Quantum chemical analysis of possible fragmentation of [M + H]+ and [M + Na]+ complexes of monosubstituted methane, cyclohexane, and benzene derivatives in mass spectrometric studies

Abstract

The formation and fragmentation energies of the proton and sodium cation complexes with monosubstituted methane, cyclohexane, and benzene derivatives in which carbon atoms are bonded to substituents (NH 2 , OH, F, Cl, Br, ONO 2 , NO 2 , COOH, CN, and Ph) were calculated by the B3LYP/6 31G(d) method. For [M+Na] + complexes, the formation energies are much lower (and differ from one another to a much lesser extent), while the dissociation energies are much higher, than the corresponding energies of the [M+H] + complexes. Na + cation shows a lower selectivity toward localization at functional groups in molecules com pared to H + . Key words: quantum chemical calculations, B3LYP/6 31G(d) method, mass spectromet ric studies, fragmentation of [M + Na] + and [M + H] + ions.The electrospray ionization (ESI) technique has been effectively used in organic mass spectrometry in the last two decades. 1 The method allows fluid samples to be in troduced into a mass spectrometer, which signifcantly enhances the potential of mass spectrometry in studies of low volatile substances and high molecular weight com pounds. 2 Therefore, significant increase in interest in com bined quantum chemical/ESI mass spectrometric studies of the structure and properties of ionic complexes formed under these conditions (see Refs 3-10) is not surprising. Indeed, the results of ab initio and semiempirical quan tum chemical calculations make it possible to gain a deeper understanding and in some cases to bridge the gap that is due to unavailability of mass spectrometry data owing to experimental limitations.Conventionally, products generated under the ESI conditions include [M + H] + and [M + Na] + ions. The detection conditions can be chosen in such a manner that the former or the latter, or both types of ions are formed. Ions generated under the ESI conditions are very stable and the mass spectra of the compounds studied almost do not exhibit fragment ion peaks. The fragment ions are

Tandem ESI mass spectrometry of 11α- and 11β-{4-[N,N-bis(2-chloroethyl)amino]phenyl}acetates of the estrane steroids

“…11 All this Using quantum chemical calculations, 13 we estimated the nature of the fragmentation of the ions [AlkX + Na] + and [AlkX + H] + (Alk = Me, cyclo C 6 H 11 , and Ph; X = -NH 2 , -OH, -F, -Cl, -Br, -NO 2 , -ONO 2 , -COOH, -CN, and -Ph). We found that the energies of formation of the [AlkX + H] + ions are substantially higher than those of the [AlkX + Na] + ions.…”

“…13) are probably responsible for intense generation of the fragments [(M + Na) -NaCl] + from the compounds studied.…”