X = F, Cl, Br and I), $\rm C\sb3H\sb6X\sp+$ (X = Cl, and Br), $\rm(C\sb2H\sb5)\sb2O\sp+C\sb2H\sb4X$ (X = Cl and Br), and $\rm C\sb3H\sb6O\sb2\sp{+\cdot}$ has been accomplished by employing tandem mass spectrometric techniques, i.e. metastable ion (MI) mass spectrometry, collision induced dissociation (CID) mass spectrometry, collision induced dissociative ionization (CIDI) mass spectrometry and neutralization reionization (NR) mass spectrometry. For $\rm C\sb2H\sb4X\sp+$ cations, apart from the $\alpha$-isomer, $\rm CH\sb3CHX\sp+,$ the cyclic ethylenehalonium ions, $\rm{\buildrel{{X\sp+}\atop\enspace}\over{CH\sb2\ CH\sb2}}$ are also stable for X = Cl, Br and I (Chapter 3). The two isomers have readily been characterized by CID mass spectrometry and their neutral counterparts have been produced and studied by NR mass spectrometry. It was found that based on an analysis of the heat of formation values of these ions and the electronegativity and polarizibility data of the X atoms, the relative stability of the two isomers is essentially controlled by the C-X bond strength. The relative stability of the cyclic species, however, is also controlled by the polarizibility of the X atom, which indicated that its polarization by the charge-centered carbon involved the outer-electrons in the X atom to form a back-donating bond with the charge-centered carbon. The isomeric halogen substituted triethyloxonium ions $\rm (C\sb2H\sb5)\sb2O\sp+C\sb2H\sb4X,$ (X = Cl and Br) were generated by appropriate gas phase ion molecular reactions between diethylether and appropriate $\rm C\sb2H\sb4XBr\sp{+\cdot}$ via Br$\sp\cdot$ loss (Chapter 4). The $\alpha$-substituted isomer, $\rm (C\sb2H\sb5)\sb2O\sp+CHXCH\sb3,$ and the $\beta$-substituted isomer, $\rm (C\sb2H\sb5)\sb2O\sp+CH\sb2CH\sb2X,$ are both stable in the gas-phase and do not interconvert on a time scale of $10\sp{-5}$s. Three $\rm C\sb3H\sb6X\sp+$ isomers (at least) were found to be stable in the gas-phase (Chapter 5). They are $\rm CH\sb3{-}\sp+CX{-}CH\sb3,\ CH\sb3{-}{\buildrel{{X\sp+}\atop\enspace}\over{CH{-}CH\sb2}},$ and $\rm{\buildrel{CH\sb2-X\sp+\atop\enspace}\over{CH\sb2{-}CH\sb2}}.$ These ions were characterized by their CID mass spectra and their different behavior in forming oxonium ions--$\rm(C\sb2H\sb5)\sb2O\sp+C\sb3H\sb6X$--with diethyl ether. The distonic radical cations $\rm \sp\cdot CH\sb2CH\sb2O\sp+CHOH$ and $\rm \sp\cdot CH\sb2CH\sb2\sp+C(OH)\sb2$, have been directly generated and characterized by their MI and CID mass spectra (Chapter 7). Comparing the dissociative process of $\rm\sp\cdot CH\sb2CH\sb2O\sp+CHOH$ to that of $\rm HCOOCH\sb2CH\sb3\sp{+\cdot}$ led to the conclusion that this distonic ion is the key intermediate in the dissociation of the latter. Thus the previous proposal, based only on the dissociation of HCOOCH$\sb2$CH$\sb3\sp{+\cdot},$ was confirmed. The heat of formation of $\rm\sp\cdot CH\sb2CH\sb2O\sp+CHOH$ was estimated from an appearance energy measurement to be 137 $\pm$ 4 kcalmol$\sp{-1};$ this is 16 kcalmol$\sp{-1}$ lower in energy than HCOOCH$\sb2$CH$\sb3\sp{+\cdot}.$ A detailed study of distonic ions is presented in Chapter 6, including a survey of the common methods to generate and characterize these ions. Properties of such ions which were considered were: stability, isomerization, bond cleavage and ring-strain. The results showed that the characteristics of distonic ions which distinguish them from their conventional counterparts result from the specific interaction of charge and radical sites.