title: On the origin of the methyl radical loss from deprotonated ferulic and isoferulic acids: electronic excitation of a transient structure.
abstract: Formation of radical fragments from even-electron ions is an exception to the "even-electron rule". In this work, ferulic acid (FA) and isoferulic acid (IFA) were used as the model compounds to probe the fragmentation mechanisms and the isomeric effects on homolytic cleavage. Elimination of methyl radical and CO2 are the two competing reactions observed in the CID-MS of [FA - H](-) and [IFA - H](-), of which losing methyl radical violates the "even-electron rule". The relative intensity of their product ions is significantly different, and thereby the two isomeric compounds can be differentiated by tandem MS. Theoretical calculations indicate that both the singlet-triplet gap and the excitation energy decrease in the transient structures, as the breaking C-O bond is lengthened. The methyl radical elimination has been rationalized as the intramolecular electronic excitation of a transient structure with an elongating C-O bond. The potential energy diagrams, completed by the addition of the energy barrier of the radical elimination, have provided a reasonable explanation of the different CID-MS behaviors of [FA - H](-) and [IFA - H](-).
authors.affiliation: Key Laboratory of Organosilicon Chemistry and Material Technology, Hangzhou Normal University, Hangzhou 310012, China.
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Zhang
authors.forename: Fei
authors.initials: F
authors.lastname: Li
authors.forename: Huiqing
authors.initials: H
authors.lastname: Lv
authors.forename: Yanqing
authors.initials: Y
authors.lastname: Wu
authors.forename: Gaofeng
authors.initials: G
authors.lastname: Bian
authors.forename: Kezhi
authors.initials: K
authors.lastname: Jiang
mesh_terms: D002934:Cinnamates / Q000737:chemistry*; D003373:Coumaric Acids / Q000737:chemistry*; D004583:Electrons; D007477:Ions / Q000737:chemistry; D007536:Isomerism; D008958:Models, Molecular; D011522:Protons; D053719:Tandem Mass Spectrometry; D013816:Thermodynamics
chemical_list: D002934:Cinnamates; D003373:Coumaric Acids; D007477:Ions; D011522:Protons; C004999:ferulic acid; C008180:isoferulic acid