The test of an analogy: Are intramolecular hydrogen bonds in .beta.-diols and .beta.-hydroxy ethers preserved in their molecular ions?

Isotopic labelling in mass spectrometry as a tool for studying reaction mechanisms of ion dissociations

“…In the first reaction, which is so fast that it is also observed in the normal mass spectrum, the H atoms of the ethyl chain become positionally equivalent. Reversible rearrangement of EA-1 into the distonic ion EA-3 followed by isomerization into the ion-dipole complex This result, obtained from a double collision experiment, 29 was unexpected, not least because earlier it had been proposed that the hydrogen bonding between the OH groups in the neutral persists in the gas-phase ion and thus directs its dissociation chemistry, 31 a very reasonable (and therefore highly suspect) extrapolation from neutral chemistry. The solution to this problem again relied upon the results of computations, 32 where it was shown that the (predictable) 1,4-D shift, from oxygen to carbon, in the rearranged molecular ion, EG-2, required a large activation energy, making way for the less energy demanding internal catalysis mechanism, shown above in EG-4 !…”

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

“…These losses also occur from authentic CH 3 O + (H)CH 2 D, although in the opposite ratio of 1.6 : 1.0. 4 The difference in the two ratios, due to the larger amount of CH 4 loss from the m/z 48 ions from 1-O-d 1 +• , strongly indicates that these ions consist of not only CH 3 O + (H)CH 2 D, but also CH 3 O + (D)CH 3 . This means that there is also a DHT pathway in which the hydroxyl hydrogen becomes bonded to oxygen.…”

“…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.…”

“…Hydrogen rearrangements have been studied extensively because of their importance for understanding the fragmentation dynamics of gas-phase ions. 1 Together with double hydrogen atom transfers (DHT), hydrogen rearrangements are known to play a significant role in the metastable-ion fragmentations of ionized molecules having two functional groups at adjacent carbon atoms, such as 1,2ethanediol, 2-6 2-mercaptoethanol, 7,8 2-aminoethanol, [9][10][11] 2methoxyethanol 4,5,12 and 1,2-dimethoxyethane.…”