Crossaemma

Comments

It's great that you have a working wiki—you did a good job. However, the wavenumber and intensity values were rounded incorrectly—please double-check those for accuracy. If you have any queries or want more detailed feedback on your computational lab, please contact Prof. Hunt.

NH₃ Molecule

Calculation Data

Key Information of NH₃ Optimisation

Name of Sumbitted log	NH3_optf.log
Molecule	NH3
Method	RB3LYP
Basis Set	6-31G(d,p)
Final Energy	-56.557769
RMS Gradient	0.000003
Point Group	C3V

Convergence

Item table confirming forces and displacements are converged.

        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

Confirming a minimum, the CoM motion is near zero

Low frequencies ---   -5.6864   -3.6131   -3.6124    0.0017    0.0048    0.0162
 Low frequencies --- 1089.3674 1693.9284 1693.9284

Optimised Molecule Image

Static Image:

Rotateable Molecule: Logfile: Media:ECROSAN_NH3_OPTF_POP.LOG

Optimised NH Molecule

Important Gemoetric Parameters

Optimised bond distance / angles of NH₃

Coord	Value
Bond Length (N-H)	1.02Â
Bond Angle θ (H-N-H)	106°

Vibrations

Vibrations of NH₃

Mode	1	2	3	4	5	6
Wavenumber (cm-1)	1089.37	1693.93	1693.93	3461.38	3589.93	3589.93
Symmetry	A1	E	E	A1	E	E
Intensity	145	14	14	1	0	0

Computed Spectrum

Questions to Answer

How many modes do you expect from the 3N-6 rule?

Using the 3N-6 rule, where N=the number of atoms: N=4 so 12-6=6, therefore we expect 6 modes.

How many bands (peaks) do you see in the computed spectrum of gaseous ammonia?

Only two vibrational bands are observed in the computed IR spectrum. While computational methods calculate all vibrational modes, only those involving a dipole moment change are IR active, and thus appear as observable peaks.

Which modes are degenerate (ie have the same energy)?

The E modes at 1694 cm^-1 and 3590 cm^-1 are degenerate, as each consists of two vibrations with the same energy.

Which modes have essentially no intensity?

The A₁ mode at 3461 cm^-1 and the E mode at 3590 cm^-1 show essentially no intensity in the IR spectrum because they do not involve a change in dipole moment, which is required for a vibration to be IR active.

What is the selection rule for IR vibrations?

A vibrational mode is IR active only if it involves a change in the molecular dipole moment. If the dipole moment of the molecule changes (the separation of positive and negative charge varies), the vibration can absorb infrared radiation and will appear in the IR spectrum.

Why are there fewer modes in the spectrum than you would predict from the 3N-6 rule?

In the spectrum, there are less than the 6 calculated modes because some modes have very low or no IR intensity, and degenerate modes (e.g the E modes) appear as a single peak.

Which modes are "bending" vibrations and which are "bond stretch" vibrations?

"Bending" vibrations correspond to lower energy modes 1, 2, and 3; "bond stretch" vibrations correspond to higher energy modes 4, 5, and 6.

One mode is known as the "umbrella" mode, which one is this?

Mode 1 is known as the umbrella mode as it resembles an umbrella folding up and down.

Why is the umbrella mode so intense?

The umbrella mode is intense because it involves a large dipole moment and is thus very IR active.

Charges

NH₃ Charges

Atom	Charge
N	-1.13
H	+0.38

Molecular Orbitals

NH₃ Molecular Orbitals

Real 2a1 MO	LCAO MO

Project Molecule: N₂F₂

Calculation Data

Information of N₂F₂ Optimisation

Name of Sumbitted log	N2F2_optf.log
Molecule	N2F2
Method	RB3LYP
Basis Set	6-31G(d,p)
Final Energy	-309.01241
RMS Gradient	0.00000
Point Group	C2V

Convergence

Data from log confirming forces and displacements are converged

         Item               Value     Threshold  Converged?
 Maximum Force            0.000001     0.000015     YES
 RMS     Force            0.000000     0.000010     YES
 Maximum Displacement     0.000001     0.000060     YES
 RMS     Displacement     0.000001     0.000040     YES

Low Frequencies from log

Low frequencies ---   -0.0017   -0.0013   -0.0005    3.2224    4.3531    5.1001
 Low frequencies ---  347.8772  561.2472  771.6105

Optimised Molecule

Static Image:

Jmol Rotateable Molecule: Logfile: Media:ECROSAN_N2F2_OPTF_POP.LOG

Optimised NF Molecule

Geometric Parameters

Optimised bond angles for N₂F₂

Optimised bond distance and angle of N₂F₂

Coord	Value
Bond Length (N-F)	1.4Â
Bond Length (N-N)	1.2Â
Bond Angle θ (N-N-F)	114°

Vibrations

Vibrations of N₂F₂

Mode	1	2	3	4	5	6
Wavenumber	347.88	561.25	771.61	949.45	987.25	1636.61
Symmetry	A1	A2	B2	A1	B2	A1
Intensity	0	0	74	75	81	20

Computed Spectrum

Questions To Answer

The molecule from the log file does not have bonds between the F and N atoms, what is going on here?

The bond distances are likely too far away for the computer program to register. Other possible reasons could be:

- Unstable simulation

- Incorrect or incomplete simulation

- Atoms are too far apart

How many vibrations do you expect from the 3N-6 rule?

4 atoms in N₂F₂

3(4)-6=6

6 vibrational modes.

Why are there only 4 peaks in the IR spectrum?

Some normal modes may be degenerate (two modes of nearly identical energy overlap into a single band) or some may be IR inactive. In the case of N₂F₂, the two lowest vibrations at 347.88 and 561.25 have very low values or no infrared, meaning they are not visible in the spectra.

Which vibration is the asymmetric N-F stretch?

Mode 4, at a frequency of 949.45 is associated with N-F stretching.

What is the nature of the highest energy vibration?

Generally, stretching vibrations occur at higher wavenumbers compared to bending ones. Stronger bonds = higher wavenumber

In this molecule, the highest energy vibration is associated with N=N stretching.

Which MOs are core orbital MOs

Orbitals 1-4 are core bonding orbitals, who are not affected by reactions.

Charges

NBO charges colour coded green for positive and red for negative. The charge range is +/- 0.215

Colour Range:

N₂F₂ Charges

Atom	Charge
N	+0.22
F	-0.22

Molecular Orbitals

Molecular orbitals (MOs) 1-4 are core orbitals.

N₂F₂ Molecular Orbitals

MO 9	LCAO MO