In the Laboratory
Measurement of the Isotopic Ratio of by 1H NMR
10B/11B
in NaBH4
W
Murray Zanger* and Guillermo Moyna Department of Chemistry & Biochemistry, University of the Sciences in Philadelphia, Philadelphia, PA 19104-4495; *
[email protected] The experiment described in this article employs nuclear magnetic resonance (NMR) spectroscopy in a novel way to determine the isotopic ratio between 10B and 11B. Even though it is unlikely that this method could be applied to any other nuclei, it does provide an unusual and relatively simple means for an undergraduate student to accurately measure the distribution of the two isotopes of boron. Several features make this experiment atypical of most NMR studies. The first is that while interest is primarily in the two boron nuclei, their effects on the protons in borohydride are used as an indirect observation probe. Second, when NMR experiments are recorded and discussed in a teaching environment, it is usually to observe nuclei with nuclear spin quantum number, I, of 1兾2. Thus, if multiplicities are observed they are due to the presence of one or more neighboring magnetically-active nuclei, and in the simplest cases, the “2In + 1 rule” applies, where n is the number of nuclei. An example is a freely rotating ethyl group (CH3CH2⫺X), which appears as a triplet for the methyl group protons (split by two neighboring methylene protons), and a quartet for the methylene protons (split by three equivalent methyl protons). In this experiment, however, the nuclei splitting the proton signals in BH4− are 10B and 11B, which have nuclear spins of 3 and 3兾2, respectively. Therefore, and as discussed in more detail below, the 10B nucleus will split the protons directly attached to it into an heptet, while the 11B nucleus will produce a quartet. Ordinarily, nuclei with I values greater than 1兾2 are avoided for a number of reasons. As mentioned above, these nuclei give rise to multiplicity patterns that are sometimes cumbersome to analyze. Moreover, most nuclei with high spin quantum number have significant quadrupole moments, and their NMR signals as well as those arising from nuclei directly attached to them are usually broadened due to quadrupolar effects (1). Table 1. Properties of Selected Magnetically-Active Nuclei
Isotope 1
H
13
C
31
P
19 10 11
F
B B
Natural Gyromaga Abundance netic Ratio (%)
Relative Sensitivityb
Larmor Frequencyc
99.99
1.00 x 10+00
400.00
6.7282
1.07
1.70 x 10᎑04
100.58
1/2 10.8394
100.00
6.66 x 10᎑02
161.92
1/2 25.1623
100.00
8.33 x 10᎑01
376.38
19.90
3.95 x 10
᎑03
42.98
1.32 x 10
᎑01
128.34
I
1/2 26.7522 1/2
3 3/2
2.8747 8.5847
80.10
a
Gyromagnetic ratios are in units of 107 rad s᎑1 T᎑1.
b
Relative sensitivities with respect to 1H.
c
Larmor frequencies in MHz at a magnetic field of 9.4 T.
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However, neither of these problems are encountered in this experiment. First, the two overlapping multiplets observed in the 1H spectrum of NaBH4 are well resolved and can be analyzed easily. Second, and despite the fact that both 10 B and 11B have relatively large quadrupole moments, quadrupolar effects are absent as a result of the tetrahedral symmetry of BH4− and sharp signals are obtained. It is also worthy of note that while 402 articles on NMR have been published in this Journal since 1957 (2), only two involved boron (3, 4). In addition, their publication in the 1960s and 1970s preceded the advent of high field NMR spectrometers. Rationale NMR is typically introduced in organic chemistry, where its powerful structure-elucidation capabilities are used. In general chemistry, the subject is not usually discussed, or if mentioned, its medical application, magnetic resonance imaging (MRI), is briefly described. Since NMR is arguably the instrumental technique that has the widest utility across all branches of chemistry, it would seem logical to expose students as early as possible to the elements of NMR, which can add to their knowledge of chemistry. In preparing for, and carrying out this experiment, the student is exposed to the fundamentals of NMR without becoming enmeshed in the complexities of structure determination or spectral interpretation of organic molecules. Although this application of NMR has limited utility for other isotopic mixtures, it does illustrate an unusual example of the broad range of uses for which NMR has demonstrated its versatility. Additionally, and depending on class size, students can get hands-on experience in the use of an NMR spectrometer, the basics of sample preparation, and the general aspects of spectral processing. It has been found that a one-hour lecture or prelaboratory presentation on the fundamentals of NMR enables freshman-level students to fully appreciate the experiment. A class of 20 to 25 students working in groups of 4 or 5 students can successfully complete the experiment in a single three-hour laboratory period. Background When NMR is introduced to a class in organic chemistry the nuclei discussed are, in order of importance, 1H, 13C, 31 P, and 19F. This is because these are the elements that occur in organic molecules, many of biological interest, and because these isotopes possess magnetic properties that allow them to be studied easily using NMR. All the nuclei cited possess a nuclear spin of 1兾2. Table 1 provides much of the information needed to understand how these and other nuclei behave when they are observed in an NMR spectrometer. A more complete discussion of the NMR phenomenon, as well
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In the Laboratory
