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, 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

Received 24 December 1998; Revised 10 February 1999; Accepted 11 February 1999 Information about the mechanism of hydrogen or skeletal rearrangement during the fragmentation of ions plays an important role in the interpretation of mass spectra. Double hydrogen atom transfer (so-called transfer of two hydrogen atoms) is observed in the mass spectra of propyl and higher alkyl esters, 1,2 isobutanol, 3-5 1,2-ethanediol, 6-8 hydroxyacetone, 9,10 methyl glycolate 6,8,11 and methyl lactate. 12 In particular, in the mass spectra of isobutanol, 1,2-ethanediol and methyl lactate, a fairly intense oxygen-protonated methanol ([CH 3 OH 2 ] , m/z 33) is generated by double hydrogen atom transfer.Recently, we have noticed that the ion at m/z 47 (m/z 33 14 u) is generated in the metastable decomposition of the ions from dimethyl acetals, dimethoxymethane, CH 2 (OCH 3 ) 2 (1) and 1,1-dimethoxyethane, CH 3 CH(OCH 3 ) 2 (2). In this short communication, we investigate the fragmentation processes of the ions at m/z 75 [HC (OCH 3 ) 2 ] from 1 and m/z 89 [CH 3 C (OCH 3 ) 2 ] from 2, with attention to the formation of CH 3 OH CH 3 (m/z 47), by a combination of mass-analyzed ion kinetic energy (MIKE) spectrometry and deuterium labeling. EXPERIMENTALThe mass and MIKE spectra were obtained using a JEOL JMS-HX 100 tandem mass spectrometer. Samples were introduced via a heated (100°C) inlet system from which they were leaked into the ion source (180°C). The electron energy was 70 eV, and the ion accelerating voltage was 5 kV. MIKE spectra were obtained by scanning the voltage of the second electric sector, to observe the decompositions in the third field-free region of the ions of interest selected by the magnetic sector.Samples 1 and 2 were research grade products from Tokyo Kasei Co. Ltd., and were used without further purification. Samples 1-d 6 , (CH 2 (OCD 3 ) 2 ), and 2-d 6 , (CH 3 CH(OCD 3 ) 2 ), were prepared from formaldehyde and acetaldehyde with CD 3 OD, respectively. RESULTS AND DISCUSSIONThe MIKE spectra of the ions at In the metastable time window, the ion at m/z 75 (HC (OCH 3 ) 2 ) from 1 eliminates, essentially exclusively, a neutral species with mass 28 u to generate the m/z 47 ion. The peak at m/z 47 shifts fully to m/z 53 in the MIKE spectrum of the ion at m/z 81 (HC (OCD 3 ) 2 ) from 1-d 6 (see Fig. 1(b)). This means that the 28 u fragment lost is CO, not C 2 H 4 . Taking into account the known fragmentation of O-protonated methyl formate (HC (OH)OCH 3, m/z 61), 6,8,12 we propose the fragmentation mechanism shown in Scheme 1(a), of HC (OCH 3 ) 2 ion at m/z 75 from 1 to m/z 47. This mechanism includes the transfer of a hydrogen atom and a methyl group to the ether oxygen followed by the loss of CO.As shown in Fig. 2(a), although its relative abundance is not so high, the m/z 47 ion is also generated by the loss of CH 2 CO from the m/z 89 ion of 2. This m/z 47 ion also shifts to m/z 53 in Fig. 2(b). This result is explained by a similar mechanism. This fragmentation is also shown in Scheme 1(b). In other words, the tra...

The unimolecular metastable decompositions of dimethoxymethane (CH(2)(OCH(3))(2), 1) and 1,1-dimethoxyethane (CH(3)CH(OCH(3))(2), 2) upon electron impact have been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry, collision-induced dissociation (CID) spectrometry and D-labeling techniques. Both molecular ions are formed at extremely low abundance. Sequential transfers of a methyl group and a hydrogen atom to an ether oxygen are observed during the decomposition of [M - H](+) ions from 1 and 2. The [M - H](+) ion from 2 also decomposes into the m/z 43 ion by the loss of dimethyl ether. Almost complete hydrogen exchange is observed prior to the loss of CH(4) from the m/z 45 ion ([M - OCH(3)](+)) of 1. The m/z 59 ions ([M - OCH(3)](+)) of 2 decompose competitively into the m/z 31 and 29 ions by the losses of C(2)H(4) and CH(2)O, respectively. The former loss occurs via two different fragmentation pathways. The relative abundances of the ions in the MIKE spectra increase with decreases in the total heat of formation (Sigma DeltaH(f)) of the ion plus the neutral fragment. Copyright 2000 John Wiley & Sons, Ltd.

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