Communication pubs.acs.org/jchemeduc
Measuring Gas-Phase Basicities Relative to the Lithium Cation by Mass Spectrometry: A Physical Chemistry Experiment Jean-François Gal,*,† Charly Mayeux,‡ Lionel Massi,† Mohamed Major,§ Laurence Charles,§ and Tõiv Haljasorg‡ †
Institut de Chimie de Nice (CNRS UMR 7272), Université de Nice-Sophia Antipolis, 06108 Nice Cedex 2, France Institut de Chimie Radicalaire (CNRS UMR 7273), Aix-Marseille University, 13397 Marseille Cedex 20, France ‡ Institute of Chemistry, University of Tartu, 50411Tartu, Estonia §
S Supporting Information *
ABSTRACT: The determination of the gas-phase basicity of organic bases (ligands) toward the lithium cation may be achieved by mass spectrometry, as described for protonic gas-phase basicity by Sunderlin et al. (J. Chem. Educ. 2005, 82, 1071−1073). The lithium cation-bound dimers, generated by electrospray ionization, are dissociated by collision-induced dissociation in a tandem mass spectrometer such as triple quadrupole or quadrupole−time-of-flight instruments. An ion trap instrument is not recommended because of solvent adduct formation from Li+ containing fragments. The relative intensities of the ionic fragments are treated along the kinetic method. This experiment is another illustration of the possibilities offered by mass spectrometry for the determination of thermochemical data on gas-phase species. In this experiment, students have the opportunity to identify and manipulate metal−cation adducts which are frequently observed in mass spectrometry. Additional pedagogical interest may arise from discussion of the bonding with Li+, the structural effects on Li+ basicity, and the specific applications of the lithium ion, in particular its role in energy storage. KEYWORDS: Graduate Education/Research, Upper-Division Undergraduate, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Acids/Bases, Coordination Compounds, Mass Spectrometry
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compared with the more covalent character of the proton binding. Adducts of alkali metal ions are present in several types of mass spectra, and discussion of the basic features of their formation is appropriate in a MS class. Other attractive examples may be found in the central role played by the lithium cation in electrochemistry with the advent of lithium ion batteries9 and in the catalytic effect of the weakly solvated (“naked”) lithium cation.10 In this communication, we propose to apply the kinetic method to the determination of relative lithium cation basicities, as an extension of the lab work described by Sunderlin et al.4
ass spectrometry (MS) is largely used as a technique for structural studies as well as for analytical purposes. It is less well understood by students that MS is also valuable for the determination of thermochemical data on ions and neutral molecules in the gas phase.1 For example, thousands of gasphase proton basicities and affinities,2 enthalpies of formation of ions, and other data pertaining to gas-phase ions3 have mostly been determined by using MS-based techniques. In 2005, Sunderlin et al. published a laboratory experiment4 dedicated to the determination of the “protonic” (Brønsted) gas-phase basicity of a series of amino acids. The measurements were based on the dissociation under collisional activation of proton-bound amino acid dimers (A1)H+(A2), in which A1 and A2 are amino acids. According to the kinetic method,1,5 the relative intensity of the so-produced fragments (A1)H+ and (A2)H+ is related to the relative basicities of the two amino acids. The attractive educational aspects of this approach are well depicted in the Sunderlin article, which also mentions applications to binding energetics of adduct formation with metal cations. Noteworthy, a scale of Li+ binding energies was established for 15 amino acids using the kinetic method.6 The gas-phase basicity relative to alkali metal cations, mostly Li+, Na+, and K+, has been thoroughly investigated.7 The study of Li+ adducts formation is interesting due to the specificity of the bonding between alkali metal ions and organic bases, which is considered to be largely electrostatic,8 as © 2012 American Chemical Society and Division of Chemical Education, Inc.
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PRINCIPLE AND TERMINOLOGY The energetic properties of the bond formed between Li+ and a Lewis base (or ligand) L may be characterized by the standard enthalpy ΔH° and the standard Gibbs energy ΔG° corresponding to the dissociation L−Li+ → L + Li+
(1)
These thermodynamic parameters are called lithium cation affinity and basicity, LiCA and LiCB, respectively, by analogy with proton affinity, PA, and gas-phase basicity, GB. Relative basicities are accessible using the kinetic method. When a Published: August 27, 2012 1476
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cation bound dimer [L1−Li+−L2] (generated by electrospray ionization, ESI) is collided with an inert gas (collision-induced dissociation, CID), two fragment ions [L1−Li+] and [L2−Li+] are formed with unimolecular dissociation rate constants k1 and k2, with k1/k2 equal to the intensity ratio I[L1−Li+]/I[L2−Li+] (Scheme 1).
Table 1. Student Data for the Kinetic Li+ Basicities and the Corresponding Experimental LiCB for the Four Ligands
Scheme 1. Dissociation of a Li+ Cation Bound Dimer into Neutral Species and 1:1 Adducts
Liganda
ln(I[L1−Li+]/I[TMP−Li+])b
LiCB(373 K)/(kJ mol−1)
TEP TMP DMA DMF
2.241 0.000 −2.435 −5.216
188.4 “unknown”c 179.1 173.7
a
The four ligands are N,N-dimethylformamide (DMF), N,Ndimethylacetamide (DMA), trimethylphosphate (TMP), and triethylphosphate (TEP). bTMP is the “unknown” in this set, and the ln(I[L1−Li+]/I[L2−Li+]) values are referenced accordingly. cExperimental value: 183.1 kJ mol−1. Measured value: 184.0 kJ mol−1 (calculated from the intercept of the calibration equation, cf. Supporting Information).
