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Third Year Computational Lab Transition States - Emily Brown

Exercise 2

Two different products are known to form in this Diels-Alder reaction, they are known as the endo and exo adducts.

Transition state 2

These GIFs make it east to see where the Endo-TS has favorable secondary orbital interactions, towards the left hand side it can be seen that the oxygens p-oribtals could interact with those of the 6-carbon ring near it in a favorable manner.

(You have endo and exo mixed up. In the diagram above, it's shown correctly, but the figures and data below it's the wrong way around Tam10 (talk) 11:17, 6 December 2016 (UTC))

Exo Transition State. Negative Frequency = -520.92 cm-1

Endo Transition State. Negative Frequency = -528.82 cm-1

Thermochemistry anaylsis 2

Electronic and Thermal free energies relative to reactants(KJ/mol) (2dp)

	Reactants	Transition state	Products
Exo	0	+159.81	-67.41
Endo	0	+167.65	-63.77

From the thermochemistry relative energy values it can be seen that the Exo product has an activation energy which is 7.84 KJ/mol lower than that of the Exo product. The Exo-Adduct is 3.64 KJ/mol more stable than the Endo-adduct making it the thermodynamicly favoured product as well as the kinetically favoured product as it has a lower energy TS.

Nf710 (talk) 22:36, 14 December 2016 (UTC)Your energies are correct but you have got them the wrong way round.

MO analysis

Nf710 (talk) 22:45, 14 December 2016 (UTC) This setion was done fairly well however you got the orbital mixed up, howeve ryou missed a few things such as explicitly showing the secondary orbital overlap

This reaction and that the reaction in exercise 2 are both Diels-Alder reactions the MO diagram from exercise 1 is still valid to be used to understand/explain the MO interactions within this reaction.

MO diagram for the Diels Alder Reaction in exercise 1

(Butadiene has no centre of inversion and so cannot undergo the symmetry operation to determine gerade/ungerade. It's easier to look at the symmetry axis that bisects the TS Tam10 (talk) 11:35, 6 December 2016 (UTC))

From looking at the molcular orbitals below one can identify which orbitals are interacting in the Transition states. For example: it can be seen that the Endo TS LUMO is formed by the overlap of the Cyclohexadiene LUMO-1 and the 1,3-Dioxole LUMO.

Nf710 (talk) 22:46, 14 December 2016 (UTC) And this would suggest?

MOs
Reactants		Transition States
1,3-Dioxole	Cyclohexadiene	Endo-TS	Exo-TS
HOMO+1	HOMO+1	HOMO+1	HOMO+1

HOMO	HOMO	HOMO	HOMO

LUMO	LUMO	LUMO	LUMO

LUMO-1	LUMO-1	LUMO-1	LUMO-1

Structure Optimisation

All optimized to the B3LYP/6-31G(d) level but initially with the semi-empirical PM6 method. Everything but the transition states was confirmed to be a true minima by lack of negative frequencies in the frequency calculations as shown below. Finding a single negative frequency confirms the structures to in fact be the end and eco transition states desired.

Reactants

Cyclohexadiene

Optimization Summary

Optimised with 6-31G(d) level