Spontaneous and catalyzed hydrogen shifts in radical cations having a phosphoryl or carbonyl group: A tandem mass spectrometry and CBS-QB3 computational study

Intermolecular and intramolecular hydrogen shifts represent a key component of a vast number of chemical reactions. This is particularly true for radical cations, whose high reactivity makes them prone to isomerization and dissociation reactions. In the context of the experimental work in this thesis, hydrogen transfers involved in both the intra- and inter-molecular isomerization of radical cations containing a phosporyl (P=O) or carbonyl (C=O) functionality have been studied.; Hydrogen shifts are often induced when electronegative atoms such as oxygen are present in the radical cation. This trend is evident in the relatively small CH₃O-P=O·+ ion, which has more than fifteen stable isomers, not including rotational or conformational isomers. This ion has a fairly low heat of formation but it is not as stable as its distonic H-shift isomer CH₂O-P-OH·+, or its “enol” isomer CH₂=P(OH)=O·+, which represents the global minimum on the CH₃O₂P ·+ potential energy-surface. Several CH₃O₂ P·+ isomers were characterized experimentally and found to display a remarkable low energy decarbonylation reaction that requires three consecutive H-shifts. A detailed computational study revealed that the complex mechanism for this reaction involves the ion-dipole complex [O=(H)-H···POH]·+ and the hydrogen bridged radical cation, [CH₂O-H···O=P] ·+ as key intermediates.; An enol radical cation is as a rule more stable than its keto isomer but this appears not to be true for the acetanilide ion. Its enol, C ₆H₅NH(OH)=CH₂·+, was calculated to be higher in energy than the keto tautomer. The enol ion was found to eliminate HNCO at low internal energy and not ketene as reported previously. This HNCO loss occurs via an intriguing skeletal rearrangement, whose mechanism was explored using isotopic labelling and computational chemistry. As the ionized enol is less stable than its keto counterpart, it is not surprising that molecule-assisted enolization reactions of ionized acetanilide cannot be realized. However, the reverse process, i.e. a molecule-assisted ketonization of the enol ion, does not take place either but this may be due to the formation of unreactive encounter complexes.; Hydrogen shifts also feature prominently in the dissociation chemistry of even-electron ions. The low energy oxonium ions CH₃CH=CH-C +(H)OCH₃, CH₂=CH-C+(CH₃ )-OCH₃, CH₂=C(CH₃)-C+(H)OCH ₃ and CH₂=CH-CH(CH₃)-OCH₂+ all abundantly lose CH₂O. The chemistries of these C₅H ₉O+ isomers are closely related but they are not identical and the distinctions become clearer when labelled analogues are examined. Elimination of CH₂O from the three C₄H₆OCH ₃+ ions is proposed to involve a largely irreversible 1,5-H shift, from the OCH₃ group to the hydrocarbon chain, followed by a dipole-assisted 1,3-H shift to give energy-rich ion-neutral complexes of 1- or 2-methallyl cations and neutral CH₂O, which dissociate. For the CH₂=CH-CH(CH₃)OCH₂+ ion, the CH₂O loss is associated with a very small kinetic energy release, suggesting that it generates the most stable C₄H ₇+ ion, 1-methallyl cation, at the thermochemical threshold. The enthalpies of formation for the key ions in this study were obtained from CBS-QB3 calculations and thermochemica...