Intramolecular proton transfers in stereoisomeric gas-phase ions and the kinetic nature of the protonation process upon chemical ionization
✍ Scribed by Vais, V.; Etinger, A.; Mandelbaum, A.
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 115 KB
- Volume
- 34
- Category
- Article
- ISSN
- 1076-5174
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✦ Synopsis
The isobutane chemical ionization (CI) mass spectra of cis-and trans-1-butyl-3-and -4-dimethylaminocyclohexanols and of their methyl ethers exhibit abundant [MH
-H 2 O] Y and [MH -MeOH] Y ions respectively. On the other hand, only the MH Y ions of the cis-isomers exhibit significant [MH -H 2 O] Y and [MH -MeOH] Y ions under collision-induced dissociation (CID) conditions. The non-occurrence of water and methanol elimination in the CID spectra of the trans-isomers indicates retention of the external proton at the dimethylamino group in the MH Y ions that survive after leaving the ion source and the first quadrupole of the triple-stage quadrupole ion separating system, and the trans-orientation of the two basic sites does not allow proton transfer from the dimethylamino group to the hydroxyl or methoxyl. Such transfer is allowed in the cis-amino alcohols and amino ethers via internal hydrogen-bonded (proton-bridged) structures, resulting in the elimination of water and methanol from the surviving MH Y ions of these particular stereoisomers upon CID. The abundant [MH -ROH] Y ions in the isobutane-CI mass spectra of the trans-isomers indicates protonation at both basic sites, affording two isomeric MH Y ions in each case, one protonated at the dimethylamino group and the other at the less basic oxygen function. These results show that the isobutane-CI protonation of the amino ethers and amino alcohols is a kinetically controlled process, occurring competitively at both basic sites of the molecules, despite the large difference between their proton affinities (∼25 and ∼35 kcal mol -1 ; 1 kcal = 4.184 kJ).