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|Title:||SPONTANEOUS AND CATALYZED HYDROGEN SHIFTS IN RADICAL CATIONS HAVING A PHOSPHORYL OR CARBONYL GROUP|
|Authors:||Heydorn, Natasha Lisa|
|Advisor:||Terlouw, Johan K.|
|Abstract:||<p>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 phosphoryl (P=O) or carbonyl (C=O) functionality have been studied. The radical cations studied were generated in the rarefied gas-phase of the mass spectrometer by common ionization methods. Their structure and reactivity was studied by a variety of tandem mass spectrometry based techniques : mestable ion (MI) and collision-induced dissociation (CID) spectrometry, neutralization-reionization mass spectrometry (NRMS), collision induced dissociativ ionization (CIDI) spectrometry and a number of hybrid techniques, including CID/CID, NR/CID and CIDI/CID. The use of deuterium, 18-oxygen and 13-carbon labelled isotopologues and quantum chemical calculations at the CBS-QB3 level of theory formed an essential component in interpretation of the results. A single hydrogen shift can lead to the isomerization of a keto radical cation ito its thermodynamically more favourable enol isomer. However, if the isomerization of a solitary ion into its more stable counterpart involved a 1,2- or 1,3-H shift, it is often prohibited by a high energy barrier. Thus, the "keto" ion (CH₃O)₂(H)=O･⁺, ionized dimethyl phosphonate (1･⁺), does not readily isomerize into its "enol" tautomer (CH₃O)₂P-OH･⁺, ionized dimethyl phosphite (1b･⁺). Instead, 1･⁺ readily isomerizes via a 1,4-H shift to the very stable distonic ion CH₂O-(CH₃O)P(H)OH･⁺ (1a･⁺) and related ion-dipole complexes, which serve as precursors for the low energy loss of CH₂=O. In fact, in the μ timeframe ions 1･⁺ have completely isomerized into distonic ions 1a･⁺ which do not significantly communicate with their more stable enol counterpart. In contrast, an ion-molecule reaction of 1･⁺ with a benzonitrile molecule readily leads to a complete enolization. This is by virtue of a benzonitrile-catalyzed lowering of the high 1,3-H shift barrier separating the isomers 1a･⁺ and 1b･⁺. In the same vein, a facile tautomerization of the cyclic ethylene phosphonate "keto" ion [-OCH₂CH₂O-]P(H)=O･⁺ (2a･⁺) into its more stable "enol" isomer, ethylene phosphite [-OCH₂CH₂O-]P-OH･⁺ (2B･⁺), is prevented by a substantial 1,2-H shift barrier, 14 kcal/mol relative to 2a･⁺. In line with this conclusion, the collision-induced dissociation (CID) and neutralization-reionization (NR) spectra of the two isomers are charactistically different. Unlike the corresponding acyclic tautomers 1･⁺ and 1b･⁺, where the phosphonate isomer rapidly loses its structure identity by a facile distonicization, the barrier for this reaction in 2a･⁺ is prohibitively high and the cyclic distonic 1,2-H shift isomer [-OCH₂CH₂O-]P(H)=O･⁺, 2c･⁺, is not directly accessible. Thus, non-dissociating "keto" ions 2a･⁺ retain their structure identity in the μs timeframe. Here too, the interaction of 2a with a benzonitrile molecule in a chemical ionization type experiment readily yields the more stable "enol" type ion 2b･⁺. Experiments with benzonitrile-d₅ show that this reaction does not involve true proton-transport catalysis but rather a quid-pro-quo mechanism, analogous to that proposed for the benzonitrile-assisted enolization of acetamide. 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 CH₃O₂P･⁺ 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=C(H)-H・・・POH]･⁺ and the hydrogen bridged radical cation, [CH₂O・・・O=P]･⁺ as key intermediates. Characterization of the various CH₃O₂P･⁺ isomers also proved to be important in the context of differentiating the isobaric ions CH₃O₂P･⁺ and CH₅NOP⁺. Both m/z 78 ions are generated in equal abundance via dissociative electron ionization of acephate. The CH₃O₂P･⁺ component was found to consist of CH₃O-P=O･⁺ and CH₂O-P-OH･⁺, whereas the N-containing component was identified as CH₃O-P-NH₂⁺. Therefore, the recovery signal at m/z 78 in the previously reported neutralization-reionization spectrum of acephate does not demonstrate that CH₃O-P=O･⁺ has a stable neutral counterpart. Collision-induced dissociative ionization experiments on the reaction C₆H₅-P(=O)OCH₃⁺ ➝ C₆H₅⁺ + CH₃O-P=O is a stable molecule in the dilute gas-phase. The CH₃O₂P･⁺ isomer CH₃P(=O)₂･⁺ is only marginally stable but its neutral counterpart is predicted by theory to lie in a deep potential well. The stability of this elusive neutral was probed by a collision induced dissociative ionization (CIDI) experiment on the reaction (C₆H₅CH₂O)₂P(=O)CH₃･⁺ ➝ (C₇H₇O)(C₆H₅CHO)P(OH)CH₃･⁺ ➝ (C₆H₅CHO)P(=O)(OH)CH₃⁺ + C₇H₇･ ➝ C₆H₅CH-OH⁺ + CH₃-P(=O)₂ and a CIDI/CID experiment confirmed its structure identity. 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, C₆H₅NH(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. When a radical cation dissociates by loss of a radical, the reaction often involves a simple bond cleavage but hidden hydrogen shifts may also occur. Theory and experiment agree that the prominent loss of CH₃･ from ionized sorbic acid, CH₃CH=CHCH=CHCOOH･⁺, is not a direct bond cleavage not a hidden H-rearrangement. Instead, this dissociation is proposed to be a displacement reaction in which a very stable cyclic product ion, protonated 2H-pyran-2-one, is generated. Hydrogen shifts also feature prominently in the dissociation chemistry of even-election ions. The low energy oxonium ions CH₃CH=CH-C⁺(H)OCH₃, CH₂=CH-C⁺ (CH₃)-OCH₃, CH₂=C(CH₃)-C⁺(H)OCH₃ and CH₂=CH-CH(CH₃)-OCH₂⁺ all abundantly lose CH₂O. The chemistries of these C₅H₉O⁺ 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 C₄H₆OCH₃⁺ 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 C₄H₇⁺ ion, 1-methallyl cation, at the thermochemical threshold. The enthalies of formation for the key ions in this study were obtained from CBS-QB3 calculations and thermochemical estimates.</p>|
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