Rep:Mod:pHenOM2
MiniProject: Investigating Ammonia-Borane and Derivatives
Here we will be investigating the coordination of ammonia (NH3 to borane (BH3), which creates the Lewis acid-base adduct ammonia-borane, or borazane. Specifically, we will be looking at the coordination of the N atom to the B atom, and seeing how this changes in response to substituting the H atoms on NH3 with Me (electron-donating), F (electron-withdrawing), and using PH3 instead. We are looking for changes in:
1)Bond coordination distance
2)Frequency/Energy of the N(P)->B bond
3)NBO analysis of the coordination bond.
Optimisation
We are optimising 4 structures here: a)H3N->BH3; b)Me3N->BH3; c)F3N->BH3; and d)H3P->BH3. In order to optimise these from a good starting point, the B<--N(P) bond lengths for all 4 structures was estimated/found in the literature. These were:
| Bond | Bond Length |
|---|---|
| H3N->BH3 | 1.65[1] |
| Me3N->BH3 | 1.62[2] |
| F3N->BH3 | 1.88 (used covalent radii) |
| H3P->BH3 | 1.57 (used covalent radii) |
The initial optimisation was carried out using the following key commands:
| Method: DFT-B3LYP |
| Basis Set: 3-21G (low level) |
| Type: OPT (optimisation) |
| Additional Keywords: opt=maxcycle=50 |
The additional keywords above relate to the fact that since we are investigating new molecules, we don't want the calculation to run forever; this sets a termination limit.
The initial optimised structures are below for a) and c):
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and here are the other calculations for b) DOI:10042/to-6729 and d) DOI:10042/to-6730 .
Notice that for a) and c) there are some bonds missing; again we need not worry about this, as Gaussview has just chosen not to draw them.
All of the initial optimisations were proved to have converged, leading onto the main optimisations. This involved using the following key commands in the calculations:
| Method: MP2 |
| Basis Set: 6-311G (d,p) |
| Type: OPT (optimisation) |
| Additional Keywords: opt=maxcycle=50 |
6-311G and MP2 are used for higher level calculations and accuracy, ideal for determining the structure of new compounds. (d,p) refer to polarisation functions used to model the skewedness of the electron density as the N(P) donates its lone pair towards the B.
The fully optimised structures are below:
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The fully optimised structures, upon looking in the .log file, were found to have converged.
Analysis and Discussion
In analysing the fully optimised structures, we are looking to perform a full frequency and population analysis. This was performed using the following key commands in the calculation:
| Method: MP2 |
| Basis Set: 6-311G |
| Type: FREQ (frequency) |
| Additional Keywords: opt=maxcycle=50 pop=(full,nbo) |
The frequency calculations for each molecule are: a) DOI:10042/to-6764 , b)DOI:10042/to-6765 , c)DOI:10042/to-6636 , d)DOI:10042/to-6766 . The vibrations for all 4 molecules were positive, indicating that minima had been found for all of them, confirming that they are the lowest energy minima.
The main data that we want to analyse is summed in a table below:
| Molecule | N(P)->B bond length/Å | Vibration Frequency/cm-1 | Vibration Intensity/arbitrary units | Symmetry | |
|---|---|---|---|---|---|
| a) | 1.65 | 683 (mode number 4) | 12 | A | |
| b) | 1.64 | 886 (mode number 11) | 54 | A' | |
| c) | 1.63 | 451 (mode number 4) | 5 | A | |
| d) | 1.94 | 550 (mode number 4) | 1 | A' |
Looking at this table, we can see that phosphine coordinates a lot less strongly towards borane than any of the ammonia derivatives, as evidenced by the much longer bond length (~0.3Å). The trimethylamine derivative coordinates more strongly than ammonia, but only a little bit, causing a 0.1Å decrease in bond length. Interestingly enough, the trifluoroamine derivative coordinates even more strongly, following the same argument, as its bond length is 0.1Å shorter than that of the trimethylamine. This possibly suggests that although F is electron-withdrawing, the availability of the lone pairs on the F is used in strengthening the coordinate bond. An opposite argument follows for observing the frequencies, as the strength of the vibrational frequency (which can correlate to the strength of the coordinate bond) decreases as the substituent attached to N becomes more electron-withdrawing. The Me groups donate electron density and thus strengthen the coordinate bond, explaining why b) has the highest vibrational frequency. Phosphine, which is not as good a coordinator as ammonia, still has a larger coordinate bond strength than the trifluoroamine.
