Talk:Mod:fjm19882

Katie, please read these comments, a considerable amount of time and effort has been spent in examining your labs and writing detailed comments in order to provide you with a good level of carefully evaluated and balanced feedback.

Part1 BH3 Your calculations and analysis are good, however you could have discussed your results in a little more detail. TM Good vibrational analysis although would have benefited from a little more detail. In your energy analysis, you should have discussed the differences between the cis and trans energies in more detail and considered how you could change the energy ordering. You should only analyse energy differences between two structures – the absolute values produced by Gaussian are dependent on the method and basis set used so are not very useful. NH3 You have calculated all the correct values, but you needed to discuss the differences in the calculations in more detail – why the calculations took different times, whether symmetry can be broken.

Mini-project Good work on carrying out your calculations at a high level of theory, not many people did, well done! Yes, the B3LYP can give lower absolute energies ... in computational chemistry you cannot compare numbers between methods because of the inherent approximations, you must always compare two energies calculated with exactly the same basis set and method. Hence the reason for computing two structures, staggered and eclipsed, it is the energy difference that is important.

Your energy difference between the staggered and eclipsed ammonia borane is much too large it should be of the order of 20 kJ/mol, without the files loaded into D-space I cannot look to see where you went wrong. How did you confirm that your optimisation was successful? Did you compute the frequencies to ensure there were no negative ones?

Your focus on the B-N bond is appropriate and your description here good, your rationalisation is also consistent with your energy results. However if you checked the bond distance between the C-C and N-B you may note that it is longer for N-B and hence a weaker bond indicating a lower rotation barrier which would be inconsistent with your results. Showing the size effect due to the electonegativity of the atoms is nicely done and your explaination shows that you understand this well. Good work here.

For the reaction, you needed to compute NaCl (neutral species) and H2 and the NH4+ and BH4- species, then to determine the direction of the reaction (thermodynamically) compute E(reactants)-E(products) to see which way is "down hill". Your description of the higher melting point of ammonia borane is good.

Out of 6 days on this module, two were assigned to the mini project, this gives you an indication of the volume of work expected, your contribution falls a little short in this respect. There are no links to D-space files and no explanation of any problems encountered in this respect.