Premium
Intramolecular proton transfers in stereoisomeric gas‐phase ions and the kinetic nature of the protonation process upon chemical ionization
Author(s) -
Vais V.,
Etinger A.,
Mandelbaum A.
Publication year - 1999
Publication title -
journal of mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 1076-5174
DOI - 10.1002/(sici)1096-9888(199907)34:7<755::aid-jms831>3.0.co;2-j
Subject(s) - chemistry , protonation , chemical ionization , isobutane , intramolecular force , proton affinity , ion , trimethylsilyl , medicinal chemistry , molecule , kinetic isotope effect , photochemistry , stereochemistry , deuterium , ionization , organic chemistry , catalysis , physics , quantum mechanics
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] + and [MH − MeOH] + ions respectively. On the other hand, only the MH + ions of the cis ‐isomers exhibit significant [MH − H 2 O] + and [MH − MeOH] + 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 + 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 + ions of these particular stereoisomers upon CID. The abundant [MH − ROH] + ions in the isobutane‐CI mass spectra of the trans ‐isomers indicates protonation at both basic sites, affording two isomeric MH + 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). Copyright © 1999 John Wiley & Sons, Ltd.