Unimolecular metastable decompositions of gem‐dimethoxyalkanes (RR′C(OCH3)2) upon electron impact. I. Dimethoxymethane and 1,1‐dimethoxyethane

Abstract

, which correspond to the expulsion of one of the substituents on the central carbon atom. These fragment ions further decompose into various ions following hydrogen or skeletal rearrangement in the metastable time window. In a review of the mass spectra of compounds of this type, Beynon et al. have emphasized that deuterium labeling would seem to offer the only means of elucidating the true fragmentation process. ) from 1,1-dimethoxyethane, (CH 3 CH(OCH 3 ) 2 , 2).6 Further, they have reported that the metastable ion at m/z 45 (CH 2 =O CH 3 ) from 1 Á decomposes into the m/z 29 ion (CH 4 loss) in preference to the m/z 19 ion (C 2 H 2 loss). 7 The structure of the m/z 59 ion) from 2 Á has been investigated by measuring the intensity ratio of m*(H 2 O) and m*(C 2 H 4 ), where m*(X) is the intensity of the metastable peak corresponding to the loss of X from the m/z 59 ion. 8 The suggestion of Beynon et al.3 prompted us to reinvestigate the fragmentation mechanims of RR'C (OCH 3 ) 2 ions by using mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. In a preliminary report, we have proposed that sequential transfers of a methyl group and a hydrogen atom occur during the fragmentations of [M À H] from 1 and 2. 9 In this paper, more detailed fragmentation pathways of 1 and 2 are reported. The relative abundances of the ions in the MIKE spectra are rationalized by considering the total heat of formation (AE DH f ) of the ion plus the neutral fragment. EXPERIMENTALThe mass and unimolecular MIKE spectra were obtained using a JEOL JMS-HX100 tandem mass spectrometer. The electron energy was 70 eV, and the ion accelerating voltage was 5 kV. The CID spectrum of protonated ethanol was obtained using a JEOL JMS-DX303 tandem mass spectrometer equipped with a chemical ionization (CI) source. The electron energy was 200 eV, and the ion accelerating voltage was 3 kV. Samples were introduced via a heated (100°C) inlet system from which they were leaked into the ion source (180°C).Samples 1, 2 and ethanol were research grade products from Tokyo Kasei Co. Ltd, and were used without further purification. Ethanol was used as a reference compound. Isobutane was used as a reagent gas for CI. Samples 1-d 6 , (CH 2 (OCD 3 ) 2 ), and 2-d 6 , (CH 3 CH(OCD 3 ) 2 ), were prepared

Unimolecular metastable decompositions of 1,1,1‐trifluoroisopropyl methyl ether [CF3(CH3)CHOCH3] upon electron ionization

“…It most probably is initiated by methyl cation migration from the methoxy group to one of the fluorine atoms of the trifluoromethyl group, because loss of CFH 3 is also observed from the m/z 113 ion from 1 + , as noted below. Recently, this type of sequential migration was observed during the decomposition of RC + (OCH 3 ) 2 (RH, CH 3 ) to form protonated dimethyl ether 11, 12. The loss of CFH 3 from the m/z 113 ion from 1 + should also be initiated by methyl cation migration from the methoxy group to one of the fluorine atoms of the trifluoromethyl group.…”

The Formation of Protonated Dimethyl Ether from the Metastable Molecular Ions of 1-Methoxy-2-Propanol, CH3OCH2CH(OH)CH3

“…+ ions, leading to the elimination of formaldehyde and the generation of m/z 45 ions. This conclusion can be reached from the MIKE spectrum [ Figure 5 , does not eliminate a methyl radical in addition to the fact that methyl loss from a methoxy group is not common 19 ). The peak at m/z 45 in Figure 5…”

“…15 This ion decomposes further by the loss of a methane molecule, in which the proton bonded to the oxygen atom does not participate, to give protonated formaldehyde at m/z 31. 4,[15][16][17][18][19] Recently, we have noticed that the metastable fragmentation of the protonated dimethyl ether species at m/z 47 generated from 1 +• cannot be explained only on the basis of the earlier proposed DHT mechanism.…”

“…It is known that protonated dimethyl ether (m/z 47), generated by the decomposition of several precursor ions 4,[15][16][17]19 or by ion-molecule reaction of CH 3 + and CH 3 OH, 18 decomposes by the loss of a methane molecule, in which the proton bonded to the oxygen atom does not participate, to give protonated formaldehyde at m/z 31. The critical energy of this reaction was estimated a few years ago.…”

“…24 Further insight is provided by the MIKE spectra of the ions at m/z 47 from . Thus, the latter ions eliminate unlabeled methane, which means that the deuterium atom transfered in the DHT is bonded to the oxygen atom 15,18,19 and is not in one of the methyl groups of the deuterated dimethyl ether species formed from 1-d 1 +• . In other words, the m/z 48 ions from 1-d 1 +• are generated by DHT in which the methine hydrogen atom becomes bonded to oxygen whereas the other hydrogen atom, either from the hydroxyl group or from the methyl group at the 3-position, becomes bonded to the original methylene group.…”