Rep:MOD:Year201390203

Borane

Basis set

B3LYP/6-31G level

Summary table

Copy of borane summary table image

Link to original LOG file

Link to log/frequency file for Borane

Convergence and low frequency verification data

Item               Value     Threshold  Converged?
 Maximum Force            0.000004     0.000450     YES
 RMS     Force            0.000003     0.000300     YES
 Maximum Displacement     0.000017     0.001800     YES
 RMS     Displacement     0.000011     0.001200     YES

Low frequencies ---   -1.1800   -1.0028   -0.0054    4.1927   11.0182   11.0637
Low frequencies --- 1162.9912 1213.1792 1213.1819

Image of borane

Borane molecule in Jmol

Vibrations of the borane molecule

Mode	Frequency	Infrared	Type
1	1162	92	Symmetric bend
2	1213	14	Symmetric wag
3	1213	14	Asymmetric wag
4	2582	0	Symmetric stretch
5	2715	126	Asymmetric stretch
6	2715	126	Asymmetric stretch

Why are there not three peaks in the spectrum even though there are six vibrations?

Although there are six vibrations for borane, according to the 3N-6 rule for non-linear molecules, because of the degeneracy of some of the vibrational modes only three peaks appear in the IR spectrum. In mode 4 there is not a change in dipole and thus violates the selection rule that there must be a change in dipole. Modes 2 and 3 are degenerate as well as modes 5 and 6 whilst mode 1 is non degenerate.

Correct explanation of the 3 visible peaks which could have been a little clearer in terms of the writing. You've correctly assigned the type of frequency for each mode but the symmetries are missing from the information presented. Smf115 (talk) 21:54, 13 May 2019 (BST)

MO diagram of BH₃ with calculated molecular orbitals

Reference: 1. Hunt, P. (2019). Lecture_4_Tut_MO_diagram_BH3. [ebook] Available at: http://www.huntresearchgroup.org.uk/teaching/teaching_comp_lab_year2a/Tut_MO_diagram_BH3.pdf [Accessed 2 May 2019].

Answer to questions on MO diagram

There are subtle differences between the LCAO MOs and the real MOs. The shapes of the MOs are consistent with the LCAO approach but looking at the real MOs we can see electron density is localised over larger regions of space and this is due to the orbital overlap. In the LCAO approach, the atomic orbitals or fragment orbitals are not merged to form a larger MO. This allows for humans to easily decipher the fragments used to create the MO. Thus, the qualitative MO diagram is a useful guide to the general shapes of the real MO diagram but this qualitative approach does not fully visualise the forms of the MOs. Nevertheless, it is a useful tool for understanding where the real MOs come from.

Ammonia

Basis set

B3LYP/6-31G level

Summary table

Link to original LOG file

Link to log/frequency file for Ammonia

Convergence and low frequency verification data

         Item               Value     Threshold  Converged?
 Maximum Force            0.000006     0.000450     YES
 RMS     Force            0.000004     0.000300     YES
 Maximum Displacement     0.000012     0.001800     YES
 RMS     Displacement     0.000008     0.001200     YES
 
Low frequencies ---   -8.5223   -8.4750   -0.0029    0.0335    0.1918   26.4067
Low frequencies --- 1089.7616 1694.1862 1694.1866

Image of ammonia

Ammonia molecule in Jmol

Borane-Ammonia adduct

Basis set

B3LYP/6-31G level

Summary table

Copy of the summary table for the borane-ammonia adduct

Link to original LOG file

Link to log/frequency file for Borane-Ammonia adduct

Convergence and low frequency verification data

Item               Value     Threshold  Converged?
 Maximum Force            0.000114     0.000450     YES
 RMS     Force            0.000063     0.000300     YES
 Maximum Displacement     0.000621     0.001800     YES
 RMS     Displacement     0.000355     0.001200     YES

Low frequencies ---   -0.0617   -0.0457   -0.0066   21.6779   21.6838   40.5363
Low frequencies ---  266.0165  632.3610  640.1359

Image of borane-ammonia adduct

Borane-Ammonia adduct molecule in Jmol

Reaction energetics

The following values are given in atomic units, au, to 8 dp to avoid rounding errors:

E(NH3)= -56.55776863

E(BH3)= -26.61532362

E(NH3BH3)= -83.22468814

The reaction energy, calculated as follows:

ΔE=E(NH3BH3)-[E(NH3)+E(BH3)]

and is calculated below:

-83.22468814 - [-26.61532362 + -56.55776863] = -0.05159589 a.u.

Now using the conversion factor of 2625.5 to convert to kJ/mol and attain -135. We find that the adduct is a stable form evidenced by the more stable energy. However, upon comparison to the B-N bond dissociation energy, tabulated as 389 kJ/mol we can suggest that the dative bond is weak.

