Generation and Dissociation of RCOOCaCl2− and other Carboxylate‐Substituted Superhalogens: CO2 Capture and Implications for Structure Analysis

Carboxylate-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.