Gas-phase protonation of arylalkylamines. A metastable ion study

Abstract

The unimolecular metastable decompositions of protonated arylalkylamines of general formula R and R 5 = H and/or CH 3 ; and n = 1-3, generated by chemical ionization with water and ammonia as ionizing reagents, have been examined via the analysis of the mass-analysed ion kinetic energy spectra of the protonated molecules. The mechanisms proposed have been probed by chemical ionization of the deuterated analogues of the amines with D 2 O and ND 3 . The results show that protonation occurs preferentially on the amino group leading to abundant ions due to ammonia (or amine) loss, although ions resulting from transfer of the proton from the amino group to the aromatic ring, to the hydroxy group para to the aliphatic chain and to the benzylic OH, are also observed. Experimental evidence for H/D exchange between the deuteronated amino group and the aromatic ring is presented, and discussed in terms of the internal energy content of the deuterated ions and structural features of the amines, such as aliphatic chain length and the presence of a second hydroxy group on the aromatic ring. # 1998 John Wiley & Sons, Ltd. Received 3 April 1998; Revised 29 April 1998; Accepted 2 May 1998The site of protonation 1-33 of polyfunctional molecules is a topic of current interest in the field of gas-phase ion chemistry. The research in this area has been focused mainly on two aspects: proton affinity measurements by equilibrium, bracketing or kinetic methods, and structural elucidation of the protonated molecules by several mass spectrometric methods. Proton affinity measurements, in general, provide information on the basicity of the molecule as a whole, and not on specific protonation sites. For polyfunctional molecules the location of the proton in the protonated molecule has been discussed in terms of the proton affinities of each functional group in the molecule 11,13,15,18 by comparison of chemical ionization mass spectra, 5-10 analysis of the mass-analysed ion kinetic energy (MIKE) and MIKE-CID (collision induced dissociation) spectra of the protonated molecules and their labelled analogues, comparison of charge stripping processes, 29 and information provided by neutralization-reionization mass spectra. 12,[30][31][32][33] The present work reports the unimolecular metastable decompositions of protonated molecules of the arylalkylamines given in Table 1, including a group of biologically important amines (biogenic amines) which play significant physiological roles. The protonated molecules of the arylalkylamines were prepared by chemical ionization with water and ammonia as ionizing reagents, in order to evaluate the effect of the exothermicity of the protonation reaction in the dissociations of the metastable MH ions. The compounds studied have essentially two basic sites, the (un)substituted aromatic ring and the aliphatic chain with a terminal amino group, so that any interchange of protons will occur between these two sites. The MIKE spectra of the MD ions of the deuterated arylalkylamines will be discussed in order to eval...

Determination of therapeutics with growth‐hormone secretagogue activity in human urine for doping control purposes

“…MS3 experiments demonstrated that the generation of m/z 225 occurs via m/z 257. The release of methylamine from m/z 288 yields the ion at m/z 257, which presumably requires the initial protonation of the precursor ion at the methoxy residue, although it bears a considerably weaker proton affinity (approximately 840 kJ/mol)38, 39 than the tertiary amino function ( vide supra ). The subsequent loss of methanol (32 Da, route o ) gives rise to m/z 225, which eliminates 28 Da to yield m/z 197 (route p ).…”

Four decades of joy in mass spectrometry

“…A.M. Cardoso aralkylamines of the general formula R 1 R 2 C 6 H 3 CHR 3 (CH 2 ) n NR 4 R 5 with R 1 = H or OH, R 2 = H, F, NO 2 , OH or OCH 3 , R 3 = H or OH, R 4 and R 5 = H, and/or CH 3 and n = 1–3, were found to be preferentially deuteronated at the amino group by D 3 O + and ND. The deuteron appeared to be transferred from the deuteronated amino group to the benzylic and p‐ hydroxyl groups, and to the aromatic ring where it exchanged with the ring hydrogens (Cardoso et al, 1998a). These aralkylamines were also studied with ions generated from dimethyl ether and trimethyl borate.…”