PCh10 15 lec 3

(5/6/20, bes)

Greetings...welcome to your last PChem II lecture for the term. I am generally pleased with the content delivered online, but not having Mathematica, Igor, and Excel at our immediate access has limited some of the more hands-on activities. I am pleased that we were able to utilize WebMO/Gaussian to keep us in "lab."

There is so much more i would like to share in this course, but out time has run out :(

I am continuing to work on an estimate for your current grade. I hope to send this ASAP (no later than 2 pm today)...it might not be fully complete...but i will try.

Ch10/15 cont.

So as i flip thru chapter 15 there are many sections that have been covered already or should be understandable based on your current experiences. For example:

Potential Energy Surfaces (Sec 15.2)

I would expect that each of you understand the idea of a rotational energy barrier (or as they say, "the plot of energy vs the torsional angle"). As shown in Figure 15.1, the rotational (energy) barrier (or activation energy) is 16 kJ/mol, but the difference in the energy between the "anti" and "gauche" states is only 3.8 kJ/mol. It is interesting to think about this in terms of the amount of energy that is provided by "temperature"...

k*T*N = 1.38e-23 J/(K*molecule) * 298 k = 4.11e-21 J/molecule * 6.023e23 molecules/mol = 2.5 kJ/mol. This thermal energy is not enough to allow this bond to be freely rotating at room temperature. The rotational energy for ethane is ~12 kJ.mol, so even ethane is not freely rotating at room temp.

Table 15.1 discusses in general the "ratio" of products to reactants (or anti vs gauche) depending on the energy difference between the two states...as you will see thes ratios are calculated from the Boltzmann Distribution (eq. 15.13).

I would also expect that all of you could run a "coordinate scan" calculation in WebMO...hint, hint.

Reaction Energies (Sec 15.4.1)

Homolytic Bond Dissociation Energies (Table 15.2)

Table 15.2 shows that the HF level of theory (ie no electron correlation) does not do a very good job at calculating the bond dissociation energy. The way one would do this calculation is to calculate the energy of the bonded atoms - the energy of the unbonded (radicals) atoms. I have not done this type of calculation since it is not in my research focus, but some people do. I am guessing (ony guessing since i have not done the calc) that you could use WebMO to calculate any of the Hartree-Fock Limit values in this table...given some instruction.

Proton Affinities of Nitrogen Bases (Table 15.4)

Table 15.4 shows the proton affinity of nitrogen bases relative to methyl amine. This is a bit more complicated computation, but it is interesting to see that these toyes of calculation might provide some insight into an organic reaction. I would not ask you to do such a calculation on the final.

Equilibrium Geometries (Sec 15.4.2)

Tables 15.5/15.6 shows bond lengths...this is certainly in your skill set. Table 15.7 shows vibrational frequencies...very similar to the HCl rot-vib lab...H-F is listed...WebMO calcs would involved a geometry optimization followed by the vibrational frequency analysis (these can be done in 1 step in WebMO/Gaussian).

Sec 15.8

- Tables 15.9/15.10 --> more on bond lengths

- Table 15.11 --> more on dissociation energies

- Tables 15.16/15.17 --> transition state/activation energies (cool stuff but not sure we could get these calcs to work).

3D Visualization (Sec 15.9)

- Molecular orbitals (Sec 15.9.1) <-- you can do that!

- Electrostatic Potential Maps (Sec 15.9.6) <--- you can do that!

End of ...last... lecture for the term :(