Fluorohydrogenate Cluster Ions in the Gas Phase: Electrospray Ionization Mass Spectrometry of the [1-Ethyl-3-methylimidazolium+][F(HF)2.3–] Ionic Liquid

New Insights into the Stability of Anhydrous 2H-Imidazolium Fluoride and its High Dissolution Capability toward a Strongly Hydrogen-Bonded Compound

Fluorides have been widely applied in pharmaceutical, medicinal, and materials science as well as in fine chemical manufacturing. The performance of fluorides, however, can be markedly affected by the water content. One previous study (Maiti, A.; et al. Phys. Chem. Chem. Phys. 2008, 10, 5050) suggested that anhydrous 1,3-dimethylimidazolium fluoride ([DMIm]F) was unstable since the fluoride undergoes a self-decomposition reaction. Herein we first show quantum-chemical calculation evidence that although gas-phase [DMIm]F is unstable, the bulk phase of anhydrous [DMIm]F is quite stable. We then demonstrate the successful synthesis of the anhydrous [DMIm]F compound via the reaction between 1,3-dimethylimidazolium iodide and silver fluoride. Importantly, we find that anhydrous [DMIm]F possesses a high dissolution capability toward 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), although it is known that TATB is hardly dissolved in many common organic solvents. Our Born-Oppenheimer molecular dynamics (BOMD) simulations further show that the high dissolving ability of anhydrous [DMIm]F toward TATB can be attributed to the chemical reaction between the F^- anion and the TATB molecules, which disrupts the strong hydrogen-bonding interaction among the TATB molecules. Alternatively, water molecules in hydrous [DMIm]F tend to form a hydration layer around the F^- anion, thereby preventing F^- from reacting with the TATB molecule. This result explains why TATB is barely dissolved in hydrous [DMIm]F.

Production of Gas-Phase Uranium Fluoroanions Via Solubilization of Uranium Oxides in the [1-Ethyl-3-Methylimidazolium]+[F(HF)2.3]- Ionic Liquid

A new methodology for gas-phase uranium ion formation is described in which UO₂ is dissolved in neat N-ethyl,N'-methylimidazolium fluorohydrogenate ionic liquid [EMIm⁺][F(HF)_2.3^-], yielding a blue-green solution. The solution was diluted with acetonitrile and then analyzed by electrospray ionization mass spectrometry. UF₆^- (a U(V) species) was observed at m/z = 352, and other than cluster ions derived from the ionic liquid, nothing else was observed. When the sample was analyzed using infusion desorption chemical ionization, UF₆^- was the base peak, and it was accompanied by a less intense UF₅^- that most likely was formed by elimination of a fluorine radical from UF₆^-. Formation of UF₆^- required dissolution of UO₂ followed by or concurrent with oxidation of uranium from the + 4 to the + 5 state and finally formation of the fluorouranate. Dissolution of UO₃ produced a bright yellow solution indicative of a U(VI) species; however, electrospray ionization did not produce abundant U-containing ions. The abundant UF₆^- provides a vehicle for accurate measurement of uranium isotopic abundances free from interference from minor isotopes of other elements and a convenient ion synthesis route that is needed gas-phase structure and reactivity studies like infrared multiphoton dissociation and ion-molecule dissociation and condensation reactions. The reactive fluorohydrogenate ionic liquid may also enable conversion of uranium in oxidic matrices into uranium fluorides that slowly oxidize to uranyl fluoride under ambient conditions, liberating the metal for facile measurement of isotope ratios without extensive chemical separations. Graphical abstract ᅟ.

Collision-Induced Dissociation Study of Strong Hydrogen-Bonded Cluster Ions Y-(HF) n (Y=F, O2) Using Atmospheric Pressure Corona Discharge Ionization Mass Spectrometry Combined with a HF Generator

Hydrogen fluoride (HF) was produced by a homemade HF generator in order to investigate the properties of strong hydrogen-bonded clusters such as (HF) _n . The HF molecules were ionized in the form of complex ions associated with the negative core ions Y^- produced by atmospheric pressure corona discharge ionization (APCDI). The use of APCDI in combination with the homemade HF generator led to the formation of negative-ion HF clusters Y^-(HF) _n (Y=F, O₂), where larger clusters with n≥4 were not detected. The mechanisms for the formation of the HF, F^-(HF) _n , and O₂^-(HF) _n species were discussed from the standpoints of the HF generator and APCDI MS. By performing energy-resolved collision-induced dissociation (CID) experiments on the cluster ions F^-(HF) _n (n=1-3), the energies for the loss of HF from F^-(HF)₃, F^-(HF)₂, and F^-(HF) were evaluated to be 1 eV or lower, 1 eV or higher, and 2 eV, respectively, on the basis of their center-of-mass energy (E_CM). These E_CM values were consistent with the values of 0.995, 1.308, and 2.048 eV, respectively, obtained by ab initio calculations. The stability of [O₂(HF) _n ]^- (n=1-4) was discussed on the basis of the bond lengths of O₂H-F^-(HF) _n and O₂^-H-F(HF) _n obtained by ab initio calculations. The calculations indicated that [O₂(HF)₄]^- separated into O₂H and F^-(HF)₃.

