Absolute configuration assignment of ortho,meta, or para isomers by mass spectrometry |
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Authors: | Luis?Alberto?B?Moraes Ad?o?A?Sabino Eduardo?C?Meurer Email author" target="_blank">Marcos?N?EberlinEmail author |
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Institution: | 1.Thomson Mass Spectrometry Laboratory, Institute of Chemistry,State University of Campinas-UNICAMP CP 6154,Campinas SP,Brazil |
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Abstract: | A general method based solely on mass spectrometric techniques for the absolute configuration assignment of ortho, meta, or para isomers of acyl nitrobenzenes and derivatives is described. Instead of comparing the mass spectra of the three intact molecules
of each positional isomer and investigating each one of the many sets of positional isomers, the method generalizes the effort
by performing structural analysis on configurationally diagnostic fragment ions that are common for a given class of compounds.
These ions must therefore retain the positional information of the parent molecules and be unequivocally distinguished. Nitrobenzoyl
cations are common and stable fragment ions of most acyl nitrobenzenes and derivatives retaining the respective ortho, meta, or para configuration of the precursor molecules. The different NO2 and CO+ ring alignments profoundly influence their collision-induced dissociation and bimolecular reactivity, and the isomeric 2-,
3-, and 4-nitrobenzoyl cations are found to be unequivocally distinguished using both approaches. Absolute ortho, meta, or para positional assignment by tandem MS of every isomeric molecule of the acyl nitrobenzene class and derivatives forming detectable
amounts of any of those diagnostic nitrobenzoyl cations is, therefore, possible. The ability to perform absolute (non-comparative)
configuration assignment using such diagnostic ions is exemplified for a single test molecule of (2R)-(−)-2-methylglycidyl
4-nitrobenzoate. The general application of this absolute MS-only method for other classes of positional isomers is discussed. |
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