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Toluene and isotoluene radical cations: alkoxymethyl substituents as a probe to study the formation, fragmentation and isomerization
Author(s) -
Harting Natascha,
Grützmacher HansFriedrich
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(199904)34:4<408::aid-jms786>3.0.co;2-j
Subject(s) - chemistry , toluene , tautomer , ketone , aldehyde , photochemistry , medicinal chemistry , isomerization , radical ion , ion , organic chemistry , catalysis
The unimolecular reactions of toluene radical cations andtautomeric isotoluene radical cations (methylenecyclohexadieneradical cations) substituted by an ROCH 2 group(R=CH 3 , C 2 H 5 , n ‐C 3 H 7 , i ‐C 3 H 7 ) were studied by mass‐analyzed ionkinetic energy (MIKE) spectrometry and collisionalactivation mass spectrometry. The alkoxymethyl‐substitutedtoluene radical cations were obtained directly by 70 eV electronionization, while the substituted isotoluene radical cations wereprepared by elimination of an aldehyde or ketone molecule fromionized bis(alkoxymethyl)benzenes. It is shown that thefragmentation reactions induced by the alkoxymethyl side‐chainin the toluene radical cations and isotoluene radical cations can beused conveniently to distinguish unambiguously between the tautomers.The substituted toluene radical cations eliminate abundantly thealkoxy group of the side‐chain as aldehyde or ketone by a wellknown fragmentation mechanism. Typical of the substituted isotolueneradical cations is an abundant elimination of alcohol from theside‐chain of metastable ions, which with the exception ofsome para ‐substituted ions surpasses the abundance ofthe loss of aldehyde/ketone. Further, the substituted tolueneradical cations typically fragment by loss of the methyl group of thearomatic ring. The analysis of the MIKE spectra reveals that<10% of the isotoluene racial cations are isomerized intothe toluene isomers if these ions are generated by energeticallyfavorable processes from the bis(alkoxymethyl)benzenes.Otherwise, extensive tautomerization between toluene andisotoluene‐like ions takes place. Thus,(substituted) toluene and isotoluene radical cations arestable species which can be detected by tandem mass spectrometry.However, the activation barrier between the tautomeric ions cannot bevery large, and energetically excited isotoluene radical cations areconverted into the more stable toluene isomer. Copyright © 1999John Wiley & Sons, Ltd.