Ammonia
Vibrational Analysis
Now we are going to calculate and visualise the vibrational spectrum for NH3. If you are doing the Symmetry and Spectroscopy course this section will be very interesting for you!
- carry out a frequency analysis for the optimised B3LYP/6-31G C3v and D3h structures.
- you should be getting to grips with Gaussview by now, as a hint, open the relevant log file, and choose Calculate then Gaussian, and then in the Job Type tab choose frequency. When you save the job give it a new name so you don't overwrite your optimisation file.
- animate the vibrational frequencies and look at the spectra for both compounds, record the vibrational frequencies and take a snap-shot of the IR spectrum to include in your wiki. Look at the individual vibrations on the screen, but don't reproduce the on your wiki ... it will take too long. However, you should notice that they look very similar to those of BH3!
- answer the following questions in your wiki:
- how many positive frequencies are there for the C3v and the D3h structures?
- draw the vibrational modes for both structures (visualise them with gaussview) and match them up, ie which vibrational modes have the same character of motion for the C3v and D3h structures
- compare the calculated frequencies for the C3v structure to those obtained experimentally (you will have to look them up). The C3v structure is the ground state structure, ground state structures always have all positive frequencies.
- The D3h structure is a transition state structure, transition states always have only one negative frequency, what is the value of negative frequency?
- which vibration in the C3v and D3h structures "follows" the inversion reaction path?