Dunnmaxi

lab marking

You have a very good working wiki. It would be good if you report values of wavenumber in your answers and the formatting of spectrum could be better. Overall a very good attempt. If you have any specific questions, do email Prof. Hunt

NH₃ molecule

Method information

name of submitted log file	dunnmaxi_NH3_optf.log
molecule	NH3
method	RB3LYP
basis set	6-31G(d,p)
final energy	-56.557769
RMS gradient	1.53e-07
point group	C3v

Convergence

         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

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

NH3

Media: DUNNMAXI NH3 OPTF.LOG

Geometric parameters

Optimised bond distance and angle for NH₃

coord	value
r(N-H)	1.02Â
θ	106°

Vibrations

Low frequency values indicate whether the geometry is fully converged, the values on the first line should not be more than ± 20cm-1.

Low frequencies ---   -5.6864   -3.6131   -3.6124    0.0012    0.0049    0.0163
Low frequencies --- 1089.3674 1693.9284 1693.9284

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 (arbitary units)	145	14	14	1	0	0

IR Spectrum

Charges

image of NBO charges colour coded red for negative and green for positive the charge range is ±1.125e

table of NH₃ charges:

atom	charge
N	-1.13
H	+0.38

Q&A

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

6

• how many bands do we see in the computed spectrum of gaseous ammonia?

Two

• 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

• some modes have essentially no intensity, which ones?

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

• what is the selection rule for IR vibrations?

A vibration must change the dipole moment of the molecule to be considered as IR active

• why are there fewer modes than we expect 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

• 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

• which mode is highly symmetric?

The A1 modes are highly symmetric

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

mode 1 is the umbrella mode

• Why is the umbrella mode so intense?

this mode is very intense because there is a large dipole moment change

N₂F₂ molecule

Method information

name of submitted log file	dunnmaxi_NH3_optf.log
molecule	N2F2
method	RB3LYP
basis set	6-31G(d,p)
final energy	-309.012413
RMS gradient	3.17e-07
point group	C2v

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

Geometric parameters

Optimised bond distance and angle for N₂F₂

coord	value
r(N-F)	1.39Â
r(N-N)	1.22Â
θ	114°

Optimised molecule

N2F2

Media:DUNNMAXI N2F2 OPTF.LOG

Q&A

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

This is because the Gauss program has specific criteria for the distances between nuclei for bonds to show in the 3D image. The bond length of 1.39Â isn't within the criteria range, even though we know there is a bond present.

Vibrations

Low frequency values indicate whether the geometry is fully converged, the values on the first line should not be more than ± 20cm-1.

Low frequencies (cm-1) ---   -0.0004   -0.0004    0.0002    3.2233    4.3533    5.0998
Low frequencies (cm-1) ---  347.8772  561.2472  771.6105

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 (arbitary units)	1	0	75	75	81	21

IR Spectrum

Q&A

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

6 vibrational modes

• Why are there only 4 peaks in the IR Spectrum?

some vibrations have an intensity very close to 0, and two of the modes are degenerate and so only appear as a single peak.

• Which vibration is the asymmetric N-F stretch?

Vibration number 3. Frequency of 771.61 and intensity of 74.7044

• what is the nature of the highest energy vibration?

The highest frequency vibration is number 6, frequency of 1636.61, intensity of 20.5391. It is an N-N stretch

Charges

image of NBO charges colour coded red for negative and green for positive the charge range is ±0.215e

table of NH₃ charges:

atom	charge
N	+0.215
F	-0.215

Molecular Orbitals

Q&A

• which MOs are core orbital MOs?

The first 4 (MO 1-4, as they are very close to the nucleus

Image of MO 9:

LCAO diagram: