title: Differentiation of isomeric 2-aryldimethyltetrahydro-5-quinolinones by electron ionization and electrospray ionization mass spectrometry.
abstract: A series of isomeric 2-aryl-6,6-dimethyltetrahydro-5-quinolinones (set I) and 2-aryl-7,7-dimethyltetrahydro-5-quinolinones (set II) were studied under positive ion electron ionization (EI) and electrospray ionization (ESI) techniques. Under EI conditions, the molecular ions were found to be less stable in set I isomers, and they resulted in abundant fragment ions, i.e., [M-CH(3)](+), [M-CO](+.), [M-HCO](+), [M-(CH(3),CO)](+), and [M-(CH(3),CH(2)O)](+), when compared with set II isomers. In addition, the set I isomers showed specific fragment ions corresponding to [M-OH](+) and [M-OCH(3)](+). The retro-Diels-Alder (RDA) product ion was always higher in set II isomers. The ESI mass spectra produced [M + H](+) ions, and their decomposition showed favorable loss of CH(3) radical, CH(4) and C(2)H(6) molecules in set I isomers. The set II isomers, however, showed predominant RDA product ions, and specific loss of H(2)O. The selectivity in EI and ESI was attributed to the instability of set I isomers by the presence of a gem-dimethyl group at the α-position, and it was supported by the data from model compounds without a gem-dimethyl group. Density functional theory (DFT) calculations successfully corroborated the fragmentation pathways for diagnostic ions. This study revealed the effect of a gem-dimethyl group located at the α-position to the carbonyl having aromatic/unsaturated carbon on the other side of the carbonyl group.
authors.affiliation: National Centre for Mass Spectrometry, Indian Institute of Chemical Technology, Hyderabad-500 007, India.
authors.forename: Ch Dinesh
authors.initials: ChD
authors.lastname: Kumar
authors.forename: V Naresh
authors.initials: VN
authors.lastname: Chary
authors.forename: A
authors.initials: A
authors.lastname: Dinesh
authors.forename: P S
authors.initials: PS
authors.lastname: Reddy
authors.forename: K
authors.initials: K
authors.lastname: Srinivas
authors.forename: G
authors.initials: G
authors.lastname: Gayatri
authors.forename: G N
authors.initials: GN
authors.lastname: Sastry
authors.forename: S
authors.initials: S
authors.lastname: Prabhakar
mesh_terms: D007477:Ions / Q000737:chemistry; D007536:Isomerism; D008956:Models, Chemical; D015363:Quinolones / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
chemical_list: D007477:Ions; D015363:Quinolones