Water bridges anchored by a C–H⋯O hydrogen bond: the role of weak interactions in molecular solvation

Hydrogen-bonded water bridges are re-directed from a polar NH bond to a weakly activated C(2)–H bond upon N-methylation. Infrared spectra, supported by ab initio calculations, provide direct evidence of the role of the C(2)–H donor in the solvation of the imidazole ring.

Iron Fluoroanions and Their Clusters by Electrospray Ionization of a Fluorinating Ionic Liquid

Metal fluoroanions are of significant interest for fundamental structure and reactivity studies and for making isotope ratio measurements that are free from isobaric overlap. Iron fluoroanions [FeF(4)](-) and [FeF(3)](-) were generated by electrospray ionization of solutions of Fe(III) and Fe(II) with the fluorinating ionic liquid 1-ethyl-3-methylimidazolium fluorohydrogenate [EMIm](+)[F(HF)(2.3)](-). Solutions containing Fe(III) salts produce predominately uncomplexed [FeF(4)](-) in the negative ion spectrum, as do solutions containing salts of Fe(II). This behavior contrasts with that of solutions of FeCl(3) and FeCl(2) (without [EMIm](+)[F(HF)(2.3)](-)) that preserve the solution-phase oxidation state by producing the gas-phase halide complexes [FeCl(4)](-) and [FeCl(3)](-), respectively. Thus, the electrospray-[EMIm](+)[F(HF)(2.3)](-) process is oxidative with respect to Fe(II). The positive ion spectra of Fe with [EMIm](+)[F(HF)(2.3)](-) displays cluster ions having the general formula [EMIm](+) (n+1)[FeF(4)](-) n, and DFT calculations predict stable complexes, both of which substantiate the conclusion that [FeF(4)](-) is present in solution stabilized by the imidazolium cation. The negative ion ESI mass spectrum of the Fe-ionic liquid solution has a very low background in the region of the [FeF(4)](-) complex, and isotope ratios measured for both [FeF(4)](-) and adventitious [SiF(5)](-) produced values in close agreement with theoretical values; this suggests that very wide isotope ratio measurements should be attainable with good accuracy and precision when the ion formation scheme is implemented on a dedicated isotope ratio mass spectrometer.

Generation of gas-phase zirconium fluoroanions by electrospray of an ionic liquid

RATIONALE

New approaches for forming anions are sought that have strong abundance and no isobaric overlap, attributes that are compatible with the measurement of isotope ratios. Fluoroanions are particularly attractive because fluorine is monoisotopic, and thus will not have overlapping isobars with the isotope of interest. Since many elements do not have positive electron affinity values, they do not form stable negative atomic ions, and hence are not compatible with isotope ratio measurement using high sensitivity isotope ratio mass spectrometers such as accelerator mass spectrometers.

METHODS

Zirconium fluoroanions were prepared using the fluorinating ionic liquid (IL) 1-ethyl-3-methylimidazolium fluorohydrogenate, which was used to generate abundant [ZrF5](-) ions using electrospray ionization. The IL was dissolved in acetonitrile, combined with a dilute solution of either Zr(4+) or ZrO(2+), and then electrosprayed. Mass analysis and collision-induced dissociation experiments were conducted using a time-of-flight mass spectrometer. Cluster structures were predicted using density functional theory calculations.

RESULTS

The fluorohydrogenate IL solutions generated abundant [ZrF5](-) ions starting from solutions of both Zr(4+) and ZrO(2+). The mass spectra also contained IL-bearing cluster ions, whose compositions indicated the presence of [ZrF6](2-) in solution, a conclusion supported by the structural calculations. Rinsing out the zirconium-IL solution with acetonitrile decreased the IL clusters, but enhanced [ZrF5](-), which was sorbed by the polymeric electrospray supply capillary, and then released upon rinsing. This reduced the ion background in the mass spectrum.

CONCLUSIONS

The fluorohydrogenate-IL solutions are a facile way to form zirconium fluoroanions in the gas phase using electrospray. The approach has potential as a source of fluoroanions for isotope ratio measurements, which would enable high-sensitivity measurement of minor zirconium isotopes without overlapping isobars caused by the charge carrier (i.e., the monoisotopic fluorine atoms).