Fundamentals of surface-induced dissociation Ion-surface interactions can be used to obtain analytical information about either gas-phase ions or surface chemistry. Such collisions are already well established for probing the surface (with secondary ion MS, for example), and these interactions are becoming more useful for probing gasphase ions via surface-induced dissociation (SID) tandem MS, but little is known about the fundamentals of these reactions. In an effort to fill in the knowledge gaps, Steven L. Bernasek and co-workers at Princeton University, American Cyanamid, and Virginia Commonwealth University used thermodynamic and experimental data to examine the sputtered-ion mechanisms of the alkylation of benzene to C7H7 during SIDMS/MS.
Sputtered radical and concerted mechanisms for the formation of C7Hf have been proposed in the past because they can be supported without observation of charged intermediates. The authors, however, noticed that the sputtered proton signal accounted for 20-40% of all ions observed and began to reconsider the role of ion sputtering in associative ion-surface reactions. They studied the formation of C7H7 by the reaction of neutralized benzene molecular ions with the potentially sputtered ions CH3, C2H3, C2U^, C3H7, and C4Hg. A gas-phase model with methane and isobutane chemical ionization of benzene was used to test the possible intermediates. C7H7 is a major product observed in both CI spectra; and stable adducts of CH£, C2HJ, C2H£, C3H^, and C4H| were all observed. Experiments with 2H6- and 13 C6-labeled benzene showed that
IR method for combinatorial libraries
monitoring resin-based reactions directly on the solid-phase support. They substituted deuterium-labeled reactants, which greatly enhanced the functional group selectivity because the C-D Fast, nondestructive analytical techniques stretching absorbance frequencies fall are key to the development of solid-phase into the 2300- to 2200-cm"1 region, which combinatorial synthetic methods. Unfortuis largely free from spectral interfernately, many powerful analytical techences. Quantitative analysis was carried niques—TLC, HPLC, GC, and MS—require the compound to be cleaved from the out with a K-Matrix calibration using first-derivative spectra, and the polystypolymeric resin support and are inherently rene derivative absorbance difference destructive. In addition, an incomplete cleavage reaction can lead to misinterpreta- was used as an internal pathlength reference to normalize for bead curvature and tion of a high-yielding synthetic reaction. inconsistent bead diameters. Keith Russell, Don E. Pivonka, and co-workers at Zeneca Pharmaceuticals IR was also used to monitor deuterated developed IR microspectroscopy as an protecting groups in peptide synthesis, alternative nondestructive technique for particularly the Af-(fluorenylmethoxycarbonyl) (Fmoc) and tertbutyloxycarbonyl (Boc) protecting groups. Unless the C-D IR absorbance is independent of the environment, IR use is limited because calibrations would be necessary for each reaction. To determine if a single calibration is possible, the authors prepared compounds in which the Boc-d9 group was present in different chemical environments. They showed that matrix effects had only a negligible effect on Boc-d9 quantitation by IR C-D derivative IR spectra for 0, 20, 50, 80, and 100% absorbance. (J. Am. Chem. deuteration of the reaction product of (aminomethyl) Soc. 1996,118, 7941-45) polystyrene resin and benzoyl chloride/benzoyl-6 chloride. 5
each of the adducts contained the entire labeled benzene molecule. The authors suggested that a likely route to C7H7 formation was the thermodynamically favorable C6Hg+ + hydrocarbon surface overlayer -> C6H6 + C3H5 -»C7H7 + C2H4. (J. Am. Chem. Soc. 1996,118, 8375-80)
SID spectrum of benzene molecular ions undergoing 30
