Satoshi Nakajima

Metastable decompositions of gem-dialkoxyalkanes upon electron impact. III. Diethoxymethane (CH2(OCH2CH3)2)

Unimolecular metastable decomposition of diethoxymethane (CH(2)(OCH(2)CH(3))(2), 1) upon electron impact has been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and theD-labeling technique in conjunction with thermochemistry. The m/z 103 ion ([M - H](+) : CH(OCH(2)CH(3)) = O(+)CH(2)CH(3)) decomposes into the m/z 47 ion (protonated formic acid, CH(OH) = O(+)H) by consecutive losses of two C(2)H(4) molecules via an m/z 75 ion. The resulting product ion at m/z 47 further decomposes into the m/z 29 and 19 ions by losses of H(2)O and CO, respectively, via an 1,3-hydroxyl hydrogen transfer, accompanied by small kinetic energy release (KER) values of 1.3 and 18.8 meV, respectively. When these two elimination reactions are suppressed by a large isotope effect, however, another 1,1-H(2)O elimination with a large KER value (518 meV) is revealed. The m/z 89 ion ([M - CH(3)](+) : CH(2)(OCH(2)CH(3))O(+) = CH(2)) decomposes into the m/z 59 ion (CH(3)CH(2)O(+) = CH(2)) by losing CH(2)O in the metastable time window. The source-generated m/z 59 ion ([M - OCH(2)CH(3)](+) : CH(2) = O(+)CH(2)CH(3)) decomposes into the m/z 41 (CH(2) = CH(+)CH(2)) and m/z 31 (CH(2) = O(+)H) ions by losses of H(2)O and C(2)H(4), respectively, with considerable hydrogen scrambling prior to decomposition. Copyright 2000 John Wiley & Sons, Ltd.

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

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.

On the chemistry following methoxy migration in the metastably decomposing (M − COOCH3)+ ions (m/z 135) from dimethyl phthalate, isophthalate and terephthalate

Abstract The metastable ion dissociations of the (M − COOCH 3 ) + ions ( m / z 135) generated upon electron ionization from dimethyl phthalate ( 1 ), isophthalate ( 2 ) and terephthalate ( 3 ), have been studied by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. These ions all show as primary fragmentation channels the losses of methyl and carbon monoxide to give the ions m / z 120 and 107, respectively. The latter ions decompose further by the losses of another molecule of carbon monoxide or a molecule of formaldehyde to generate the ions at m / z 79 and 77, respectively. An additional dissociation channel is observed for the (M − COOCH 3 ) + ions from 2 + and 3 + , the loss of methanol to give the ions m / z 103. The dissociation mechanism of the (M − COOCH 3 ) + ions from 1 + is shown to be identical to that of the (M − CH 3 ) + ions from ionized 2-methoxyacetophenone. The latter ions have the 2-methoxybenzoyl cation structure demonstrating a methoxy migration in the (M − COOCH 3 ) + ions from 1 + precedes their metastable ion dissociations. Part of the (M − COOCH 3 ) + ions from 2 + and 3 + has also rearranged to the 2-methoxybenzoyl cation structure prior to dissociation, but the significantly enhanced loss of methyl and the loss of methanol from these ions occur from the unrearranged structures as indicated by comparison with the dissociation behavior of the (M − CH 3 ) + ions from ionized 3- and 4-methoxyacetophenones.

Loss of HF from C2H4FO+ produced from 2-hydroxy-2-trifluoromethylpropanoic acid upon electron ionization

Abstract Metastable C 2 H 4 FO + ions ( m / z 63) produced by electron ionization of 2-hydroxy-2-trifluoromethylpropanoic acid CH 3 C(CF 3 )(OH)COOH ( 1 ) decompose into CH 3 CO + ( m / z 43) and HF. The corresponding peak in the MIKE spectrum is composite with translational energy release values of 0.83xa0eV and m / z 63 ion from 1 + consists of two isomers, i.e., CH 3 C + FOH and CH 3 CO + ⋯FH, with the former (latter) being responsible for the broad (narrow) component of the peak. The decomposition processes of the precursor ion CH 3 C + (CF 3 )OH are also explored in some detail.

