* not completed

. One of the following reactions is unimolecular, while the others are bimolecular. Identify the unimolecular reaction.

C₆H₅SNa + C₂H₅Cl → C₆H₅SC₂H₅ + NaCl correct incorrect

NO₂⁺ + C₆H₆ → [C₆H₆-NO₂]⁺ correct incorrect

CH₃-CH(BH₂)-CH₃ → CH₃-CH₂-CH₂-BH₂ correct incorrect

CH₃-CH=CH₂ + O₃ → ozonide correct incorrect

* not completed

. Which of the following reactions or processes would necessarily involve a potential energy barrier? (The others could involve uniformly attractive potential energy surfaces.)

Cr(CO)₅ + PPh₃ → (OC)₅Cr−PPh₃ correct incorrect

CH₃ + C₂H₅ → CH₃-CH₂-CH₃ correct incorrect

CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH correct incorrect

CH₃NH₃⁺ + CH₃OH → CH₃NH₃⁺…HOCH₃ correct incorrect

* not completed

. To characterize the potential energy surface for the rearrangement of hydrocyanic acid HCN to hydrogen isocyanide HNC, ab initio calculations are performed for different values of the H-C distance, the C-N distance, and the H-C-N angle. Assuming 5 different C-N distances, 15 different H-C distances, and 15 different angles are considered, how many calculations are needed in total?

1,125 correct incorrect

35 correct incorrect

90 correct incorrect

232 correct incorrect

* not completed

. Considering the steepest descent optimization algorithm and second-order methods such as that suggested by Broyden, Fletcher, Goldfard and Shanno (BFGS), which of the following statements is not correct?

Both the steepest descent and BFGS method require access to the energy and the gradient of the energy. correct incorrect

The BFGS method is so complicated that in most cases, it is easier just to use the steepest descent method despite the fact that it is less efficient. correct incorrect

The steepest descent method does not rapidly converge to a point very close to the minimum in most cases. correct incorrect

The steepest descent method is simpler and thereby easier to program. correct incorrect

* not completed

. Geometry optimization can be carried out using Cartesian coordinates, a minimal set of internal coordinates, or redundant internal coordinates. In the case of ethanol C₂H₅OH, what are the minimum number of coordinates needed to define the internal structure, the number of Cartesian coordinates, and the number of redundant internal coordinates, respectively?

21 internal coordinates, 27 Cartesian coordinates and 120 redundant internal coordinates, respectively. correct incorrect

27, 27 and 27. correct incorrect

21, 27 and 33. correct incorrect

21, 27 and 27. correct incorrect

* not completed

. For a non-linear molecule with N atoms, there are 3N−6 vibrational frequencies. At a transition state structure, one of these 'frequencies' corresponds to the reaction coordinate, so there is one fewer vibrational frequency. How many vibrational frequencies will there be for the TS of the Diels-Alder cycloaddition of ethene and 1,3-butadiene?

16 correct incorrect

41 correct incorrect

48 correct incorrect

9 correct incorrect

* not completed

. Which of the following is not a method to locate transition states structures?

Quasi-synchronous transit method correct incorrect

Relaxed scan method correct incorrect

Nudged elastic band method correct incorrect

Periodic boundary conditions correct incorrect

* not completed

. There are three different minima on the potential energy surface for n-butane, due to rotation around the central C^B-C^C bond (we neglect chemically identical conformers obtained by rotating around the C^A-C^B and C^C-C^D bonds). How many different conformers do you expect for hexane?

9 correct incorrect

27 correct incorrect

3 correct incorrect

18 correct incorrect

* not completed

. In order to locate transition state structures, a scan along some coordinates, as shown in Fig. 5.15, is frequently used. Consider the Cope rearrangement of 1,5-hexadiene H₂C^A=C^BH-C^CH₂-C^DH₂-HC^E=C^FH₂ to an isomeric form H₂C^D=C^EH-C^FH₂-C^AH₂-HC^B=C^CH₂. Which of the following coordinates could be scanned in order to locate the region of the transition state?

The distance between C^A and C^B correct incorrect

The distance between C^A and C^C correct incorrect

The distance between C^A and C^E correct incorrect

The distance between C^A and C^F correct incorrect

* not completed

. Which of the following is not a property of transition state structures on a potential energy surface?

TSs are relatively low energy points separating one low-energy region on the surface from another region. correct incorrect

Multiple different transition state structures can exist separating two low-energy regions. correct incorrect

The curvature of the potential energy surface is positive in all directions at the TS structure. correct incorrect

The Hessian matrix has one negative eigenvalue at the TS structure. correct incorrect