User:Xv114

Introduction

A transition state is a transient species with the highest energy in the reaction profile that cannot be isolated or detected by spectroscopic methods. As a result it is the maximum point in the potential energy surface with zero gradient. This wiki page would discuss about transition states of cycloaddition reaction between different reactants using Gaussview for transition state optimisation followed by IRC and frequency calculation to determine the energy of reactants, transition states and products to determine favourable reaction route.

Nf710 (talk) 16:43, 20 January 2017 (UTC) This is not enough detail you need to talk about second derivatives etc.

Exercise 1: Reaction between butadiene and ethylene

Reaction Scheme

Butadiene and ethylene react in Diels Alder as shown in Scheme 1:

Scheme 1: Reaction between butadiene and ethylene, producing a cyclohexene via a cyclic transition state

Molecular orbital

The highest occupied molecular orbitals (HOMO) and lowest occupied molecular orbitals (LUMO) of both butadiene and ethylene shown in Figure 1, can overlap to a give 4 different transition state MOs, which correlates with those illustrated on the MO diagram, Figure 2.

Figure 1: HOMOs and LUMOs of butadiene and ethylene

Figure 2 Molecular orbitals of transition state with MO2 and MO3 being the transition state HOMO and LUMO respectively

From the MO diagram, it is the LUMO of butadiene that interacts with HOMO of ethylene.

Figure 3: Molecular orbital for butadiene, ethylene and transition state, symmetry of the orbitals are either symmetric (S) or asymmetric (A)

From Figure 2 and 3, it can be concluded that symmetry of MO of both reactants needs to be the same in order for overlap to occur and in turn the reaction would be 'allowed'. I.e. Only symmetric orbital can overlap with another symmetric orbital, not with asymmetric. This explains why only LUMO of butadiene and HOMO of ethylene can overlap with each other due to both having the same symmetry.Thus, the orbital overlap integral for a symmetric-asymmetric interaction would be zero, whereas that for symmetric-symmetric or asymmetric-asymmetric interaction would be non-zero.

Bond Length

Changes in bond length can be monitored to determine whether reactants have undergone a reaction.

Table 1: C-C bond length

Typical carbon-carbon bond length :	Å
Sp3-sp3	1.54
Sp3-sp2	1.50
Sp2-sp2	1.34

Van der waal radius for carbon: 1.70 Å

Figure 4:There is a change in bond length as reactant progress through transition state to product

Table 2: Change in C-C bond length (Å) from reactant to transition state and product

	Bond length (Å)
Bond number	Reactant	Transition state	Product
1	1.32	1.38	1.54
2	0	2.11	1.54
3	1.33	1.38	1.50
4	1.47	1.41	1.33
5	1.33	1.38	1.50
6	0	2.11	1.54

The bond lengths in both reactants and product in Figure 4 correlate with those in Table 2. However the C=C bonds lengthen in transition state to a length that is between a typical sp³-sp²  and sp²-sp² C-C bond length before finally becoming single C-C bond in product.

Transition state vibrations

According to Figure 5, the vibration of transition state at -949 Hz indicate that the formation of two bonds appear to be synchronous with both carbons of diene and dienophile approaching each other simultaneously. In contrast to vibration at 145 Hz the formation of bond is asynchronous.

Figure 5: Vibration of transition state at negative frequency

Figure 6: Vibration of transition state at the lowest positive frequency

IRC

Figure 7: IRC for the reaction

Nf710 (talk) 17:08, 20 January 2017 (UTC) Key points are there, minimal explanation.

Exercise 2: Reaction of Cyclohexadiene and 1,3-Dioxole

Reaction Scheme

Cyclohexadiene and 1,3-dioxole can react with each other in Diels Alder reaction as shown in Scheme 2:

Scheme 2: Diels Alder reaction between cyclohexadiene and 1,3 dioxole

Molecular orbitals

The MO diagram for reaction between cyclohexadiene and 1,3-dioxole in Figure 8 below:

Figure 8: MO diagram of reaction between cyclohexadiene and 1,3-dioxole

The three dimensional transition state orbitals visualised in Gaussview in Figure 10-13 correlate with those in the MO diagram, Figure 8. The presence of oxygen lone pair would raise the energy levels of dioxole, including its HOMO. As a result energetically the HOMO of dioxole would interact better with LUMO of cyclohexadiene. Furthermore presence of mirror plane extending along the length of the transition state is consistent with that for a cycloaddition reaction with an inverse electron demand as illustrated in Figure 14 confirming that the reaction has an inverse electron demand.

