Marshaluke
Comments
You've done a great job overall well done! However, your answers could have been elaborated more thoroughly. If you have any queries or want more detailed feedback on your computational lab, please contact Prof. Hunt.
NH3 Molecule
Calculation Data
| Log file name | LAM_OPT_NH3_POP.LOG |
| Molecule | NH3 |
| Method | RB3LYP |
| Basis Set | 6-31G(d,p) |
| Final Energy | -56.557769 |
| RMS Gradient | 1.53e-07 |
| Point Group | C3v |
Item Table
Item Value Threshold Converged?
Maximum Force 0.000000 0.000015 YES
RMS Force 0.000000 0.000010 YES
Maximum Displacement 0.000003 0.000060 YES
RMS Displacement 0.000001 0.000040 YES
Low Frequencies-- -5.6864, -3.6131, -3.6124, -0.0008, 0.0047, 0.0163 Low Frequencies-- 1089.3674, 1693.9284, 1693.9284
Optimised Structure Image
Jmol Rotateable Structure
logfile: Media:LAM_OPT_NH3_POP.LOG
Optimised NH3 Molecule |
Important Geometric Parameters
Optimised bond angle and distance for NH3
| Coord | Value |
| r(N-H) | 1.02Å |
| θ | 106° |
Vibrations
Table of vibrations:
| Mode | 1 | 2 | 3 | 4 | 5 | 6 |
| Wavenumber(cm-1) | 1089 | 1694 | 1694 | 3461 | 3590 | 3590 |
| Symmetry | A1 | E | E | A1 | E | E |
| Intensity(arbitrary units) | 145 | 14 | 14 | 1 | 0 | 0 |
Gaussian copy of IR spectra:
Questions and Answers
1. how many modes do you expect from the 3N-6 rule?
3N-6 where N=the number of atoms, N=4 so 12-6=6, therefore we expect 6 modes which we have
2. how many bands (peaks) do you see in the computed spectrum of gaseous ammonia?
only TWO vibrational bands in the spectrum! We calculate ALL the vibrations, unlike experiments where you can only "see" some of the bands due to specific vibrations.
3. which modes are degenerate (ie have the same energy)?
the E modes at 1694 and 3590 cm-1 each have 2 components which are degenerate, ie have the same energy
4. which modes have essentially no intensity?
the A1 3461 cm-1 and E 3590 cm-1 modes have essentially no intensity. Modes must include a dipole moment change to be "allowed" in an IR spectrum, both of these vibrations must have no dipole moment change
5. why are there fewer modes in the spectrum than you would predict from the 3N-6 rule?
some vibrations have an essentially zero intensity, and two of the modes are degenerate and so only appear as a single peak
6. which modes are "bending" vibrations and which are "bond stretch" vibrations?
stretches => high energy these are modes 4,5,6, bends => low energy these are modes 1,2,3
7. one mode is known as the "umbrella" mode, which one is this?
mode 1 is the umbrella mode, for obvious reasons!
8. why is the umbrella mode so intense?
this mode is very intense because there is a large dipole moment change
Charges
Image of NBO charges colour coded, red for negative, green for positive. Next to it is the scale and reference for the charge distribution. The scale is -1.125 to 1.125
Table of NH3 charges:
| Atom | Charge |
| N | -1.13 |
| H | 0.38 |
Molecular Orbitals
| Real 2A1 Orbital | LCAO Orbital |
 |
|
Project Molecule: N2F2
Calculation Data
| Log file name | LAM_N2F2_OPT_POP_fine.LOG |
| Molecule | N2F2 |
| Method | RB3LYP |
| Basis Set | 6-31G(d,p) |
| Final Energy | -309.012413 |
| RMS Gradient | 0.000003 |
| Point Group | C2v |
Item Table
Item Value Threshold Converged?
Maximum Force 0.000007 0.000015 YES
RMS Force 0.000004 0.000010 YES
Maximum Displacement 0.000014 0.000060 YES
RMS Displacement 0.000010 0.000040 YES
Low Frequencies-- -0.0023, -0.0018, -0.0016, 3.7873, 4.1532, 5.0933 Low Frequencies-- 347.8869, 561.2422, 771.6024
Optimised Structure Image
Why no bond between N and F?
Because Gaussview has a calculated distance for which a bond is formed, the calculated optimal distance between the fluorine and nitrogen is greater than the Gaussview specification removing the bond. The bond is still able to form just not within Gaussviews limits
Jmol Rotateable Structure
logfile: Media:LAM N2F2 OPT POP fine.LOG
Optimised N2F2 Molecule |
Important Geometric Parameters
Optimised bond angle and distance for N2F2
| Coord | Value |
| r(N=N) | 1.22Å |
| r(N-F) | 1.39Å |
| θ(F-N=N) | 114° |
| θ(F-N=N-F) | 0° |
Vibrations
Table of vibrations:
| Mode | 1 | 2 | 3 | 4 | 5 | 6 |
| Wavenumber(cm-1) | 348 | 561 | 772 | 949 | 987 | 1637 |
| Symmetry | A1 | A2 | B2 | A1 | B2 | A1 |
| Intensity(arbitrary units) | 1 | 0 | 75 | 75 | 81 | 21 |
Gaussian copy of IR spectra:
Vibrations Questions and Answers
1. How many vibrations are expected from the 3N-6 rule?
Where N is the number of atoms it's (3x4)-6= 6. Which is accurate to the rest of the results
2. Why are there only 4 peaks in the IR spectrum?
Because, two of the vibrations have very muted intensities making the peaks inseparable from the baseline.
3. Which vibration is the asymmetric N-F stretch?
Mode 3 with B2 symmetry is the asymmetric N-F stretching vibration.
4. What is the nature of the highest energy vibration?
It is stretching occurring between the two nitrogen atoms as they are the strongest bond as well as stretching being a high energy vibration results in this vibration being the highest in energy.
Charges
Image of NBO charges colour coded, red for negative, green for positive. Next to it is the scale and reference for the charge distribution. The scale is -0.215 to 0.215
Table of N2F2 charges:
| Atom | Charge |
| N | 0.22 |
| F | -0.22 |
Molecular Orbitals
| Real Ninth Orbital | LCAO Orbital |
 |
|
Orbital Questions and Answers
Which MO's are core orbital MO's?
Molecular orbitals 1-4 are core molecular orbitals as they don't merge with any other orbital, suggesting no bonding taking place which is characteristic of a core MO.
