In this exercise we will characterize a reaction path on the B3LYP/6-31G* potential energy surface for the unimolecular elimination of hydrogen from amine-borane. We will begin with the determination of the transition state structure and then proceed to confirm that the transition state structure leads to H₂NBH₂ and H₂ in the forward direction and H₃BNH₃ in the reverse direction. Finally, the energy barriers will be computed for the forward and reverse reactions.

Procedure:

1. Build a model of the eclipsed conformer of amine-borane, a model of H₂ and a model of aminoborane. Optimized each structure at the B3LYP/6-31G* level. Be sure to compute IR frequencies and print thermodynamic quantities for each structure to the output file.





2. Propose and build a model of the transition state structure for the elimination of H₂ from amine-borane. Use the B3LYP method with the 6-31G* basis set to optimize the "Transition State Geometry." Compute the IR frequencies and print the thermodynamic quantities to the output file. See the Tutorial for the Construction of a Transition State Structure.


Questions:


1. How many imaginary frequencies (frequencies with i or a negative values) did you find?
2. Does the normal mode of vibration for the imaginary frequency resemble the motion of the atoms along the reactions path?




3. Use the "Spartan Coordinate Drive Program" to calculate an Energy Profile and confirm that the transition state leads to H₂NBH₂ and H₂ in the forward direction and reforms H₃ BNH₃ in the reverse direction. Make a copy of the transition state file from Procedure B and labeled it h_elimin_ts_ep_f.Spartan. Constrain the distance between the nitrogen and hydrogen atoms so that this distance may be varied incrementally between the value found for the transition state and 2.000 Å in eight steps. Perform an "Energy Profile" calculation with the B3LYP method and the 6-31G* basis set. The "Spartan Coordinate Drive Program" optimizes the geometry and calculates the electronic energy at each of the eight constrained N-C distances. Next, prepare a "Spreadsheet" that lists the electronic energy (kJ/mol) for each of the eight steps in one column and the corresponding "constrain distance" in another column. "Plot" utilizes the data in the "Spreadsheet" to construct a graph of the potential energy along the reaction coordinate. Label the y-axis of the plot E (kJ/mol) and the x-axis Constraint (N-H distances). Finally, view the animation of the reaction coordinate to confirm that the transition state does eliminate H₂ and form H₂NBH₂. Follow a similar procedure to verify that amine-borane is formed from the activated complex in the reverse direction. See the Tutorial for the Energy Profile Calculation.


Questions:


1. Does the transition state found in Procedure B eliminate H₂ and form H₂BNH₂? Please include a printout of the potential energy plot for the forward direction.
2. Is the H₂BNH₂ molecule structurely similar to the optimized geometry found for aminoborane in the Procedure A?
3. Does the transition state found in Procedure B form the eclipsed conformer of H₃BNH₃ in the reverse direction? Please include a printout of the potential energy plot for the reverse direction.
4. Is the H₃BNH₃ molecule structurely similar to the optimized geometry found for the eclipsed conformer of amine-borane in the Procedure A exercise?




4. Calculate the energy barrier for the elimination of H₂ from amine-borane in the forward and reverse directions.




Use the conversion factor 2625.4997 kJ/mol-hartree to express the electronic energies, E_e , in kJ/mol. Also, the zero-point energies, E₀⁰ must be scaled. The scaling factor is 0.9804. Please include your values in the table below.




Species	Ee (hartrees)	Ee (kJ/mol)	E00	E00 (scaled)
H2
H2BNH2
H3BNH3
Transition State



Questions:


1. What is the energy barrier to the forward reaction (elimination of H₂ from H₃BNH₃)?
2. What is the energy barrier to the reverse reaction (addition of H₂ to H₂BNH₂)?