NNO and NH3

A glowing hot barof metal

Mirror moves back andforth causing each wavelength to interfereat different times

IR light intensity from ALL FREQUENCIES AT ONCEinterfering with each other at a particular time

Interferogram

The Fourier Transform of the interferogram provides the spectrum of the light.

Fourier Transform is the technical term for nothing more than:All the overlap integrals of cos(2πνt) with the interferogram.

The amplitude of the overlap integral for a particular frequency = intensity of light at that frequency

The Interferometer

2.4 miles

0 500 1000 1500 2000 2500 3000 3500 40000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

H2OStretching

H2OBending

Air only (transmittance)BackgroundThis is what is subtracted

Tran

smitt

ance

Time

CO2CO2 Bending

Transmittance Spectrum of air only from our IR spectrophotometer

Wavenumbers cm-1

0 500 1000 1500 2000 2500 3000 3500 40000.0

0.5

1.0

1.5Air onlyBackgroundThis is what is subtracted

Abso

rban

ce

Wavenumbers

H2OBending

CO2

H2OStretching

CO2 Bending

Absorbance Spectrum of air only from our IR spectrophotometer

N2O and CO2N=N=O O=C=O

How different are these?Is the bonding really the same?

Degenerate

The 3N-5 = 4 normal modes for the linear moleculesNNO and CO2

Ab initio computation of vibrational modes and frequencies

CO2 vs. N2O

OCO vs. NNOBoth have 22 electrons

atomic number of C = 6atomic number of N = 7atomic number of O = 8

Start with OCOTake a proton out of left O and give to C

= NNO

1000 2000 3000

-1

0

1

2

3

4

5

6

0

time

0

NNO low pressure

500 1000 1500 2000 2500 3000 3500 4000

0.0

0.2

0.4

Abso

rban

ce

Wavenumbers

NNO higher pressure

600 800 1000 1200

0.00

0.05

0.10

0.15

Abso

rban

ce

Wavenumbers

NNO higher pressure

600 800 1000 1200

0.00

0.05

0.10

0.15

Abso

rban

ce

Wavenumbers

NNO higher pressure

Q branch: ∆J=0 for rotational motionWhy is this so intense and sharp???

Angular momentum must be conserved!The PHOTON has angular momentum = 1 h/2πSo, the molecule must increase or decrease by the

same amount.

d) break down of ∆n = + or – 1 for harmonic oscillator (recall HCl)

so many transitions with almost same ∆EJ’=J”

For a linear molecule like N2O ∆J rotational can be + or – 1 because the angular momentum change can bein the degenerate BENDING VIBRATIONS

Assignments for N2O

700 800 900 1000 1100 1200 130

0.0

0.5

1.0

1.5

2.0

2.5

3.0

NH3

abso

rban

ce

wavenumbers

Tunelling