as detailed definitions of the parameters presented in this table, are given in the Supplemental Material.W The experiment is carried out by observing the proton NMR spectrum of sodium borohydride. As seen in Table 1, the two isotopes of boron would be observed at widely differing frequencies (128.34 and 42.98 MHz), so it is not possible to record spectra for both isotopes in a single one-dimensional (1D) NMR experiment. It is important to note that in 1D NMR experiments only one nucleus is observed. Both 10B and 11B NMR have been studied separately (3, 4), but it is only in coupling to a common nucleus, for example, 1H, that the effects of both nuclei can be observed simultaneously in a single spectrum. The boron isotopes have different natural abundances and sensitivities in NMR, but their only effects on 1H nuclei are based on their spin quantum number, which dictates the multiplicities of the peaks in the 1H spectra and the relative contribution of each isotope to the 1H signal. The isotope 10B, which has I = 3, splits the coupled 1H nucleus into an heptet (2 × 3 × 1 + 1), while 11B, with I = 3兾2, produces a quartet (2 × 3兾2 × 1 + 1). Both multiplets have identical 1 H chemical shifts of ᎑0.20 ppm. However, due to the markedly different coupling constants between a proton and the two boron isotopes (1J10BH = 27.1 Hz and 1J11BH = 80.5 Hz), all 11 peaks are well-resolved in a high-resolution spectrum and their individual integrations are straightforward. By comparing the sum of the areas of the septet (10B) to the combined peak areas of the quartet (11B), the isotopic distribution between the two boron isotopes can be estimated easily and accurately (Figure 1). The Experiment A 5% solution of sodium borohydride (NaBH4) in deuterium oxide (D2O or 2H2O) is prepared, and a 0.7 mL aliquot is placed in an NMR tube and closed with a vented cap. D2O is used in place of water for two reasons for which the students should be made aware. First, the intensity of the signal from the water protons would otherwise overwhelm the peaks that arise from the borohydride sample. Second, most modern NMR spectrometers require deuterated solvents for field-frequency lock. It should also be noted that NaBH4 will react with D2O producing deuterated boric acid, sodium hydroxide, and hydrogen gas: NaBH4 + D2O BH3 1 B H + 3D O 2 2 2 6
NaBH3 + 4D2O
BH3 + NaOD + HD 1 B H 2 2 6
B(OD)3 + 3HD B(OD)3 + NaOD + 4HD
The initial hydrolysis step, however, is both rate-limiting and slow, and no significant signals are observed for the short-lived borane (BH3) or diborane (B2H6) intermediates. The spectrum of the NaBH4 sample is obtained on a highresolution NMR instrument along with the integrals for each peak of the multiplet at ᎑0.20 ppm (Figure 1). The individual areas of the quartet are added together as are those of the www.JCE.DivCHED.org
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Figure1. 1H NMR spectrum of sodium borohydride (NaBH4) in deuterium oxide (D2O) recorded at (A) 400 MHz and (B) 90 MHz. Integration areas are shown next to the signals. The four large peaks are the splitting from the 11B and the seven small peaks are the splitting from the 10B.
septet. From these totals, the relative percentages for each isotope are calculated. In this experiment, useful spectra were obtained on either 90 MHz or 400 MHz instruments. Students can receive either paper copies of the integrated spectrum or their data can be saved for offline processing. Hazards While only minute quantities of NaBH4 are employed in this laboratory experiment, care should be exercised when handling this reducing agent. Lab instructors also have to stress that only vented caps be used, otherwise the pressure buildup owing to hydrogen gas evolution inside the NMR tube could cause the caps to pop or the tubes to break inside the NMR probe. Results Typical results using an Anasazi Eft-90 90 MHz spectrometer gave an isotopic distribution of 21.14% for 10B and 78.86% for 11B. Using a Bruker AVANCE 400 400 MHz spectrometer values of 17.88% for 10B and 82.12% for 11B were obtained. These results compare well with the known ratio of 19.90% 10B to 80.10% 11B (5).
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In the Laboratory W
Supplemental Material
An expanded NMR background section and instructions for this experiment are available in this issue of JCE Online.
Literature Cited
Acknowledgments This work was supported by a matching grant from the National Science Foundation used towards the acquisition of the Bruker AVANCE 400 NMR spectrometer (DUE9952264). We greatly appreciate the help of Alfonso R. Gennaro in suggesting several changes that improved the clarity of the manuscript. We also acknowledge the reviewers, whose insightful comments and criticisms led to a much improved article. Finally, we wish to thank the Principles of
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Chemistry Laboratory II classes of 2001 and 2002, who willingly served as “guinea pigs” in the implementation of this laboratory experiment and collected all the data employed in the preparation of this report.
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1. Claridge, T. D. W. High-Resolution NMR Techniques in Organic Chemistry; Pergamon Press: Oxford, 1999. 2. JCE Online Journal Search. http://www.jce.divched.org/Journal/ Search/index.html (accessed Jun 2005). 3. Eaton, G. R. J. Chem. Educ. 1969, 46, 547. 4. Smith, W. L. J. Chem. Educ. 1977, 54, 469. 5. Coursey, J. S.; Schwab, D. J.; Dragoset, R. A. NIST Atomic Weights and Isotopic Compositions Home Page http:// physics.nist.gov/Comp (accessed Jun 2005).
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