We can also look at the key coordinate bond in the NBO analysis of each molecule. Below are the key texts for a)-d):
a)
4. (1.99129) BD ( 1) B 1 - N 5
( 17.14%) 0.4140* B 1 s( 16.63%)p 4.99( 83.04%)d 0.02( 0.34%)
0.0001 0.4038 0.0564 -0.0008 -0.9092
-0.0592 0.0171 0.0000 0.0000 0.0000
0.0000 0.0000 0.0000 0.0000 0.0000
0.0000 0.0502 -0.0290
( 82.86%) 0.9103* N 5 s( 37.02%)p 1.70( 62.97%)d 0.00( 0.01%)
0.0000 0.6083 -0.0103 0.0001 0.7923
0.0453 0.0001 0.0000 0.0000 0.0000
0.0000 0.0000 0.0000 0.0000 0.0000
0.0000 0.0082 -0.0048
b) 4. (1.96054) BD ( 1) B 1 - N 5
( 16.13%) 0.4016* B 1 s( 16.93%)p 4.88( 82.60%)d 0.03( 0.48%)
0.0007 0.4113 0.0087 -0.0051 -0.2808
-0.0131 0.0034 -0.8633 -0.0402 0.0105
0.0000 0.0000 0.0000 0.0352 0.0000
0.0000 -0.0483 -0.0345
( 83.87%) 0.9158* N 5 s( 28.88%)p 2.46( 71.10%)d 0.00( 0.02%)
0.0000 0.5373 -0.0114 -0.0003 0.2606
-0.0105 -0.0002 0.8012 -0.0324 -0.0008
0.0000 0.0000 0.0000 0.0068 0.0000
0.0000 -0.0094 -0.0067
c)
4. (1.99459) BD ( 1) B 1 - N 5
( 15.16%) 0.3894* B 1 s( 14.19%)p 6.02( 85.35%)d 0.03( 0.47%)
0.0002 0.3720 0.0589 -0.0017 -0.9220
-0.0567 0.0160 0.0000 0.0000 0.0000
0.0000 0.0000 0.0000 0.0000 0.0000
0.0000 0.0591 -0.0341
( 84.84%) 0.9211* N 5 s( 47.63%)p 1.10( 52.36%)d 0.00( 0.01%)
0.0001 0.6893 -0.0345 0.0001 0.7215
-0.0550 0.0080 0.0000 0.0000 0.0000
0.0000 0.0000 0.0000 0.0000 0.0000
0.0000 0.0100 -0.0057
d) 4. (1.98647) BD ( 1) B 1 - P 5
( 33.87%) 0.5819* B 1 s( 18.04%)p 4.53( 81.80%)d 0.01( 0.16%)
0.0004 0.4239 0.0273 -0.0006 0.0000
0.0000 0.0000 0.9043 0.0024 -0.0161
0.0000 0.0000 0.0000 0.0000 0.0000
0.0000 -0.0350 -0.0202
( 66.13%) 0.8132* P 5 s( 40.32%)p 1.47( 59.35%)d 0.01( 0.33%)
0.0000 0.0006 0.6349 0.0076 -0.0027
0.0000 0.0000 0.0000 0.0000 0.0000
0.0000 0.0000 -0.7702 -0.0107 0.0096
-0.0002 0.0000 0.0000 0.0000 0.0000
0.0000 0.0000 0.0000 0.0000 -0.0501
-0.0289
What these values show us is that in general, P contributes less to the coordinate bond than does N. P has a bond contribution of 66.13%, compared to the other three N-based Lewis bases, which all contribute around 83%. The hybridisation of the coordinating Lewis basic atom is similar for phosphine/ammonia (midway between sp and sp2), however for the trimethylamine it skews beyond sp2 slightly towards sp3,and for the trifluoroamine it does the reverse, skewing to an almost sp hybridised N centre. The effect with trimethylamine shows the electron-donating groups promoting the formation of another bond from the N, leading towards 4 bonds and an sp3 hybridisation. The reverse is true for trifluoroamine.
Conclusion
To conclude, ammonia-borane is a fairly stable structure that has generated large interest as a source of H2 fuel, due to the Lewis acidic nature of borane and the Lewis basic nature of ammonia. Derivitising the ammonia Lewis base so that the lone pair on the N is less/more available for coordination influences the relative stability of the coordinate bond and thus of the Lewis acid-base adduct. Too strong a coordinate bond could lead to its subsequent ability to bind H2 to be relinquished, but too weak a coordinate bond would cause the adduct to only be stable for short periods of time; not ideal for a H2 fuel store.
References
- ↑ C.R. Miranda and G. Ceder, J. Chem. Phys., 126, 184703, (2007) DOI:10.1063/1.2730785
- ↑ B. Rice, R.J. Galiano, and W.J. Lehmann, J. Phys. Chem., 1957, 61 (9), 1222-1226 DOI:10.1021/j150555a020