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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 NH Molecule

Important Geometric Parameters

Optimised bond angle and distance for NH₃

Coord	Value
r(N-H)	1.02&Acirc
&theta	106&degree

Vibrations

Table of vibrations:

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(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? With N being the no. of atoms present, this being 4 in this case. Means that the modes of this molecule would be equal to (3x4)-6 = 6. Suggesting correctly that there are 6 modes

2. how many bands (peaks) do you see in the computed spectrum of gaseous ammonia? There are only two bands present as the other modes have little to no intensity

3. which modes are degenerate (ie have the same energy)? Modes 2 & 3, as well as 5 & 6 are degenerate.

4. which modes have essentially no intensity? Modes 4-6 have intensities lesser than 2 which renders them insignificant on the spectrum

5. why are there fewer modes in the spectrum than you would predict from the 3N-6 rule? Because the degeneracy of some of them cause them to overlap and intensify as well as some of them not being intense enough to visualise

6. which modes are "bending" vibrations and which are "bond stretch" vibrations? 1-3 are bending vibrations, while 4-6 are stretching vibrations as stretching is high energy and rare.

7. one mode is known as the "umbrella" mode, which one is this? This is mode 1 as it's vibration looks similar to that of an umbrella caught in wind and at normal state.

8. why is the umbrella mode so intense? It causes 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