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Gas-phase ion-molecule chemistry of chromium nitrosyl complex CpCr(NO)₂CH₃ and coulomb interaction between ions in fourier transform ion cyclotron resonance mass spectrometry Chen, Shu-Ping


This work is devoted to application and performance modifications of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR): (1) gas-phase chemistry of chromium nitrosyl; (2) Coulomb interactions between ions in ion cyclotron motion. Chromium nitrosyl CpCr(NO)2CH3 (Cp = η5-C5H5) produces a series of ions which has been observed to fifth kinetic order. The ions of CpCr(NO)2CH3 show many products in which the oxygen of the NO ligand is retained and the nitrogen is lost as part of a neutral product. An empirical method was proposed for calibrating nominal pressures of transition metal complexes to determine real rate constants. The Cr+ ions in an excited state can be quenched in collision with moleculesN2,H2,H20, NH3, and CH4. The ground state Cr+ ions prefer charge transfer reactions which result in different products from those of the condensation reactions of the excited state Cr+ ions. H2, H2O, NH3 and CH4 also can react with the nitrosyl ligand in CpCr(NO)2CH3 to produce the ammine ligand. A point model and a line model, which correspond more closely to physical reality than some prior models, are proposed to account for the Coulomb-induced frequency shifts observed in FT-ICR. The first model consists of two point charges which undergo cyclotron orbits with the same orbit centers at their respective frequencies. The model predicts that each excited cyclotron motion should induce a negative frequency shift in the other’s cyclotron motion. The line model, created by extension of the point model, gives rise to a position-dependent frequency shift which is synonymous with inhomogeneous Coulomb broadening. A disk model for the Coulomb shifting, unlike the point model, has a finite average radial Coulomb force. It consists of a uniformly charged disk, whose excited cyclotron motion is perturbed by a second excited, uniformly charged disk. The average radial force is found to be a function of ratio of the cyclotron radius to the disk radius. This allows characterization in terms of an “apparent Coulomb distance”. This distance, when applied in a charged-cylinder model, accounts for Coulomb-induced line broadening and frequency shifting. The charged-cylinder model agrees with experiments. Absolute mass calibration of FT-ICR spectra is enhanced by the Coulomb correction. The charged-point and charged-disk models are valid when the Coulomb interaction is much smaller than the Lorentz force. When the Coulomb force is comparable to the Lorentz force, a strong coupling interaction arises. A strong coupling Coulomb interaction for small spatial separations between two ion species is developed using a Taylor’s expansion method based on two tetragonal ion clouds. Under strong coupling, the two ion mass peaks will merge.

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