Mechanism of carbon dioxide elimination from ionized methyl haloacetates in the gas phase. Formation of CH3XCH2+.cntdot. and CH3XCHX+.cntdot. (X = Cl, Br) halonium radical ions

Abstract

significant, and showed no trend with dielectric constant. Thus, it appears that solvent effects are not important in those cases in which there are no changes in rotamer population^.^^ ConclusionsGood correlations of the orthobenzylic coupling constants in methyl aromatic compounds and unsaturated compounds (having vinylic methyl groups) are found with the *-bond order q, the square of q, and the mutual atom-atom polarizability A~~. However, the linear dependence on 4 (which has no theoretical basis) gives values which are much too high in the region of very low bond orders.In the extension of the study to molecules in which both conformational and bond order effects are important to the Job, it is found that the generalization of the relationship of coupling constant to A to include angular factors gives results which are in very gooi'agreement with the experimental results. The agreement is an improvement on the results based on the IND-0-FPT methodS4 and is much better than the correlation with qz. It seems likely that the inadequacy of the latter is due to the overestimation of J".The experimental and theoretical results presented here provide an important method for obtaining useful structural information in complex molecules. (X = H), 10239-86-8; 49 (X = CH,), 84959-62-6; 49 (X = CH(CO,CHS),), 84944-79-6; 49 (X = C&5), 84944-80-9; 84944-83-2; 49 (X = OH), 84944-84-3; 49 (X OCOCH,), 84944-85-4; 50 (X = H), 4208-97-3; 50 (X = CHS), 84944-86-5; 50 (X = CH-(C02CH,)2), 84944-87-6; 50 (X = C,H5), 84944-88-7; 50 (X = NH,), = OH), 84944-91-2; 50 (X = OCOCHS), 84944-92-3. 49 (X = NH2), 84944-81-0; 49 (X = Br), 84944-82-1; 49 (X = CI), 84944-89-8; 50 (X = Br), 29094-68-6; 50 (X = Cl), 84944-90-1; 50 (X Supplementary Material Available: Data concerned with general experimental procedures, synthesis of compounds, and spectral analyses (1 12 pages). Ordering information is given on any current masthead page.Abstract: The molecular ions of the methyl esters of chloro-, bromo-, dichloro-, and dibromoacetic acids undergo unimolecular elimination of C 0 2 in the gas phase. Collisional activation (CA) mass spectrometry suggests that the resulting [M -C02]+. ions possess novel types of hypervalent structures, Le., CHSXCH2+. and CH3XCHX+. (X = C1, Br). MNDO and ab initio calculations show that the methylmethylenechloronium radical ion CH3CICH2+. and the isomeric chloroethane cation radical CH3CH2CI+. exist in a potential minimum. MNDO predicts that CH3C1CH2+. is more stable than CH3CH2C1+. by 10.1 kcal mol-', whereas according to 6-31G' and UMP2/6-31G* CHSCH2C1+. is more stable by 9.1 and 6.4 kcal mol-', respectively. CzHSX+* (X = C1, Br) ions obtained after COz elimination from

Electron ionisation induced fragmentation of ethyl 5(1H)‐oxo‐ and 7(1H)‐oxo‐1‐aryl‐2,3‐dihydroimidazo[1,2‐a]‐pyrimidine‐6‐carboxylates: evidence for an unusually regioselective rearrangement of M+• ions

“…Later, the CO 2 elimination from the M +· of methyl haloacetates was shown to proceed via a five‐centre mechanism (Scheme ) and result in the formation of hypervalent halonium radical cations 10. The heats of formation of the transition states calculated by the MNDO method suggested the overall activation energy of 45 kcal/mol for the consecutive methyl migration and CO 2 elimination from the M +· of methyl chloroacetate, although the authors argued that this value should be lower by ca.…”

Free Radicals in Mass Spectrometry. Part I. General Fragmentation Rules for Organic and Organometallic Compounds under Electron Impact

“…It was noticed 5 that CO 2 elimination does not occur in X 3 CCOOMe compounds (X=F, Cl, Br). The authors 78,79 do not explain the formation of [M -45] + and 45 + ( + COOH) ions in halogenated esters and why methyl migration takes place only in this class, but not in most other halogenated compounds. Finally, the following isomerization and fragmentation of ionized XCH 2 COOMe molecules is proposed: 4-6 (Equation 45):…”

“…•+ and [M -HCO 2 ] + ions was noticed in esters XCH 2 COOMe and X 2 CHCOOMe. 5,78,79 In Reference 79, CO 2 elimination was theoretically explained by a "routine" CH 3 shift to halogen. It was noticed 5 that CO 2 elimination does not occur in X 3 CCOOMe compounds (X=F, Cl, Br).…”

The ethyl halides: Stable neutral and radical cation isomers [C2, H2, X] where X = F, Cl, Br, I

“…Indeed, the earlier identification of the ions required deuterium labelling to differentiate their CID mass spectra from those of the conventional isomer. 21 The NR mass spectra of the other ylidions [CH,CHClH]+' and [CH3CHBrH]+', shown in Fig. 6 with the corresponding CID mass spectra, contain signals corresponding to surviving neutral counterparts.…”