Unimolecular gas-phase reactions of methyl and ethyl trifluoroacetoacetates upon electron ionization

Abstract The unimolecular metastable decompositions of methyl and ethyl trifluoroacetoacetates, CF 3 COCH 2 COOCH 3 (MW: 170 ( 1 )) and CF 3 COCH 2 COOCH 2 CH 3 (MW: 184 ( 2 )) induced by electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. In the metastable time window, the molecular ions 1 + decompose to give exclusively the ions at m / z 101 [ M −CF 3 ] + . However, the metastably decomposing ions 2 + lead not only to the formation of the major fragment ion m / z 115 [ M −CF 3 ] + , but also to three minor fragment ions m / z 164 [ M −HF] + , m / z 156 [ M −C 2 H 4 ] + and m / z 87. A large part of the metastably decomposing ions 1 + and 2 + has the enol form. The loss of CO 2 from the ions m / z 101 and m / z 115 occurs through migration of the methyl and ethyl groups, respectively. The source-generated m / z 69 ions from 1 + and 2 + are most abundant and consist of both CF 3 + and OCCHCO + . The latter ion, a protonated carbon suboxide, is generated by at least three and four different fragmentation routes from 1 + and 2 + , respectively. The m / z 43 ion, C 2 H 3 O + , from 2 + is formed by at least two different routes.

Formation and decomposition of the m/z 75 fragment ions from the molecular ion of ethyl lactate, CH3CH(OH)COOCH2CH3

Abstract The m/z 75 fragment ions are formed by the loss of a neutral species with 43 Da from the molecular ions of ethyl lactate [CH 3 CH(OH)COOCH 2 CH 3 ]. In contrast to the previously reported results, it is shown that the population of these ions consists of at least three different ionic species, which are CH + (OH)OCH 2 CH 3 (protonated ethyl formate), CH 3 CH(OH)O + =CH 2 , and CH + (OH)COOH (protonated glyoxylic acid). The protonated ethyl formate species decomposes into the m/z 47 ion (protonated formic acid) by the loss of ethylene, the second ion decomposes into the m/z 45 ion (protonated acetaldehyde) by the loss of formaldehyde and the protonated glyoxylic acid eliminates two molecules of carbon monoxide to generate the m/z 19 ion (protonated water).

The formation of protonated dimethyl ether from the metastable molecular ions of 1-methoxy-2-propanol, CH 3 OCH 2 CH(OH)CH 3

The unimolecular metastable decomposition of 1-methoxy-2-propanol, CH3OCH2CH(OH)CH3 (mol. wt. 90) induced by electron ionization, has been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling in conjunction with thermochemical data. In the metastable time window, the molecular ions decompose almost exclusively into ions at m/z 47 [i.e. protonated dimethyl ether, CH3O+(H)CH3] by the loss of a C2H3O radical species following a double hydrogen atom transfer (DHT). Until now, only one DHT mechanism has been proposed, involving and accounting for the loss of an acetyl radical, C2H3O. In the present study it is shown that more DHT mechanisms are operative, leading to the losses of isomeric C2H3O radicals. The results obtained are best explained by the formation of the key intermediate ion–molecule complexes [CH3OCH3+•, CH3CHO] and [CH3OCH3, CH2=C(H)OH+•] following unimolecular metastable dissociation of the molecular ion. Subsequent hydrogen atom abstraction channels by CH3OCH3+• in the former complex and proton abstraction channels by CH3OCH3 in the latter complex lead eventually to the formation of protonated dimethyl ether with m/z 47.

Unimolecular gas-phase reactions of diethyl phthalate, isophthalate, and terephthalate upon electron ionization

Unimolecular gas-phase reactions of diethyl phthalate (1), isophthalate (2), and terephthalate (3), upon electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and deuterium labelling. The metastable molecular ions (1)+ decompose to give exclusively the ions m/z 176 ([M – CH3CH2OH]+) and not the ions by the loss of CH3CH2O as proposed earlier in the literature. The metastable molecular ions (2)+ and (3)+ fragment differently from (1)+ and lead not only to the formation of the major fragment ions m/z 194 ([M − CH2CH2]+) via a McLafferty rearrangement but also to minor fragment ions m/z 193 ([M – CH2CH3]+). Yet, molecular ions decomposing in the ion source all show as primary fragmentation channel the loss of CH3CH2O to give the ions at m/z 177, which further dissociate to give the ions at m/z 149 through the loss of C2H4 or CO, indicating the resulting ions are +COC6H4COOH and +C6H4COOCH2CH3. The +COC6H4COOH ions decompose into the m/z 121, 93, and 65 ions by the consecutive losses of three carbon monoxide molecules, respectively. Prior to the second CO loss, a migration of the OH group to the benzene ring occurs. During the metastable fragmentation of the +C6H4COOCH2CH3 ions no ethoxy migration occurs, in contrast to the methoxy migration taking place in the metastable decomposition of the lower homologue +C6H4COOCH3 ions.