Fluoride affinities of fluorinated alanes
Graphical abstract
Research highlights
▶ Fluorinated aluminate anions (AlHmF4−m−) can be formed in the gas phase by ionization of diethylmethylamine-alane with molecular fluorine. ▶ Energy-resolved collision-induced dissociation is used to measure fluoride affinities of the alanes. ▶ The measured fluoride affinities agree with predictions obtained from coupled-cluster calculations with very large basis sets. ▶ Fluoride affinities reflect the extent of positive charge character on the aluminum in the alanes.
Introduction
Thermochemical properties provide fundamental insight into the nature of molecular structure and bonding [1]. There are a wide variety of commonly measured thermochemical properties such as proton affinities, gas-phase acidities, and electron and hydride affinities. Fluoride affinity (FA) is also an important thermochemical property, serving as a measure of the Lewis acidity. The measurements of FAs have been determined experimentally using a variety of techniques [2], [3], [4]. Haartz and McDaniel used ion cyclotron resonance spectroscopy to determine the relative order of the fluoride affinity to be SF4, SF5 < SO2, HCl, AsF3 < SiF4 < BF3 < PF5 < BCl3 < ASF5 [2]. Larson and McMahon related fluoride affinities to the hydrogen bond strengths of various chemical species [3], which enabled them to create a wide-range fluoride affinity scale. The scale was fixed at the low end with the fluoride affinity of H2O (23.3 kcal/mol) and at the high end with the fluoride affinity of HCO2H (45.3 kcal/mol). Energy-resolved collision-induced dissociation (CID) has led to revisions of the fluoride affinity scale allowing the addition of new values [4], [5], [6].
Despite the advancement in FA measurements, very few aluminum containing systems have been studied. In fact, few thermochemical properties are known for any aluminum containing molecules, alanes in particular. Aluminates such as aluminum hydride are important because they are commonly used as reducing agents and have been proposed as a means for hydrogen storage [7]. In this study, we determined the fluoride affinities of fluorinated alanes using a flowing afterglow-triple quadrupole instrument.
Section snippets
Instrumental description and data analysis
The fluoride affinities of fluorinated alanes were determined using a flowing afterglow-triple quadrupole mass spectrometer that has been previously described elsewhere [8]. Fluoride ions were generated by 70 eV electron ionization of F2 (5% in helium). Helium buffer gas (P = 400 mTorr) was used to carry the ions through the flow tube at a flow rate of ca. 190 STP cm−3/s. Ions were then allowed to undergo ion-molecule reactions with the neutral reagent (dimethylethylamine-alane, DMEAA) added through
Results
Fluorinated aluminate ions were generated by chemical ionization (CI) of the Lewis acid–base complex dimethylethylamine-alane, DMEAA, with F− generated from F2 as the CI reagent Eq. (4). Among the products observed are AlHmF4−m−, where m = 0–3. By changing the source conditions, more of the higher masses can be formed (m = 1, 2). The fourth ion, AlF4− could not be formed in sufficient abundance for CID studies. AlH3F− is likely generated by substitution of the dimethylethylamine in DMEAA. The more
Discussion
The experimentally measured FAs are in good agreement with the theoretical predicted values, with only the B3LYP value for AlHF2 at the edge of the assigned error limit. As noted above, the FA values are larger for alanes with more fluorine substituents, reflecting the greater Lewis acidity for these substrates. The increased Lewis acidity results from increased positive charge density on the aluminum in the alane when replacing a hydrogen atom with a more electronegative fluorine. Charge
Conclusions
Fluorination of alanes increases the fluoride affinity, and hence the Lewis acidity, by increasing the extent of positive charge character on the aluminum atom. The increase in fluoride affinity is not linear because the energies of fluorinated alanes are not linear with increased number of fluorines. The measured fluoride affinities agree well with theoretically predicted values.
Acknowledgements
This work was supported by the National Science Foundation (CHE04-54874 and CHE08-08964). Thanks also to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support. Calculations were carried out using the resources of the Center for Computational Studies of Open-Shell and Electronically Excited Species (iopenshell.usc.edu), supported by the National Science Foundation through the CRIF:CRF program.
References (23)
- et al.
The fluoride affinity of SO2
Int. J. Mass Spectrom.
(2003) - et al.
Gas-phase ion chemistry and ion thermochemistry of phenyltrifluorosilane
Int. J. Mass Spectrom.
(2003) - et al.
A tandem selected ion flow tube-triple quadrupole instrument
Int. J. Mass Spectrom. Ion Processes
(1994) - et al.
Thermochemistry of titanium(1+)-hydrocarbon bonds: translational energy dependence of the reactions of Ti+ with ethane propane, and trans-2-butene
Int. J. Mass Spectrom. Ion Processes
(1989) Toward the systematic decomposition of benzene
Angew. Chem. Int. Ed.
(2005)- et al.
Fluoride ion affinity of some Lewis acids
J. Am. Chem. Soc.
(1973) - et al.
Strong hydrogen bonding in gas-phase anions. An ion cyclotron resonance determination of fluoride binding energetics to bronsted acids from gas-phase fluoride exchange equilibria measurements
J. Am. Chem. Soc.
(1983) - et al.
Bond dissociation energies of F2− and HF2−. A gas phase experimental and G2 theoretical study
J. Phys. Chem.
(1995) - et al.
Aluminum hydride: a reversible material for hydrogen storage
Chem. Commun.
(2009) - et al.
Dipole effects on cation-pi interactions: absolute bond dissociation energies of complexes of alkali metal cations to n-methylaniline and n,n-dimethyaniline
J. Phys. Chem. A
(2008)
Translational energy dependence of Ar+ + XY → ArX+ + Y (XY = H2, D2, HD) from thermal to 30 eV cm
J. Chem. Phys.
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