Lewis Acid-Base, Molecular Modeling and Isotopic Labeling. The objective of this experiment is to analyze and compare infrared spectroscopy to characterize the two adducts formed in the laboratory, H3B-NH2(C4H9) and D3B-NH2(C4H9). Changes in the spectra d

Lewis Acid-Base, Molecular Modeling and Isotopic Labeling

Introduction

The objective of this experiment is to analyze and compare infrared spectroscopy to characterize the two adducts formed in the laboratory, H3B-NH2(C4H9) and D3B-NH2(C4H9). Changes in the spectra due to isotopic labeling will be noted and compared to values calculated using the equation .

According to Hooke’s law, V(x) = 1/22kx, the stretching frequency of a diatomic bond in a molecule is related to the mass, m, and the force constant of the bond, k, by the formula1

where k is the force constant, ν is the frequency of the stretching vibration and, where MA is the mass of one atom and MB is the mass of the second atom; this is known as the reduced mass.

In Hooke's law it is shown that the frequency for a stretching vibration would change if one of the atoms were replaced with an atom of a different mass (such as an isotope) or if the bond strength were changed. A reasonable estimation of the changed IR band can be determined using the diatomic molecule vibration frequency model2

where ν is the vibrational frequency of the stretching bond and µ=, which is the reduced mass. If the ratio of the reduced masses of the original diatomic bond and the isotopic diatomic bond is greater than 1, the isotope replacement will cause vibrational frequency wavenumbers to increase (shift to the left); if it is less than 1, the wavenumbers will decrease (shift to the right).

In this experiment, an adduct labeled with deuterium is prepared. This replacement of hydrogen with the isotope, deuterium, is known as deuterium labeling. This reaction will be carried out using NH2(C4H9)+ Cl-, the Lewis base, and NaBH4, the Lewis acid. Then in a second reaction, deuterated NaBH4, NaBD4, will be used to replace NaBH4. A Lewis acid acts as an electron pair acceptor and a Lewis base act as a an electron pair donor. A Lewis acid is denoted by A and a Lewis base is denoted by :B, often omitting any other lone pairs that may be present. The Lewis base has this unshared lone pair that occupies the HOMO, or Highest Occupied Molecular Orbital. The Lewis acid will have an empty valence orbital known as the LUMO, or Lowest Unoccupied Molecular Orbital. The chemistry of a chemical reaction takes place at the HOMOs and LUMOs of molecules. The fundamental reaction of Lewis acids and bases is the formation of a complex or adduct, A—B, where A and B bond together by sharing the electron pair supplied by the base.3

The following reactions will take place in these experiments:

and

Experimental

In a round bottom flask, NH3(tBu)+ Cl- (1.316 g) and THF (15 mL) were mixed. With steady stirring, powdered NaBH4 (0.20 g) was added to the suspension. Additional THF (10 mL) was added to the solution. The solution was then stirred at room temperature for 2 hrs. Then the solution was filtered with a suction filter apparatus. The filtered THF solution was evaporated using a rotary evaporator. The melting point of the product was determined. The percent yield of the product was calculated. The IR spectrum ...

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The following reactions will take place in these experiments:

and

Experimental

Results

The theoretical yield was calculated for H3B-NH2(C4H9):

And the percent yield was calculated for H3B-NH2(C4H9):

The theoretical yield was calculated for D3B-NH2(C4H9):

And the percent yield was calculated for D3B-NH2(C4H9):

The melting point for H3B-NH2(C4H9) was recorded to be between 95ºC and 100ºC. The melting point D3B-NH2(C4H9) was recorded to be between 101ºC and 103ºC. Both adducts were white, clumpy powders.

Bond stretching frequencies were observed (νobserved) when measuring the IR spectrum of H3B-NH2(C4H9) and D3B-NH2(C4H9). Theoretical bond stretching frequencies (νtheoretical) were determined using Spartan software. The frequencies between H3B-NH2(C4H9) and D3B-NH2(C4H9) were then compared:

Table 1: Comparison of νobserved and νtheoretical IR data

For each product, an IR was measured in CDCl3:

Figure 1: IR spectrum of H3B-NH2(C4H9) adduct.

N-H, B-H, and B-N observed frequencies are indicated.

Figure 2: IR spectrum of D3B-NH2(C4H9) adduct.

N-H, B-H, and B-N observed frequencies are indicated.

The reduced mass differences were determined for the B-H stretch of H3B-NH2(C4H9):

and for the B-D stretch of the deuterated species:

The structures HOMO (Figure 3) and LUMO (Figure 4) of BH3 and the HOMO (Figure 5) and LUMO (Figure 6) of NH2(C4H9) were determined using Spartan software. Using the same software, the energies of the HOMOs and LUMOs were recorded. For BH3, the energy of the HOMO was recorded as -1144.51 kJ/mol and the energy of the LUMO was recorded as 154.36 kJ/mol. For NH2(C4H9), the energy of the HOMO was recorded as -948.84 kJ/mol and the energy of the LUMO was recorded as 340.60 kJ/mol.

