Brosnanate

lab marking

You have a good working wiki. It would be good if you report all the method information and round off reported values to correct significant figures. Overall, a good attempt. If you have any specific questions, do email Prof. Hunt

NH₃ Molecule

Calculation Data

Name of Submitted Log File	NATEBROSNAHAN_nh3_optf.log
Molecule	NH3
Basis Set	6-31G (d,p)
Final Energy	-56.557769 Hartree
RMS Gradient	0.000000
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

Optimised molecule image

Jmol Rotateable Molecule

logfile: Media:NATEBROSNAHAN_NH3_OPTIMISATION.LOG

Optimised NH3 Molecule

Important Geometric Parameters

Optimised bond distance and angle for NH₃
r(N-H)=1.018Â
θ(H-N-H)=106°

Low Frequencies (cm^-1)

Low frequencies ---   -5.6864   -3.6131   -3.6124   -0.0014    0.0045    0.0162
Low frequencies --- 1089.3674 1693.9284 1693.9284

Wavenumber, Symmetry, Intensity Table

Number	Vibration (cm^-1)	Symmetry	Intensities (km/mol)
1	1089	A1	145
2	1694	E	14
3	1694	E	14
4	3461	A1	1
5	3590	E	0
6	3590	E	0

IR Spectrum

Atomic Charge and Color Range

Atomic Charge Table

Atom	Atomic Charge (e)
Nitrogen	-1.13
Hydrogen	0.38
Hydrogen	0.38
Hydrogen	0.38

Project Molecule N₂F₂

Calculation Data

Name of Submitted File	NATEBROSNAHAN_N2F2_OPTIMISATION.LOG
Molecule	N2F2
Method	RB3LYP
Basis Set	6-31G(d,p)
Final Energy	-309.012413 Hartree
RMS Gradient	0.000000
Point group	C2v

Item Table

 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

Optimised N₂F₂ image

Jmol Rotateable N₂F₂ image

logfile: Media:NATEBROSNAHAN N2F2 OPTIMISATION.LOG

Optimised N2F2 Molecule

Important Geometric Paramaters for my Optimised N₂F₂

Optimised bond distance and angle for N₂F₂
r(N=N)=1.220Â
r(F-N)=1.391Â
θ(F-N-N)=114°
θ(F-N-N-F)=0°

Low Frequencies (cm^-1) for N₂F₂

Low frequencies ---   -0.0007    0.0011    0.0015    3.2225    4.3532    5.1001
Low frequencies ---  347.8772  561.2472  771.6105

Wavenumber, Symmetry, Intensity Table for my Optimised N₂F₂

Number	Vibration (cm^-1)	Symmetry	Intensities (km/mol)
1	348	A1	1
2	561	A2	0
3	772	B2	75
4	949	A1	75
5	987	B2	81
6	1637	A1	21

IR Spectrum for my Optimised N₂F₂

Questions:

How many vibrations are expected from the 3N-6 rule?

For a molecule with 4 atoms (N₂F₂), the number of vibrational modes is given by 3N-6. Substituting N=4, we get 3(4)- 6 = 6, so molecule has 6 vibrational modes.

Why are there only 4 peaks in the IR spectrum?

Although 6 vibrational modes are predicted, only 4 peaks appear, as two modes are essentially undetectable. Mode 2 at 561cm^-1(A2) has zero intensity thus IR-inactive. Mode 1 at 348cm^-1 (A1) has an intensity of 1, which is so low its barely visible (red spot on spectrum). Hence only 4 peaks seen.

Which vibration is the asymmetric N-F stretch?

The asymmetric N-F stretch corresponds to Mode 5 at 987cm^-1. This is identified as the asymmetric stetch as it has a higher intensity compared to the symmetric N-F stretch, which appears at 949cm^-1 (Mode 4). Mode 5 has B2 symmetry, meaning the two N-F bonds vibrate out of phase with each other. In contast, the symmetric stretch involves both bonds moving in phase. This out-of-phase motion produces a larger change in dipole moment, leading to a stronger IR signal and a slightly higher vibrational frequency.

What is the nature of the highest energy vibration?

The highest energy vibration corresponds to the N=N vibrational mode. This is because it involves the strongest bond in the molecule, the rigid N=N double bond, thus has largest force constant. The vibration has A1 symmetry, meaning it is fully symmetric with respect to the molecules symmetry elements. Its moderate IR intensity (21) indicates a relatively small change in dipole moment during the motion.

Atomic Charge and Color Range for my Optimised N₂F₂

Question:

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

Gaussview determines bonds based on typical interatomic distances. So, the N-F bond distance has become longer than the programs default threshold for displaying a bond, so no bond is shown even though an interaction may still exist. The N=N bond remains shorter and falls within detecting limit, hence visible.

Atomic Charge Table

Atom	Atomic Charge (e)
Nitrogen	0.22
Nitrogen	0.22
Fluorine	-0.22
Fluorine	-0.22

MO Analysis of Optimised N₂F₂

Image of Molecular Orbital 9

Question:

Which MOs are core orbital MOs?

MOs 1-4 are the core orbitals because they originate from the 1s atomic orbitals of each atom. The 1s atomic orbitals are very low in energy and remain highly localised near the nuclei. Thus do not significantly contribute to bonding and are effectively non-bonding

LCAO Diagram for MO9