This ratio is related to the relative basicity through the basic equation of the kinetic method: ln(I[L1−Li+]/I[L2−Li+])
which offers attracting features already described by Sunderlin et al.4 An additional pedagogical benefit of the study of adducts of alkali metal ions is that they are omnipresent in ESI spectra, and their observation will familiarize students with the problems that may be faced in the interpretation of mass spectra.
1 ≈ (LiCB[L] − LiCB[L2])/RTeff
= ΔLiCB/RTeff
(2)
where the effective temperature Teff is an adjustable parameter reflecting the degree of excitation of the dissociating dimer and R is the gas constant. A more detailed treatment of the kinetic method is included in the Supporting Information. The experiment consists of measuring ln(I[L1−Li+]/I[L2−Li+]) with different pairs of L1 and L2, connecting the values and calibrating them using known LiCB values.7 The complete procedure, including instructions for student lab work, is described in the Supporting Information.
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ASSOCIATED CONTENT
S Supporting Information *
Instructor notes, outline of procedure, optimization method, and references. This material is available via the Internet at http://pubs.acs.org.
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EXPERIMENTAL CONSIDERATIONS Basic knowledge of mass spectrometric techniques and ESI is required. As explained in the Supporting Information, the CID measurements should be done on triple quadrupole or quadrupole−time-of-flight instruments equipped with an ESI source (ion trap instruments are not recommended, because adduct formation with solvent vapors may occur). Hazards are associated with the manipulation of methanol and other lab chemicals. To save both supervisor and instrument time, it is advisable to work with a small homogeneous group of 2−3 students. Typical student lab time may be estimated as about a half-day shift: preparation of ligands and lithium salt solutions and their mixtures (concentrations are noncritical) ∼30 min; mass spectrometry ∼2 h; data transcription, calculations ∼1 h. The actual experiments were performed on a set of four ligands (N,N-dimethylformamide, N,N-dimethylacetamide, trimethylphosphate, triethylphosphate) selected for their relatively close basicity toward Li+.7 One ligand is arbitrarily chosen as the “unknown” to be determined. As example, a series of ln(I[L1− Li+]/I[L 2−Li+]) measurements by a student, with the corresponding LiCB, is shown in Table 1. Calculated LiCB of unknowns are in good agreement with tabulated values; see footnote c of Table 1. The experiment was qualified on two different instruments with consistent results. Other pairs of ligands having sufficiently close basicities may be tested. If time permits, structural effect on basicity and comparison with other alkali metal cations may be discussed, as suggested in the Supporting Information.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS L.C. acknowledges support from Spectropole (Analytical Facility, Aix-Marseille University), for giving access to instruments purchased with European Funding (FEDER OBJ21423341). We are grateful to Ivo Leito, University of Tartu, Estonia, for sharing test results, and to Christian Laurence, University of Nantes, France, for his suggestions.
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REFERENCES
(1) Ervin, K. M. Chem. Rev. 2001, 101, 391−444; Chem. Rev. 2002, 102, 855 additions and corrections. (2) Hunter, E. P.; Lias, S. G. J. Phys. Chem. Ref. Data 1998, 27, 413− 656. (3) NIST Chemistry WebBook; NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD; http://webbook.nist. gov (accessed Aug 2012). (4) Sunderlin, L. S.; Ryzhov, V.; Keller, L. M. M.; Gaillard, E. R. J. Chem. Educ. 2005, 82, 1071−1073. (5) Cooks, R. G.; Patrick, J. S.; Kotiaho, T.; McLuckey, S. Mass Spectrom. Rev. 1994, 13, 287−339. (6) Feng, W. Y.; Gronert, S.; Lebrilla, C. J. Phys. Chem. A 2003, 107, 405−410. (7) Laurence, C.; Gal, J.-F. Lewis Basicity and Affinity Scales: Data and Measurement; Wiley: Chichester, U.K., 2010. (8) Drago, R. S.; Ferris, D. C.; Wong, N. J. Am. Chem. Soc. 1990, 112, 8953−8961.
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CONCLUSIONS The measurement of basicities relative to Li+ constitutes an interesting extension of the application of the kinetic method, 1477
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(9) (a) Treptow, R. S. J. Chem. Educ. 2003, 80, 1015−1120; J. Chem. Educ. 2003, 80, 1383 correction. (b) Collins, S. J. Chem. Educ. 2010, 87, 1017−1018. (10) (a) Moss, S.; King, B. T.; de Meijere, A.; Kozhushkov, S. I.; Eaton, P. E.; Michl, J. Org. Lett. 2001, 3, 2375−2377. (b) Vyakaranam, K.; Barbour, J. B.; Michl, J. J. Am. Chem. Soc. 2006, 128, 5610−5611. (c) Volkis, V.; Mei, H.; Shoemaker, R. K.; Michl, J. J. Am. Chem. Soc. 2009, 131, 3132−3133.
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