Discussion

The percent yield was calculated to be 232% for H3B-NH2(C4H9); the percent yield was calculated to be 137% for D3B-NH2(C4H9). Both of these yields are very high. It is very likely that the products were still very moist after being taken out of the rotary evaporator and that this contributed to the high actual and percent yields. The products were clumpy powders, rather than soft powders, and this was an indication that the product was most likely still too wet before the IR was measured.

The melting point for H3B-NH2(C4H9) was recorded to be between 95ºC and 100ºC. This is in accordance with expected values of about 96º C4. The melting point D3B-NH2(C4H9) was recorded to be between 101ºC and 103ºC. This makes sense, since the deuterated species of H3B-NH2(C4H9) is ever so slightly heavier, and would then boil at a higher temperature.

The reduced masses were calculated for H3B-NH2(C4H9), 0.92 amu, and for the deuterated species, 1.7 amu. The ratio of the reduced masses, 0.92/1.7, was less than 1, 0.54. This indicates that the isotopic replacement of deuterium for hydrogen causes the wavenumbers of the frequencies of the deuterated species to be lower than those of H3B-NH2(C4H9). This also makes sense when comparing the theoretical values in Table 1, where the frequencies for D3B-NH2(C4H9) were lower than those of H3B-NH2(C4H9). The observed frequencies, however, did not differ as they should have. This is probably due to impurities that may have been present in the IR sample, as well as scratches and the presence of other compounds on the salt plates used for the IR, which could have skewed the reading and the spectra.

The greatest downward shift of frequencies should be seen between the B—H bond of H3B-NH2(C4H9) and the B—D bond of D3B-NH2(C4H9). In the theoretical values, this is evident. The B—H frequency is expected to occur between 2350 cm-1 and 2650 cm-1.5 The observed frequency is just below this at 2256 cm-1 and the theoretical value is well above this at 3184 cm-1. Using the equation,
, the values of B—D stretching are estimated to occur between 1731 cm-1 and 1944 cm-1. The observed frequency is well above the value, as is the theoretical frequency. B—N bands typically occur from 1430 to 1335 cm-1. 6 The observed values in each species were far below this range, both at 925 cm-1. The theoretical values were well below this range, at 874 cm-1 and 844 cm-1. In IR spectra, N—H stretches typically occur between 330 and 3500 cm-1. 7 The observed frequencies fell just below this range, and the theoretical frequencies were within the range. Regardless of the discrepancies between accepted values, observed values, and theoretical values, the theoretical values still followed the expected trend in that the deuterated specie occurred at a lower frequency than that of the hydrogen specie.

The recorded energy of the HOMO of BH3 was lower than the recorded energy of the LUMO. The same trend was observed for the HOMO and LUMO of NH2(C4H9). Conventionally, the LUMO is higher in energy than the HOMO, which can be observed in a molecular orbital diagram as the LUMO is drawn above the HOMO and the HOMO is completely occupied, making it much more stable than the incompletely occupied LUMO.

Conclusion

The theoretical frequencies followed the expected trend of a downward shift or decrease in the deuterated adduct’s stretching frequencies, most notable in the B—H and B—D bands. The observed frequencies were not as expected and did not follow the same trend as the theoretical frequencies. To avoid these discrepancies in the future, the products should be left in the rotary evaporator for a longer period of time and/or they should be thoroughly dried in an oven to ensure that all moisture is lost. Also it is recommended that clean IR salt plates be used with minimal or no scratches.

References

(1) University of Colorado. Infrared Spectroscopy: Theory. .

edu/hndbksupport/irtutor/IRtheory.pdf (accessed Oct 12, 2010).

(2) Nataro, Chip; et al. Lewis Acid-Base, Molecular Modeling, and Isotopic Labeling in a Sophomore Inorganic Chemistry Laboratory. J. Chem. Education 2004, 81, pp 722-724.

(3) Atkins, P.; Overtone, T.; Rourke, J.; Weller, M.; Armstrong, F. Inorganic Chemistry, 4th ed.; Oxford University Press: Great Britain, 2006.

(4) Feigerle, J.; et al. Thermal decomposition of t-butylamine borane studied by in situ solid state NMR. J. Ceramic Transactions 2010, 78.

(5) Nakanishi, K; Solomon, P. H. Infrared Absorption Spectroscopy, 2nd ed.; Holden-Day, Inc.: Oakland, CA, 1977; pp 55.

(6) Pouchert, C.J. The Aldrich Library of Infrared Spectra, 2nd ed.; Aldrich Chemical Company, Inc.: Milwaukee, WI, 1975; pp 1323.

(7) University of Puget Sound Characteristic IR Absorption Frequencies of Organic Functional Groups http://www2.ups.edu/faculty/hanson/Spectroscopy/IR/

IRfrequencies.html (accessed Oct 12, 2010).

Figures

Figure 3: HOMO structure of BH3

Figure 4: LUMO structure of BH3

Figure 5: HOMO structure of NH2(C4H9)

Figure 6: LUMO structure of NH2(C4H9)