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Theoretical Ir Spectrum


The goal of this experiment is to characterize NaBD4 and NaBH4 using infrared (IR) spectroscopy and melting points. The changes in the spectra due to isotopic labeling are compared to calculated values from the simple harmonic oscillator model. In addition, the specific molecular vibration nodes are visualized using computational chemistry software.


The Lewis acid-base is defined simply: acids accept electrons, and bases donate electrons. Lewis acids and bases are important in inorganic syntheses because bonds are usually formed between an electron-deficient Lewis base and electron-rich Lewis acid in the absence of compounds that are unable to donate protons.

There are two types of boron-hydrogen bonds, in which the classical B-H bond is a 2-center-2-electron bond similar to the carbon-hydrogen bond. The non-classical bond, however, has two electrons formed between two boron atoms and one hydrogen atom, making it a 3-center-2-electron bond1. Although this bonding is unique, boron hydrides are common reagents in organic syntheses1.

The simplest boron hydride is BH3 and is a Lewis acid because it contains six electrons. This molecule is a strong acid and exists in equilibrium with B2H62. Lewis bases react with BH3 to form an adduct. B2H6 spontaneously combusts in air, reacts with minute amounts of water, and is toxic, so NaBH4 is synthesized alternatively2.

The simple diatomic oscillator model allows scientists to understand complex nodes of vibration, the motion of atoms, and much more3. The energy of an ideal chemical bond is quantized using the equation:

where is vibrational frequency (=) and is reduced, or effective, mass3. For simple diatomic molecules, is defined as:

To calculate the IR bands of the new B-D compound, the following equation can be used:

The values are the reduced masses of B-D and B-H and is the frequency of the respective bands in the IR spectrum. Although this method is not completely accurate because only one bond is taken into consideration, it does aid in estimation of shifts in the IR spectra when labeling isotopes3. Experimental Procedure

Materials and instrumentation All of the materials were obtained from the Indiana University laboratory preparatory room and were used without further purification. The melting point was determined with the MelTemp apparatus and the residual material tested was done with Indiana University’s rotary evaporator. The IR spectra were obtained from the PerkinElmer Spectrum 100 spectrometer and all computational calculations were done using Spartan 2014 v1.1.8.

Synthesis First, a round-bottom flask was loaded with tert-butylammonium chloride (1.2997 grams) and THF (10 milliliters). After stirring, powdered NaBH4 (0.2071 grams) was added to the mixture, in which the evolution of hydrogen gas was observed. Another portion of THF (10 milliliters) was added to the reaction mixture, and then stirred at room temperature for one and a half hours. The mixture was then filtered using a suction filtration apparatus. Using the filtrate, the IR spectrum was obtained and the rest of the filtrate was added to a clean round-bottom flask and attached to the rotary evaporator to purify the residual material. Using the capillary tubes, a small portion of the residual material was used to identify the melting point.

Results and Discussion

In this experiment, the tert-butylamine adduct of BH3 and BD3 were prepared and characterized with IR spectroscopy. As shown in Figure 1, the mechanisms for these proteo- and deutero-adducts are similar. In the protium mechanism, H2 is removed to produce H3B-NH2tBu (tert-butylamine borane). However, in the deuterium mechanism, H-D is removed to obtain the same product. The tert-butylamine in both mechanisms are Bronsted acids because the molecule donates protons. However, since the BH4- and BD4- anions accept protons they are Bronsted bases. In addition, the anions donate electrons, so they are also considered Lewis bases. The hydride in BH4- is actually donating electrons, so it acts as the base instead of the boron. Tert-butylamine accepts electrons from the respective anions, so the molecule is a Lewis acid.

Figure 1. Overall reaction mechanisms for both the proteo- and deutero-adduct formations.

Figure 2 displays the LUMO energies of BH3 and BF3. BH3’s energy is much lower than BF3, which indicates that BH3 is much more stable and is a better Lewis acid. Because of the pi back bonding in BF3, it is a weaker Lewis acid. BH3 better participates in chemical reactions by accepting an electron pair. As a result of the vacant p orbital on boron, BH3 is an acid. Thus, two BH3 molecules could react with one another and produce B2H6.

Figure 2. LUMOs of BH3 and BF3 with the corresponding energy levels.

Tert-butylamine was found to have much lower highest occupied molecular orbital (HOMO) energy than trimethylamine, as shown in Figure 3. Thus, tert-butylamine cannot be ionized readily. WHY LOWER IN ENERGY?

Figure 3. HOMOs of tBuNH2 and (CH3)3N with the corresponding energy levels.

The electrons in the simplified MO diagram in Figure 4 are donated into an empty non-bonding orbital of the boron complex. BH3 has a LUMO energy of -1.78 eV and tBuNH2 has a HOMO energy of -6.25 eV. The energy difference between the HOMO and LUMO generally indicates the lowest electronic excitation possible in a molecule and is important in organic reactivity4. The lower the HOMO-LUMO energy gap, the stronger the interaction. In this case, the gap is -1.78 eV – (-6.25 eV), which produces a gap of 4.47 eV. Therefore, the interaction between these two molecules is not strong. This does, however, imply high stability4.

Figure 4. Simplified MO diagram using the LUMO of BH3 and HOMO of tBuNH2.

Two factors influence the frequency of stretching vibrations depends: the mass of the atoms and the stiffness of the bond5. In the IR spectrum of BH4, stretches are present at 2287 cm-1 and 2293 cm-1, as shown in Figure 5. However, in the IR spectrum of BD4, stretches are present at 879 cm-1, 1622 cm-1, and 1700 cm-1, as seen in Figure 6. The higher wavenumbers in BH4 indicate that there is quicker stretching frequency than in BD4, which also implies that BH4 contains stronger bonds. The broad peak seen in Figure 6 at 3425 cm-1 might be the presence of water or THF. Compared to the theoretical IR spectrum in Figure 7, sharper peaks are present in the experimental IR spectrum in Figure 5.

This could be due to contamination or excess THF. The theoretical and experimental IR spectra (Figures 6 and 8) are similar, except for the broad peak in the experimental BD4, which indicate minimal contamination and errors. The calculated values from Spartan assisted in band assignment. The ratio of BD/BH should be approximately three times the value of BH/BD. The value predicted from the simple diatomic oscillator model and the observed value in the experimental spectra demonstrate that the theoretical result is in agreement with the experimental result, as shown in Figure 9.

Figure 5. Experimental IR spectrum of BH4. Stretches are present at 2287 cm-1 and 2293 cm-1.

Figure 6. Experimental IR spectrum of BD4. Stretches are present at 879 cm-1, 1622 cm-1, and 1700 cm-1.

Figure 7. Theoretical IR spectrum of BH4.

Figure 8. Theoretical IR spectrum of BD4.

Figure 9. Equations and calculations for the simple diatomic oscillator model.

The melting point of BH4 was found to be 104C, whereas the melting point for BD4 was found to be 54C. Since deuterium is heavier, the melting point for BD4 was expected to be higher. However, since BH4 is more stable its melting point could be higher than BD4. The 50C difference in melting points are most likely a result of contamination of either one or both the residues.


Orbital energies, bond stretching, melting points, and the simple diatomic oscillator model collectively demonstrate the strength of the B-H bond in comparison to other bonds, such as B-F and B-D. The non-classical boron-hydrogen bonds reveal unique properties that demonstrate the usefulness of Lewis acids and bases.

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