No abstract

The solvation energies of the pyridine*+ radical cation by 1-4 H2O molecules were determined by equilibrium measurements in a drift cell. The binding energies of the pyridine*+(H2O)n clusters are similar to the binding energies of protonated pyridine-water clusters, (C5H5NH+)(H2O)n, which involve NH+..OH2 bonds and different from those of the solvated benzene radical cation-water clusters, C6H6*+(H2O)n, which involve CHdelta+..OH2 bonds. These relations indicate that the observed pyridine*+ ions have the distonic *C5H4NH+ structures that can form NH+..OH2 bonds. The observed thermochemistry and ab initio calculations show that these bonds are not affected significantly by an unpaired electron at another site of the ion. Similar observations also identify the 2-fluoropyridine*+ distonic ion. The distonic structure is also consistent with the reactivity of pyridine*+ in H atom transfer, intra-cluster proton transfer and deprotonation reactions. The results present the first measured stepwise solvation energies of distonic ions, and demonstrate that cluster thermochemistry can identify distonic structures.

Differential mobility spectrometry or field asymmetric waveform ion mobility spectrometry (FAIMS) is gaining broad acceptance for analyses of gas-phase ions, especially in conjunction with largely orthogonal separation methods such as mass spectrometry (MS) and/or conventional (drift tube) ion mobility spectrometry. In FAIMS, ions are filtered while passing through a gap between two electrodes that may have planar or curved (in particular, cylindrical) geometry. Despite substantial inherent advantages of the planar configuration and its near-universal adoption in current stand-alone FAIMS devices, commercial FAIMS/MS systems have employed curved FAIMS geometries that can be more effectively interfaced to MS. Here we report a new planar (p-) FAIMS design with slit-shaped entrance and exit apertures that substantially increase ion transmission in and out of the analyzer. The entrance slit interface effectively couples p-FAIMS to multi-emitter electrospray ionization (ESI) sources, improving greatly the ion current introduced to the device and allowing liquid flow rates up to ϳ50 L/min. The exit slit interface increases the transmission of ribbon-shaped ion beams output by the p-FAIMS to downstream stages such as a MS. Overall, the ion signal in ESI/FAIMS/MS analyses increases by over an order of magnitude without affecting FAIMS resolution. (J Am Soc Mass Spectrom 2009, 20, 1768 -1774) © 2009 American Society for Mass Spectrometry F ield asymmetric waveform ion mobility spectrometry (FAIMS) is increasingly recognized as a powerful separation technique that can process ions in gases in milliseconds at ambient conditions [1][2][3][4][5][6] and is readily combined with other common analytical tools, including liquid chromatography (LC) [7][8][9] and/or mass spectrometry (MS) [9 -13]. The operation of FAIMS has been understood fundamentally [2,5,14] and modeled mathematically and in simulations [10,[15][16][17]. Briefly, the mobility of an ion in a gas mainly depends on E/N, where E is the electric field intensity and N is the gas number density [18]:where K(0) is the mobility in zero-field limit and f(E/N) is a function of field strength. This allows sorting ions by the difference of their mobility at high and low E/N [that is, the form of f(E/N)], regardless of K(0). On the contrary, the conventional drift tube ion mobility spectrometry (DTIMS) separates ions by absolute mobility, and the field is commonly weak enough to equate K(E/N) to K(0). In FAIMS, ions experience alternating strong and weak fields of opposite polarity, generated by a periodic asymmetric waveform [1] applied in the gap between two electrodes in the direction perpendicular to a gas flow. Ions carried through FAIMS by the flow are displaced toward one of the electrodes in each waveform cycle and would eventually be lost. To prevent this for species with a specific mobility difference between high and low E/N and keep them stable in the gap, the net displacement induced by the waveform is offset by adding to it an appropriate dc potential (the com...

The application of ion mobility to separate the electronic states of first-, second-, and third-row transition metal cations generated by the laser vaporization/ionization (LVI) technique is presented. The mobility measurements for most of the laser-generated transition metal cations reveal the presence of two or three mobility peaks that correspond to ground and excited states of different electronic configurations. The similarity of the measured reduced mobilities for the metal cations generated by LVI, electron impact, and glow discharge ion sources indicates that the same electronic configurations are produced regardless of the ion source. However, in comparison with electron impact of volatile organometallic compounds, the LVI populates fewer excited states due to the thermal nature of the process. Significant contributions to the production and populations of excited states of Ni+, Nb+, and Pt+ cations have been observed in the presence of argon during the LVI process and attributed to the Penning ionization mechanism. The origin of the mobility difference between the ground and the excited states is mainly due to the different interaction with helium. The ratio of the reduced mobilities of the excited and ground states decreases as one goes from the first- to the second- to the third-row transition metal cations. This trend is attributed to the ion size, which increases in the order 6sd(n-1) > 5sd(n-1) > 4sd(n-1). This work helps to understand the mechanisms by which transition metal cations react in the gas phase by identifying the ground and excited states that can be responsible for their reactivity.

The binding energies of the first 5 H2O molecules to c-C3H3+ were determined by equilibrium measurements. The measured binding energies of the hydrated clusters of 9-12 kcal/mol are typical of carbon-based CH+...X hydrogen bonds. The ion solvation with the more polar CH3CN molecules results in stronger bonds consistent with the increased ion-dipole interaction. Ab initio calculations show that the lowest energy isomer of the c-C3H3+(H2O)4 cluster consists of a cyclic water tetramer interacting with the c-C3H3+ ion, which suggests the presence of orientational restraint of the water molecules consistent with the observed large entropy loss. The c-C3H3+ ion is deprotonated by 3 or more H2O molecules, driven energetically by the association of the solvent molecules to form strongly hydrogen bonded (H2O)nH+ clusters. The kinetics of the associative proton transfer (APT) reaction C3H3+ + nH2O --> (H2O)nH+ + C3H2* exhibits an unusually steep negative temperature coefficient of k = cT(-63+/-4) (or activation energy of -37 +/- 1 kcal mol(-1)). The behavior of the C3H3+/water system is exactly analogous to the benzene+*/water system, suggesting that the mechanism, kinetics and large negative temperature coefficients may be general to multibody APT reactions. These reactions can become fast at low temperatures, allowing ionized polycyclic aromatics to initiate ice formation in cold astrochemical environments.