After creating the basic structure, it was optimised by using DFT B3LYP and LanL2DZ (medium level basis set). The point group (D3h) was maintained via setting a tolerance of 0.0001, giving the
optimised structure
, with TlBr bond lengths
2.65
Å and angles
120.0\'[1]
Summary of optimised structure
Now that we have a molecule believed to be at a minimum energy, the vibrational spectrum was derived to confirm (as non positive frequencies indicate the molecule is not at a minimum)[2]. This was done via a frequency calculation (using the same method as above) that included a full analysis of the electron density and orbitals.
The result gives the following low frequencies:
-3.4213 -0.0026 -0.0004 0.0015 3.9367 3.9367
which are, as expected, close to zero compared to the vibrational frequencies below:
The computed vibrational data agrees with experimentally found data [3] indicating that the bond length and angles have been calculated correctly. The face that there are two peaks on the IR is caused by the overlap of the first three peaks due to their similar values, and the final two. The fourth peak does not give an IR reading due to its fully symmetric nature, resulting in no change in the dipole.
A feature noticed during the initial optimisation step, was the apparant lack of a bond between the Ti and Br atoms in gaussview. This is due to gaussviews way of determining where a bond exists. In reality a chemical bond between two atoms is a result of the sharing of electron density between the two nuclei and the attraction between oppositely charged atoms. The forces involved are effected by the distance between the nuclei and have an optimum distance. If the distance between atoms is significantly higher than pre-determined values gaussview has stored, it does not display a bond. However, this is a cosmetic effect and does not effect the calculations involved.
BH3
Summary of optimised structure
*optimise step*
1.19 angstroms, closer than TiBr3 due to smaller molecule and greature difference in electronegativity, 120.0' due to same symmetry
MO
The clear and easily assignable computed orbitals compared to those predicted by the LCAO MO's demonstrates the value and use qualitative MO methods as a tool to make predictions.
A vibrational analysis was done, as above, to ensure the molecule was at a minimum point in energy. The symmetry of the vibrations are the same as the above molecule due to both having D3h symmetry. As all value are positive, it is confirmed that the molecule is at a minimum point.
This section demonstrates the different stabilities and IR spectra of cis and trans conformers of Mo(CO)4L2.
After forming a base molecule for both the cis and trans forms, a rough approximation was found using the DFT B3LYP method and LANL2MB basis set, using the "opt=loose" keyword to increase the likelihood of finding the optimal configuration. Afterwards, the geometry was slightly adjusted (the cis C-PCL3 groups relative torsion = 180', with the trans groups eclipsed) then a more specific optimisation was done using the more accurate LANL2DZ basis set and the keywords "int=ultrafine scf=conver=9". The resulting optimised molecules then underwent a frequency analysis to ensure they were at a minimum point
The energy difference is 2.74kj/mol-1, favouring the cis formation. However, this energy value is comparatively small, indicating that both neither is a particularly favoured product. The trans structure maintains standard octahedral structure angle wise, with the larger P groups further than the carbons, but closer than their counterparts in the cis conformer. This is due to the repulsive effect between the two PCl3groups, also shown by the higher P-C-P angle squashing the smaller equatorial C-C-C angle.
The table clearly shows that there are four distinct C=O vibrations (>1900) in the cis isomer. The trans isomer contains 2, however the stretches are very similar (~0.5 without integer rounding), and have a very similar intensity. These two peaks can be seen as degenerate bending, but are slightly off due to imperfect optimisation. Four C=O stretches for the cis and a single for the trans is as expected due to their symmetries.
miniproject - AlCl2Br dimers
The effects of a bromine atom substituting a chlorine atom preceeding the dimer formation of Al2Cl6 is analysed to find what would be the optimal isomer from a thermodynamic view, and how to differentiate between the products via IR.
The five compared forms studied are:
The first step was optimisation. The initial Al2Cl6 was modelled, then an approximate optimisation was found using the DFT B3LYP method and LANL2MB basis set, using the "opt=loose" keyword to increase the likelihood of finding the optimal minimum point. Then a more accurate optimisation using the LANL2DZ basis set and the keywords "int=ultrafine scf=conver=9" was performed as above, with he resulting optimised molecules undergoing a frequency analysis to ensure they were at a minimum point.
The most obvious effects from the Br substitution are from its increased size causing increased bond lengths and steric repulsion (ie increased Al-Al distance, Br-Br cis higher than Cl-Cl etc). The relative energies correlates with the proximity of the Br atoms. Using the highest energy isomer (central Br, with the shortest Br-Br distance) as a comparison, the mixed molecule is 15kJ/mol (all energies to nearest 5kJ/mol) more stable, while the cis and trans isomers are both 30Kj/mol more stable. This suggests that if a mixture of AlCl2Br would form predominantly cis and trans isomers, with both central atoms being chlorine atoms.
The vibrational analysis lacks any negative frequencies, ensuring that a minimum point has been located. The vibrational spectra for the cis and trans isomers are very similar, while the other two differ substantially and would be easily noticed in an IR spectra. As analysed above, the main thermodynamic proucts are the cis and trans isomers. However, the main identifying difference between the two is a low intensity peak at 434 cm-1 due to the higher dipole cis isomer, however, by analysing the other products, which would likely exist in smaller concentrations, it can be seen that there is a very high intensity peak from the central Br isomer just 10cm-1 less at 424cm-1. THis makes IR an impractical tool for identifying between the two primary isomers. A much more accurate analysis would be Cl NMR, as that would both give less likely to overlap spectra, but also allow the product ratio's judged much more easily.
