The Raman and infrared spectra of a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ([C 2-4 MIM]PF 6 ) ionic liquids have been recorded and analyzed using density functional theory (DFT) and RHF methods at the 6-311+G(2d,p) computational level. The DFT calculations reproduce the vibrational spectra of 1-ethyl-3-methyl imidazolium hexafluorophosphate [EMIM]PF 6 , 1-propyl-3-methyl imidazolium hexafluorophosphate [PMIM]PF 6 , and 1-butyl-3-methyl imidazolium hexafluorophosphate [BMIM]PF 6 using correction factors of 0.964-0.967 with correlation coefficients R 2 of 0.999. The vibrational spectra calculated at the RHF/6-311+G-(2d,p) level require a correction factor of 0.89 and a correlation coefficient R 2 of 0.999 using the fully optimized structures. The 1-alkyl-3-methyl hexafluorophosphate ionic liquids have common Raman C-H stretching frequencies that may serve as possible probes in studies of ionic liquid interactions. The DFT (B3LYP) and RHF gas-phase molecular structures of the [C 2-4 MIM]PF 6 ion pairs indicate hydrogen bonding interactions between the fluorine atoms of the PF 6anion and the C2 hydrogen on the imidazolium ring. Additional interactions are observed between PF 6and the H atoms on the adjacent alkyl side chains.

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Electrospray ionization was used to generate gas phase complexes of Ag+ with selected alpha-amino acids. Following storage (isolation without collisional activation) in an ion trap mass spectrometer, the mass spectra produced from the complexes of Ag+ with alpha-amino acids such as alanine, valine and tert-leucine contained peaks consistent with the formation of water or methanol molecule adduct ions. The same adduct ions were not present, however, in the mass spectra generated from the Ag+ complexes with phenylalanine, tyrosine and tryptophan following isolation and storage under similar conditions. For those complexes that showed reactivity, the uptake of water and methanol increased with longer storage times in the ion trap. A preliminary molecular modeling study using phenylalanine demonstrated that the aromatic ring coordinates the Ag+ ion, and the interaction between the metal ion and pi-system, in part, is assumed to prohibit the binding of water or methanol during isolation in the gas phase. This conclusion is supported by a comparison of the adduct formation by the Ag+ complexes with phenylalanine, 4-fluorophenylalanine and alpha-aminocyclohexanepropionic acid. In addition, collision induced dissociation experiments involving the Ag+ complexes of phenylalanine, tyrosine and tryptophan suggest that limiting the coordination of the Ag ion by the complexing molecule (i.e. by loss of a coordinating functional group and/or change in structure due to dissociation) results in the binding of a water or methanol molecule during storage in the ion trap. Surprisingly, the bare Ag+ ion, when trapped and stored under identical experimental conditions, formed neither adduct species, suggesting that the attachment of water or methanol may be due to interactions with a molecular orbital within the Ag+/molecule complex.

During our ongoing investigation of the formation and reactivity of gas-phase complex ions composed of Ag(I) and various R-amino acids, we discovered that the mass-to-charge ratio for the major collision-induced dissociation (CID) product generated from a binary Ag + complex with phenylalanine was consistent with the formation of an Ag + complex with an aldehyde. In this study we investigated and compared the fragmentation pathways for complexes of Ag + with phenylalanine, phenylalanine with exchangeable protium replaced with deuterium, phenylalanine with the carboxylic acid group labeled with 13 C, and phenylalanine with the benzylic group labeled with deuterium. The reaction pathways were determined using multidimensional dissociation steps in an ion-trap mass spectrometer. The dissociation experiments provide clear evidence for the formation of several novel product species, including the Ag + complex with phenylacetaldehyde, as well as the formation of an Ag + complex with either a benzyl carbene or styrene. These dissociation products are markedly different from those observed following the fragmentation of other transition and alkali metal adducts of phenylalanine. On the basis of the dissociation of the various isotope-exchanged and -labeled versions of phenylalanine, we propose several reaction pathways that implicate the formation of an Ag + complex with an aziridinone (Rlactam), for which a peak at the correct mass-to-charge ratio was observed in the MS/MS spectrum of the (M + Ag) + ion. A comparison of the apparent reactivity toward water and methanol in the ion-trap mass spectrometer of the Ag + -containing product ions to Ag + complexes with various low-mass organic molecules provided further evidence to support the proposed formation of the aldehyde and styrene complexes with Ag + ions. For instance, the apparent reactivity of the Ag + /aldehyde product ion generated from the CID of the (M + Ag) + ion is identical to that observed for a complex produced by the electrospray ionization of a solution containing Ag + ions and neat phenylacetaldehyde. Similar results were obtained for a dissociation product ion assumed to be a complex composed of Ag + ions and styrene.