Nicely presented energy values, correct calculation and consideration of the accuracy of the final reported value. To improve you should have a reference for the literature value! Smf115 (talk) 21:50, 13 May 2019 (BST)

NI₃

Basis set

B3LYP/GEN

Summary table

Copy of the summary table for the NI3

Link to original LOG file

Link to log/frequency file for NI3

Convergence and low frequency verification data

         Item               Value     Threshold  Converged?
 Maximum Force            0.000061     0.000450     YES
 RMS     Force            0.000037     0.000300     YES
 Maximum Displacement     0.000606     0.001800     YES
 RMS     Displacement     0.000357     0.001200     YES
Low frequencies ---   -5.5587   -5.4901   -0.0003   -0.0002    0.0004    6.5130
Low frequencies ---  101.1565  101.2827  148.4562

Image of NI₃

NI3 molecule in Jmol

Al₂Cl₄Br₂

Isomerism of Al₂Cl₄Br₂

The five isomers of Al₂Cl₄Br₂ can be seen below, drawn with Chemdraw. The isomer with both bromine in the bridging positions will be analysed, as well as the isomer with the bromines occupying the terminal trans positions.

Convergence and low frequency verification data

        Item               Value     Threshold  Converged?
 Maximum Force            0.000084     0.000450     YES
 RMS     Force            0.000033     0.000300     YES
 Maximum Displacement     0.001205     0.001800     YES
 RMS     Displacement     0.000690     0.001200     YES
Low frequencies ---   -0.0009    0.0008    0.0019    1.3337    2.3401    4.2420
Low frequencies ---  119.7829  132.7164  182.6113

Summary table

Copy of the summary table for trans Alcl₂Br

Link to original LOG file

Link to log/frequency file

Image of Alcl₂Br

AlClBr in Jmol

Energetics and comparison to the monomer, AlCl₂Br

The following values are given in atomic units, au, to 8 dp to avoid rounding errors:

E(Trans isomer of Al₂Cl₄Br₂)= -571.43791902 au

E(Al₂Cl₄Br₂ isomer with bromines in bridging position)= -571.43283091 au

Energy difference between:

E(Trans isomer of Al₂Cl₄Br₂) - E(Al₂Cl₄Br₂ isomer with bromines in bridging position)

= (-571.43791902) - (-571.43283091)

= -0.00508811 au

In kJ/mol this is -13 kJ/mol. This suggests that the trans isomer is more stable than the isomer with the bromines in the bridging positions. A possible explanation for this observation is that there is significant steric hindrance of the bromines in the bridging position due to its large molecular size. This will make the trans isomer more stable as the bromines are on different faces and maximally distant from eachother. Furthermore, with the chlorines in the bridging positions, given that chlorine has more compact orbitals this will maximise the overlap integral S_ab, bromine with larger and more diffuse orbitals will have a poorer overlap integral. Thus, stabilisation will be worse and hence why the isomer with the bromines trans and the chlorines in the bridging position is favoured.

E(AlCl₂Br)= -275.70300109 au

2xE(AlCl₂Br)= -571.4060022 au

The energy difference is calculated as follows:

ΔE=2xE(AlCl₂Br)-E(Trans isomer of Al₂Cl₄Br₂)]

and is calculated below:

(-571.4060022) - [-571.43791902] = 0.03191684 au

This equates to 84 kJ/mol, which implies the monomer is significantly less stable than the dimer. Aluminium is electron deficient and although it benefits from hyperconjugation between its empty p orbital and the filled p orbitals of the bromine and chlorine it is bonded to, it is still significantly electron deficient. However, upon dimerization the atoms in the bridging ligand provide stabilisation to the molecule as they form molecular orbitals with the AlClBr fragments which stabilise the molecule significantly more than would if the bromines were bridging for the same reasons as above. The overlap integral will be poorer due to the more diffuse orbitals and thus resulting in weaker interactions and less stabilisation when forming the MO.

Correct calculation of the dissociation energy and great attempt at justifying the resulting stability of the dimer. The ideas raised are good with technical terms used but haven't been explained too well in places - the Cl/Br and Al in the monomer would still form MOs, so the idea presented need a little more clarity. Smf115 (talk) 21:59, 13 May 2019 (BST)

Analysis of molecular orbitals

MO 11

MO 20

MO 21

Nice range of MOs selected, technical terms are used well and the overall character has been evaluated correctly. The presentation lets the information down in places with the LCAOs for MO20, in particular, not being too clear and Al seems to be omitted. Smf115 (talk)

Basis set

B3LYP/GEN

Summary table

Copy of the summary table for trans Al₂Cl₄Br₂

Link to original LOG file

Link to log/frequency file for Al₂Cl₄Br₂

Convergence and low frequency verification data

         Item               Value     Threshold  Converged?
 Maximum Force            0.000094     0.000450     YES
 RMS     Force            0.000027     0.000300     YES
 Maximum Displacement     0.001461     0.001800     YES
 RMS     Displacement     0.000457     0.001200     YES

Low frequencies ---   -4.5471   -2.4668    0.0011    0.0018    0.0019    2.2399
Low frequencies ---   18.6151   47.5783   71.2358

Image of Al₂Cl₄Br₂

AlClBr in Jmol

Good inclusion of a pseudopotential in the structure calculations presented, however, the pseudopotential should have only been used on the Br and not the Cl, resulting in incorrect energies. You are also missing the structure information and log file for the D2h isomer. Smf115 (talk) 22:07, 13 May 2019 (BST)

Overall, a good report! Very good first section and the project section is just let down by some missing sections Smf115 (talk) 22:07, 13 May 2019 (BST)