Figure 9: HOMOs and LUMOs of 1,3-dioxole and cyclohexadiene

(You're using the PM6 orbitals here. They are quite different to B3LYP Tam10 (talk) 14:14, 4 January 2017 (UTC))

Figure 10: HOMO of transition state in exo-Diels Alder

Figure 11: HOMO of transition state in endo-Diels Alder

Figure 12: LUMO of transition state in endo-Diels Alder

(This is the endo product Tam10 (talk) 14:14, 4 January 2017 (UTC))

Figure 13: LUMO of transition state in exo-Diels Alder

Figure 14: Symmetry plane, indicated by red dash line, is present only in the transition state that correspond with an inverse electron demand interaction which correlates. The presence of symmetry plane in the transition state MO confirms that it is an inverse electron demand cycloaddition

Table 2: Activation energy and Gibbs free energy for both endo- and exo- Diels Alder reactio between 1,3-dioxole and cyclohexadiene

Reaction	Activation energy (kJ/mol)	Gibbs free energy (kJ/mol)
Endo-Diels Alder	151	-76.2
Exo-Diels Alder	159	-72.3

Figure 15: In the exo Diel Alder there is only primary orbital interaction whereas in endo Diels Alder, the presence of secondary orbital interaction stabilise the transition state

Secondary orbital interaction is observed in the circled region between oxygen lone pair and π* orbital as shown in Figure 16. The presence of secondary and primary orbital interactions, as shown in Figure 16 and 15, and from the lower activation energy and Gibbs free energy calculated in Table 2 means the endo-Diels Alder reaction is both the thermodynamic and kinetic product. This also suggests that there is a lower steric interaction in endo- than exo-Diels Alder reaction.

Figure 16:On the left diagram the orbitals coloured in red within the circled region are overlapped with each other indicating secondary orbital overlap, whereas on the right diagram there is no secondary orbital overlap

Nf710 (talk) 17:57, 20 January 2017 (UTC) Good second section your energies are slightly out but you have got the correct conclusion, good description of SSO

Exercise 3: Diels Alder vs Cheletropic

Reaction Scheme

Xylene and sulphur dioxide can undergo either Diels-Alder or cheletropic reaction, forming six membered ring and a five membered ring respectively, as shown in the scheme below.

Scheme 3: Reaction scheme for exo- and endo- Diels Alder and cheletropic reaction of xylene and sulphur dioxide

From Table 3 and Figure 17-19, it can be deduced that endo-Diels Alder reaction has an activation barrier that is lower than that of exo-Diels Alder reaction by 4 kJ/mol, however the endo-Diels Alder product is higher in energy than its exo counterpart. Cheletropic product on the other hand is has the greatest activation barrier as well as Gibbs free energy. This suggests that kinetically endo-Diels Alder is the preferred reaction route but cheletropic reaction produces the most thermodynamically stable product.

Table 3: The activation energy and Gibbs free energy of different types of reaction listed in Scheme 2

Reaction type	Activation barrier (kJ/mol)	Gibbs Free energy (kJ/mol)
Endo- Diels Alder	60	-117
Exo-Diels Alder	64	-120
Cheletropic	82	-176

From Figure ? below, it is also evident between the two types of Diels-Alder reactions, the endo-Diels Alder product is the kinetic product and exo product is the thermodynamic product.

Figure 17: Reaction profile of exo-Diels Alder reaction

Figure 18: Reaction profile of endo-Diels Alder reaction

Figure 19: Reaction profile of cheletropic reaction

(Best to put these on the same coordinate and normalising to the reactants ie setting them to 0 Tam10 (talk) 14:14, 4 January 2017 (UTC))

IRC

The IRC calculations below were obtained after PM6 optimisation of the transition states for each reaction shown in Scheme 3.

Figure 20: Cheletropic IRC

Figure 21: Exo IRC

Figure 22: Endo IRC

For all three IRC calculations shown above in Figure 21-22, the energy of IRC is highest when the first partial double bond is formed in the cyclohexadiene ring before the the formation of C-S and C-O sigma bonds, this is then followed by formation of partial bond in the exocyclic double bond and SO2, and full aromaticity is established at the end of the reaction to produce a benzene ring. Figure 21 shown is different from Figure 20 and 22 by starting off with a higher energy, because IRC calculation begin with the exo product and ends with the breaking of 2 sigma bonds to give the reactants.