Generation and Dissociation of RCOOCaCl2− and other Carboxylate‐Substituted Superhalogens: CO2 Capture and Implications for Structure Analysis
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abstract
AbstractCarboxylate‐substituted superhalogens of the type RCOOMX2− (M=Mg, Ca, Sr, Ba, Mn, Co, Ni, Zn; X=Cl, Br) are easily accessible in the gas phase by electrospray ionisation. Their collision‐induced dissociation (CID) characteristics have been probed by using ion‐trap and triple‐quadrupole mass analysers with particular emphasis on the behaviour of RCOOCaCl2−‐type ions. In the ion trap these appear to react readily with residual water to yield HOCaCl2− as the hydrolysis product. In the absence of water, a collision‐induced McLafferty‐type rearrangement takes over to produce HCaCl2− with the expulsion of an olefin and CO2. A brief computational analysis using the CBS‐QB3 model chemistry provides a satisfactory rationale for these observations. If complexed with MX2 (M=Mg, Ca, Sr, Ba), long‐chain unsaturated aliphatic carboxylate anions undergo various backbone cleavages upon collision. These lead to structure‐diagnostic olefin losses because the position of the double bonds remains intact. Such cleavages are absent in the bare ion RCOO−. The long‐chain ions RCOOMX2− also produce the intriguing species [CO2]MX2−.. These have been characterised by CID experiments, and theory indicates that they may be viewed as a CO2 molecule captured by the salt anion MX2−.. Finally, it is shown that the CID spectra of RCOOCaCl2− ions derived from all‐trans retinoic acid, a compound of current interest in biochemistry and medicine, show a unique structure‐diagnostic dissociation that may greatly aid its qualitative and quantitative analysis.
